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Subsurface Exploration

Purpose and Scope of Soil Exploration 1. Definition of soil exploration

Field and laboratory investigations to obtain physical properties of soils involved 2. Influence of soil conditions on exploratory program

- Choose the method of soil exploration in accordance with the type of soil profile at site - Simple/regular ground condition

more of less parallel boundaries between strata considerable amount of soil testing of excellent quality samples relatively accurate forecast of magnitude and rate of settlement eliminate the danger of differential settlement by appropriate distribution of loads adjustment of subbasement depth deep foundation

- Erratic ground condition more or less irregular or not well-defined pattern of boundaries between strata deposition by ice, landslides, torrential streams etc. difficult to determine average values of soil properties little information from index properties of several representative samples extracted

from exploratory drill holes less expensive subsurface soundings along closely spaced vertical lines discovery of weak or compressible spots is more important than an accurate

knowledge of properties of random samples - Stiff clays and soils with great cohesion

associated with secondary structure development after soil deposition cracks & joints due to exposure to atmosphere after deposition slickensides due to volume change by chemical process or due to deformations by

gravity or tectonic forces results of laboratory test or field penetration test may give erroneous conception of

its mechanical properties better to rely on experience or on large scale field tests

- Degree of variation of physical properties considerable in vertical direction & smaller variation in horizontal direction

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Variation in wn of clay which appears homogeneous visually

Variation in wn of composite shore deposit

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3. Influence of project size on exploratory program - Small-scale construction

can not afford large expenditure small number of exploratory borings and a few classification tests on representative

soil samples lack of accurate information is compensated by a liberal factor of safety in design

- Large-scale construction extensive subsoil investigations cost of thorough and elaborate subsoil investigation is small compared to the savings by utilizing the results in design and construction the expenditures arising from a failure due to erroneous design assumptions

- To perform exploratory program to satisfy the requirements of a given job and to obtain essential data at minimum cost and time, engineer in charge must be familiar with; tools and processes for exploring soils methods for analyzing results of laboratory and field tests uncertainties involved in the results obtained by different methods of soil exploration

4. Cause of misjudgement of subsoil conditions

- Reliability of design depends on the differences between the real and idealized subsoil conditions

- Idealization of subsoil conditions construct a soil profile by interpolation between drill holes or soundings divide the subsoil in zones of materials with similar engineering properties estimate the average values of pertinent soil parameters for each zone

- Cause of misjudgement differences between test and field conditions laboratory tests on excessively disturbed soil specimens failure to recognize the most unfavorable subsoil conditions inadequate communication between designer and construction engineer

5. Observational procedure (정보화시공)

- Design based on the most unfavorable assumptions safe but uneconomical

- If permit modification of design with observational procedure achieve economical design on the basis of the most probable rather than the most

unfavorable possibilities

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the design is modified as a sequence of observations during construction - Requirements for observational procedure

must know the presence and characteristics of weak or compressible zones before construction must have special provisions to secure quantitative information on undesirable

characteristics of weak zone during construction field monitoring of pore pressure, load and stress, vertical and horizontal

displacements, quantity of water flow - References

(a) Casagrande (1965) "Role of the 'Calculated Risk' in Earthwork and Foundation Engineering", J. SMFE, ASCE, 91(4), pp. 1-40.

(b) Peck (1969) "Advantages and Limitations of the Observational Method in Applied Soil Mechanics", Geotechnique, 19(1), pp. 171-187.

Methods of Soil Exploration 1. Principal procedures

- Review of all available information on geological and subsurface conditions - Investigations

drilling sampling and laboratory test field tests geophysical method

- Report 2. Considerations on the tools for the site investigation

- Selection of tools best to have - Checklist of features

how the tool is used critical dimensions characteristics of exploratory tool what does it well? what does it do poorly? what it can not do at all?

accessibility of the tools to the site

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3. Boring (1) Wash boring - casing, weight, derrick, wash pipe, chopping bit, pump

- Procedure drive a casing into ground lower the wash pipe into top of casing water circulation

tub→pump→swivel head→wash pipe→chopping bit→annular space b/w wash pipe & casing→tee→tub

advance wash pipe by churning, rotating, and washing - Logging

observe the color and general appearance of the mixture of soil & water sampling by split spoon sampler every 1.5 m detect changes in materials character by feel of wash pipe and by color of wash water

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(2) Rotary drilling - Similar essential features with wash boring

- Mechanical rotation of drill rod and cutting bit - Circulation of drilling mud

a suspension of bentonite with density of 1.09-1.15 Mg/m3 higher density than water helps the removel of cuttings thixotropy prevents accumulation of cuttings at the bottom of hole thin layer of mud on borehole wall prevent caving

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(3) Percussion drilling (4) Auger boring - can not auger sand below water table due to lack of adherence to auger - cohesive soils

destroyed soil structure greater w/c than that of soil in place

- difficult to determine real position of boundary between strata - can auger to depth greater than 50 m - continuous flight auger - hollow stem auger

sampling without removing auger no casing

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(5) hammer drilling - Becker hammer drill - Drilling through deposits of cobbles & boulders - Drive double-walled casing into ground by a closed end diesel hammer - Circulation of compressed air through annulus between pipes

rise through inner pipe with choppings cyclone separator

(6) test pit/inspection shafts - For soils standing stable such as stiff cohesive soils or stable sands above water table - Inspection and sampling in hole - Inspection cage to protect inspector - Air sampling and ventilation

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4. Sampling (1) Purpose of sampling - Recovery of samples containing all the constituents of materials in their proper

proportions - Evaluation of appropriate engineering properties by laboratory tests on fairly intact or

undisturbed samples - Cost of sampling depends on;

degree of disturbance size of specimen depth of sampling

(2) Split spoon sampling - Disturbed sampling with sampling spoon attached to the bottom of wash pipe or drill rod

in place of bit - Typical split spoon sampler

2 inch OD & 30 inch long 2 short pieces of threaded pipe & 2 halves of the spoon

- Disturbed samples from split spoon Visual inspection and the determination of index properties clay samples retain at least part of the characteristics of undisturbed soil

samples with high permeability almost always thoroughly compacted fail to inform the relative density, which is far more significant than the

character of soil grains themselves cohesionless sand below water table likely to lost a sample when lifted from the bottom of drill hole use bailers, core catchers, scraper bucket

gravel stratum no recovery of gravel samples with diameter as small as 60 mm

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(3) Thin wall tube samplers

- disturbance depends on; ⓐ manner in which the sampler is forced into soil

best if pushed in the ground at a high and constant speed greatest disturbance by driving the sampler by successive blows of hammer

ⓑ dimensions of the sampler

Area ratio

1002

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×−

=i

ieR D

DDA

AR of split spoon sampler = 112% AR of 50mm thin wall tube = 13.8%

AR of well designed sample tube ≈ 10% Inside clearance ratio

100×−

=i

iitR D

DDC

Dit < Di to reduce the friction between soil and inner wall of tube - Sampling procedure with thin wall tube

push the sampler with one rapid continuous movement up to 15cm less than tube length Initially, rise of soil into the tube faster than the rate of tube penetration Later, adhesion and friction b/w tube and sample oppose the rise of sample

Rotate to shear the end of sample Remove Sealing metal disk + microfine wax mechanical seal packer + capping + masking tape

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- Effect of sampling disturbance on unconfined compressive curves of Chicago clay

(4) Piston sampler - Piston prevents;

the rise of soil with faster rate initially the top of sample from pulling away from piston without creating vacuum

- Excellent sampling tool for cohesive soils, even very soft and sensitive

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(5) Sampling combined with coring - When to use this technique

when soil is too stiff or compact to penetrate with damaging the cutting edge or buckling the tube when expect too much disturbance in brittle soils when sample the deposits containing successive soft and hard layers

- Pitcher sampler Rock coring technique + tube sampling Sampling procedure While lowering into hole, circulation of drilling fluid flushes the cuttings from

the bottom of the hole Rotating cutter barrel cut around the sampler and cuttings are carried upward by

fluid circulation sampling in soft soil cutting edge of tube well below the cutter barrel, like tube sampling

sampling in hard soil cutting edge of tube above the level of cutter barrel bottom, like over-coring

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- Denison sampler good for stiff to hard cohesive soils including soft shales, and slightly cohesive or

silty sands may not be good for sampling of clean sand below water table similar with rock-coring technique with double core barrel 0.6m length & 150mm inner diameter thin cylindrical lining spring core catcher

(6) Hand carved samples in clay - Obtain a large diameter undisturbed clay sample in excavations of shafts, cuts or tunnels - No mechanical disturbance, but still not perfect sampling due to stress relief - Procedure

carefully carve around the specimen with piano wire (soft clay) or spatula (stiffer clay) place the metal can on top of the carved pedestal trim pedestal to the diameter of container and force down container cut off pedestal below the can by means of piano wire trim the soil flush with the end of container fill the void between sample and container with microfine wax sealing

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(7) Block sampling in bore holes - Reduce the stress-relief effect associated with carving block samples - Block sampling in boreholes by Sherbrooke or Laval sampler - Sherbrooke sampler

Lefebvre & Poulin (1979) “A New Method of Sampling in Sensitive Clay”, CGJ, 16, 226-233. Pc is less affected by sampling disturbance Properties in the low stress range are much affected by tube sampling even with

large diameter samples

specimen of diameter 250mm and height 350mm good for quick clay Sampling procedure lower sampler to bottom of 400 mm diameter fluid-filled borehole three cutting tools carved out a cylindrical annulus about 50 mm thick with

rotation of drill rod circulation of drilling fluid removes cuttings components of horizontal diaphragm are activated to cut off specimen at it

bottom withdraw the specimen from slurry-filled drill hole with the support of closed

diaphragm

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- Laval sampler La Rochelle et al. (1981) “Causes of Sampling Disturbance and Design of A New

Sampler for Sensitive Soils”, CGJ, 18, 52-66. Samples of excellent quality in sensitive clays 200 mm diameter samples by overcoring in a flurry-filled hole Design principle no inside clearance rigorous tolerances of the cylindricity of the inside of the tube very sharp angle of cutting edge no piston no suction or negative stresses at all stages of the sampling sufficiently large diameter to reduce disturbed annulus around intact core

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Operational procedure

Lower the sampler assembly into the borehole, with head valve open When coring tube reaches the bottom of the borehole, push down tube sampler

by a continuous thrust After closing head valve, coring operation is carried out by rotating the core tube

while injecting pressurized bentonite mud Stop coring at 20 mm below the edge of sampler and rotate sampler 90 degrees

(8) Quality of samples / Degree of sample disturbance

(a) Anderson & Kolstad (1979) Express a quantitative measure of the quality of samples with the volumetric strain

during recompression of oedometer specimen to in-situ σvo’ Sample Quality Designation (SQD)

SQD εrecompression

(%) Note

A <1 block sample, Sherbrooke or Laval sampler

B 1~2 54mm, 95mm fixed piston samplers with AR<10%

C 2~4

D 4~8

E >8

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(b) Holtz, Jamilkowski, and Lancellotta (1986) Montalto di Castro clay (stiff clay)

σp’(measured)/ σp’(best estimate) > 0.8 at εv(recompression) < 1.0%

(c) Jamilkowski et al. (1985) Orinoco clay (soft clay)

σp’(measured)/ σp’(best estimate) > 0.9 at εv(recompression) < 6~7%

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(9) Sampling in sands (a) Sands above GWL likely to cause a volume change of material that enters the sampler sampling spoons with core catcher obtain disturbed sample use for identification

thin-wall piston sampler possibly take less disturbed sample use for GSD and stratification

carving the sand samples from a shaft use sample to estimate natural void ratio level the surface of bench in sand at the bottom of shaft place a cylindrical metal shell and push it into sand for its full length trim away surrounding sand and seal the top of sample with a metal cap cut off the sample several centimeters below shell by shovel invert the sample and remove surplus sand seal with metal cap

(b) Sands below GWL usual samplers can not retain sand samples below GWL piston samplers may recover satisfactory samples if cohesive zone plug the bottom

of the tube excavation of shaft in drained sand after lowering GWT by pumping from well

points Freezing Adachi (1989) “Sampling of Cohesionless and Gravelly Soils”, The Art and

Science of Geotechnical Engineering, Volume honoring Ralph B. Peck, 206-220 Undisturbed samples of water bearing cohesionless sands and gravels Procedure

• rotary drilling with bentonite mud and install a steel pipe • plug the bottom and remove drilling fluid from steel pipe • install a copper pipe inside the steel pipe • supply liquid nitrogen through copper pipe to freeze the soil around steel pipe • coring frozen soil column with chilled bentonite mud • cut frozen sample into blocks and wrap them in heavy vinyl sheet • transport to laboratory in a refrigerated truck

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Others impregnation of chemical agents such as Agar, asphalt emulsion freezing a plug in a lower end of sampling tube

5. Storage of Samples

(1) Wrapping (a) waxing shrinkage when cooling ordinary waxes; good for two days Beeswax lasts less than a week mixture of paraffin and petroleum oil lasts 2~3 weeks submerging the sample in petroleum oil lasts about 3 years

(b) plastic caps + masking tape (2) Storage in humidity room - As 100% humidity is never really achieved, will lose water in the sample - moisture migration from disturbed to undisturbed zone

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(3) Extraction of sample from tube (a) push the soil with close fitting plunger push it smoothly and rapidly in a manner for the sample to move continuously with respect to the tube in the same

direction as it entered if excessive side friction during ejection, cut the tube in 150mm sections and eject

the soil (b) split the sampling tube in longitudinal direction (c) immediate transfer into testing equipment the shorter the time from removing the sample from ground to testing, the less the

disturbance (d) Effect of unloading increases with time change in effective stress causes the redistribution of water within the sample if outside of sample was trimmed 10 days ago, the shear strength will be lower than

if it wasn't trimmed when thin outside layer is remolded, water migrates to inside of sample w/c of specimen > in-situ w/c

(e) Wet paper towels on samples O.K. for rock or granular soils Detrimental for clayey soils sample develops menisci at surface and applies positive effective stress all

around wet towel will release those stresses and the effective stress becomes zero sample will swell and cause disturbance

6. Subsurface Soundings

(1) Purpose of subsurface soundings - Exploring layers of soil with an erratic structure - Variation in the penetration resistance reflects the changes in the character of the subsoil - Identify exceptionally soft spots between drill holes - Get indirect information on type and consistency or relative density of soil (2) Static sounding methods - Static push of sounding rod or casing with a drive point into the ground by static pressure - Original Dutch cone

useful for rapid detailed surveys of erratic deposits of soft clays, silts, and peats push 0.5m into ground at the rate of 10mm/sec

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can measure qc by measuring pressure exerted on the rod

- Begemann Dutch cone friction-cone penetrometer push point 40 mm at a rate of 20mm/sec and measure qc further push will engage friction sleeve and measure fs+qc fs and qc are not determined at exactly same depths

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- Electric cone (CPT) Fugro penetrometer Useful for investigation on land and underwater Measure fs and qc simultaneously Continuous record

- Piezocone (CPTU) CPT + porous filter to measure pore water pressure Location of porous filter pore water pressure differs significantly at different filter location standard; porous filter behind the shoulder filter at conical point; sensitive but may get damage easily

Capable penetration few meters in dense sand up to 40 meters or more in soft soils

Inclinometer included to check verticality

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(3) Dynamic sounding methods - Dynamic drive of sounding rod or casing with a drive point into the ground by the impact

Measure number of blows to achieve a given penetration - SPT

Number of blows to advance the split spoon sampler 30 cm by impacting 63.5 kg hammer from falling height of 76 cm

Try to obtain at least some information on the degree of compactness or the stiffness

of the soils in-situ Unsuitable for investigating deposits consisting mainly of cobbles or boulders Correction of N value N values were influenced by several factors that determine the rod energy ratio Factors interfering free fall of hammer, therefore the velocity immediately

before impact

• type of hammer; donut, safety, auto, trip etc. • release mechanism; rope+pulley+cathead, trigger mechanism Factors affecting energy loss at impact

• anvil

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combined efficiency (ratio of energy reaching rod to potential energy of hammer)

• Most correlations based on N values by old standard • about 60% of potential energy → N60

Other corrections rod length, liner, borehole diameter (Skempton, 1986) Dilatancy of saturated fine or silty sands of moderate to high density Overburden pressure

Relation between CPT qc and SPT N

- Drive point penetrometer

obtain general information for deposits of erratic composition such as river or shore deposits sacrificial drive points of conical shape

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- Becker hammer drill For very coarse-grained deposits Use plugged bit instead of open bit, which gives erratic results For granular materials, correlations between NBC and N60

7. In-Situ Tests

- will be covered in class “In-situ testing in Geotechnical Engineering” 8. Groundwater Investigations

- will be covered in class “In-situ testing in Geotechnical Engineering” 9. Geophysical Methods

- will be covered in class “In-situ testing in Geotechnical Engineering”

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Program for Subsoil Exploration 1. Type and Sequence of Operations

(1) Why geology is important? - subsoil were formed by geological processes - geological factors have decisive influence on the sequence, shape and continuity of the

soil strata - With clear understanding of general geological character of site, more efficient program

for soil exploration can be laid out (2) Steps in subsoil exploration - need to know what exactly is needed to fine out from the exploration - need to choose appropriate methods to get information we want

(a) Office study collection of all available information on site air photo, quadrangle(1/5,000~50,000), geology map/report, hydrological data

(rainfall, max. flood level), existing boring logs layout soil exploration program

(b) Site visit confirmation of the pre-reviewed information preliminary investigation geology and geomorphology constructed/constructing facilities nearby; deep excavations, cutting

slopes, foundations of nearby structures, streams running nearby, wells for GW condition structures expected to affect my structure; tunnel, buried structures expected problems during construction; deep excavation, dewatering etc.

layout more efficient exploration program depth, location, type of explorations tools to be used for the explorations plans for in-situ tests/sampling/laboratory tests

(c) Exploratory boring/Subsurface sounding furnish more specific information on general character and thickness of the

individual strata in-situ tests sampling for laboratory tests

(d) Further investigation

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depends on the size of projects or the complexity of the soil profile (e) Summarized report Geology of site Results of exploratory boring and subsurface soundings Results of in-situ test and laboratory tests Idealization of subsurface condition stratification strength parameters & engineering properties of each strata

2. Spacing and Depth of Exploratory Borings

(1) Convention - building; borings spaced at 15~20 m in both principal directions - subway and dam; 30~60 m spacing

(2) Rational approach (a) Perform subsurface sounding at each point where convention would call for drill hole (b) If found a simple soil profile

one or two exploratory drill holes where average conditions prevail few drill holes where sounding results show maximum deviations from average

conditions (c) If site involves the possible geological disruptions, such as channel filling and

compressible inclusion, perform supplementary soundings If sounding encounters inclusions, perform exploratory drill hole to determine

the type and character of soil consisting inclusions (d) If sounding reveals erratic geological conditions, perform intermediate soundings and

get complete information on the general shape and trend between the soil boundaries Require few drill holes to determine the types of soil

(3) Depth of exploratory drill holes - No general rules and it depends on;

soil profile existence of soft layer type of foundation topography of rock surface regional characteristics of weathering and geology