File 3 Method of Investigation

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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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