construction technology lecture-2

7/30/2019 Construction Technology Lecture-2

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Construction Technology -1


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Contents

Site Analysis

Site Investigation

Soil

Soil Investigation

Foundations


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Site Analysis


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prior to purchasing a building site it is essential to

conduct a thorough survey to ascertain whether the sitecharacteristics suit the development concept. Thefollowing guidance forms a basic checklist:

Site Analysis.


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Refer to Survey maps to determine adjacent features,

location, roads, facilities, footpaths and rights of way.

Conduct a measurement survey to establish sitedimensions and levels.

Site Analysis.


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Conduct a measurement survey to establish site

dimensions and levels.

Observe surface characteristics, i.e. trees, steepslopes, existing buildings, rock outcrops, wells.

Site Analysis.


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Investigate subsoil. Use trial holes and borings todetermine soil quality and water table level.

Consider flood potential, possibilities for drainage ofwater table, capping of springs, filling of ponds,diversion of streams and rivers.

Site Analysis.


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Consult local utilities providers for underground andoverhead services, proximity to site and whether theycross the site.

Note suspicious factors such as filled ground, cracksin the ground, subsidence due to mining and any cracksin existing buildings.

Site Analysis.


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Regard neighborhood scale and character of buildingswith respect to proposed new development.

Decide on best location for building (if space permits)with regard to cut and fill, land slope, exposure to sunand prevailing conditions, practical use and access.

Site Analysis.


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Regard neighborhood scale and character of buildingswith respect to proposed new development.

Decide on best location for building (if space permits)with regard to cut and fill, land slope, exposure to sunand prevailing conditions, practical use and access.

Site Analysis.


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Site Investigation


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Site Investigation

Site Investigation For New Works ~

the basic objective of this form of siteinvestigation is to collect systematically and

record all the necessary data which will beneeded or will help in the design and constructionprocesses of the proposed work.


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Site Investigation

Procedures ~

1. Desk study

2. Field study

3. Laboratory analysis


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Site Investigation Desk Study ~ collection of known data, Include:

Ordnance Survey maps historical and modern,note grid reference.

Geological maps subsoil types

Site history green-field/ brown-field.

Previous planning applications/ approvals.

Current planning applications in the area.

Development restrictions conservation orders.

Utilities location of services on and near thesite.

Aerial photographs.

Ecology factors protected wildlife.

Local knowledge rights of way.

Proximity of local land fill sites.


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Soils


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Soil..

Most soils consist of a mixture of solid particles, water andair. In addition, the soil near the surface of the ground willalso contain organic material and this top soil must neverbe used as a base for a foundation. It varies greatly in

volume due to changes in water content and is veryunstable due to the amount of organic material it contains.Before construction commences the top soil(about 150-300mm thick) should be removed from the siteor stockpiled for future landscaping or off-site disposal.


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Soil..

Below the top soil lies the subsoil (often just referred to assoil) , which actually supports the building.

Apart from organic soils and rock there are two broadcategories of soil type: cohesive and non-cohesive.


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Soil..

Cohesive soils are those, such as clay and silt, withmicroscopic particle size and which can contain very highlevels of water. The shape of the tiny particles makes

them cling together trapping water in between and it is thiswhich gives clays their characteristic smooth, sticky feel.


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Soil..

Non-cohesive soils such as sands and gravels containnegligible amounts of water and have large particle sizeswhich are held together mainly by their weight.

Unlike clays they do not stick together and the soils can bevery loose in structure.


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Soil..

Soils under Load Increasing the pressure on a soil byapplying the load of a building squeezes some of thewater out of the soil causing it to consolidate andallowing slight settlement of the structure above.


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Soil..

Cohesive soils such as clay lose their water much moregradually and buildings may slowly settle for many yearsbefore equilibrium is reached. The softer clays contain

large amounts of water and thus permit extensive settlement.


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Soil..

In non-cohesive soils such as sands, water movementsare rapid and a building will normally complete itssettlement during its construction. In addition, becausesands only contain a very small amount of water any

settlement will be minimal.


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Soil.. some of the common soils and their suitability for house

foundations.


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Soil Investigation


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Soil Investigation..

Site Investigation

~ this is an all embracing term covering every

aspect of the site under investigation.

Soil Investigation

~ specifically related to the subsoil beneath the

site under investigation and could be part of orseparate from the site investigation.


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Purpose of Soil Investigation

1. Determine the suitability of the site for theproposed project.

2. Determine an adequate and economicfoundation design.3. Determine the difficulties which may arise

during the construction process and period4. Determine the occurrence and/ or cause of all

changes in subsoil conditions.


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The above purposes can usually be assessed byestablishing the physical, chemical and generalcharacteristics of the subsoil by obtaining subsoil

samples which should be taken from positions onthe site which are truly representative of the areabut are not taken from the actual position of theproposed foundations.


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Soil Samples~ these can be obtained as disturbed or asundisturbed samples.


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Disturbed Soil Samples

~ these are soil samples obtained from bore holesand trial pits. The method of extraction disturbsthe natural structure of the subsoil but such

samples are suitable for visual grading,establishing the moisture content and somelaboratory tests. Disturbed soil samples shouldbe stored in labeled airtight jars.


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Undisturbed Soil Samples~ these are soil samples obtained using coringtools which preserve the natural structure and

properties of the subsoil. The extractedundisturbed soil samples are labeled and laid inwooden boxes for dispatch to a laboratory fortesting. This method of obtaining soil samples issuitable for rock and clay subsoils but difficulties

can be experienced in trying to obtain undisturbedsoil samples in other types of subsoil..


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Bore Hole Data ~ the information obtained fromtrial pits or bore holes can be recorded on a proforma sheet or on a drawing showing the position

and data from each trial pit or bore hole thus:-


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Trial Pits and Hand Auger Holes

Purpose ~ primarily to obtain subsoil samplesfor identification, classification and ascertainingthe subsoils characteristics and properties.

Trial pits and augered holes may also be usedto establish the presence of any geologicalfaults and the upper or lower limits of the watertable.


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Trial Pits

General use ~ dry ground which requires little orno temporary support to sides of excavation.

Subsidiary use ~ to expose and/ or locate

underground services.Advantages ~ subsoil can be visually examinedin-situ both disturbed and undisturbedsamples can be obtained.


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Site Investigation

Hand Auger Hole

General use ~ dry ground but liner tubes couldbe used if required to extract subsoil samples at

a depth beyond the economic limit of trial holes.Advantages ~ generally a cheaper and simplermethod of obtaining subsoil samples than thetrial pit method.


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As a general guide the most suitable methods interms of investigation depth are

1. Foundations up to 6 000 deep trial pits/AugerHoles

2. Foundations up to 30 000 deep borings.


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As a general guide the cost of site and soilinvestigations should not exceed 1% of estimatedproject costs.


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Soil Investigation Methods

~ method chosen will depend on several factors

1. Size of contract.2. Type of proposed foundation.3. Type of sample required.4. Type of subsoils which may be encountered.


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Soil Classification

~ soils may be classified in many ways such as

geological origin, physical properties, chemicalcomposition and particle size. It has been foundthat the particle size and physical properties of asoil are closely linked and are therefore ofparticular importance and interest to a designer.


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Particle Size Distribution

~ this is the percentages of the various particle

sizes present in a soil sample as determined bysieving or sedimentation. BS 1377 divides particlesizes into groups as follows:-


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Gravel particles over 2 mmSand particles between 2 mm and 0 06 mmSilt particles between 0 06 mm and 0 002 mm

Clay particles less than 0 002 mm


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Particle Size Distribution Chart


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classifications can be further divided thus:- Theresults of a sieve analysis can be plotted as


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Site Soil Tests ~ these tests are designed toevaluate the density or shear strength of soils andare very valuable since they do not disturb the soil

under test. Three such tests are the standardpenetration test, the vane test and the unconfinedcompression test all of which are fully described inBS 1377; Methods of test for soils for civilengineering purposes.


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Standard Penetration Test

~ this test measures the resistance of a soil to the

penetration of a split spoon or split barrel samplerdriven into the bottom of a bore hole. The sampleris driven into the soil to a depth of 150 mm by afalling standard weight of 65 kg falling through adistance of 760 mm. The sampler is then driven

into the soil a further 300 mm and the number ofblows counted up to a maximum of 50 blows. Thistest establishes the relative density of the soil.


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Soil Investigation..Soil types as per SPT


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Vane Test

this test measures the shear strength of softcohesive soils. The steel vane is pushed intothe soft clay soil and rotated by hand at a

constant rate. The amount of torque necessaryfor rotation is measured and the soil shearstrength


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Unconfined Compression Test

~ this test can be used to establish the shearstrength of a cohesive soil sample usingportable apparatus either on site or in a

laboratory.


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Laboratory Testing ~ tests for identifying andclassifying soils with regard to moisture content,liquid limit, plastic limit, particle size distributionand bulk density are given in BS 1377.


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Bulk Density

this is the mass per unit volume which includesmass of air or water in the voids and is essentialinformation required for the design of retaining

structures where the weight of the retainedearth is an important factor.


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Shear Strength

this can be defined as the resistance offered bya soil to the sliding of one particle over another.


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Consolidation of Soil

this property is very important in calculating themovement of a soil under a foundation.


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Foundation Introduction


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Foundation -Introduction.

This chapter explains the principlesand practice of domesticfoundations. The chapter is divided

into two sections; the firstconcentrates on the general factorswhich affect the choice of foundationand the second considersappropriate solutions for specificground conditions.


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For most houses the design andconstruction of foundations is arelatively straightforward exercise.The simplest, and most common,form of foundation comprises a stripof concrete under all the loadbearing walls. The depth and widthof the concrete strip is determinedby the nature of the ground and theload of the building.


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The foundations of houses mustcarry the dead loads of the walls,roof and floors etc, together with theimposed loads of occupants andfurniture, and transmit them safelyinto the ground. They must bedesigned so that settlement issufficiently controlled to keep anydistortion (and possibly cracking) towithin acceptable limits.


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Foundation Some Definitions

Bearing pressure - The pressure onthe soil caused by the building load.

Bearing capacity - The load whichthe ground can carry.

Subsidence -Downward movement of

the ground caused by the impositionof internal forces, eg water, miningworks.


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Foundation Definitions ..

Settlement - Downward movementof the ground, or any structure on it,due to soil consolidation, normallycaused by the load applied by thestructure.

Compaction - The act of increasing

the density and strength of amaterial by the application of impactforces, eg a heavy roller.


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Foundation Definitions ..

Consolidation -The act of increasingthe density and strength of the soilby the expulsion of water and airunder self-weight and constantexternal loading.


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Foundation Introduction ..

Building Load For a typical modern three-bedroom detached house the total of thedead and imposed loads is about 120tonnes and most types of ground caneasily carry this load using simplefoundations.


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Thank You

Senarath Bandara

construction technology lecture-2

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