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Subsurface Investigations PDCA Professor’s Driven Pile Institute

Loren R. Anderson

Utah State University

June 27, 2013

Ralph B. Peck (1962)

Subsurface engineering is an art; soil

mechanics is an engineering science…. We

would do well to recall and examine the

attributes necessary for the successful

practice of subsurface engineering. There

are at least three: knowledge of precedents,

familiarity with soil mechanics, and a

working knowledge of geology....

Geotechnical Engineering

Process (Worth, 1972)

1. Define the Project Concept

2. Project Site Reconnaissance.

3. Develop a “Working Hypothesis” of the

Subsurface Conditions.

4. Plan a Field Investigation to Test the

“Working Hypothesis.”

5. Develop a Model for Analysis.

Geotechnical Engineering

Process (cont)

6. Evaluate Alternative Schemes.

7. Make Specific Recommendations

8. Prepare Plans and Specifications

9. Construction Inspection and Consultation

10. Performance Feedback

PDCA Design and Construction

of Driven Pile Foundations ,

Volume I

• Chapter 4,

“Subsurface

Explorations”

• Chapter 5, “In-Situ

Testing”

• Chapter 6,

“Laboratory Testing”

Subsurface Exploration

• Planning the

Exploration (Office)

• Field Reconnaissance

Survey

• Detailed Subsurface

Exploration

Subsurface Investigation

What do we need? • Pile capacity (shear

failure)

• Lateral capacity

• Driving resistance

• Pile length

• Pile group

settlement

Subsurface Investigation

Guidelines – Minimum Program • Number and location of borings – may change

• Drilling method

• Depth of borings – may change

• Type of samples to take

• Standard Penetration Test – ASTM T206, also consider liquefaction – Engineer is in charge

• Extend boring into rock – say 10 ft.

• Proper field logging

• Water level – at time of drilling and future

• Backfill borings

Subsurface Exploration,

In Situ Testing and Laboratory

Testing

Reference Manual Chapters 4, 5, & 6

Lesson 3

Subsurface Explorations

Foundation design requires adequate

knowledge of the subsurface conditions

With the appropriate information, an

economical system can be designed

Subsurface Explorations

Absence of thorough geotechnical data often

leads to:

• Large factor of safety, generally more

expensive and/or difficult to construct

• An unsafe foundation

• Construction disputes & claims

A thorough foundation study

consists of:

• Subsurface exploration program

• Laboratory testing

• Geotechnical analysis of all data

• Design recommendations

Boring Plan Guidelines • One boring / substructure unit

• More borings for substructures > 30 m (100 ft)

• Stagger boring locations

• Confirm suitability of boring depth for design purpose early

• Extend through unsuitable layers, terminate in hard or dense materials

Boring Plan Guidelines (cont.)

• Thoroughly explore the affected depth

• SPT samples at 1.5 m (5 ft) intervals and at

strata changes

• Undisturbed sample locations/frequency to

meet project needs

• NX size rock core obtained for a depth of 3

m (10 ft) where rock is encountered

Boring Plan Guidelines (cont.)

• Crews should maintain a field drilling log

• Samples should be properly labeled, sealed,

and transported

• Water level readings made and recorded

• Bore holes properly backfilled & sealed

Soils Samples

Disturbed

Undisturbed

Identification and

Classification Tests

Consolidation,

Shear Strength, and

Permeability Tests

SPT Split-Barrel Sampler

(a) (a)

Standard Penetration Test

Advantages

Disadvantages

• Widely used

• Substantial data available

• Simple & inexpensive

• Provides disturbed soil samples

• Highly variable

• N-value determined from test is

influenced by many factors

SPT Hammer Types

Donut Safet

y

Automatic

Automatic Safety

SPT N Value

Comparison

for Safety

and

Automatic

Hammers

SPT Hammer

Calibration

N60 = N

(ETR/60)

ETR = EMX / 350 ft-

lbs

SPT N Values

SPT hammer type & operational

characteristics have a significant influence

Type of hammer should be clearly noted on

all boring logs

SPT Error Sources

• Effect of overburden pressure

• Variations in hammer drop heights

• Interference with hammer free-fall

• Damaged or worn sampler drive shoe

• Failure to properly seat sampler at bottom

of hole

SPT Error Sources • Inadequate cleaning of loose material at

bottom of hole

• Failure to balance hydrostatic pressures

inside & outside of borehole

• Unreliable results in gravelly soils

• Samples from dilatant soils may cause

plugging

• Careless work by drill crew

Figure 4.3 Chart for Correction of N-values in Sand for Influence of EffectiveOverburden Pressure (after Peck et al., 1974)

Figure 4.3 Chart for Correction of N-values in Sand for Influence of EffectiveOverburden Pressure (after Peck et al., 1974)

N’ = CN (N)

N’ = corrected SPT N value

CN = correction factor for overburden pressure

N = uncorrected or field SPT N value

TABLE 4-5 EMPIRICAL VALUES FOR , Dr, AND UNIT WEIGHT OF GRANULAR SOILS BASED ON

CORRECTED N' (after Bowles, 1977)

Description Very Loose Loose Medium Dense Very Dense

Relative density

Dr

0 - 0.15

0.15 - 0.35

0.35 - 0.65

0.65 - 0.85

0.85 - 1.00

Corrected

Standard

Penetration

N' value

0 to 4

4 to 10

10 to 30

30 to 50

50+

Approximate

angle of

internal

friction *

25 - 30˚

27 - 32˚

30 – 35˚

35 - 40˚

38 - 43˚

Approximate

range of moist

unit weight,

, kN/m3

(lb/ft3)

11.0 - 15.7

(70 - 100)

14.1 - 18.1

(90 - 115)

17.3 - 20.4

(110 - 130)

17.3 - 22.0

(110 - 140)

20.4 - 23.6

(130 - 150)

Correlations may be unreliable in soils containing gravel. See discussion in Section 9.5

of Chapter 9.

* Use larger values for granular material with 5% or less fine sand and silt.

TABLE 4-6 EMPIRICAL VALUES FOR UNCONFINED COMPRESSIVE STRENGTH (qu) AND CONSISTENCY OF COHESIVE SOILS BASED ON UNCORRECTED N

(after Bowles, 1977) Consistency Very Soft Soft Medium Stiff Very Stiff Hard

qu, kPa

(ksf)

0 – 24

(0 – 0.5)

24 – 48

(0.5 – 1.0)

48 – 96

(1.0 – 2.0)

96 – 192

(2.0 – 4.0)

192 – 384

(4.0 – 8.0)

384+

(8.0+)

Standard

Penetration

N value

0 - 2 2 - 4 4 – 8 8 - 16 16 - 32 32+

(saturated),

kN/m3

(lb/ft3)

15.8 - 18.8

(100 – 120)

15.8 - 18.8

(100 – 120)

17.3 - 20.4

(110 – 130)

18.8 - 22.0

(120 – 140)

18.8 - 22.0

(120 – 140)

18.8 - 22.0

(120 – 140)

The undrained shear strength is 1/2 of the unconfined compressive strength.

Note: Correlations are unreliable. Use for preliminary estimates only.

Undisturbed Samplers

• Thin wall open shelby tube

• Piston

• Hydraulic piston

• Pitcher

• Denison core barrel

Shelby Tube Samplers

Ground Water Monitoring

• Observation well

• Piezometer

Soil Profile Development

• Visual presentation of subsurface conditions

• Uncertainties in profile indicate need for

more borings & lab tests

• Develop in stages as data is available

• Identify soil layers, then lab test

determinations

Soil Profile Development

• Final soil profile includes:

– Soil physical properties (, qu, Cc, , etc.)

– Visual soil description

• Note ground water, boulders, voids, artesian

pressures, etc.

• Well developed soil profile is necessary to

design a cost effective foundation

IN-SITU TESTING

Reference Manual Chapter 5

In Situ Testing

• Provides design parameters in conditions

where high quality undisturbed samples

cannot be obtained

• In-situ tests are performed to obtain

foundation design parameters

Primary In-Situ Tests

• CPT – Cone Penetration Test

• CPTU – CPT with pore pressure measurement

• PMT – Pressuremeter Test

• DMT – Dilatometer Test

• VST – Vane Shear Test

Interpretation of CPT/CPTU Results

• Provides a continuous profile of subsurface

stratigraphy

• Soil classification from cone tip resistance &

friction ratio

• CPT correlations for evaluation of:

– Dr

–

– qu

A Series of Exploration Events

Setup at the Site


Push the Cone


Into the ground


Induce a Shear Wave


Interpret the Results


Leave the Site


Plug the hole from China


Drive a Pipe


It’s dark and the water flows


We even used our pants

Advantages of CPT/CPTU

• Rapid & economic development of

continuous profile of subsurface conditions

• Determination of in situ strength parameters

• Reduce the number of conventional borings,

or focus attention on discrete zones for

sampling & testing

CPT/CPTU Disadvantages

• Cannot be pushed in dense soils

• CPT must be pushed in borehole advance

through dense deposits

• Soil samples not recovered

• Local correlations are important in data

interpretation

Pressuremeter and Dilatometer

• Limited applicability for vertically loaded

pile foundation design

• Pressuremeter useful for p-y curve

determination for lateral load designs

• Dilatometer has potential usefulness for

lateral load design

Vane Shear Testing

• In situ test to determine undrained shear

strength of soft to medium clays

• Measures peak & remolded strength

• Most accurate method for qu < 50 kPa (1 ksf)

• Very useful data for driveability analysis &

soil setup evaluation

LABORATORY TESTING

Reference Manual Chapter 6

Laboratory Testing

The trend to higher capacity piles & greater

pile penetration depths for special design

events reinforces the importance of accurate

determination of soil shear strength &

compressibility.

Laboratory Testing

• Quality of results far more important than

quantity of test results

• Inaccurate results may lead to design

misjudgements &/or construction problems

Laboratory Testing Cohesionless Soils:

SPT & CPT primary tools for and Dr,

complimented by index testing

Cohesive Soils:

Traditional tests on undisturbed samples yield best

results for qu and Cc

Lab QC Procedures • Sample handling & storage

• Sample prep

• Adherence to procedures

• Equipment calibration

• Qualification of personnel

• Result review & checking

• Reporting of test results

Types of Lab Tests

• Soil classification & index

• Shear strength

• Consolidation

• Electro chemical classification

Classification & Index Tests

• Moisture content

• Particle size analysis

• Atterberg limits

• Unit weight

Shear Strength Tests

• Unconfined compression

• Direct shear

• Triaxial compression

Consolidation Tests

• Amount of settlement

• Time rate of settlement

Electro Chemical Classification

• pH

• Resistivity

• Sulfate content

• Chloride content

Lab Tests for

Driveability Evaluations

• Remolded shear strength of cohesive soils

– Sensitivity St = qu-undist / qu-remold

( Sensitivity qualitative not quantitative indicator of soil setup )

• Gradation of cohesionless soils

– Fine content

– Angularity

Lesson 3 – Learning Outcomes

You will be able to:

• Explain what field & lab test results are needed

to develop a design soil profile for pile design.

• Identify SPT hammer types and their influence

on “N” values.

• Discuss the importance of quality subsurface

information & lab test results in pile design.

About the Site

USU Drainage Farm

• Infrastructure Engineering

• Earthquake Engineering

• Dynamic Testing

• Rehabilitation with

composite materials

• Wind Engineering

Pile Test Site

Any Questions

Recruiting Strategy • High School Bridge

Contest

– 10 to 19 schools

– Promotes USU and

Engineering

• Hospitality Tours

– Coordinate with

Math/Science Teachers

– Follow up letter

– 5 games, 16 HS, 500 +

students

• High School visits and

tutoring (Envir)

Purpose of the I-15 Testbed

• I-15 Corridor – Window

of opportunity

• Old Structures – a

destructive testing

opportunity

• New construction

– Behavior of soft clays

– Performance of innovative

structures

– Instrumentation

Sometimes we worked late

Sometimes it was overwhelming

There was always a bright side

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