site investigation

Site Investigation

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Goal:�Estimate geometry of soil strata and ground water

�Field investigation � identify materials and layering, retrievesamples and engineering properties through in situ testing

�Laboratory testing � Determine engineering properties fromsamples

Site reconnaissance: All sites should be visited by anexperienced professional to collect first-hand information �geology, terrain and equipment access, existing structure andtheir condition, existing utilities and potentially hazardousconditions.





Check list:�Examine exposure of soils and rocks in cuts (highway, railroads), building excavations, gravel pits, stream banks etc), and on the surface and note effluent groundwater seepage.

�Examine slopes for sign of instability

�Examine existing structures and pavements for signs of distress

�Note evidence of flood levels along streams

�Contact local well drillers for information on ground water condition�Contact local public officials for building code data and information on foundations and soil conditions

�Note site conditions imposing constraints on access for exploration equipment





Desk study prior to field investigation

�Air photos � vegetation (water table depth), ground color (soil or rock type), Slides, faults

�Maps � Topographic map, geologic map (rock types on surface, deposition of soils, geology

�Nearby structure

�Literature � previous reports and published work

�Local knowledge





Planning for subsurface soil exploration

Three steps:

Boring - refers to drilling or advancing a hole in the ground (75 � 600 mm dia)

Sampling - refers to removing soil from the hole

Testing � refers to determining characteristics or properties pf the soil.





Determine:

�Number of boreholes, borehole spacing and location, depth of borehole

�Type of sampling device

�Sampling interval

�Additional in-situ test

�Relationship with laboratory testing programInstrumentation





Borehole spacing

Spacing depends on project, surface conditions, etc.

�Building � approximately 15 m (50?) apart in both direction, focus on corners and key interior locations

�Site development � every 50 to 150 m apart for preliminary studies

�Soil wall or cut � every 30 m interval

�Earth dam � every 60 m across foundation

Road � every 100 m interval





Borehole depth�Buildings � to depth where vertical pressure is less than 10% of the pressure at the surface (~2 to 3B) but not less than 10 m and 3m into rock. If soft soil strata exists go deep enough to define geometry

�Cuts � ¾ to 1 times the base width

�Excavation � 2H or 2B

�Earth dams � ½ times the base width

�Piles � 3 to 5 m below the pile tip





Sampling interval

Every stratum or every 1m to 3m

Typically every 1.5 m (observe cutting and sample more if necessary)





Goal - identify materials and layering, retrieve samples and engineering properties through in situ testing

Primary tools are borehole sampling, Cone penetration test (CPT), seismic testing

Clay materials � CPT, Undisturbed samples from drilling, field vane

Sands � CPT, SPT and disturbed samples





Boring and sampling:

Field engineer should record all relevant information (time, date, weather, type of equipment, casing used, drilling mud or dry, depth at which water table 1st encountered, loss of drilling fluid, water level at the end of the boring, anything unusual).





Boring equipment:�Hand auger � small jobs only, shallow borings

�Truck mounted auger � above water table in sands, silts and clays (may collapse in soft soils or below water table)

�Rotary wash drilling � Most conditions, requires truck/drill rig and water for drilling mud

�Wire-line drilling � deep holes and off shore drilling

�Percussion drills (Becker) � gravels or soils with boulders Casing � used to support the hole and prevent caving





Cased AugerRotary wash





Sampling equipment (sampler):

Disturbed samples (thick walled) - Used for routine jobs to identify soils and determine index properties�Split barrel SPT sampler�Augured samples

Undisturbed samples � More complex jobs or where clays exist. Samples used for laboratory testing for strength and consolidation.





( )%10100

2

220 á

-= x

D

DDA

i

ir

( )%1

100á

-=

e

eir D

DDC

Requirements of sampler for undisturbed sample:

Area ratio,

Inside clearance ratio,





�Hydraulic piston � Thin walled or Shelby tube pushed ahead of device

�Pitcher/Denison Barrel � hard/dense soils removed by advancing tube while cutting

Hand cut block samples � highest quality, most expensive, only retrieve from excavation





In-situ Testing

Extensive use, especially in ground improvement evaluation(SPT, CPT, Geophysical testing, vane shear)

Advantages:�Use in soil that can not sampled easily�Tests in correct environment�Continuous profile (CPT)�Cost effective

Disadvantages:�No samples for identification�Some unknown test conditions�Need empirical correlationTests only current conditions





Standard penetration testing (SPT)

�Oldest and most widely used�Good for most soils�Useful correlations � especially for ?, Dr and E�Standardized: SPT Number, N = no. of blows/ 300 mm using 65 KGs hammer falling from 750 mm height, Sampler � 600 mm long, 50 mm outside diameter and 35 mm inside diameter.Requires corrections





NCN n=1

'

100

vzs=

Overburden correction

Where N is the field N value, and N1 is corrected N value afteroverburden correctionCN = correction factor for overburden

?vz? = overburden pressure at depth z in kPa





N-value Approximate relative density

Approximate angle of internal friction, f

0 � 5 0 � 5 26 - 30

5 � 10 5 � 30 28 � 35

10 � 30 30 � 60 35 � 42

30 � 50 60 � 95 38 - 46





Relation of Es with SPT No. in cohesionless soil:

Non-plastic silty sand mixture 4 N t/ft2 or 400 N kPa

Clean fine to medium sand 7 N t/ft2 or 700 N kPa

Coarse sands and with a little gravel 10 N t/ft2 or 1000 N kPa

Sandy gravel and Gravel 12 N t/ft2 1200 N kPa





Relation of SPT No. with consistency of clayey soil

SPT Value (N) Consistency Unconfined compressive strength, qu, inkN/m2

0 � 2 Very soft 0 - 25

2 � 5 Soft 25 - 50

5 � 10 Medium stiff 50 - 100

10 � 20 Stiff 100 - 200

20 � 30 Very stiff 200 - 400

> 30 Hard > 400





Cone Penetration Test

Newer methodGood for developing profile (continuous) but do not get sampleFaster and less expensive than drilling and SPTUse in conjunction with SPTMeasures friction and cohesion also can measure pore pressureand shear wave velocity (with piezocone).





Other methods:

Vane shear

Geophysical-seismic refraction, cross hole, down hole, up hole,electrical resistivity etc.

Pressure meter etc.





Geophysical method :

Boring and test pits is time consuming and expensiveGives subsurface conditions only at boring or test pit locations

Geophysical method is quick, less expensive, can cover greater areathoroughly. But results are more in qualitative nature.

Seismic methodElectrical resistivity method





Seismic method: Elastic waves are generated by some energysource. There are mainly three different types of elastic waves.

1. Compression or Primary wave (P-wave) has the highestvelocity,

2. Shear wave (S-wave) is 0.58 VP for well consolidated soil and0.45 VP for poorly consolidated soil.

3. Rayleigh wave (R-wave) its effect is generally near surface andamplitude attenuate rapidly with depth. VR = 0.9VP.





Seismic refraction method: First arrival time of P waves arerecorded through a seismogram, then travel time vs traveldistance plot. From the plot one can obtain Thickness of thelayers and velocity of waves through each layer which is relatedto the shear modulus/elastic modulus of the soils.





Seismic direct method:

Up hole surveys

Down hole surveys

Cross hole surveys

Seismic reflection method: off shore





Electrical resistivity method:Please purchase PPT to Flash on http://www.verydoc.com to remove this watermark.




Material Resistance (ohm-meter)

Clay soil, wet to moist 1.5 � 3.0

Silty clay and silty soil 3 - 15

Silty sand and sandy soil 15 - 150

Bed rock 150 - 300





site investigation

Documents

soil conditions

borehole sampling

ground color soil

hole testing

surface conditions

geometry of soil strata

ground water condition

soft soil strata