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Method of geotechnical investigation

14:00-14:50

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Preliminaryinvestigation

Detailedinvestigation

General investigation

Simple methods

Low cost

Wide area

Specific problems

Sophisticated methods

High cost

Specific area/soil type

Very detailed

investigation

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Which investigation methods do

you know?

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Preliminary investigation

Literature search

Existing data of boreholes, wells, and other soil investigations

Information on nearby structures such as foundation types, sizes, depths,settlements, and inclinations

Reconnaissance site visits

Geological reconnaissance (observation of outcrops at cut slopes along roads andexposed rocks on river beds; identifying problematic geology for foundation construction)

Terrain observation for landslides and problematic topography for bridge

construction

Geophysical investigations

Seismic prospecting for depths to the bed rock and for extent of weathering anddevelopment of cracks

Echo sounding for depths to sea bed

Borehole investigation and trial pits excavation

Boreholes for deep bearing strata; to study soil stratification and depth ofgroundwater table; to select suitable bearing strata based on , as an example, the

standard penetration tests (SPT); to estimate permeability and compressibility of theground based on various laboratory soil tests performed on SPT soil samples.

Trial pit excavations for shallow bearing strata

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Detailed investigation

Borehole investigationSoil sampling

SoundingSoil/rock tests

Groundwater investigationLoad tests

Geophysical prospecting and loggingPoisonous gas and Oxygen poor air

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Method of Soil Investigation

Boring

Standard Penetration Test (SPT)

Undisturbed soil sampling

Rock Coring

Full scale tests

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SPT

Standard Penetration Test

Standard split sampler (50mm in diameter)

Standard hammer (65.0kg, free falling from

a height of 760mm)

Blow counts (150+300+50mm)

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Sand Cohesionless soil,

granular soil)

SPT Relativedensity

0-4 Very loose

4-10 Loose

10-30 Medium

30-50 Dense

50- Very dense

Terzaghi, Peck and Mesri 1996) BS Classification

SPT Consistency Unconfinedcompressive

strength

kPa)

Consistency

Unconfined

compressiv

e strength

kPa)

0-2 Very soft Less than 50 Very soft Less than40

2-4 Soft 50-100 Soft 40-80

4-8 Medium 100-200 Firm 80-150

8-15 Stiff 200-400 Stiff 150-300

15-30 Verystiff

400-800 Very stiffor hard

Greaterthan 300

30- Hard Greater than800

Clay Cohesive soil) by

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Undisturbed soil sampling

What does undisturbed mean?

What are problems of disturbed soil

samples?

How to take undisturbed soil samples?

Soft clay, stiff clay

Loose sand, dense sand

Evaluation methods

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Rock Coring

Cylindrical pieces of rocks are retrieved during drilling in rocks. The rockcoringisthe act of taking these pieces of rocks with intention of full recovery in goodcondition for visual, physical and chemical examination.A tool to take the rock pieces consists of a bit and steel tube. The bit cuts the rockinto cylindrical form. The tube retains the rock pieces inside. The rock pieces arethen called coreor rockcore.The tool is called corebarrel.The most simple type ofthe core barrel is single core barrel which is essentially a steel tube with a bitattached to one end. There are double tube and triple tube core barrels, which canretain more rock cores than the single tube core barrel.Core recovery ratio is a measure for rock quality. It is the ratio of the cumulativelength of the cores recovered to the length of actual core-run. It is affected not onlyby the rock quality but also the tool used to take the cores and the skill of drillingoperator.Another measures for rock quality is RQD (Rock quality designation). The RQD is

the ratio of the sum of the length of intact rock pieces that are more than 10cm ineach length, to the length of the actual core-run. RQD is 100% if the rock cores arefully recovered with each piece longer than 10cm. However, RQD is 0% if each pieceis shorter than 10cm even if the recovery is 100%.Rock quality is important factor in design where tensile conditions are induced inrock mass such as excavation and tunneling. However, the rock quality is less

important where the rocks are under compressive stresses.

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Rock core

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Full scale tests such as pile load tests, platebearing tests, pull-out tests of anchors, testembankments on soft ground are conductedin two aspects; one as a part of design andthe other for quality assurance duringconstruction.

Full scale tests

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