Standard for GeotechnicalSite and Route SurveysMinimum Requirements for the Foundationof Offshore Wind Turbines

Dr. Roland AtzlerNautik Nord GmbH, Pohnsdorf

Dr. Alexander BartholomäForschungsinstitut Senckenberg am Meer,Wilhelmshaven

Prof. Dipl.-Ing. Horst BellmerProf. Bellmer Ingenieurgruppe GmbH, Bremen

Dipl.-Ing. Jost BergmannDet Norske Veritas, Hamburg

Dipl.-Ing. Ulrich HachmannGermanischer Lloyd Offshore and Industrial Services GmbH, Hamburg

Prof. Dr.-Ing. Harry HarderInstitut für Geotechnik, Hochschule Bremen

Dipl.-Ing. Marcus KloseGermanischer Lloyd WindEnergie GmbH, Hamburg

Dr. habil. Wolfram LemkeInstitut für Ostseeforschung, Rostock-Warnemünde

Dr.-Ing. Kerstin LesnyInstitut für Grundbau und Bodenmechanik,Universität Duisburg-Essen

Dr. Klaus MichelsOSAE Offshore Survey and Engineering,Gesellschaft für Seevermessung mbH, Bremen

Peter Petersen, M.Sc.Det Norske Veritas, Hellerup (Dänemark)

Dr. Lutz ReinhardBundesanstalt für Geowissenschaften und Rohstoffe,Hannover

Prof. Dr.-Ing. Werner RichwienInstitut für Grundbau und Bodenmechanik,Universität Duisburg-Essen

Knut Ronold, Ph.D.Det Norske Veritas, Høvik (Norwegen)

Dr. Klaus SchwarzerInstitut für Geowissenschaften, Universität Kiel

Prof. Dr. Volkhard SpiessFachbereich Geowissenschaften, Universität Bremen

Dr. Hansjörg StreifNiedersächsisches Landesamt für Bodenforschung,Hannover

Dipl.-Ing. Jörn UeckerIMS Ingenieurgesellschaft, Hamburg

Standard for Geotechnical Site andRoute SurveysMinimum Requirements for the Foundation of Offshore WindTurbines and Power Cable Route Burial Assessments

Status: August 1st, 2003

Issued by Bundesamt für Seeschifffahrt und Hydrographie (BSH) in co-operation with

© Bundesamt für Seeschifffahrt und Hydrographie (BSH)Hamburg und Rostock 2003www.bsh.de

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system,or transmitted in any form or by any means, electronic, mechanical, photocopying, recording orotherwise, without the prior written permission of the BSH.

C o n t e n t s 3

Section A Introduction 7

A.1 Preface 7

A.2 General 7

A.3 Standard for Geotechnical Site Surveys for Offshore Wind Turbines 8

A.4 Deviation from the Standard 11

A.5 Updating 11

Section B Minimum Requirements forthe Geological-GeophysicalSurveying 12

B.1 General 12

B.2 Quality Assurance 12

B.3 Time Schedule 13

B.4 Objectives 13

B.4.1 Preliminary Reconnaissance Phase 13

B.4.2 Planning Phase 13

B.4.3 Monitoring Phase 14

B.5 Technical Instructions 14

B.6 Power Cable Route Burial Assessment 20

B.7 Geological Report 20

B.7.1 Purpose 20

B.7.2 Contents 20

B.7.3 Requirements 20

C o n t e n t s

C o n t e n t s4

Section C Minimum Requirements for theGeotechnical Site Surveying as aBasis for Planning/Designing theOffshore Wind Farm 21

C.1 General 21

C.2 Quality Assurance 21

C.3 Requirements for Field Testing 21

C.3.1 Drilling and Penetration Tests 21

C.3.2 Field Testing for Pile Foundations 22

C.3.3 Field Testing for Gravity Foundations 23

C.3.4 Requirements for Soil Sampling 24

C.4 Requirements for Laboratory Tests 25

C.4.1 Laboratory Tests on Board Survey Vessels 25

C.4.2 Onshore Laboratory Tests 25

C.5 Geotechnical Site Survey Report 28

C.5.1 Contents of the Geotechnical Site Survey Report 28

C.5.2 Presentation of the Results 28

C.5.2.1 Field Tests 28

C.5.2.2 Laboratory Tests 29

C.5.3 Geotechnical Site Description 29

C.5.4 General Geotechnical Site Assessment 29

C.6 Soil and Foundation Expertise 29

C.6.1 Contents of the Soil and Foundation Expertise 29

C.6.2 Information Provided by the Soil and Foundation Expertise 29

C.7 Supplementary Investigation (Tendering Phase) 30

C.8 Monitoring in the Construction Phase 30

C.9 Monitoring in the Operation Phase 30

C.10 Literature 31

C o n t e n t s 5

Figures

Fig. A.1 Overview of the Individual Phases and ReconnaissanceMeasures 9

Fig. C.1 Cyclic Direct Shear Test 27

Fig. C.2 Cyclic Compression Test 27

Fig. C.3 Cyclic Triaxial Test 28

Tables

Table A.1 Type and Minimum Scope of Reconnaissance for the Planning,Construction, and Operation Phases 10

Table B.1 Requirements for Echosounder Surveys (Single-Beam andMultibeam Echosounding) 15

Table B.2 Requirements for Side Scan Sonar (SSS) Surveys 16

Table B.3 Requirements for Seismic Surveys 17

Table B.4 Requirements for Borehole Drilling 18

Table B.5 Requirements for Magnetometers and Active Metal Detection Systems(Recommended) 18

Table B.6 Requirements for the Power Cable Route Survey 19

Table C.1 Field Testing for Pile Foundation 22

Table C.2 Exploration Depth and Sampling in Field Testing for Pile Foundations(Minimum Requirements) 23

Table C.3 Exploration Depth and Sampling in Field Testing for Gravity Foundations(Minimum Requirements) 24

Table C.4 Laboratory Tests for Evaluation of Cohesionless Soils 26

Table C.5 Laboratory Tests for Evaluation of Cohesive Soils 26

S e c t i o n A 7

Section A Introduction

A.1 Preface

Within the framework of the approval procedures foroffshore wind farms in the Exclusive Economic Zone(EEZ), developers submitting applications are re-quired to furnish proof of the installations' structuralsafety and use of approved state-of-the-art technolo-gy (cf. Art. 5, para. 2, Marine Ordance Facility(Seeanlagenverordnung, SeeAnlV)). Up to now, geo-technical site investigations as a technical prerequi-site to the construction of foundations for offshorewind turbines have been subject to standards issuedby classification companies and to a number of tech-nical instructions, which differed, however, at least inpart aspects. As, on the one hand, the high cost ofsuch installations had caused companies to discusspotential conflicts between safety requirements andeconomic viability and, on the other hand, the ap-proval authority considered the safety of some instal-lations unsatisfactory – depending on the standardused – the present standard has been developedunder the guidance of the BSH and introduced as atechnical standard according to Art. 4, para. 2(SeeAnlV). Compliance with the standard will berequired in the approvals granted under SeeAnlV. Thepurpose of the standardisation of requirements is toensure legal certainty and investment security, and toprovide the approval authority being committed to theprinciple of equal treatment, with a valuable tool in itsdecisionmaking process regarding offshore wind farmapprovals.

The present Standard for Geotechnical Site Surveysfor offshore windfarms, which has been issued by theapproval authority, specifies the minimum require-ments to be met when performing the mandatory geo-logical and geophysical site investigations. The stand-ard is based on a well-researched proposal made bya group of engineers and geoscience experts fromvarious universities, authorities, engineering andclassification companies. It takes into account experi-ence that has been gained with offshore wind farmsin other European countries (Denmark and Sweden)as well as preliminary results of the offshore wind

farm research group “Gigawind“, which has beenfunded by the Federal Ministry of Education andResearch (BMBF). It defines the thematic and techni-cal minimum requirements for geotechnical site inves-tigations prior to the erection of offshore wind turbinesand their foundations.

It should be noted that this standard represents theoutcome of discussions characterised by a high levelof commitment and expertise. If individual opinionsand concepts discussed in the course of the deci-sionmaking process have not been taken into consid-eration, this does not imply any criticism of suchopinions but only means that the approval authority,after consultation with the experts, has chosen one ofseveral possible solutions or has allowed alternativesconsidered suitable for the procedure.

The present standard is open to development, i.e. theconsiderable number of new experience and findingsexpected to be available in the near future will beevaluated and incorporated in the standard as requir-ed.

A.2 General

Offshore wind turbines are construction projects in-volving a high level of geotechnical difficulty. Apartfrom constructional aspects and load cases, the soilconditions play an crucial role. Contrary to commonopinion, the seabed in the EEZ in the North Sea andBaltic is not a homogeneous body of sediment butmay be highly heterogeneous on regional and localscales. Unlike the steel or concrete structure of theturbines, the soil properties of the seabed cannot bechanged to fit the structure. Therefore, exact knowl-edge of the geological and geotechnical properties ofthe seabed are indispensable if a wind farm project isto be successfully implemented.

A geological model of the seabed structure has to bedeveloped as a basis for subsequent phases of siteinvestigation, planning, and project implementation. Acomprehensive geological/geophysical survey pro-gramme may allow reconnaissance measures in theplanning phase to be reduced and allows the identifi-

S e c t i o n A8

cation of alternative sites for offshore turbines in seaareas with unfavourable soil properties.

The design of offshore wind turbine foundations re-quires comprehensive knowledge of the seabed properties and geotechnical parameters at each indi-vidual site. It must be ensured by the scope of thegeotechnical site investigations that all soil propertiesrelevant to planning are determined well before instal-lation of the turbines. Therefore, a geotechnical siteinvestigation and evaluation by qualified experts ismandatory.

The present standard prescribes a investigation pro-gramme for the planning and construction of offshorewind turbines which is graded in type and scope tomeet the requirements of the individual planning andconstruction phases and defines the minimum re-quirements.

Fig. A.1 illustrates the integration of appropriatephases of the site survey and assessment into theplanning and approval process of offshore windturbines.

The following phases are distinguished:

• Preliminary activities ➱ Preliminary Geological Report• Preliminary reconnaissance ➱ Geological Report• Site investigation ➱ Geotechnical Site Survey Report

Update of Geological Report• Foundation design ➱ Soil and Foundation Expertise• Invitation to bid for foundation work• Construction of foundations ➱ Records and evaluation• Monitoring of foundation work ➱ Reports and evaluation• Monitoring of plant operation ➱ Reports and evaluation

A.3 Standard for Geotechnical Site Surveysfor Offshore Wind Turbines

The Standard is based on geological, geophysical,and geotechnical field and laboratory studies. It has tobe adjusted to the planned foundation concept, withadequate consideration of the difficulties of foundationdesign, on the one hand, and soil properties andother conditions, on the other hand. The scope ofgeotechnical site surveys shall be such that all soilproperties that are relevant to planning are deter-mined well before the turbine installation. The investi-gation methods used comprise:

• geophysical methods and• geotechnical methods.

Geophysical methods comprise indirect methods(sonar, seismics, echosounding, etc). Geotechnicalmethods comprise direct exploration methods, mainly

bottom sampling (drilling) and indirect explorationmethods (penetration tests), field testing such as thevane tests, borehole pressiometer tests as well aspile-driving tests or pile test loading.

The soil characteristics are generally determined inlaboratory tests, rarely in field testing. The type andscope of investigations to be carried out in the indi-vidual planning and construction phases have beencompiled in Table A.1.

The concept presented in this guideline defines theminimum scope of investigations that is normallyrequired.

Requirements for geotechnical site investigations foroffshore windfarms have been defined in DIN 4020(drafted in 2002). Besides, requirements for geotech-nical site investigations are part of the relevant stand-ards for offshore structures (Det Norske Veritas

S e c t i o n A 9

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S e c t i o n A10

Table A.1: Type and Minimum Scope of Reconnaissance for the Planning, Constructionand Operation Phases

Phase Purpose of reconnaissance Type of reconnaissance

1 Preliminaryreconnaissanceof the area

• Site selection and preliminary planningof the structures

• The purpose of preliminaryreconnaissance is decisionmaking onwhether or not the soil properties in aparticular area are suitable for theerection of the planned wind farmstructures, determination of generalrequirements for the foundationconcepts, design and construction, andof the required site reconnaissancemeasures

• Survey and evaluation of availabledocumentation

• Geophysical reconnaissance in theentire wind farm area

• Spot reconnaissance through direct andindirect exploration (rough grid coveringthe wind farm area)

• Spot checks of relevant foundation soilcharacteristics and properties

2 Sitereconnaissance

• The scope of soil reconnaissance andinvestigation and the methods to beused depend on the type, size, andimportance of wind turbine design,homogeneity of the foundation soil,morphology of the seabed andsediment types encountered

• When investigating the area, possiblealterations of site planning must betaken into account

• The soil composition and sedimentcharacteristics must be determined ateach site

• Survey and evaluation of availabledocumentation

• Geophysical site reconnaissance

• Direct exploration by drilling boreholesat the turbine sites

• Indirect exploration by performingpenetration tests at the turbine sites

• Laboratory testing of sediment samplesfrom the sites

3 Foundationdesign

• Design of structure

• The investigation requirements dependon the foundation type chosen. Typeand scope of the investigations must besuitable for the determination of alldimensions of the foundation and forfurnishing complete proof of turbinestability and serviceability

• Additional direct exploration at the sitesof the foundation elements

• Additional indirect exploration at thesites of the foundation elements

• Laboratory testing of sediment samplesfrom the sites

4 Invitation to bidfor foundationwork

• Contract specification must contain acomplete, detailed description of thefoundation work to be performed

• Field tests, e.g. pile driving and pileloading tests may be included

5 Construction offoundations

• Production of foundation elements

• Checking of foundation soil propertiesfor compliance with the design;monitoring of production of foundationstructures; monitoring of buildup andreduction of excess pore waterpressure; monitoring of settling andtilting of foundation structures

• Pile driving record or report, productionreport in the case of site-mixedconcrete piles

• Deformation measurements

• Measurement of excess pore waterpressure

6 Monitoring ofturbineoperation

• Checking of turbine behaviour underoperational loads

• Creation of possibility to counter-actturbine behaviour that is not incompliance with the design

• Monitoring of sediment dynamics oncable routes inside and outside thewind farm area

• Deformation measurements onselected turbines in the wind farm

• Monitoring of scouring at all foundationstructures at regular intervals

The soil characteristics are generally determined in laboratory tests, rarely in field testing.

The type and scope of investigations to be carried out in the individual planning and

construction phases have been compiled in Table A.1.

S e c t i o n A 11

Classification A/S, 1992; Det Norske Veritas, 2003;American Petroleum Institute, 1993; GermanischerLloyd, 1999). The present Standard for GeotechnicalSite Surveys refers to the above standards.

The minimum requirements for site surveys, de-pending on the foundation type used, are specifiedin sections B and C.

A.4 Deviations from the Standard

The geotechnical engineer in charge may submit anapplication to the approval authority requesting anexemption from the requirements and indicating thereasons for such exemption, if it has become appar-ent in the course of the reconnaissance that parts ofthe survey programme at a particular site are eitherinadequate or dispensable for particular reasons, orcannot be implemented as planned or would involvedisproportionate effort and expense, with a satisfac-tory explanation to be provided in each case. Theapproval authority reserves the right to adjust thereconnaissance programme in general or in the indi-vidual case.

A.5 Updating

The present Standard represents state-of-the-artknowledge in the area of offshore geotechnical sitesurveys. As new knowledge will be gained from imple-mented projects, and technological innovations are tobe expected as well, the present Standard is notstatic but will be updated at suitable intervals to keepup with latest developments.

To make sure that new knowledge gained in practicaloperation will be taken into account in future updates,relevant comments and suggestions should beaddressed to

Bundesamt für Seeschifffahrt und Hydrographie(Federal Maritime and Hydrographic Agency)Dr. Manfred ZeilerBernhard-Nocht-Straße 7820359 Hamburg – GERMANYTel: +49 / 40 / 3190-3521Fax: +49 / 40 / 3190-5000E-Mail: manfred.zeiler@bsh.de

stating as reference: Standard for Geotechnical SiteSurveys (Offshore Wind Farms)

S e c t i o n B12

Section B Minimum Require-ments for the Geolo-gical-GeophysicalSurveying

B.1 General

Geological-geophysical reconnaissance is indispen-sable for the identification of soil types, quantificationof their properties, and evaluation of their suitabilityfor particular types of construction. It makes usestate-of-the-art, high-performance hydroacousticmethods whose results are then verified by means ofdirect methods (drilling). As the seabed is not easilyaccessible, hydroacoustic techniques have proved tobe useful and time-efficient methods for obtaining anoverview of existing soil properties, and thus of tecto-nic elements and sediment distribution, in order toidentify, e.g., risk areas.

Properly applied state-of-the-art geophysical methodsallow an optimisation of the number and location ofdrill holes and, in case of homogeneous sedimentproperties, a reduction of expenses for subsequentdrilling and pressure sounding.

As standards and regulations for geological-geophysi-cal surveying do not yet exist, the present standardspecifies requirements for state-of-the-art geoscien-tific reconnaissance which are expected to contributeto the successful realisation of offshore wind farmprojects by compiling the complete data and informa-tion available.

Geological-geophysical reconnaissance consists ofthree phases:

The purpose of preliminary reconnaissance is todetermine the general suitability of an area and, onthe basis of an adequate number of survey transects,to provide a detailed geological interpretation of thewind farm area on a scale of several kilometres.

The site investigation is to provide a small-scalesurvey, i.e. from a few metres to tens of metres, oflocal conditions at all wind turbine locations in orderto be able to present a risk analysis and, if necessary,

to propose alternative sites or an optimisation of indi-vidual wind turbine locations.

In the monitoring phase following construction of theunits, the individual sites have to be monitored forscouring, and the cables for possible free-spanningdue to sediment dynamics. Use of state-of-the-artgeophysical methods like, e.g., echosounders andside scan sonars is mandatory in order to obtain relia-ble data on the structures' local impact on the sea-bed. The results have to be compiled in a InspectionReport and presented to the approval authority.

B.2 Quality Assurance

• The persons in charge of reconnaissance must be adequately qualified and able to prove that they have sufficient experience. Their names shall be listed in the Geological Report.

• The data and their evaluation must be correct and verifiable.

• Measurement records shall be kept stating, among others, the environmental conditions during meas-urements (e.g. wind and wave conditions, stratifica-tion of the water body, algal bloom), the ship used, measuring instruments, configuration of measure-ments, and names of persons in charge.

• The position and depth accuracy must conform to the requirements of the International Hydrographic Organization, IHO Standards for Hydrographic Surveys, Order 1 Surveys. Details are provided in Tables B.1, B.2, and B.6.

• The measuring conditions shall ensure full compli-ance with the quality requirements. Past experiencehas shown that no adequate data quality is ob-tained at sea states•M5.

• The geological structure of the seabed in the plan-ning area shall be investigated to a depth of 20 m below the foundation depth using suitable geophys-ical measurement methods. In areas with gas or basin effect, where seismic methods may fail, the geological structure has to be explored by means of boreholes drilled at representative locations.

• The results of the geophysical surveys have to be confirmed by drilling. The seismic units have to be

S e c t i o n B 13

correlated with the lithological profiles of the seabed.

• A minimum resolution of 1 m is required for the upper sediment layers.

• The raw data should be stored digitally if possible, and a Geological Report shall be provided which correlates and evaluates the geophysical and geo-logical surveys. Maps showing the location of tran-sects and boreholes, survey tracks etc. must be provided digitally, in a GIS or CAD format.

• The applicant has to ensure long-term data archiv-ing.

B.3 Time Schedule

1. A comprehensive literature study has to be made for a complete compilation of all relevant informa-tion on water depths, geological and hydrographic conditions, existing cables and pipelines, other structures, fishery activities, shipping traffic, leisure activities, existing nature reserves and prohibited areas within the planned wind turbines and in their vicinity. The result of the literature search shall be submitted as a Preliminary Geological Report be-fore approval can be granted. It usually has to be included in the application documents, and its re-vised version is provided as part of the Environ-mental Impact Assessment (EIA) if required.

2. Bathymetric and geophysical surveys (echosound-ing and side scan sonar) have to be performed before approval can be granted (in connection with the Environmental Impact Assessment). These sur-veys usually can be performed in connection with the seismic survey of the planning area, so that first geological details of the seabed are available before the approval is granted.

3. Representative borehole drilling to calibrate the seismic data is mandatory. In exceptional cases, provided that plausible reasons have been given, the drilling may be performed after approval has been granted. The type and scope of such drilling has to be co-ordinated with the geotechnical recon-naissance as appropriate.

4. It must be ensured in each case that prior to the planning phase a complete geophysical survey hasbeen carried out, and representative boreholes have been drilled.

5. The Geological Report shall be submitted to the approval authority after completion of all prelimi-nary-phase investigations, together with the appli-cation.

6. Any additional geophysical and geological results from the planning phase have to be added to the Geological Report as a supplement and shall be submitted to the approval authority together with the application for the operating permit.

7. The results of the geological/geophysical investiga-tions during the monitoring phase shall be submit-ted as a Inspection Report.

B.4 Objectives

B.4.1 Preliminary Reconnaissance Phase

The purpose of preliminary reconnaissance is a de-tailed investigation of the lithological and tectonicstructures in the planning area and its general bed-ding conditions as well as a geological evaluation ofthe soil conditions. The geophysical profiles shouldcover the individual wind turbine sites. Apart fromgeological units, also obstructions such as unchartedwrecks, war ammunition, and sea cables have to bedocumented.

B.4.2 Planning Phase

In case findings during the planning phase deviatefrom the data compiled in the preliminary reconnais-sance phase, necessitating a relocation of turbinesites, the geological suitability of such new sites hasto be checked by additional geophysical surveys anddrilling.

S e c t i o n B14

B.4.3 Monitoring Phase

After completion of the turbine construction phase,the seabed in the area of the structures shall bechecked for scouring, and the submarine cables forfree spanning. Besides, safety measures such asrockfills using so-called “gravel bags” shall be inspect-ed as well. In the first two years following completionof the wind farm, inspection has to be performedtwice a year, in spring (immediately after the stormyseason) and late summer. The geophysical data shallbe provided in an annex to the Geological Report andcompiled in a Inspection Report which has to be sub-mitted to the approval authority for review by the endof the calendar year. After the first two years, inspec-

tion once a year (in spring) will probably be sufficient;after four years, an application may be filed for longerinspection intervals.

B.5 Technical Instructions

In the following tables B.1 to B.6, the targets, scope,time schedule, methods, and presentation of resultsare specified for each individual method used, com-plete with all technical details, providing a synopticview of the requirements to be met by the geologi-cal/geophysical survey in the preliminary reconnais-sance, planning, and monitoring phases.

S e c t i o n B 15

Table B.1: Requirements for Echosounding Surveys (Single-Beam and MultibeamEchosounders)

Preliminary reconnaissance /Planning phase

MonitoringOperating phase

Targets • Survey of bathymetric conditions • Recording of local depth changes(scouring)

Scope • Each turbine site shall be surveyed at leastonce

• Line spacing max. 500 m

• Along the turbine sites (longitudinal lineson both sides)

• At least 200 m on either side

Timeschedule

• Once • In the first 2 years after completion, twice ayear in spring and late summer toinvestigate sediment dynamics in thestormy and calm seasons

• Thereafter, once a year

• If a single-beam sonar system has beenused in the preliminaryreconnaissance/planning phase, areference survey using a multibeamechosounder has to be made beforeconstruction of the turbines

Method • Single-beam echosounder if the sea floor isrelatively smooth, or multi-beam system if itis rough

• Positioning better than 5 m + 5 % of thewater depth *

• Accuracy for reduced depths acc. to IHOStandards for Hydrographic Surveys, Order1 Surveys (~0.75 m)*

• Multi-beam echosounder

• Positioning better than 2 m

• Accuracy for reduced depths acc. to IHOStandards for Hydrographic Surveys, Order1 Surveys (~0.75 m)*

Presentationof results

• Bathymetric chart of surveyed areas

• Reported water depths must be soundvelocity corrected and related to chartdatum (tidal correction)

• The data must also be provided in digitalform

• Bathymetric chart of surveyed areas

• Reported water depths must be soundvelocity corrected and related to chartdatum (tidal correction)

• The data must also be provided in digitalform

* International Hydrographic Organization, IHO Standards for Hydrographic Surveys, Order 1 Surveys,

Special Publication No 44, 4th

Edition, April 1998

S e c t i o n B16

Table B.2: Requirements for Side Scan Sonar (SSS) Surveys

Preliminary reconnaissance /Planning phase

MonitoringOperating phase

Targets • Survey of sediment types and structures

• Verification or calibration of grab sampleinterpretations (soil truthing)

• Complete mapping of all sediment typesand structures in the area

• Recording of erosion areas, scouring, andobstructions

• Verification and/or calibration of grabsample interpretations (soil truthing)

Scope • Each turbine site shall be surveyed at leastonce

• Max. line spacing of 500 m onhomogeneous sea floor

• Complete coverage of the area if the seafloor is heterogeneous

• Complete coverage

Timeschedule

• Once • In the first 2 years after completion, twice ayear in spring and late summer toinvestigate sediment dynamics in thestormy and calm seasons

• Thereafter, once a year

Method • Frequency 100 kHz or higher

• Coverage max. 2 x 100 m

• Recognition of cubic features of 2 m

• Analogous or digital recording

• Cruise speed max. 4 knots

• Equipment positioning better than 10 m

• Analogous recording with continuousmarking of frequency, range, position, timeand date

• Frequency 100 kHz or higher

• Coverage max. 2 x 75 m

• Recognition of cubic features of 2 m

• Digital and analogous recording

• Cruise speed max. 4 knots

• Equipment positioning better than 10 m

• Analogous recording with continuousmarking of frequency, range, position, timeand date

Presentationof results

• Map with interpretation

• Digital SSS mosaic (recommended)

• Digital SSS mosaic

• Map with interpretation

* International Hydrographic Organization, IHO Standards for Hydrographic Surveys, Order 1 Surveys,

Special Publication No 44, 4th

Edition, April 1998

S e c t i o n B 17

Table B.3: Requirements for Seismic Surveys

Preliminary reconnaissance /Planning phase

MonitoringOperating phase

Targets • Determination of type and location ofgeological units

• Determination of type and location ofgeological units (in case wind turbine siteshave to be changed)

Scope • Each turbine site shall be surveyed at leastonce

• Max. (longitudinal) line spacing of 500 m

• Max. spacing of cross sections 2000 m

• Data from preliminary reconnaissancemust be taken into account

• Mandatory: 1 longitudinal and 1 crosssection across each site

• Recommended: longitudinal sections with a10 m spacing across the total width of thefoundation and 1 cross section

Timeschedule

• Once • Once

Method • Boomer or alternative systems ofcomparable or better performance andsufficient signal penetration. Near-surfaceresolution min. 1 m

• Possibly supplemented by subbottomprofiler or chirp sonar in near-surface areas(e.g. along planned cable routes),resolution min. 0.5 m

• Cruise speed: max. 4 kn

• Deployment up to sea state max. 4

• Boomer or alternative systems ofcomparable or better performance andsufficient signal penetration. Near-surfaceresolution min. 1 m

• Possibly supplemented by subbottomprofiler or chirp sonar in near-surface areas(e.g. along planned cable routes),resolution min. 0.5 m

• Cruise speed: max. 4 kn

• Deployment up to sea state max. 4

Presentationof results

• Profiles and profile interpretation

• Map showing spatial location of boundariesbetween geological units and structuralelements (e.g. isolines map)

• Profiles and profile interpretation

• Map showing spatial location of boundariesbetween geological units and structuralelements (e.g. isolines map)

S e c t i o n B18

Table B.4: Requirements for Borehole Drilling

Planning

Targets • Verification and/or calibration of the seismic survey results

Scope • Number and location of boreholes to be determined on the basis of thegeophysical data obtained

• A guiding value for the number of boreholes is 10 % of the wind turbine sites

• All seismic units up to the required signal penetration depth must be recorded

Time schedule • As required

Method • Drilling methods to obtain suitable materials for the required laboratory tests

• Vibration corer (for power cable route burial assessment)

Presentation ofresults

• Sediment core decription acc. to DIN

• Laboratory report

Notes:

• It is recommended to investigate the drilling site by means of a magnetometre or activemetal detection system before using a corer

• Scope, time schedule, and method should be co-ordinated with the geotechnicalengineer in charge!

Table B.5: Requirements for Magnetometers and Active Metal DetectionSystems(Recommended)

Preliminary reconnaissance

Targets • General testing of the investigation area for wrecks, active and inactive cables,ammunition, and other metal parts

Scope • As required, based on the results of the desk study

• Always in areas with ammunition

Time schedule • As required

Method • Magnetometer

• Active metal detection system

Presentation ofresults

• Map showing investigation results

S e c t i o n B 19

Table B.6: Requirements for the Power Cable Route Survey

Preliminary reconnaissance / Planning Monitoring

Targets • Bathymetric and morphologicalinvestigation of the planned cable route

• Mapping of wrecks, other obstructions, andammunition

• Investigation of sediment composition,geological stratification, and geotechnicalproperties of the upper sediment layers

• Exact mapping of existing cables andpipelines

• Determination of cable route and length

• Detection of possible free spanning ofcable

• Checking of rockfills and comparable cablesafety features

Scope • Complete coverage of 500 m wide corridorof planned cable route using side scansonar and multi-beam echosounder

• Survey of planned cable route usinggeological, geophysical, and geotechnicalmethods

• Complete survey of the planned cableroute

Timeschedule

• Once • During the first years, once a year

Methods • Multi-beam; positioning better than 2 m,accuracy for reduced depths acc. to IHOStandards for Hydrographic Surveys, Order1 Surveys (~0.75 m)

• Side scan sonar; frequency 100 kHz orhigher; measuring range max. 2 x 100 m;recognition of cubic features >2 m* ; digitaland/or analogue recording; cruise speedmax 4 kn; sonar positioning better than10 m; analogue record marked with fre-quency, range, position, time, date

• Sub-bottom profiler, chirp sonar oralternative systems of comparable or betterperformance

• One cone penetration test (CPT) per kmcable route, down to the planned cableburial depth, or more frequently in case ofunclear sediment stratification

• Magnetometer or active metal detectionsystem

• Cable tracking system or other suitablemethod for cables buried in sediment

• Multibeam if applicable; positioning betterthan 2 m, accuracy for reduced depths acc.to IHO Standards for HydrographicSurveys, Order 1 Surveys (~0.75 m)

• Side scan sonar if applicable; frequency100 kHz or higher; measuring range max.2 x 100 m; recognition of cubic features >2 m*; digital and/or analogue recording;cruise speed max 4 kn; sonar positioningbetter than 10 m; analogue record markedwith frequency, range, position, time, date

Presentationof results

• Map (scale 1 : 5000 or larger) showing allsurvey results

• Map (scale 1 : 5000 or larger) showing allsurvey results

* International Hydrographic Organization, IHO Standards for Hydrographic Surveys, Order 1 Surveys,

Special Publication No 44, 4th

Edition, April 1998

S e c t i o n B20

B.6 Power Cable Route Burial Assessment

Different requirements apply to the cable routesregarding horizontal cover and penetration depth inthe study area. With regard to the required cable buri-al depth, the seabed's hardness or trenchability is ofparticular interest. The state-of-the-art cable buryingmethod is ploughing or water jet trenching; if plough-ing or trenching is not feasible, the cable must becovered, e.g., by rockfills using so-called “gravelbags”. Therefore, in the monitoring phase, the pipe-line's burial depth and condition of cover has to bechecked using suitable methods. Technical details ofthe requirements are listed in Table B.6.

B.7 Geological Report

B.7.1 Purpose

In the Geological Report, the results of the geophysi-cal surveys and drilling have to be compiled andassessed. The Report is the basis for further planningand contains a geological description of the seabedsoils on which the structures are to be erected. Itshould include engineering details and, in conjunctionwith the Geotechnical Report, provide a reliable databasis for the verification of planned locations and forthe selection of suitable foundation designs.

B.7.2 Contents

The Geological Report contains at least the followinginformation:

• Brief project description• Purpose of the investigations• Office and persons in charge• Period of work at sea and in the laboratory• Description of all measuring systems and equip

ment used• Relevant information from the measuring records,

e.g. ambient conditions, sound profiles in water• Data processing• Data evaluation (e.g. sound velocities used in

sediment)• Comparison of acoustic description of the sediment

units with lithological description from the stratifica-tion record (e.g. in the form of a table)

• Location of sections and boreholes, stratification record acc. to DIN, grain size analyses and other geotechnial parameters determined acc. to DIN(in an annex as appropriate)

• Presentation of results in the form of maps of suita-ble scale (plan view and sections)

• Evaluation of results• Summary, and• CD with digital chart in CAD or GIS format

B.7.3 Requirements

Reference system: World Geodetic System 1984 (WGS 84)

Projection: Gauss-Krüger (GK) or Universal Transverse Mercator (UTM)

Depth data: Related to chart datum

S e c t i o n C 21

Section C Minimum Require-ments for the Geotech-nical Site Surveying as a Basis for Planning/Designing the OffshoreWind Farm

C.1 General

According to DIN 1054 (2003) and DIN 4020 (draft2002), offshore structures have been allocated to geo-technical category 3 as they involve a high degree ofdifficulty, i.e. difficult construction and/or difficult soilproperties and unusual load cases.

C.2 Quality Assurance

For planning and implemention of site reconnaissanceand investigation as well as for foundation planningand implementation,

• a certified geotechnical engineer• with proven experience in handling projects of com-

parable difficulty

has to be contracted. The tasks of the expert include:

• development of the site investigation concept• commissioning of site reconnaissance and investi-

gation• monitoring of site reconnaissance and investigation• presentation of the results in the form of site investi-

gation reports• preparation of the soil and foundation expertise.

As a matter of principle, the planning, implementation,documentation, and evaluation of the field and labora-tory tests shall be performed in compliance with appli-cable DIN standards and guidelines or, if this is notpossible, as close as possible to such DIN standardsand guidelines (see C.10 Literature).

The minimum requirements for field and laboratorytesting within the framework of the designing and bid-ding phases, depending on the foundation type to beused, have been specified in the following.

C.3 Requirements for Field Testing

C.3.1 Drilling and Penetration Tests

Borehole drilling must be performed in accordancewith DIN 4021 (1990). Sediment core descriptions ac-cording to DIN 4022, part 1 (1987), are required fornon-continuous soil cores, and according to DIN 4022,part 3 (1982), for continuous soil cores.

In cone penetration tests (CPT) the cone tip resistanceand local sleeve friction are measured. Special tests(CPTu) additionally measure the pore water pressure.The requirements for performing and evaluating conepenetration tests are specified in DIN 4094, part 1(2002). Cone penetration tests always have to be car-ried out in connection with at least one borehole in or-der to be able to calibrate the penetration test dataagainst the borehole data.

In the standard penetration test (SPT), a sampler isdriven into a borehole. Performed according to ASTMD 1586-99 (1999), the test is used to obtain samplesfrom cohesive and cohesionless soils by driving asampler into the soil by means of a hydraulic ram andmeasuring the number of blows required. According toDIN 4094, part 2 (draft 2002), the sampler head is re-placed by a conical tip and soil samples cannot betaken.

S e c t i o n C22

C.3.2 Field Testing for Pile Foundations

In the case of pile foundations, the soil investigationhas to provide at least the parameters for the followingcalculations:

• ultimate bearing capacity of piles for axial tension and compression

• load-deformation relation for axial and horizontalloads (bedding)

• expected pile resistance during driving, obstacles todriving, and

• mudmat bearing capacity in the case of a jacket foundation.

The minimum drilling and penetration test require-ments for the different types of pile foundation arelisted in Table C.1.

Where cohesive sediment layers are relevant for thefoundation design due to their location and thicknessor where soil conditions at particular locations arehighly inhomogeneous or otherwise unfavourable, theminimum programme according to Table C.1 is supple-mented by additional core drilling at some or all of thefoundation sites in order to be able to determine thesoil-mechanical parameters that are required for thefoundation design with sufficient reliability. The scope

of such additional investigations will be determined bythe geotechnical engineer in charge in co-ordinationwith the foundation design engineer, considering eachindividual case and taking into account the geologicalstructure and homogeneity of the foundation soil.

The guideline values in Table C.2 (based on Fugro-McClelland Ltd., 1993; McClelland & Reifel, 1986;American Petroleum Institute, 1982; Det Norske Veri-tas Classification A/S, 1992; Classification A/S 2003;DIN 4020, 2002 draft) should be used to determine theexploration depth and sampling frequency.

The soil investigation programme according to TableC.2 constitutes the minimum scope of investigationsrequired. The borehole and penetration test data, inconjunction with the results of the geological/geophysi-cal results, must allow a reliable assessment of thefoundation soil properties as a basis for the foundationdesign.

The drilling and penetration test activities must be su-pervised by experts, and the responsible geotechnicalengineer in charge will decide whether additional in-vestigations are required. Additional, or different, in-vestigations may have to be carried out according tohis instructions, which is to be expected especially ifthe foundation soil is inhomogeneous.

Table C.1: Field Testing for Pile Foundations

Type of pilefoundation

Scope of exploration

Monopile

Tripod

Jacket

• Unless the geophysical reconnaissance data and cone penetration data suggestdifferent locations that are more suitable, one borehole has to be drilled at eachcorner of the wind farm area (mostly four corners) and one borehole at its centre

• In case the wind farm dimensions are particularly large, additional boreholes have tobe drilled with a spacing to ensure that about 10 % of locations are covered (guidingvalue)

• One cone penetration test per foundation site. For tripod and jacket foundations, onecone penetration test per leg has to be carried out if the foundation soil properties areunfavourable or highly inhomogeneous (to be assessed on the basis of thegeophysical site investigation)

• To calibrate the penetration tests, one cone pressure test must be performed in thevicinity of each borehole

S e c t i o n C 23

C.3.3 Field Testing for Gravity Foundations

In the case of gravity foundations, soil reconnaissanceand investigation has to provide at least the parame-ters for the following computations:

• resistance to soil failure under axial and inclinedloads

• sliding stability• settling and tilting of the foundation, and • skirt penetration resistance

The following minimum requirements are determinedfor drillings and penetration tests:

• one cone penetration test (CPT) per foundation site• the drillings are co-ordinated with the geological/

geophysical preliminary reconnaissance as to type, scope, and location. Under geotechnical aspects, drillings in all corners of the wind farm (4 corners in

most cases) and in the middle of the area are advis-able unless available reconnaissance data necessi-tate or suggest a different arrangement. In the case of very large wind farms, additional boreholes have to be drilled in between those above. The guiding value is 10 % of the number of wind turbines.

• to ensure reliable calibration of the CPTs, each of the boreholes has to be drilled beside the location ofa CPT.

Where cohesive sediment strata determine the foun-dation design due to their location and thickness, orwhere soil conditions at a particular location are highlyinhomogeneous or otherwise unfavourable, the mini-mum programme has to be supplemented by addition-al core drilling to ensure that the soil-mechanical para-meters required for the foundation design are deter-mined with sufficient reliability. The scope of such addi-tional investigations will be determined by the geotech-nical engineer in co-ordination with the engineer who

Table C.2: Exploration Depth and Sampling in Field Testing for Pile Foundations

(Minimum Requirements)

Investigationmethod

Exploration depth Sampling

Drilling • With predominantly lateral loads, one pilediameter below pile base

• With predominantly axial loads, up to

• 2 – 3 times the pile diameter below pilebase

• Up to 12 m below the sea floor, sampling at1 m intervals

• Below that, to a depth of about 60 m, atleast 3 m intervals

• Below 60 m depth, 8 m intervals

• At least one sample from each soil stratum

Conepenetrationtest

• Continuous cone penetration testing

• With predominantly lateral loads, up to onepile diameter below pile base and to thereconnaissance depth for calibration drilling

• With predominantly vertical loads, up totwo – three times the pile diameter belowpile base

• If the required exploration depth cannot bereached by continuous cone penetrationtesting, pre-drilling should be applied andCPT continued from the borehole bottom

S e c t i o n C24

has designed the foundation design, considering eachindividual case and taking into account the geologicalstructure and homogeneity of the foundation soil.

In cohesive soils, the cone penetration tests may haveto be supplemented by measurements of pore waterexcess pressure (CPTu), and the drillings by vaneshear tests in the borehole. Moreover, depending onthe specific demands on the foundation, pressureme-tre tests or dilatometre tests may have to be carriedout.

The exploration depth and sampling frequency aresubject to the guideline values in Table C.4 (based onFugro-McClelland Ltd., 1993; Det Norske Veritas Clas-sification A/S, 1992; DIN 4020, draft 2002).

The soil investigation programme according to TableC.3 constitutes the minimum scope of investigationsrequired. The borehole and sounding data, in conjunc-tion with the results of the geological/geophysical in-vestigations, must allow a reliable assessment of thefoundation soil properties as a basis for the foundationdesign.

C.3.4 Requirements for Soil Sampling

The drilling methods chosen must be suitable for ob-taining soil samples of at least quality class 2 (GK 2)according to DIN 4021 (1990). The samples must beundisturbed at least in their composition, and in thecase of cohesive oils also in their water content anddensity. If GK 2 quality cannot be reached using thedrilling methods available, there is a possibility of tak-ing special samples from the borehole bottom in ac-cordance with DIN 4021 (1990) (normally in cohesivesoil types).

The methods to be used in order to obtain soil samplesof the required quality depend on the type of soil. DIN4021 (1990) provides an overview of suitable onshoredrilling methods. Offshore drilling methods are describ-ed in McClelland & Reifel (1986).

To obtain special samples, it is common practice touse samplers deployed at the bottom of the borehole,which are either driven into the soil using the percus-sion method (percussion core) or pressed into the soilapplying even pressure (push core). As a matter ofprinciple, it must be ensured that the sampling tools

Table C.3: Exploration Depth and Sampling in Field Testing for Gravity Foundations

(Minimum Requirements)

Investigationmethod Exploration depth Sampling

Drilling • At least to a depth corresponding to thefoundation diameter or the longer side ofthe foundation

• Up to 12 m below the sea floor, sampling at1-m intervals

• Below that, 3-m intervals as a minimum

• At least one sample from each soil stratum

Conepenetrationtest

• Continuous cone penetration testing

• Up to a depth corresponding to thefoundation diameter or the longerfoundation side and to the reconnaissancedepth required for calibration drilling

• If the required reconnaissance depthcannot be reached by continuous conepenetration testing, pre-drilling should beapplied and CPT continued from theborehole bottom

S e c t i o n C 25

are in perfect operating condition, and especially thecutting edges should not be blunt or deformed. Experi-ence has shown that percussion core samples gene-rally are more significantly disturbed than push coresamples. A sampler that has been developed espe-cially for seafloor sampling is the “seafloor jack”, whichcan also be used for penetration tests.

Experience has shown that thin wall tube samplers arethe most suitable tool for taking soil samples both fromcohesive and cohesionless soils; soft, cohesive soillayers are sampled best using samplers provided withliners.

Care should be taken in sampling that the quality, num-ber, size, and quantity of the samples is sufficient forlaboratory testing. The core diameter optimally shouldbe 100 mm. The minimum requirements are as follows:

• minimum diameter of 70 mm for push core samples• minimum diameter of 80 mm for percussion core

samples• minimum diameter of 114 mm for special samples

Additional requirements are specified in DIN 4021(1990).

If possible, a larger number of soil samples should betaken than is expected to be required for the laboratorytests.

C.4 Requirements for Laboratory Tests

C.4.1 Laboratory Tests on Board Survey Vessels

It is recommended to perform a limited range of labo-ratory tests while still on board the survey vessels (e.g.determination of undrained shear resistance, density,and water content). Some tests such as the determina-tion of grain size distribution in fine-grained sedimentsmay be more difficult on board a vessel than in a land-based laboratory because of the ship movement andvibration, or may not be feasible at all, but neverthelessshipboard laboratory tests provide information regard-ing the selection and scope of further field testing,which can then be carried out without major loss oftime.

C.4.2 Onshore Laboratory Tests

In Tables C.4 and C.5, the laboratory tests suitable fordescribing the soil status and determining the requiredsoil mechanical parameters have been compiled. Thegeotechnical engineer in charge will decide on thetests to be carried out in the individual case.

To examine the influence of cyclic tension changes inthe soil resulting from waves, currents, and wind (seeGermanischer Lloyd, 1999), cyclic triaxial tests or cy-clic direct shear tests, or cyclic compression testsshould be performed for the soil layers which essen-tially contribute to the load transference (Figs. C.1 toC.3).

S e c t i o n C26

Table C.4: Laboratory Tests for Evaluatation of Cohesionless Soils

Type of test Standard Quality class of sample(DIN 4021)

Soil mechanical parameter

Classification and status description

Grain size distribution DIN 18123(1996)

Min. quality class 4 Degree of nonuniformity,coefficient of gradation(DIN 18196, 1988)

Compactness DIN 18126(1996)

Min. quality class 2 Loosest and densest state

Density DIN 18125,part 1(1997)

Min. quality class 4, for thedetermination of porosity at leastquality class 2

Density, buoyant density

Calcium content DIN 18129(1996)

Min. quality class 4 (5) Calcium content

Deformation behaviour

Compression test(oedometer test)

DIN 18135(1999 draft)

Quality class 4, but mountedsample has the initial in-situdensity

Stiffness modulus, coefficient ofconsolidation, coefficient ofsecondary compression, derived:coefficient of water permeability

Drained and undrainedtriaxial tests

DIN 18137,part 2(1990)

Quality class 1, but also treatedsamples

Shear stress-strain curves,volume change and axialdeformation

Strength

Direct shear test DIN 18137,part 3(1990)

Quality class 1, but also treatedsamples

Friction angle (c‘ = 0)

Drained and undrainedtriaxial tests

DIN 18137,part 2(1990)

Quality class 1, but also treatedsamples

Friction angle (c‘ = 0)

Table C.5: Laboratory Tests for Evaluation of Cohesive Soils

Type of test Standard Quality class of sample(DIN 4021)

Soil mechanical parameter

Classification and status description

Grain size distribution DIN 18123(1996) Min. quality class 4

Degree of nonuniformity,coefficient of gradation(DIN 18196, 1988)

Water content DIN 18121,part 1 (1998)DIN 18121part 2 (2001)

Min. quality class 3 Water content of soil

Water permeability DIN 18130,part 1 (1998)

Min. quality class 2; qualityclass 4 if sample has beenadjusted to the requireddensity using Proctorcompacting equipment

Coefficient of permeability

Density DIN 18125, Min. quality class 4, for the Density, buoyant density

S e c t i o n C 27

part 1 (1997) determination of porosity atleast quality class 2

Consistency limits DIN 18122,part 1(1997)DIN 18122,part 2 (2000)

Min. quality class 4Liquid limit, plastic limit,shrinkage limit, plasticity index,consistency index

Deformation behaviour

Compression test(oedometer test)

DIN 18135(1999)

Quality class 1 Stiffness modulus, prestressingof soil, coefficients ofconsolidation, secondarycompression, and waterpermeability

Undrained triaxial tests DIN 18137,part 2 (1990)

Quality class 1, but alsotreated samples

Shear stress-strain curves,volume change and axialdeformation

Strength

Laboratory vane test Laboratorytests notstandardised

Quality class 1 if possible, butalso disturbed samples aresuitable

Undrained shear strength cu

Direct shear test DIN 18137,part 3 (1990)

Quality class 1, but alsotreated samples

Effective friction angle ‘,effective cohesion c‘

Undrained triaxial tests DIN 18137,part 2 (1990)

Quality class 1, but alsotreated samples

Shear parameters depending ontype of test:UU test: cu, u

CU test: c‘, ‘CCV test: c‘, ‘

Based on the results of the laboratory tests, a prediction is derived on potential changes in

the strength or stiffness of the soil as a result of cyclic loading. Besides, also the liquefaction

potential of the in-situ soils may have to be analysed if required.

Fig. C.1: Cyclic Direct Shear Test

Bodenprobe = soil sample

Fig. C.2: Cyclic Compression Test

S e c t i o n C28

Fig. C.3: Cyclic Triaxial Test

Bodenprobe = soil sample

C.5 Geotechnical Site Survey Report

C.5.1 Contents of the Geotechnical Site Survey Report

Apart from a precise definition of the objective of theinvestigation and a compilation of the project docu-ments made available, the Geotechnical Site SurveyReport must include at least:

• general information on the construction task• data on the geological conditions• drilling and sounding data, results of laboratory

tests and model tests if any• clearly structured compilation of the test results• synoptic description of the foundation soil• general assessment of the foundation soil

C.5.2 Presentation of the Results of Field and Laboratory Tests

C.5.2.1 Field Tests

The exact location of explorations and field tests per-formed should be entered in an accurate scale mapwhich also shows the contours of the planned struc-tures. Also reference dimensions in relation to fixedpoints or reference lines should be included. Theimplementation dates of the field tests and any spe-

cial occurrences during drilling surveillance should benoted.

The exploration and penetration test methods usedhave to be explained in the Geotechnical Site SurveyReport. If standardised methods have been used,reference to the standard is sufficient. In case ofdeviations from standard, reasons have to be provid-ed, and the method has to be described.

If the Geotechnical Site Survey Report does not in-clude the core records of the drillings according toDIN 4022, a note must be added indicating that andwhere they can be inspected. The latter also appliesto the soil samples taken.

If core samples have been taken, colour photos of thedrilling cores may be enclosed. Such colour photosdo not replace an analysis and evaluation of the soilsamples by an expert at the laboratory.

The results of penetration tests have to be document-ed taking into account DIN 4094, parts 1 to 5. It isrecommended to plot the penetration test dataagainst the local well logs using a common referencesystem for height data (i.e. chart datum).

S e c t i o n C 29

C.5.2.2 Laboratory Tests

The results of laboratory tests should be documentedand described in detail for each typical soil property(e.g. grading curves, compression test results, sheartest results), so that any reader will be able to inter-pret the results. The test set-up has to be described ineach case. If standardised tests have been used,reference to the standard is sufficient.

The results of compression tests have to be providedin the form of pressure settlement curves and timesettlement curves, with indication of load stages andconsolidation times. Documentation of the results alsohas to include data on equipment dimensions and theway of mounting the soil samples in the equipment.

The results of shear tests have to be shown in con-formity with applicable standards.

The results of the laboratory tests have to be provid-ed in the form of tables, sorted by drillings, samplingdepths, and sample numbers.

C.5.3 Geotechnical Site Description

The results of the in-situ and laboratory tests shouldbe compiled in a geotechnical site description as partof the Geotechnical Site Survey Report. Gradingcurves have to be combined to gradation bands of themain soil types. For the main soil types, the rangesand characteristic data of the soil mechanical para-meters should be indicated.

C.5.4 General Geotechnical Site Assessment

The general geotechnical site assessment shouldinclude an evaluation of the soil and subsoil pro-perties at the project site with respect to the site'ssuitability for the establishment of an offshore windfarm, both with regard to its load carrying capacityand feasibility of different foundation concepts.

C.6 Soil and Foundation Expertise

C.6.1 Contents of the Soil and Foundation Expertise

The soil and foundation expertises must include as aminimum requirement:

• the geological/geotechnical site description• the main technical data of the structures as founda-

tion criteria• the geotechnical site assessment with reference to

the specifica construction project• determination of the soil characteristics and, if

necessary, of the computation methods• if required, information about driving obstacles and

suitable methods for piling and the installation of mudmats

• description of possible foundation designs includingtheir geotechnical evaluation

• proposed foundation design including the results ofrelevant static computations of overturning moments and/or settlement

• if required, information on earthquake hazards

C.6.2 Information Provided in the Soil and Foundation Expertise

The results compiled in the Geotechnical Site SurveyReport and in the (updated) Geological Report pro-vide the basis for the soil and foundation expertise tobe prepared by the geotechnical engineer.

It contains a synoptic description of the geologicalstructure, properties of the soil strata identified and ofthe physical soil characteristics, and an evaluation ofthe soil under statics/engineering aspects as well ascivil engineering aspects. An indispensable part of theexpertise is information about grain size distribution,compactness of cohesionless soils, condition of cohe-sive soils, and evaluation of the shear parameters andcoefficients of stiffness in the Geotechnical SiteSurvey Report with regard to the requirements to bemet.

S e c t i o n C30

The Soil and Foundation Expertise specifies thecharacteristic soil parameters that are relevant to thestatic analysis, at least the densities, stiffness moduli,and shear parameters. The geotechnical engineermay first discuss and agree these values with the cli-ent, the design engineer, the responsible constructionsupervising authority or agency, possibly also with theconstruction company taking into accounts the tasksand requirements.

The Soil and Foundation Expertise must include aclassification of soils by soil groups according to DIN18196 and soil classes according to DIN 18300.

In earthquake-prone areas, the geotechnical engineeralso has to determine the applicable strong motiondata, possibly in co-operation with an expert in thisspecial field.

Also an evaluation of soil properties with respect topiling and the installation of mudmats should be partof the soil and foundation expertise. If the scope ofinvestigations performed does not allow such an eval-uation, this should be pointed out in a note, and suchadditional investigations should be proposed and per-formed at a later date.

Finally, the Soil and Foundation Expertise should alsoevaluate the risk of encountering obstacles during driving operations. In this context, not only the resultsof drillings and penetration tests should be taken intoaccount but particularly the results of the geological-geophysical study.

C.7 Supplementary Investigations(Tendering Phase)

In the course of design engineering, geotechnicalexamination of the foundation design or tendering,especially in case special proposals have been made,supplementary reconnaissance or in-situ testing suchas drive sampling or loading tests may be necessary.

The same requirements apply to such investigationsas to the site investigation.

C.8 Monitoring in the Construction Phase

The geotechnical elements of the construction workhave to be inspected and checked in compliance withapplicable regulations, the results have to be record-ed and evaluated in final reports to be prepared bythe geotechnical engineer (acceptance).

C.9 Monitoring in the Operation Phase

If particular elements of the structural stability andserviceability documentation are not based on pre-viously performed calculations or testing of compo-nent parts or experience in general or in a particularcase, proof of which can be furnished, suitable in-spection instruments have to be provided and put intooperation.

The inspection concept in such cases is part of thestructural stability documentation and constitutes amandatory element of inspection during the operationphase. The results have to be evaluated at regularintervals by the geotechnical expert, who decideswhether the performance of the installation is in com-pliance with the design. Type and scope of the investi-gations and length of the intervals as well as toleran-ces are determined by the geotechnical engineer inco-ordination with the client, the design engineer, theresponsible supervising authority or agency, and pos-sibly with the construction company taking intoaccount the tasks to be performed and the require-ments. The results and their evaluation by the geo-technical engineer have to be submitted to the ap-proval authority regularly on agreed dates.

S e c t i o n C 31

C.10 Literature

American Petroleum Institute, API (1982): Recommended Practice for Planning, Designing, and Constructing Fixed Offshore Platforms. API RP 2A, 13th Edition, Dallas.

ASTM D 1586-99 (1999): Standard Test Method for Penetration Test and Split-barrel Sampling of Soils. American Society for Testing and Materials.

Det Norske Veritas, DNV (2003): Design of Offshore Wind Turbine Structures, Offshore Standard Draft, Høvik, Norway.

Det Norske Veritas Classification A/S (1992): Foundations.Classification Notes No. 30.4, Høvik.

DIN 1054 (2003): Baugrund; Sicherheitsnachweise im Erd- und Grundbau.

DIN EN 1536 (1999): Bohrpfähle; Ausführung von besonderen geo-technischen Arbeiten (Spezialtiefbau).

DIN 4014 (1990): Bohrpfähle; Herstellung, Bemessung und Tragverhalten.

DIN 4020 (Entwurf 2002): Geotechnische Untersuchungen für bau-technische Zwecke.

DIN 4021 (1990): Baugrund; Aufschluss durch Schürfe und Bohrungen sowie Entnahme von Proben.

DIN 4022 Teil 1 (1987): Baugrund und Grundwasser; Benennen und Beschreiben von Boden und Fels – Schichtenverzeichnis für Bohrungen ohne durchgehende Gewinnung von gekernten Proben im Boden und im Fels.

DIN 4022 Teil 3 (1982): Baugrund und Grundwasser; Benennen und Beschreiben von Boden und Fels – Schichtenverzeichnis für Bohrungen mit durchgehender Gewinnung von gekernten Proben im Boden (Lockergestein).

DIN 4026 (1975): Rammpfähle; Herstellung, Bemessung und zu-lässige Belastung.

DIN 4094 Teil 1 (2002): Baugrund; Felduntersuchungen – Drucksondierungen.

DIN 4094 Teil 2 (Entwurf 2002): Baugrund; Felduntersuchungen – Bohrlochrammsondierung.

DIN 4094 Teil 3 (2002): Baugrund; Felduntersuchungen – Rammsondierungen.

DIN 4094 Teil 4 (2002): Baugrund; Felduntersuchungen – Flügelscherversuche.

DIN 4094 Teil 5 (2001): Baugrund; Felduntersuchungen – Bohrlochaufweitungsversuche.

DIN 18121 Teil 1 (1998): Untersuchung von Bodenproben;Wassergehalt – Bestimmung durch Ofentrocknung.

DIN 18121 Teil 2 (2001): Untersuchung von Bodenproben;Wassergehalt – Bestimmung durch Schnellverfahren.

DIN 18122 Teil 1 (1997): Baugrund – Untersuchung von Bodenproben; Zustandsgrenzen (Konsistenzgrenzen) – Bestimmung der Fließ- und Ausrollgrenze.

DIN 18122 Teil 2 (2000): Baugrund – Untersuchung von Bodenproben; Zustandsgrenzen (Konsistenzgrenzen) – Bestimmung der Schrumpfgrenze.

DIN 18123 (1996): Baugrund – Untersuchung von Bodenproben;Bestimmung der Korngrößenverteilung.

DIN 18125 Teil 1 (1997): Baugrund – Untersuchung von Bodenproben; Bestimmung der Dichte des Bodens – Laborversuche.

DIN 18126 (1996): Baugrund – Untersuchung von Bodenproben;Bestimmung der Dichte nichtbindiger Böden bei lockerster und dichtester Lagerung.

DIN 18129 (1996): Baugrund – Untersuchung von Bodenproben;Kalkgehaltsbestimmung.

DIN 18130 Teil 1 (1998): Baugrund – Untersuchung von Bodenproben; Bestimmung des Wasserdurchlässigkeitsbei-werts – Laborversuche.

DIN 18135 (Entwurf 1999): Baugrund – Untersuchung von Bodenproben; Eindimensionaler Kompressionsversuch.

DIN 18137 Teil 2 (1990): Baugrund – Untersuchung von Bodenproben; Bestimmung der Scherfestigkeit – Triaxialversuch.

DIN 18137 Teil 3 (1990): Baugrund – Untersuchung von Bodenproben; Bestimmung der Scherfestigkeit – direkter Scherversuch.

DIN 18196 (1988): Erd- und Grundbau; Bodenklassifikation für bau-technische Zwecke.

DIN 18300 (2000): VOB Verdingungsordnung für BauleistungenTeil C: Allgemeine Technische Vertragsbedingungen für Bauleistungen (ATV), Erdarbeiten.

EAU (1996): Empfehlungen des Arbeitsausschusses „Ufereinfassungen“ Häfen und Wasserstraßen, 9. Auflage,Ernst und Sohn, Berlin.

Fugro-McClelland Ltd. (1993): UK Offshore Site Investigation and Foundation Practices, FML Report No. 92/2549-1(03).In: Health and Safety Executive (1993): Offshore Technology Report – OTO 93024, Sheffield.

Germanischer Lloyd, GL (1999): Rules and Regulations – IV – Non-Marine Technology – Part 2 – Offshore Wind Energy Converters (GLOWE) und Rules for Classification and Construction – III – Offshore Technology – Part 2 – Offshore Installations (GLOT), Hamburg.

McClelland, B. & Reifel, M. D. (1986): Planning and Design of FixedOffshore Platforms. Van Nostrand Reinhold Company Inc., New York.

Wiemann, J.; Lesny, K. & Richwien, W. (2002): Gründung von Offshore-Windenergieanlagen – Gründungskonzepte und geo-technische Grundlagen. Mitteilungen aus dem Fachgebiet Grundbau und Bodenmechanik, Heft 29,Herausgeber Prof. Dr.-Ing. W. Richwien, Verlag Glückauf, Essen.