Site Exploration and Characterization

“Subsurface material properties cannot be specified; they must be deduced through exploration.”

Charles Dowding (1979)

Objectives

Determine location and thickness of soil and rock strata (subsurface soil profile)

Determine location of groundwater tableRecover samples for laboratory testingConduct lab and/or field testingIdentify special problems and concerns

Project AssessmentLiterature SearchField ReconnaissanceSubsurface Drilling and SamplingLaboratory Testing of Soil Samples

Site Exploration; General Strategy

Type, location and approximate dimensions of the proposed development

Existing topography and any proposed grading

Any previous developments

Project Assessment

Geologic Maps (e.g. USGS, Bureau of Economic Geology)

Soil Survey Reports (USDA Soil Conservation Service)

Geotechnical ReportsHistoric Groundwater Data

Literature Search

Conventional Aerial Photographs Geologic features (landslides, faults),

topography, drainage patterns Site history

Infrared Aerial Photographs Springs, seepage zones Useful in slope stability investigations

Remote Sensing

Any previous developments, grading etc.

Site topography, any signs of slope instability (landslides, soil creep)

Site drainage conditionsRock outcropsSite access

Field Reconnaissance

Field Exploration

Site Boring LayoutTest Borings or Test Pits?

It depends on the type of materials, and what you want to know.

Number and Frequency of BoringsDepth, Sampling Methods and

Field Testing

III. Subsurface Exploration/Sampling

Borehole Spacing Site conditions/uniformity type of structure (bridge, building,

landfill) typically one borehole/2500 ft2

Also see Table 4.1 (p.108)Borehole Depths

Magnitude of loading/soil conditions Also see Table 4.3 (p.109)

How Many Borings?

How Deep?

Dial Before You Dig

One Call SystemUnderground Utility Locator System

In Texas:1-800-344-83771-800-DIG-TESS

Look Up and Live!

Safety AwarenessRegular Emphasis

Subsurface Drilling Auger Drilling

Solid Stem AugerHollow Stem Auger

Rotary Drilling Bucket Auger Percussion (or Cable Tool) Drilling

Drilling and Sampling of Soils

Auger Drilling

Hollow Stem Auger Casing with outer spiral Inner rod with plug/or pilot

assembly For sampling, remove pilot

assembly and insert sampler Typically 5ft sections, keyed,

box & pin connections Maximum depth 60-150ft

Auger Drilling

Hollow-Stem Augers

Bit at the end of drill rod rotated and advanced

Soil/rock cuttings removed by circulating drilling fluid

Common drilling fluid; bentonite in water with slurry density 68-72pcf

Air may be used as drilling fluid

Rotary Drilling

Not common in US in geotechnical explorations

Heavy impact drilling tools lifted and dropped

Impact loosens soil and rock Cuttings removed with a

bailer Slow process; Used in

environmental explorations where drilling fluid is not permitted

Cable Tool Drilling

Rock Drilling, Coring, Augering

http://www.globaldrilsup.com/cat-index.html

Soil and Rock Sampling

Disturbed samples In-place structure is not preserved Okay for determination of soil index

properties

“…Estimating the nature of the formation from the cuttings is like identifying the cow from the hamburgers.”

G.F. Sowers

Soil and Rock Sampling

Undisturbed samples Minimizes effects from potential

disturbance Needed for determination of in-situ

density, in-situ permeability, soil shear strength and compressibility

Soil Samplers

Standard Split Spoon Samplers Shelby Tube SamplersPiston SamplersHeavy Wall Samplers

Standard Split Spoon Samplers

Thick wall (0.25in) cylinderSampling tube is split along the

lengthHammered into the ground

Standard Split Spoon Sampler

Shelby Tube (Thin-wall) Samplers

Thin wall (1/16in) sampling tube

Sampler pushed into the ground hydraulically

Sample extruded from tube

Piston Samplers

Minimizes sample disturbance caused by back-pressure

Heavy-Wall Samplers

Thicker walls provide better strength & durability

However, it creates more disturbance

Sampler pounded into the ground

Groundwater Monitoring

Groundwater level must be determined during geotechnical investigation

Can be accomplished by leaving selected soil borings open

In-situ Testing

When it is difficult to obtain “undisturbed” samples

Cohesionless soils, Sensitive claysIn-situ Test Methods

Standard Penetration Test (SPT) Cone Penetration Test (CPT) Pressuremeter Test Flat Plate Dilatometer Test

Standard Penetration Test (SPT)

140 lb (63.5 kg) Hammer 30in (76 cm) free fall Drive sampler over 18 inchesRecord no. of blows per each 6 inch

penetrationSPT blow count=blows for 2nd 6 inch

penetration + blows for 3rd 6inch penetration

Standard Penetration Test (SPT)

Types of SPT Hammers

SPT

SPT

Corrections to SPT blow Counts

Factors affecting SPT blow count: Hammer Efficiency (See Table 4.3)Borehole diameter (See Table 4.4)Type of sampler (See Table 4.4)Rod length (See Table 4.4)

SPT Correction Factors

60.0

NCCCEN RSBm

60

hammer efficiency (Em) …. Table 4.3 bore hole diameter (CB)…….Table 4.4. sampler correction (CS) ……Table 4.4 rod length (CR) ………Table 4.4

SPT Overburden Correction

)(Customary /2000

)(2

60601z

ftlbNN

(SI) 100

)( 60601z

kPaNN

Use of SPT Data

To Determine Relative Density, Dr

From AASHTO Chart From Eq. (4.3) p.122

To determine From Figure 4.11 (p.123)

To determine C From AASHTO Chart