two lectures of piling - aalto

GEO-E2080 Foundation Engineering and Ground Improvement 1

Two lectures of Piling Contents

1 Pile definitions 2 External loads on pile 3 Pile classifications 4 Functional pile types 5 Piling design and pile driving formulas 6 Selection of pile type 7 Execution of piling 8 Pile slab and pile cap structures

1 Pile definitions

Pile is a bearing structure. The maximum diameter of a pile is approximately 2000 mm and for displacement piles (maata syrjäyttävät paalut) the minimum length of 3 m, although special piles can be shorter. Piles are usually vertical, but can be inclined as well. However, the inclination of these so called Inclined piles (vinopaalut) when driven (lyötävät) should be no more than 4:1 (usually 6:1).

Large diameter pile (suurpaalu) is a pile with a minimum diameter of 300 mm and the bearing capacity is greater than 1 500 kN. Micropiles or minipiles (pienpaalu) are made from steel or ductile iron (pallografiittirauta) and have a maximum diameter of 300 mm. Diameter of a drilled pile must be at least 30 mm and at least 60 mm for piles installed using other methods. For a micropile, bearing capacity in serviceability limit state is generally around 50 kN. Piles are usually compressed structures. If tensile stressed piles are used, their design requires AA qualification.

2 External loads on piles External loads on piles are:

Vertical load: The aim is to keep the bending stresses low

Horizontal load and moment, reinforced concrete driven piles tolerate only a limited amount

Side load (sivukuormitus): Causes bending stress, must be taken into account in the structural design.

Negative skin friction (negatiivinen vaippahankaus): Occurs when the ground around the pile settles more than the piles (eg fills.). Emphasizes on individual piles. Pile groups are designed with LPO-2005 and PO-2011

Also work phase situations can be critical with respect to design.


3 Pile classifications Piles can be classified based on:

Installation method: o driven piles (lyötävät) o drilled piles (porattavat) o press-in piles (puristettavat) o grouted piles (injektoitavat)

Diameter o micropiles (pienpaalut) (d<300mm) o large diamater piles (suurpaalut) (d>300mm)

Geometric profile o square profile; reinforced concrete piles (teräsbetonipaalut, TB-paalut) o pipe profile piles; open, closed o other profiles e.g. X-piles, steel rails, welded I, H- etc. beams o sheet piles (teräspontit) and combinations of sheet piles and piles o special piles incl. large diameter piles o timber piles

Functional basis o end bearing pile (tukipaalut) o friction pile (kitkapaalut) o cohesion pile (koheesiopaalut)

Displacement piles or replacement piles

According to Eurocode (and PO-2011), piles are divided into displacement piles (maata syrjäyttävät paalut) and replacement (non-displacement) piles (maata syrjäyttämättömät paalut). Displacement piles are installed into the ground without excavating or without systematic removal of the material. Soil might be removed in some parts in order to e.g. decrease the vibrations or rising of ground surface or in order to ease up the penetration by removing obstacles such as boulders. Installation methods of displacement piles are; driven to the ground (lyömällä), vibration (täryttämällä), press-in method (puristamalla), screwing (ruuvaamalla) or a combination of these. Driven piles are most commonly made of reinforced concrete or steel. Also, ductile iron, plaster (laasti), timber and combinations of materials above have been used. Timber is the oldest material used as piles. Replacement piles are installed with a casing (suojaputki) or by excavating or drilling a pile excavation, which is filled with reinforced or unreinforced concrete. Open pipe profiles can also be made by drilling or excavating from inside of the pipe. A pile material is often steel (e.g. drilled piles), reinforced concrete, and reinforced concrete with special reinforcement such as steel pipes, profiles or fibers. Piling instructions (Paalutusohje PO-2011) covers only such construction methods which enable constructing a designed cross profile that fits the given tolerances.


Other pile types Large diameter piles

steel pipe piles cast-in-site pile/caisson piles

(kaivinpaalu) franki piles (Franki-paalut) vibrex piles (Vibrex-paalut) others

Special piles drilled steel pipe piles element piles (”pätkäpaalut”)

installation: - pressing (Mega) - driving (West) - drilling

timber driven piles: slope piles and stand piles

4 Functional pile types end bearing piles (tukipaalut) friction piles (kitkapaalut) cohesion piles (koheesiopaalut)

End bearing pile Friction pile Friction pile Cohesion pile P is compressive resistance (or bearing capacity) of a pile (and load) Pk base/tip resistance (kärkikantavuus) Pv shaft resistance (vaippakantavuus) In Finland, the most common functional pile types are end bearing and frictional piles. End bearing pile is usually the recommended functional pile type, since only end bearing pile foundation is a non-settling structure.


5 Piling design and pile driving formulas

5.1 Introduction Bearing capacity (or resistance as in Eurocode terminology) of piles consists of structural capacity and geotechnical bearing capacity. Structural capacity is safety against structural failure as well as allowed deformations of the pile. The geotechnical bearing capacity includes the safety against soil failure and allowed settlements. Bearing capacity of a single pile is generally the resistance parallel to the longitudinal axis, the smallest structural or geotechnical bearing capacity. Geotechnical bearing capacity is often the most critical. Bearing capacity of the pile group is determined by either the sum of the individual bearing capacities of piles (eg. end bearing piles) or the bearing capacity of the ground formed between them.

Things that has to be taken into account when designing piling:

Bearing capacity of single pile Minimum center distance (keskiöetäisyys) between the piles (values from tables) Structural capacity of the pile Geotechnical bearing capacity of the pile group (in practice can be critical only with

friction piles) Negative skin friction

o calculation of (fillings) settlements o calculation of negative skin friction for a single pile o calculation of negative skin friction for the pile group

Settlement of single pile (not needed in PTL 2) Connections to upper structures


Other factors that need to be taken into account in design:

• Short piles (minimum. 3 meter); for less than 5 meters long piles capacity has to be reduced • Small pile groups – sensitive to damages of single piles • Buckling risk of the pile, pile partially or completely in air, water or soil layer of which su

≤ 8 kPa • Piling in demanding soil or filling which contains stones or boulders

Design methods for piles according to piling instructions PO-2011 and Eurocode are:

static load test results (calculations and empirical knowledge support the design) dynamic load test results with end bearing piles without static additional loading test

(factors tabulated) empirical or analytical calculation methods of which accuracy has been verified by

static or dynamic load tests in similar circumstances or generally accepted analytical calculation method without load testing

observed behavior of a similar pile foundation, if the results of the ground investigations and other tests support this procedure

load tests can be done for installed trial piles, or operational piles (part of the foundation). 5.2 Calculation methods Bearing capacity of a pile changes during the installation as soil conditions change as well, and also depends on the installation method. Tip/base resistance and shaft resistance depend on each other, and as such, separate analytical calculations lead to somewhat inaccurate result. Analytical methods usually lead to quite a conservative result. Thus analytical methods should not be used alone in the determination of pile bearing capacity. Pile tip failure mechanism Piling is classified as a deep foundation when its tip depth D divided with diameter B is greater than 4. According to classical bearing formulas of deep foundations, failure surface doesn’t reach to the ground surface but stays in the ground. As such, when estimating the base resistance, only failure surface area can be taken into account. The area is usually assumed to be not greater than 10 times the pile diameter B. Base/tip resistance or capacity (kärkikantavuus) of a pile in clay is: qb = Nq x ’p x Ap

Where:’p = effective pressure on the level of the base (from the level 10 x B to level of base) Nq = bearing capacity factor (below there is nomogram showed in piling instructions PO-2011, from which it is possible to define value of bearing capacity factor for frictional soils. Nomogram is based on Finnish bearing capacity measurements on large diameter piles) Ap = cross-sectional area of the pile


Nomogram for determining bearing capacity factor based on friction angle of soil (maan sisäinen kitkakulma). Friction pile – Bearing capacity Bearing capacity of a friction pile can be calculated with the static bearing capacity formula, consisting of the base resistance (or tip resistance) (first term) and shaft resistance (second term). where Pm is failure load Nq on bearing capacity factor (stiff clay 9, soft clay 1, frictional soils >10, see PO-2011 nomogram) p’ effective vertical stress at the level of the base Ap cross-sectional area in the level of the head K0 coefficient of lateral earth pressure at rest (= 1-sin) friction angle between pile surface and soil L length of the part of the pile in the ground uz perimeter (piiri) of the pile cross section z’ effective vertical stress at depth z

L

zzppqm dzuKANP0

0 'tan'


Dynamic pile driving formula (paalutuskaava) for driven piles In theory pile driving formula is:

Where QH = weight of the ram block (pudotusjärkäle), kN H = drop height, m Pm = failure load of the soil, kN e = permanent settlement of the pile, m C = elasticity (jousto) of the top of the pile during driving, m When operating efficiency and equipment are taken into account:

where k1 = efficiency (hyötysuhde) of the ram block (0,8 drop ram (vaijerijärkäle), 0,9 hydraulic ram ja 1 =accelerated ram (kiihdytetty järkäle))

k2 = coefficient of pile cap (iskusuoja) (0,85, if no measured information) k3 = coefficient of failure load of the pile (moraine 0,7…0,85; bedrock 0,85…1,0)

Friction and cohesion piles pile driving formulas drilling resistance based

methods load tests

static, e.g. dynamic


5.3 Pile design The design of the bearing capacity of a pile is done in accordance with the new PO-2011 (Paalutusohje 2011) guidelines based on the Eurocodes and the National Annexes of Finland (environment ministry). New piling classes (paalutustyöluokat PL1 …PL3) are presented in section 4 in the instructions. New piling instructions (paalutusohje) are published later this year. The piling classes are based on consequence classes (seuraamusluokka) and geotechnical classes (geotekninen luokka) of Eurocode. The most challenging projects belong to class PL3 and the easiest in PL1. The highest class can be achieved by strict control of pile driving. Piling class is defined by the projects characteristics, ground investigation results and the quality of the piles. Class sets the requirements for example for equipment, performance of the work, supervising and quality control procedures.

The highest piling class (PL3) requires inspected driving equipment and a qualified user. In the 2011 instructions the main responsibility for the definition of the end-of-driving criteria (loppulyöntiehdot) is transferred to the manufacturer of the pile. The principle is that the manufacturer provides the criteria, but they are checked by the piling committee of SGY (Finnish Geotechnical Society). The manufacturer must also provide the recommended maximum loads in serviceability state for end bearing piles and ensuring the geotechnical bearing capacity of piles based on dynamic load test results.

Determination of resistance (bearing capacity) according to Eurocode is based on the geology of Central Europe. In Central Europe the soil layers are thick, and clear bearing stiff layer / intact bedrock is usually at great depth. As such, in Central Europe friction piles are much more common compared to end bearing piles, which are the most common functional pile type in Nordic countries, also Finland. In Finland, friction piles can be used in some areas; Eurocode design approaches can be used in those cases.

According to Eurocode (EN-1997) dynamic load test can be used in order to estimate the compressive resistance of a pile if the amount of ground investigations is sufficient and the method has been calibrated using static load tests in similar subsoil conditions with similar piles (same type, length and cross-sectional area). In Eurocode, design methods are similar for all pile types and materials.


As such, Eurocode emphasizes the static load test as a reference test. However, in the case of end bearing piles, dynamic load test is reliable enough without additional static load tests on trial piles at the site.

The piling instructions (both 2011 version and 2016/2017 version) are divided into two parts: 1) Design (mainly based on Eurocode) 2) Execution of piling work (based on standards)

Empirically proven good methods and knowledge concerning piling are kept in the new Finnish piling instructions if they are not in conflict with European standards. Piling design is part of geotechnical design. It is important to study the subsoil conditions using sufficient amount of ground investigations. One should also examine nearby structures and their foundation methods and condition. When also the loads coming from superstructure are known the pile type (point-bearing piles or friction piles) and piling method can be selected. Fore mentioned and other design phases are listed below.


Pile design according to Eurocode is presented in the figure (Figure from Veli-Matti Uotinen) below.

In this method, either design approach DA2 or DA2* is used. In the method, loads are calculated first (kuormien laskenta). The characteristic values of loads (lower index k) are transformed into design values (index d) based on reliability class and partial factors. The final design resistance (kestävyys) of the pile is selected as the smallest result (either geotechnical resistance GEO or structural resistance STR). Corrosion has to be taken into account when designing steel piles. Usually it is considered by overdesigning so the entire steel piles material isn’t taken into account on design. Amount of the corrosion can also be affected by casting the inside of the pipe pile. Amount of corrosion depends on soil conditions (see figure below; Corrosion of steel pipes. EN 1993-5, PPO-2007, Pohjarakennusohjeet RIL 121-2004).


5.4 Compression resistance of a pile based on ground tests

Either design approach DA or DA* is selected. (A1 ”+” M1 ”+” R2). The principle is simple, the design load Fc;d has to be equal or smaller than to design resistance (bearing capacity) Rc;d:

Fc;d ≤ Rc;d

The calculation can be done using one of two methods; the model pile method or an alternative method. The geotechnical resistance of the pile Rc; d has to be calculated by summing the design shaft resistance R d to the design base (tip) resistance Rb; d. a) Model pole method

Geotechnical resistance is the sum of base resistance Ab ·qb;k and shaft resistance π · d · Hfriction · qs;k when the pile is circular:

Rc;k = Rb;k + Rs;k = Ab ·qb;k + π · d · Hfriction · qs;k

The characteristic values of base and shaft resistances are calculated with the formula

Where: Rs;cal = calculated shaft resistance (in ULS) based on ground test results Rb;cal = calculated base (tip) resistance (in ULS) based on ground test results calculated; determined for the profile of ground test results under consideration (’model pile’) Rc = geotechnical compressive resistance of the pile in ULS Rc;cal = calculated value of Rc Rc;d = design value of Rc Rc;k = characteristic value of Rc ξ3 ,ξ4 = correlation factors that depend on the number of test profiles, n

For structures having sufficient stiffness and strength to transfer loads from “weak” to “strong” piles, the values ξ3 and ξ4 may be divided by 1,1. (However ξ3 must be at least 1,0).


Homogeneity of ground test results – If needed, the area must be divided into homogeneous partial areas.

Note that the table used in Finland (Table A.10(FIN)) is from National Annex (Kansallinen liite) Correlation factors in the original Eurocode are slightly different

b) Alternative methods

In alternative method, ground tests are treated as one combined data from which characteristic values of base and shaft resistances are estimated. Model factors are used instead of correlation factors. The principle is shown below.

Note: The ’model pile’ method is recommended as it is more polished. ’Alternative’ method is acceptable if the model factors are selected correctly.

When the characteristic resistance has been calculated, the design value of compressive resistance is derived:

Rc;d = Rc;k /γt

or

Rc;d = Rb;k /γb + Rs;k /γs

The partial factors are listed in the table below (Source: Finnish National Annex, translated version).


5.5 Load tests

Static load test step-by-step loading; settlement and time are observed constant strain rate; load is observed

Interpretation of the static load test:

Static load test equipment and another result graph:

Dynamic load test (PDA-testing) Dynamic load test (also known as PDA-measurement, Pile Driving Analysis) is based on the shock waves traveling inside the pile, the measurement and analysis of it. The measurement is done during piling, and can be done to operative pile (as opposed to static load test). The results will be: Static failure load Value which represents the intactness of the pile


Maximum compressive and tensile forces affecting the pile Maximum displacement in the top end (yläpää) of the pile Energy transferring into pile which can be used to estimate efficiency of the driving

equipment

The principle is presented in figure above; in soft layer the settlement is large. In the case of bearing base, settlement is small or non-existent

Next, examples on results are presented. In the graphs, x-axis is time in milliseconds and c is speed of the chock wave traveling in the pile [m/s]. (Speed of sound in a pile 3300….3700 m/s).

If there is no base resistance (kärkivastus), for example when driving into clay:

Big base resistance (left) and big shaft resistance (right):


Interpretation either with CASE-method or CAPWAP-method

CAPWAP-method

Computer program which does signal modelling => for example bearing capacity can be determined.

More accurate results than CASE-method. One can define for example:

- the distribution between base resistance and shaft resistance - driving stresses in the pile (compression, tensile stress) - damping factor (vaimennuskerroin) - end-of-driving criteria for the pile

Characteristic value of compressive resistance based on dynamic load test According to Eurocode (EN 1997-1):

Correlation factors given in Finnish National Annex:


End bearing piles – embedding depth and design

Principle: End bearing piles are aimed to drive to a certain depth, so that bearing capacity is as high as possible, but so that the piles do not get damaged during the driving.

The embedding depth (tunkeutumissyvyys) of piles depends on: • Non-homogenic conditions, fills • Properties of the the bearing layer

Soil type Denseness and compaction Amount of stones/boulders

• Properties of the layers to be penetrated affect especially shaft friction (resistance) Soil type Thickness

• Size and material of the pile (greater embedding depth with smaller piles) • Pile installation equipment

The embedding depth is estimated based on ground investigations. The depth is until (at minimum) weight sounding depth (after blows or vibration drilling). However, dynamic probing test (heijarikairaus) or static-dynamic penetration test (puristin-heijarikairaus) give more accurate estimation. In the estimation, the most difficult cases are subsoils with thick silt or loose sand layers, loose moraine with boulders and subsoil conditions that change considerably.

Piling instructions PO-2011 list some recommended sounding methods for different piles (Table, Taulukko 3.1).


Design of end-bearing piles is based on selected piling class (PTL, paalutustyöluokka) and end-of-driving criteria (loppulyöntivaatimukset) given by pile manufacturers. Criteria states the maximum penetration of the pile [mm] during 10 blows (10 lyönnin sarja). When the criteria is met during pile driving, the final embedding depth - and as such the designed compressive srength - of the pile has been reached. However, the final geotechnical bearing capacity is defined based on PDA-measurements. Next, reading the end-of-driving criteria is instructed.


6 Selection of pile type

Reinforced concrete normal piles: • The most general sizes 250 x 250 sq. mm ja 300 x 300 sq. mm. Also sizes 350 x 350 sq.

mm and 400 x 400 sq. mm are manufactured. • Length approximately 3 m…12 m with rounded meters • Various rigid joints are used to extend the piles • Rock shoe (kalliokärki) or normal shoe (laatikkokärki, normaalikenkä)

Optionals of the reinforced concrete pile are:

- pile shoe (steel casing in the bottom) - steel rim (teräsvanne) (top end)

- pile joint (paalujatkos) - rock shoe

Reinforced concrete piles parts on the right.

Connecting pile to the structures with pile footing (paaluantura):

Driven reinforced concrete piles as end bearing piles are the most commonly used pile type in Finland. Availability of piles is good throughout the country. Piles need medium-heavy or heavy installation equipment. Penetration of reinforced concrete piles into stony fills or subsoil is quite poor. Fills often require a pre-drill hole for the pile. In addition, piles are easily damaged especially in stony subsoil/fill conditions or if the driving equipment is not suitable. This is due to the fact that tensile strength of reinforced concrete piles is relatively small and it is easily exceeded during pile driving. Pile driving causes vibration and it displaces larger soil mass compared to a steel pile with same bearing capacity.


Top of the reinforced concrete pile is usually made cast-in situ pile footing

Piling will be monitored at the site. Design of pile footing is necessary to check if the piles have large deviations in location. Driven steel pipe piles (teräsputkipaalut) Driven steel piles of SSAB, previously Rautaruukki (RR-piles) are manufactured from different steel grades (S355J2H, S440J2H and S550J2H) and diameters vary between RR75…RR1200 or 75 mm…1200 mm. Availability of RR-piles is rather good and they have a wide range of different lengths and dimensions. Intactness and straightness of the pile is easily checkable from inside of the pipe. For small steel piles has been developed effective splice (jatkos) technics, so there is no unnecessary waste of material and there is no need for welding on site (RR75-RR220). Splice technic for large diamater piles is usually welding. Steel pipe piles are displacing ground only a little and bearing capacity is large compared with the volume of the pile. Especially with micropiles the advantage is that the installation equipment can be relatively lightweight and driving equipment is based on either hydraulic ram (hydraulivasara) or hammer equipment (järkälekalusto). RR micropile parts are presented below:


Shoe types for large diameter RR piles are presented below:

Steel pipe piles are suitable for sites where there are no particularly large stones in the surface layers or other obstacles such as old timber piles, which can cause curving of the pile. Steel pipe piles are also suitable for sites where below the soft layers there is a very dense bearing moraine


layer or when piles have to be installed to bedrock through the moraine layer. Designing and optimizing of the pile size is easier for steel piles especially if designed pile loads are small or too large for traditional driven piles. Because steel pipe pile has a very good bearing capacity-volume ratio they are good option when there are nearby structures that are vulnerable to vibration, or structures that don't tolerate soil displacement or horizontal displacement or piling would cause excessive increase in pore pressure and possibly too low regional/local stability (soft clays). In piling sites with loose saturated silt and sand layers, driving of reinforced concrete piles might cause too much compaction with respect to nearby structure, and as such, steel pipe piles might be a better option. Also in sites where work space is limited and short pile elements must be used, steel pipe piles are more suitable. Large diameter RR steel piles are particularly competitive when:

Foundation has large concentrated vertical or horizontal loads, so large capacity of piles can be exploited in design.

Displacement of the soil, compaction or vibration is not a significant risk. Layers to be penetrated contain some rock and boulders Soil is very soft – buckling risk

Drilled piles (RD piles of SSAB) SSAB has significantly developed the drilled steel piles. Pile elements with wide range of dimensions and lengths are available. RD are easily and quickly spliced using external threaded sleeves (kierreholkkijatkos). Another splicing method is welding. Steel grade for RD piles is S440J2H and for RDs piles S550J2H. Also steel specially developed for piling use is available. Installing drilled piles is possible in all subsoil conditions. Installing the piles, drilling, doesn’t cause vibrations or it is very light for small piles. Drilled piles are always installed vertically. Drilled piling is an expensive design solution. Advantages of drilled piles is wide range of available dimensions and lengths. They can be installed in challenging soil conditions and reliable pile tip (base) contact and straight piling are achieved easily. Large diameter RD piles give high bearing capacity, bending resistance and stiffness. Another advantage is that the piles can be installed with strict position and inclination offset requirements. Other pile types Steel micropipes:

G-piles RR-piles OSD-piles X-piles (not manufactured in Finland anymore) Groutable bored piles (e.g. Titan) ar used as anchors.

Large diameter piles:

Franki-piles (combination of replacement pile and displacement pile) special case Franki-mixte pile

Vibrex-piles (displacement pile) Special piles:

Mega-pile (press-in pile), short pile, foundation underpinning (vahvistaminen) Titan pile, foundation underpinning Screw pile


Drilled pile (to bedrock) G-pile (Gustavsberg) (left) and X-piles (right):

Large diameter steel piles:


A new application is also drilled pile wall, which may result in greater load carrying capacity, the greater the stiffness of the retaining wall structures. The structures can be made watertight. Clearly, a more expensive solution Replacement piles, excavation piles (kaivinpaalut): allowed 5...8 MN/m2 tai allowed 8 MN/m2 depending for example: - checking the contact between pile and the bedrock - thickness of coarse grained soil layer on top of the bedrock - bedrock quality diameter target 0,5...1,5 m

7 Execution of the piling: piling equipment In pile driving usually either drop or hydraulic hammer (pudotus- tai hydraulijärkäle) is used. Nowadays hydraulic hammer is most commonly used because of its efficiency. When using hydraulic ram (hydraulivasara) the pile can penetrate far deeper than has been assumed in design in some frictional soils. Driving can be executed also with rapid percussion pile driver (nopeaiskuinen juntti) and vibrator (tärytin, täryjuntti) especially when installing sheet piles or steel piles. Piling equipment has also a driving helmet / pile cap / drive cap (iskusuoja), which holds the pile in place and centers the stroke. It also decreases stress peaks. Between drive cap and pile, there can also be pile cushion (iskutyyny). Pile cushion is not usually needed when driving steel piles. Sometimes an auxiliary pile is needed, if the top end of the pile has penetrated into the soil or the guides of the machine are above. The use of auxiliary pile is not always allowed.


7 Execution of the piling, environmental impacts Manufacturers have developed the end-of-driving criteria (loppulyöntiohjeet) for piles which depend on the pile material, piling equipment, piling class and the length of the pile. These are available online. In piling execution one must take into account:

Work and the Quality Plan (Työ- ja laatusuunnitelma) - piling executor prepares Piling work supervisor (paalutustyönjohtaja) must be named At site there may also be a assistant of piling work supervisor Transport and storage of the piles shall be made according to the manufacturers'

instructions The quality of the piles must be checked (CE-marking) Monitoring of work and realisation (työn ja toteuman seuranta) and documentation

( measurements e.g. PDA measurements, intactness measuring) Preparation of control documents (tarkastusasiakirjat) Cutting of the piles according to the plan Area required by the piling equipment (also in height) In design, one has to take into account the work bedding (työpeti, työalusta) (New

guidelines for the thickness of work bedding by SGY) Constructing a bedding for piling work ensures safe and effective piling work. Another objective of bedding is to ensure that nearby structures and apparatus are recognized and removed if needed (protection). Design of bedding is based on

Strength of the subsoil Installation equipment and method to be used (dimensions of the machine, stresses caused

by the weight (pohjapaine), needed space for safe installation Needed space (Note; battered (inclined) pile require more space)

The most common and recommended material for bedding is crushed rock (murske). Thickness of bedding varies from 200 mm to 1500 mm. In some cases rafts made of logs can be used. Special issues that need to be taken into account in bedding design:

If lightweight installation equipment is used, site traffic (trucks etc) might be more critical when it comes to bedding design

In challenging conditions (very soft soil) usage of geotextiles or deep stabilization might be required besides bedding stability

If scaffolds (teline) are used, the design of such structures much be checked (loads caused by pile installation equipment and pile installation itself)

Piling causes a variety of local environmental impacts. Construction of replacement piles might cause problems such as movements in soil layers and compaction (heave or settlement of ground, horizontal displacements, difficulties in the penetration through the subsoil). Piling can also cause disturbing of the soil and an increase in pore pressure (total stability, nearby structures). This can be a problem especially in tightly built areas or in areas with ground supported structures. Especially pile installation via driving causes vibration and noise. Noise is also caused by other


installation methods. In some cases, contamination of soil, water or air caused by piling work has been detected. In addition, the environmental impacts are caused by pile material, its preparation and installation work. 8 Pile raft and pile cap structures

Pile raft

The selection of the founding method is made based technical and economic comparison, taking into account the local circumstances and environmental factors and the width of the road section especially in urban areas. Pile raft (or piled raft) (paalulaatta) and pile cap (paaluhattu) structures are typically used in soft normally or slightly overconsolidated clay areas and other soft soil areas.

Pile raft structure is a continuous reinforced concrete slab founded on piles with the embankment on top. It is a non-settling structure for founding structures on soft and settling subsoils. Pile raft structure consists of the slab, piles and protective layer situated on top of the slab. Regularly used slab types are constant thickness, flat raft (tasapaksulaatta), pile raft thickened at pile connections (“mushroom raft”, sienilaatta) and beam raft (palkkilaatta). Flat pile raft can be reinforced against shearing or not (leikkausradoitettu/-raudoittamaton). In “mushroom raft” the intermediate slab can be reinforced in one or two planes. Next figures display the schematic drawings of the differnrt pile raft types. The structure and quality requirements are displayed in InfraRYL2010.

Pile raft is used in conditions where pile foundation is needed but pile cap structure is not possible due to technical or economical reasons. In difficult subsoil conditions pile raft has to be used.


Constant thickness slab (left) and “mushroom slab” (right) (Tiehallinto 2008):

Beam slab (Tiehallinto 2008):


The selection of pile raft type depends on material costs, timetable, shape of the slab, need for form work (muotitustarve), difficulty of the reinforcement work (raudoitustyö) and the possibility of differential settlement of the pile raft. Typical sites for pile raft are:

- Soft subsoil, thin dry crust (kuivakuori) - Inclined ground surface - Approach embankments (abutments) of bridges (tulopenkereet, keilat) - Deep soft soil basins → Pile raft foundation usually more economical - Low embankments (Note; the usage of reinforcements) - Slopes with gentle angles, fills next to the structure (usage of vertical piles) - Preparation (varaus) for widening or load increase → additional vertical piles at the sides

of the embankment (relatively easy to execute)

Pile cap structures

Pile cap structure (Figure 4) consists of separate pile caps. Pile cap structure consists of pile caps and piles. On top of the caps there is a layer that distributes loads and of which lower layer acts as a protective layer (suojakerros) for pile caps. Schematic drawing of pile cap structure is presented below (Tiehallinto 2008).

Typical pile cap structure projects are sites on stiff clay areas, where the shear strength of soft layer is sufficient to prevent the sinking of the soil between the caps and to support the piles and pile caps in the construction phase and after. Pile rafts created from separate smaller slab elements founded on three or four piles are also considered pile cap structures. These are used in railway repair and improvement, especially laterally moving bridges backing structures.

Pile cap structure can be used as a foundation method for a normal embankment if the shear strength of clay is at least 15 kPa and there is at least 1 m of dry crust (kuivakuori) under the shear plane. If there is no dry crust, the shear strength of clay must be over 20 kPa. The safety factor against failure calculated for the embankment without pile caps and with characteristic values must be at least 1,0. The subsoil under the pile caps must not settle more than 100 mm after the pile cap installation. Pile cap structure should not be used in peat sites.


The pile cap structure can be used in counter embankments and inner abutments (keila) of a bridge if the shear stregth of the clay under the dry crust is at least 20 kPa (Tiehallinto 2008). Pile cap structure should not be used in peat or gyttja (lieju) sites.

Pile cap structures cannot transfer significant horizontal loads. Battered piles (vinolaalut) transfer some of the loads into the ground, but structures containing significant horizontal loads must be designed as pile rafts. In railroad structures pile caps can only be used in repair or expanding of existing pile cap structures due to particularly high horizontal loads (breaking loads).

Requirements for using pile cap structures are usually symmetrical cross-section of the road and mostly flat and horizontal surface. Pile cap structure are not suitable for projects in which the widening of the road is expected. Schematic drawings for pile caps are presented in 2007; InfraRYL 2006:n täydennysosa: Tienrakennustöiden yleiset laatuvaatimukset ja työselitykset Perustamis- ja vahvistamistyöt.

Arching (holvaantuminen)

When dealing with pile cap structures it is crucial to ensure sufficient arching. Arching of soil is concept that is hard to explain. Roughly it is arch-like distribution of stresses in soil, contrary to the concept of equally distributed stresses. When part of the soil stays in place for example on pile cap and the part adjacent to it sinks, arching usually occurs. The internal shear strength of the soil tries to keep the soil in the original place. Shearing force is created between the downward moving soil and the soil that stays in place, and this force tries to resist the deformation and displacement of the ground. When part of the soil yields, arching is created in the unyielding parts. The earth pressure distributes more on the unyielding parts than the yielding parts. (Terzaghi 1947.) Principle of arching is showed in the middle of picture below (Zhuang et al. 2010). In the picture the arching is thought to develop as semicircular arc. Design can also incorporate the friction in the vertical interfaces presented on the left in the picture. On the right is presented the settling between the pile caps.

Coarse grained soil has larger shear strength compared to fine grained soil and can transfer the earth pressure downwards by arching. Arching is an advantage in construction because it distributes the earth pressure more evenly on unevenly load bearing ground and as such decreases differential settlements. Arching also strengthens the soil. If the arching fails or it doesn’t happen at all, differential settlements occur as the settlement is larger in yielding parts of the structure. (Ranin 2009.)


Principles of load distribution in an embankment reinforced with geotextile (upper Van Eekelen & al. 2008, lower Van Eekelen & al. 2008):

Arching begins, when soft subsoil between the piles starts to settle. This creates shear stresses in the layer on top of the piles and creates arches between the piles. Part A of the load is part of the load above the arching which is transferred straight to the piles. Part B of the load is the part of the load under the arching that isn’t transferred to the piles by the arching. Load B creates vertical load to the geotextile (dotted line). Note: In this structure the reinforcement textile is straight above the pile cap. Vertical load creates small displacements in the mesh which creates tensile stresses. These stresses are transferred by the textile to the piles. The load below the geotextile, part C of the load, is the part of the load that the subsoil layers carries.

The internal shear strength of the soil enables the arching. If the soil wouldn’t have shear strength it couldn’t have shear stresses because the stresses imposed to the ground would create displacements immediately. In soil that has arched the stress field travels from support to support. (Terzaghi 1947).

The arching is advanced by the increasing height H of the embankment, because the higher the embankment is the easier the soil grains can find an arch-like form over the span. The bigger the angle of friction in the ground ɸ or effective angle of friction ɸ’, the bigger is the internal shear strength. The effective angle of friction doesn’t take into account the effect of pore pressure in the angle of friction. Soil with bigger internal shear strength can bear greater loads without failure, this kind of soil also arches better. The distance between the supports b also affects the arching, naturally the smaller the distance between the support the easier it is for the soil to arch. The area and shape of the pile caps also affects the arching. Square shaped caps are better for arching


compared to circularly shaped caps. Even the shapes of the caps can be taken into account in the designing (Corbet et al. 2010). Arching can also be encouraged by geotextiles, because they distribute the loads to the columns or piles and prevent the soil masses from sinking. (van Eekelen 2010). The unit weight of the soil γ directly affects the amount of embankment load. Amount of load directly affects the sinking, so lighter soil is beneficial to arching. Though the unit weight of the fill material rarely varies, thus this doesn’t create significant results in aiding the arching. The angle of friction directly affects the internal shear strength so the bigger the internal angle of friction the more arching occurs.

The arching of soil fails, if the differential settlement between the piles and the soil between them is too large. Arching is easily dissolved (purkaa) by dynamic loads, like vibrations caused by railroad traffic. Heavy rain and the freezing and thawing of the ground also dissolves the arches. (Ranin 2009.)

Not all design methods take into account the bearing capacity of the subsoil. (Van Eekelen & al. 2011). Parts A, B and C are created by the vertical loads, in addition to this there are horizontal loads in the embankment which are created by the lateral earth pressure. (Tanska & al. 1995). The increase in the tensile stresses created by lateral earth pressure and the tensile stress created by the loads B and C together creates the tensile stress in the geotextile. (Van Eekelen & al. 2008.) In the picture below is presented van Eekeles (2010) load distribution which is used ie. in EBGEO design method.

British standard BS8006 (see figure below) is a method suggested before EBGEO. In BS8006 the load distribution is even on the loading strips. In this method, it is assumed, that the subsoil does not bear any loads (so load C is 0). Furthermore, it is assumed that when the height of the embankment is high enough, the arching is perfect, and all the loads above the arches are transferred into the piles. In this case the traffic loads are not assumed to cause loading to geotextile. Assuming perfect arching especially in shallow embankments can lead to too small design loads for the geotextile. This is taken into account in the method by stating that it must be checked that the load carried by the geotextile is at least 15% of the load created by the embankments. Tensile stress created in the geotextile is calculated with the load centered between the strips between the piles. (Van Eekelen & al. 2011.) This method is considerably more conservative than the model presented by EBGEO.


Literature

Corbet, S. P., Jenner, C. & Horgan, G. 2010. Revisions to BS8006 for reinforced soil – what do these mean for the industry? Ground engineering. April 2011.

Liikennevirasto, 2011, Eurokoodin soveltamisohje Geotekninen suunnittelu-NCCI7, 2011

Liikennevirasto, 2013, Paalulaatat ja paaluhatturakenteet, Suunnitteluohje luonnos 4.3.2013

Rakennusinsinööriliitto, 2005, RIL 223-2005 Lyöntipaalutusohje LPO-2005

Rakennusinsinööriliitto, 2011, RIL 254-2011 Paalutusohje 2011

Ranin, L. 2009. Junaratojen vahvistus suihkuinjektoinnilla. Diplomityö, Espoo, 81 s. civil.tkk.fi/en/research/soil/theses/d_laura_ranin.pdf

Tanska, H., Slunga, E., Forsman, J., Hoikkala, S. & Saarinen, R. 1995. Geovahvisteiden käyttö: Suunnittelu ja rakentaminen. Otaniemi: Teknillinen korkeakoulu. 120 s. ISBN 951-22-4461-6.

Terzaghi, K. von 1947. Theoretical soil mechanics. Fourth printing. Printed in the USA. 510 p.

Tiehallinto, 2001, Teiden pohjarakenteiden suunnitteluperusteet

Tiehallinto 2007; InfraRYL 2006:n täydennysosassa: Tienrakennustöiden yleiset laatuvaatimukset ja työselitykset Perustamis- ja vahvistamistyöt.

Tiehallinto 2008, Paalulaattojen ja paaluhatturakenteiden suunnitteluohje

Tielaitos, 1999, Pohjarakennusohjeet sillansuunnittelussa

Van Eekelen, S.J.M. & Van der Stoel, A.M.C. 2008. Piled embankments. [Verkkodokumentti] 91 esityskuvaa. [Viitattu: 15.3.2011] Saatavissa: http://www.delftcluster.nl/website/files/files_org/piled_embankments.pdf Van Eekelen, S.J.M. 2010. Piled embankments; Dutsch Standard. 51 s. (102 esityskuvaa). Technical University of Delft

two lectures of piling - aalto

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