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Subsurface Exploration

PART 1

Topics Covered

• I. SUBSURFACE EXPLORATION PROGRAM

• II. EXPLORATORY BORINGS IN THE FIELD

• III. PROCEDURES FOR SAMPLING SOIL

• IV. OBSERVATION OF WATER LEVELS

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SUBSURFACE EXPLORATION

• Soil exploration is a part of SITE INVESTIGATION.

• Site investigation, in general deals with determining in general, THE SUITABILITY OF THE SITE FOR THE PROPOSED CONSTRUCTION.

Site Investigation

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Introduction:

WHAT?

• Attempt at understanding the subsurface conditions such as:• SOIL AND ROCK PROFILE

• GEOLOGICAL FEATURES OF THE REGION

• POSITION AND VARIATION OF GROUND WATER TABLE

• PHYSICAL PROPERTIES OF SOIL AND ROCK

• CONTAMINATION, IF ANY

• GENERAL DATA OF ADJACENT STRUCTURES, HYDROLOGICAL DATA, TOPOGRAPHY, SOIL MAPS, SEISMICITY, ETC.

Introduction (Cont’d)…

WHY?

• To DETERMINE THE TYPE OF FOUNDATIONrequired for the proposed project at the site, i.e. shallow foundation or deep foundation.

• To make RECOMMENDATIONS REGARDING THE SAFE BEARING CAPACITY or pile load capacity since it is the subsoil that provides the ultimate support for the structures.

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TO AVOID THIS!

THIS!

Foundation Weakening Due to Soil

Liquefaction in Adapazari, Turkey

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THIS!A sinkhole covers a street intersection in

downtown Guatemala City, Wednesday,

June 2, 2010. Authorities blamed heavy

rains caused by tropical storm Agatha as

the cause of the crater that swallowed a a

three-story building but now say they will

be conducting further studies to determine

the cause.

AND THESE!

SINKHOLES

LEANING TOWER OF PISA

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Introduction (Cont’d)…

HOW?

• The three important aspect are PLANNING, EXECUTIONand report writing.

• PLANNING• To minimize cost of explorations and yet give

reliable data.

• Decide on quantity and quality depending on type, size and importance of project and whether investigation is preliminary or detailed.

Introduction (Cont’d)…

• EXECUTION:• COLLECTION OF DISTURBED

AND/OR UNDISTURBED SAMPLESof subsurface strata from field.

• CONDUCTING IN-SITU TESTS OF SUBSURFACE material and obtaining properties directly or indirectly.

• STUDY OF GROUND WATER CONDITIONS and collection of sample for chemical analysis.

• Geophysical exploration, if necessary.

• Laboratory testing on samples

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Introduction (Cont’d)…

A complete site investigation will consist of:

• PRELIMINARY WORK

• Collecting general information and already existing data such as study of geologic events , seismic maps, etc. at or near site.

• Study site history – if previously used as quarry, agricultural land, industrial unit, etc.

• SITE RECONNAISSANCE: ACTUAL SITE INSPECTION.

• To judge general suitability

• Decide exploration techniques

Introduction (Cont’d)…

• EXPLORATION• PRELIMINARY INVESTIGATIONS: Exploratory borings or

shallow test pits, representative sampling, geophysical investigations, etc

• DETAILED INVESTIGATIONS: Deep boreholes, extensive sampling, in-situ testing, lab testing, etc.

• DEPTH AND SPACING: In general, depth of investigation should be such that any/all strata that are likely to experience settlement or failure due to loading. Spacing depends upon degree of variation of surface topography and subsurface strata in horizontal direction.

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PHILIPPINE SEISMIC MAP

Depth of Boring

• The approximate required minimum depth of the borings should be predetermined.

• The estimated depths can be changed during the drilling operation, depending on the subsoil encountered.

• To determine the approximate minimum depth of boring, engineers may use the following rule:

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Determining the approximate minimum depth of boring

• Depth > 2B (Strength concern)

• Depth D1 at (Δσv / q) < 0.1

• Depth D2 at (Δσv / σ’v0) < 0.05 Minimum Depth = min(D1, D2)

• In deep excavations, depth > 1.5 depth of excavation

Depth of Boring

For hospitals and office buildings, the following rule could be use to determine boring depth

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DEPTH OF BORING

No. of stories Boring depth (m)

1 3.5

2 6

3 10

4 16

5 24

• Approximate depths of borings for buildings with a width of 30m

SPACING OF BOREHOLES

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METHODS OF INVESTIGATION(SOIL BORING)

METHODS OF INVESTIGATION(SOIL BORING)

• Test pits: • Permits visual inspection

of subsurface conditions in natural state.

• Max. depth limited to 18 -20 feet.

• Especially useful for gravelly soil where boreholes may be difficult.

• Sampling/testing done on exposed surfaces.

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MAKING OF A TEST PIT

Stratigraphy and Finds

Layer Soil Soil Colour Finds Chronology

L1 Sandy soil Gray 7.5YR 5/1 Modern Rubbish (filled soil) 1980s

L2 Sandy soil Pinkish white 7.5YR 8/2 Modern rubbish (filled soil) 1980s

L3 Sandy soil Reddish yellow 7.5YR 7/6 Modern rubbish (filled soil) 1980s

L4 Sandy soil Gray 7.5YR 6/1 Modern rubbish (filled soil) 1980s

L5 Loamy soil Reddish yellow 5YR 6/6 Nil (original decomposed soil)

L6 Loamy soil Reddish yellow 5YR 6/8 Nil (original decomposed soil)

L7 Loamy soil, with

some decomposed

bed rock texture

Light red 2.5YR 6/8 Nil (original decomposed soil)

Test Pit Wall Photograph

Western Wall Section

Test Pit Wall Drawing

Western Wall Section Drawing

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BOREHOLES

•AUGER BORINGS: • Simplest method of exploration and sampling.

• Power driven or hand operated.

• Max. depth 10 m

• Suitable in all soils above GWT but only in cohesive soil below GWT

• Hollow stem augers are used for sampling or conducting Standard Penetration Tests.

Hand operated augers

• Post hole auger • Helical auger• Portable power-

driven helical

augers

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Power driven augers

• Continuous-flight auger

COMPONENTS OF A HOLLOW AUGER

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Hand Operated Auger In Action

Methods of Boring

• WASH BORING: • A casing is driven with a drop hammer. A hollow drill

rod with chopping bit is inserted inside the casing.

• Soil is loosened and removed from the borehole using water or a drilling mud jetted under pressure.

• The water is jetted in the hole through the bottom of a wash pipe and leaves the hole along with the loose soil, from the annual space between the hole and wash pipe.

• The water reaches the ground level where the soil in suspension is allowed to settle and mud is re-circulated.

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Schematic for wash boring

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Wash Boring

Methods of Boring

Another example of wash boring is called mud rotary drilling (soil) or core drilling (rock).

• MUD ROTARY(ROTARY DRILLING)• Hollow drill rods with a drill bit is rotated into

the soil. Drilling mud is continuously pumped into the hole. The bit grinds the soil and the return flow brings the cuttings to the surface.

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ROTARY DRILLING

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Methods of Boring

• PERCUSSION DRILLING:

• often used to penetrate hard rock for subsurface exploration or for the purpose of drilling wells.

PERCUSSION DRILLING

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SAMPLES

DISTURBED UNDISTURBED

(The structure of the soil is disturbed (The true in-situ structure and water content

to a considerable degree) is retained as closely as possible)

Remoulded Representative Block Drive

Wash

PROCEDURES FOR SOIL SAMPLING

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• DISTURBED SAMPLES used for determining index properties (ex. gsd, consistency, etc.)

• UNDISTURBED SAMPLES used for determining engineering properties (ex. Density, water content, shear strength parameters, etc.)

• WASH SAMPLES obtained from wash boring water or mud.

• REPRESENTATIVE SAMPLE retains all constituents of the soil, but is disturbed from natural state and structure. (ex. Split spoon sampler)

• BLOCK SAMPLES are carved out form sides or bottoms of excavations, sealed in a box and taken to lab.

• OPEN DRIVE SAMPLERS consist of thin walled tubes which are driven or pushed into the soil at the bottom of the hole. (ex. Shelby Tube sampler)

Undisturbed Samples

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SPLIT SPOON SAMPLING

• Is the most common

way to collect disturbed

samples but still

representative

SKETCH OF SPLIT SPOON SAMPLER

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Good quality samples necessary.

AR<10%

sampling tube

soil

(%) 100..

....2

22

DI

DIDOAR

area ratio

Thicker the wall, greater the disturbance.

DEGREE OF DISTURBANCE

SPLIT SPOON SAMPLING

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Methods of Sampling

• Requirements to minimize disturbance to samples• Area ratio = (D2

w-D2e) x 100%

D2e

• Area ratio should be as low as possible. (<10%).

• It represents the amount of soil displaced.

• Inside clearance = (Ds-De) x 100%

ratio De

• The inside clearance allows elastic expansion of the sample and minimizes frictional drag on the sample.

• The inside clearance should be between 0.5% to 3%.

• The outside clearance = (Dw-Dt) x 100%

ratio Dt

• Outside clearance is necessary to reduce the driving force and resistance to withdrawal.

• The outside clearance should be between 0% and 2%.

• Diameter of samples should not be less than 38mm. (Generally between 50-150mm).

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FIELD STANDARD PENETRATION NUMBER

N60 = where:

N60 = standard penetration number corrected for field conditions

N = measured penetration number

ηH = hammer efficiency %

ηB = correction for borehole diameter

ηS = sampler correction

ηR = correction for rod length

FIELD STANDARD PENETRATION NUMBER

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UNDRAINED SHEAR STRENGTH OF CLAY

cu = KN60

where:

K = constant = 3.5 – 6.5 kN/m2

N60 = standard penetration number obtained from the field

For frictional soils (sands, gravels, silty sands, etc), the

strength of the soil increases with the effective overburden

stresses σ’v. The greater the depth of the soil, the greater

σ’v and the greater the strength of the soil will usually be

(within some limits). Thus the greater the depth, the greater

the N value usually is in the SPT. Thus the raw N value is

not just a soil property, but also a depth property.

Therefore we need to correct N for frictional soils,

(N1)60 = CNN60

where: (N1)60 = corrected N value to a standard value of

95.6kPa (2ksf).

CN = correction factor

N60 = N value obtained from the field

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RELATIONSHIP BETWEEN RELATIVE DENSITY (DR) AND (N1)60 FOR FINE TO MEDIUM SAND:

Dr (%) = 1.55(N1)60 + 40 [for 0 ≤ (N1)60 ≤ 25]

Dr (%) = 0.84(N1)60 + 58.8 [for 25 ≤ (N1)60 ≤ 50]

FOR FINE TO MEDIUM SAND WITH FINES (THAT IS, % PASSING NO.200 SIEVE, FC) BETWEEN 15% AND 20%,

(N1)60 = (N60 +12.9)(

EFFECTIVE PEAK ANGLE OF FRICTION OF GRANULAR SOILS,

ϕ' (deg) = 27.1 + 0.3(N1)60 – 0.00054[(N1)60]2

CORRELATION AMONG N60, Φ',

ϕ' = tan-1

where:

N60 = field standard penetration number

= effective overburden pressure

Pa = atmospheric pressure in the same unit as σ’o

(≈100 kN/m2)

ϕ' = soil friction angle

SIMPLE CORRELATION BETWEEN Φ' AND (N1)60:

ϕ' =

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RELATIVE DENSITY OF SANDS VS. N

SPLIT SPOON SAMPLING

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THIN WALL TUBE

• Thin-wall open-tube samplers

(Shelby Tube Samplers) are used

for soils that are particularly

sensitive to sampling disturbance.

• They are suitable for fine soils up

to a firm consistency, and free

from large particles.

• Shelby Tubes are available in

carbon steel and in stainless steel.

Usual diameters are 3" or 4" OD.

Types of Samplers (Undisturbed)

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THIN WALL TUBE (SHELBY TUBE SAMPLING)

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OBSERVATION ON WATER LEVELS

Presence of a water table near a foundation

significantly affects a foundation’s load-

bearing capacity and settlement. The water

level will change seasonally. In many cases,

establishing the highest and lowest possible

levels of water during the life of a project

might be necessary.

SIGNIFICANCE

• Determine flow directions

• Identify changes in gradients

• Measurements for aquifer

testing

• Determine the volume of water

or drawdown in the well casing

for proper purging

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HOW DO WE MEASURE WATER LEVEL?

MANUAL METHODS

• The usual basic measurement in groundwater studies is that of water levels in wells.

• If water is encountered in a borehole during a field exploration, it should be recorded. In soils with high hydraulic conductivity, the level of water should stabilize about 24 hours after completion of the boring.

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CHALKED STEEL TAPE

• One of the instruments that gives most accurate measurement

• This method utilizes a graduated tape with weight attached to its end. The lower 3-4 ft. is coated with carpenter’s chalk.

• Steel tape should have limited elasticity

• Weight should be brass or stainless steal

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ELECTRIC MEASURING TAPES

• Consist of a pair of insulated wires whose exposed ends are separated by an air gap in an electrode and containing a source of power.

• When the electrode contacts the water surface, a current flows through the tape circuit and is indicated by an ammeter-needle deflection, light, and (or) audible signal.

• The “hold” depth against the reference point on the well is read directly from the tape as depth to water.

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Air Line Method

• A small diameter air-type tube of known length is installed from the surface to a depth below the lowest water level expected.

• Compressed air (compressor, bottled air, or tire pump) is used to purge the water from the tube. The pressure, in pounds per square inch (psi), needed to purge the water from the air line multiplied by 2.31 (feet of water or one psi) equals the length in feet of submerged airline.

• The depth to water below the center of the pressure gage can be easily calculated by subtracting the length of air line below the water surface from the total length of the air line (assuming the air line is essentially straight). Accuracy depends on the precision to which the pressure can be read.

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PRESSURE TRANSDUCERS

• Pressure transducers are temperature sensitive and cables are subject to stretching with time.

end