Ce2305 Foundation Engg Unit 1

PLANNING FOR AN SUBSOIL EXPLORATION

The planning of the site exploration program involves location

and depth of borings, test pits or other methods to be used,

and methods of sampling and tests to be carried out. The

purpose of the exploration program is to determine, within

practical limits, the stratification and engineering properties of

the soils underlying the site. The principal properties of

interest will be the strength, deformation, and hydraulic

characteristics. The program should be planned so that the

maximum amount of information can be obtained at minimum

cost. In the earlier stages of an investigation, the information

available is often inadequate to allow a firm and detailed plan

to be made. The investigation is therefore performed in the

following phases:

Stages in sub soil exploration

1.Reconnaissance

2. Preliminary exploration

3. Detailed exploration

Reconnaisance

It is the first step in a subsurface exploration programme it includes visit to the site and to study the maps and other relevant records.

Study of topographic features getting useful information about the soil and ground water condition.

The type of construction and condition of existing structure and look for settlements cracks on exterior walls

Reconnaisance

The soil conditions and ground water level and depth of rocks the exploration program and the best foundation type for the proposed adjacent structure.

It helps in deciding the methods of exploration to be adopted and types of samples to be taken and laboratory testing and in situ testing.

Preliminary Exploration

The aim of the Preliminary Exploration is to determine the composition of each soil stratum at the site. The depth of bed rock and ground water table is also determined

It is done in the form of few borings or test bit to collect the soil.

Test are conducted with cone penetrometers and sounding rods to obtain information about strength and compressibility of soils.

Preliminary Exploration

Geophysical method also used in preliminary exploration for locating the boundaries of different strata

Detailed exploration

The purpose of detailed exploration is to determine the properties of soil in different strata.

It includes an extensive boring programme sampling and testing of samples in laboratory.

Field test such as vane shear test, plate load test are conducted for heavy structures such as bridges, dams, and multi storey buildings it is conducted to find the natural properties of soil.

Detailed exploration

For small projects especially at sites where the soil structure is uniform detailed investigation is not required.

Methods of soil Exploration

Open excavation

Borings

Sub-surface soundings

Geophysical methods

These available methods of exploration can be broadly

classified into two categories:

Direct methods

Indirect methods

Boring Methods

Open Excavatio

nDirect

Methods

Electric Resistivit

y Method.

Seismic Method

Indirect methods

Direct methods of soil exploration

Test pits and trenches can be used for all types of soils.

Soils can be inspected in their natural condition and samples disturbed and undisturbed can be conveniently taken.

Test pit of size 1.2 x 1.2 x 3m is usually taken. It is suitable for shallow depths upto 3m. For

greater depths lateral support especially below ground water table, the lateral support in the form of sheeting and bracing is required.

Direct methods of soil exploration

For depths greater than 6m bore holes are more economic than open pits.

Boring Methods

Auger boring

Wash boring

Rotary drilling

Percussion drilling

Core boring

Auger Boring

Augers are used in cohesive and other type of soils above water table. Hand augers are used up to depth of 6m.

Mechanically operated augers are used for greater depths and they can also be used in gravely soils.

The hand augers is attached to the lower end of a pipe of about 18mm dia. The pipe is provided with a cross area at its top. The hole is advanced by turning the cross area manually and at the same time applying thrust in downward direction.

Auger Boring

When the auger is filled with soil it is taken out the samples recovered from the soil brought up by augers are useful for identification purpose only.

It is particularly useful for subsurface investigation of highways, railways and airfield where the depth of exploration is small.

The main disadvantage of the auger boring is that the soil samples are highly disturbed further it becomes difficult to locate the exact changes in the soil strata.

Wash boring

Fast and simple method for advancing holes in all types of soils.

Boulders and rock cannot be penetrated by this method.

In this method, the hole is drilled by first driving a casing through which a hollow drill rod with a sharp chisel or chopping bit at the lower end is inserted.

Water is forced under pressure through the drill rod which is alternatively raised and dropped and also rotated.

Wash boring

This results in disintegration of soil by water, which forced up the slurry (soil-water) through the space between drill rod and casing.

The change of soil strata could be guessed from the rate of progress and the colour of the wash water.

The method is not suitable for taking good quality undisturbed sample above ground water table as the wash water enters the strata below the bottom of the hole and cause increase in its water content.

Rotary drilling

In this method the bore hole is advanced by a hollow drill rod which has a cutting bit at its lower end.

It consists of an rotary mechanism and an arrangement for applying downward pressure.

As the drilling rod is rotated, the cutting bit shears off chips of the material penetrated.

Drilling fluid under pressure is introduced through the drilling rod to the bottom of the hole.

Rotary Drilling

The fluid carries the cutting material through the annular space between the drilling rod and walls of the hole.

The drilling fluid also cools the drilling bit. Rotary drilling can be used in clays, sand

and rocks. Bore holes of diameter 50mm to 200mm can be made easily with this method

This method is not suitable for in use of materials containing larger particles of gravel size and larger.

Rotary Drilling

• This method is also known as MUD ROTARY DRILLING and the hole is usually require no casing.

Percussion drilling

In this method soil and rock formations are broken by repeated blows of heavy chisel or bit suspended by cable or drill rod.

Water is added to the hole during boring and the slurry material is bailed out at intervals

This method is suitable for advancing hole in all types of soil, boulders and rock.

The soil sample is get disturbed by the impact.

Soil Samples

Soil samples are generally classified into two categories

1. Disturbed Sample 2. Undisturbed Sample

Disturbed Sample

These are the soils in which the natural structure of the soil gets disturbed during sampling

Disturbed samples can be used to determine the index properties of the soil such as grain size plasticity characteristics, specific gravity

Undisturbed sample

These are the samples in which the natural structure of the soil and the water content are retained.

These samples are used for determining the shear strength and permeability.

In case of loosy , sandy and soft soil below water table it may not be possible to take undisturbed sample. Suitable methods may be adopted for exploration.

Requirement of good sampling process

Inside clearance ratio Ci = (Ds – Dc/Dc) x 100 The soil is under great stress as it enters the sampler

and has a tendency to laterally expand. The inside clearance should be large enough to allow a part of lateral expansion to take place, but it should not be so large that it permits excessive deformations and causes disturbances of the sample.

For good sampling process, the inside clearance ratio should be within 0.5 to 3 %. For sands silts and clays, the ratio should be 0.5 % and for stiff and hard clays (below water table), it should be 1.5 %. For stiff expansive type of clays, it should be 3.0 %.


Area ratio (Ar) =( Dw2 – Dt2 /Dt2) x 100

The penetration resistance of the sampler, the possibility of entrance of excess soil and danger of disturbance of sample all increase with increase in area ratio. The allowable area ratio intended for obtaining undisturbed samples depends on diameter, design and method of operation of the sampler. The area ratio should be preferably be less than 10 %, but it is possible that the greater area ratio can be tolerated when the sampler is provided with a stationary piston and / or the cutting edge having very small angle of taper


Outside Clearance ratio = Co = (Dw – Dt)/ (Dt) x 100

For good sampling process, the ratio should be within 0-2 %.

Dt = Outer dia of the sampling tube.Ds = Inside dia of the sampling tubeDw = Outer dia of the cutting edgeDc = Inner dia of the cutting edge.


Wall friction can be reduced by suitable inside clearance, smooth finish and oiling.

The non-returned wall should have large orifice to allow air and water to escape.

Soil Samplings and Samplers

Standard split spoon

It consists of tool-steel driving shoe at the bottom, a steel tube (that is split longitudinally in to halves) in the middle, and a coupling at the top. The steel tube in the middle has inside and out side diameters of 34.9mm and 50.8mm, respectively.

When the bore hole is advanced to a desired depth, the drilling tools are removed. The split-spoon sampler is attached to the drilling rod and then lowered to the bottom of the bore hole. The sampler is driven in to the soil at the bottom of the bore hole by means of hammer blows.


The hammer blows occur at the top of the drilling rod. The hammer weights 623N. For each blow, the hammer drops a distance of 0.762m. The number of blows required for driving the sampler through three 152.4mm interval is recorded.

The sum of the number of blows required for driving the last two 152.4mm intervals is referred to as the standard penetration number; N. it is also commonly called the blow count


After driving is completed, the sampler is with drawn and the shoe and coupling are removed. The soil sample collected inside the split tube is then removed and transferred to the laboratory in small glass jars.

Determination of the standard penetration number and collection of split-spoon samples are usually done at 1.5m.

Split Spoon Sampler

Clay samples

SHELPY TUBES AND THIN WALLED SAMPLERS

SHELPY TUBES – DRIVE HEAD

SHELPY TUBE - SAMPLERS

Shelby tubes are thin walled tube samplers made of seamless steel. The outside diameter of the tube may be between 40 to 125mm.

The bottom of the tube is sharpened and bevelled which act as a cutting edge

The area ratio is less than 15% and the inside clearance is between 0.5 to 3%

The length of the tube is 5 to 10 times the diameter for sandy soils and 10 to 15times the diameter for clayey soils.

SHELPY TUBE - SAMPLERS

The sampler tube is attached to the drilling rod and lowered to the bottom of the bore hole.

It is then pushed into the soil by a continuous rapid motion without impact or twisting.

The tube is pushed to the length provided for the sample after that the tube is turned 2 revolutions to shear the sample off at the bottom before it is withdrawn.

The tube is taken out and its ends are sealed before transportation. These shelby tubes are used for collecting undisturbed samples of clay.

Piston Samplers

Piston samplers consists of a thin walled tube with a piston inside. The piston keeps the lower end of the sampling tube closed when the sampler is lowered to the bottom of the hole.

The sampler has been lowered to the desired depth, the piston is prevented from moving downward direction by a suitable arrangement, which differs in different types of piston samplers.

PISTON SAMPLERS

The thin tube sampler is pushed past the piston is obtain the sample. The piston remains contact with the top of the sample.

Piston Samplers

A vacuum is created on the top of the sample which helps in retaining the sample,

Piston sampler are used for getting undisturbed soil samples from soft and sensitive clays

COLLECTION OF SAMPLE

In-situ tests

The in situ tests in the field have the advantage of testing the soils in their natural, undisturbed condition. Laboratory tests, on the other hand, make use of small size samples obtained from boreholes through samplers and therefore the reliability of these depends on the quality of the so called ‘undisturbed' samples. Further, obtaining undisturbed samples from non-cohesive, granular soils is not easy, if not impossible. Therefore, it is common practice to rely more on laboratory tests where cohesive soils are concerned. Further, in such soils, the field tests being short duration tests, fail to yield meaningful consolidation settlement data in any case. Where the subsoil strata are essentially non-cohesive in character, the bias is most definitely towards field tests. The data from field tests is used in empirical, but time-tested correlations to predict settlement of foundations. The field tests commonly used in subsurface investigation are:

Penetrometer test Pressuremeter test Vane shear test Plate load test Geophysical methods

Penetrometer Tests

Standard penetration test (SPT) Static cone penetration test (CPT) Dynamic cone penetration test

(DCPT)

Standard penetration test

The standard penetration test is carried out in a borehole, while the DCPT and SCPT are carried out without a borehole. All the three tests measure the resistance of the soil strata to penetration by a penetrometer. Useful empirical correlations between penetration resistance and soil properties are available for use in foundation design.

This is the most extensively used penetrometer test and employs a split-spoon sampler, which consists of a driving shoe, a split-barrel of circular cross-section which is longitudinally split into two parts and a coupling. IS: 2131-1981 gives the standard for carrying out the test.

Test procedure - SPT

The borehole is advanced to the required depth and the bottom cleaned.

The split-spoon sampler, attached to standard drill rods of required length is lowered into the borehole and rested at the bottom.

The split-spoon sampler is driven into the soil for a distance of 450mm by blows of a drop hammer (monkey) of 65 kg falling vertically and freely from a height of 750 mm.


The number of blows required to penetrate every 150 mm is recorded while driving the sampler.

The number of blows required for the last 300 mm of penetration is added together and recorded as the N value at that particular depth of the borehole.

The number of blows required to effect the first 150mm of penetration, called the seating drive, is disregarded.


The split-spoon sampler is then withdrawn and is detached from the drill rods. The split-barrel is disconnected from the cutting shoe and the coupling. The soil sample collected inside the split barrel is carefully collected so as to preserve the natural moisture content and transported to the laboratory for tests.

Sometimes, a thin liner is inserted within the split-barrel so that at the end of the SPT, the liner containing the soil sample is sealed with molten wax at both its ends before it is taken away to the laboratory.


The SPT is carried out at every 0.75 m vertical intervals in a borehole. This can be increased to 1.50 m if the depth of borehole is large. Due to the presence of boulders or rocks, it may not be possible to drive the sampler to a distance of 450 mm

Precautions in SPT

The drill rods should be of standard specification and should not be in bent condition.

The split spoon sampler must be in good condition and the cutting shoe must be free from wear and tear.

The drop hammer must be of the right weight and the fall should be free, frictionless and vertical.

The height of fall must be exactly 750 mm. Any change from this will seriously affect the N value.

The bottom of the borehole must be properly cleaned before the test is carried out. If this is not done, the test gets carried out in the loose, disturbed soil and not in the undisturbed soil.

Precautions in SPT

When a casing is used in borehole, it should be ensured that the casing is driven just short of the level at which the SPT is to be carried out. Otherwise, the test gets carried out in a soil plug enclosed at the bottom of the casing.

When the test is carried out in a sandy soil below the water table, it must be ensured that the water level in the borehole is always maintained slightly above the ground water level. If the water level in the borehole is lower than the ground water level, ‘quick' condition may develop in the soil and very low N values may be recorded.

Precautions in SPT

In spite of all these imperfections, SPT is still extensively used because

The test is simple and relatively economical. It is the only test that provides representative soil

samples both for visual inspection in the field and for natural moisture content and classification tests in the laboratory.

SPT values obtained in the field for sand have to be corrected before they are used in empirical correlations and design charts. IS: 2131-1981 recommends that the field value of N be corrected for two effects, namely, (a) effect of overburden pressure, and (b) effect of dilatancy.

Correction for overburden pressure - SPT

Several investigators have found that the penetration

resistance or the N value in a granular soil is influenced by the overburden pressure. Of two granular soils possessing the same relative density but having different confining pressures, the one with a higher confining pressure gives a higher N value.

Since the confining pressure (which is directly proportional to the overburden pressure) increases with depth, the N values at shallow depths are underestimated and the N values at larger depths are overestimated. To allow for this, N values recorded from field tests at different effective overburden pressures are corrected to a standard effective overburden pressure.

Spt - test

The corrected N values given by N’ = CnN

In which N’ corrected value of observed N; = Cn correction factor for overburden pressure.

(b) Correction for dilatancy Di1atancy correction is to be applied when obtained after

overburden correction, exceeds 15 in saturated fine sands and silts. IS: 2131-1981 incorporates the Terzaghi and Peck recommended dilatancy correction (when N’ > 15) using the equation

N”=15+0.5 ( N’— 15)

SPT - TEST

N’ >15 is an indication of a dense sand. In such a soil, the fast rate of application of shear through the blows of a drop hammer, is likely to induce negative pore water pressure in a saturated fine sand under undrained condition of loading. Consequently, a transient increase in shear resistance will occur, leading to a SPT value higher than the actual one.

STATIC CONE PENETRATION TEST - SCPT

INTRODUCTION At field SCPT is widely used of recording

variation in the in-situ penetration resistance of soil in cases where in-situ density is disturbed by boring method & SPT is unreliable below water table. The test is very useful for soft clays, soft silts, medium sands & fine sands.

SCPT - PROCEDURE

By this test basically by pushing the standard cone at the rate of 10 to 20 mm/sec in to the soil and noting the friction, the strength is determined.

After installing the equipment as per IS-4968, part III the sounding rod is pushed in to the soil and the driving is operated at the steady rate of 10 mm/sec approximately so as to advance the cone only by external loading to the depth which a cone assembly available.

For finding combine cone friction resistance, the shearing strength of the soil qs , and tip resistance qc is noted in gauge & added to get the total strength.

SCPT - Limitations

This test is unsuitable for gravelly soil & soil for having SPT N value greater than 50. Also in dense sand anchorage becomes to cumbersome & expensive & for such cases Dynamic SPT can be used. This test is also unsuitable for field operation since erroneous value obtained due to presence of brick bats, loose stones etc.

SCPT - TEST

Correction in SCPT test

Cn = (m+nm1) x 10kN/M2

Here

m = mass o cone = 1.1 Kg.

m1= mass of each sounding roads = 1.5 Kg

n = No. of rods used.

SCPT correlation

Friction ratio Fr = Qs/Qc x 100 Fr = Friction ratio Qs = measured site/slip friction Qc = tip resistance/point resistance

Then, Sensitivity of soil is measured St = 10/fr (Fr in %) where, St = Sensitivity of soil

For cohesive soil (undrained shear strength) Su = qc – Po/Nk Po= overburden pressure = y Z Nk= cone factor = 15 to 20 (depends on the plasticity index of soil) Sarvac & opovic Here Ic = consistency index of soil qc = is measured in Kpa

Schmertman qc,ocr/qc,ncr = 1+k(ko,ocr/ko,Nc - 1) (K=0.75) ko,ocr/ko,Nc = (OCR)n (n = 0.32 to 0.52)

Relation between angle of internal friction & undrained shear strength For gravelly silt = (29+root of qc)+5degree For silty sand = (29+root of qc) – 5degree

SCPT - TEST

SPT & SCPT relation qc = K.N

Here K value varies 0.1 to 1.0. It depends of soil type

Sr. No Soil type K(qc/N60)

1 Silt, sand silt & slightly cohesive sand silt mix = 0.1 to 0.2

2 Clean fine sand to medium sand & slightly silty sand = 0.3 to 0.4

3 Coarse sand & sand with gravel = 0.5 to 0.7

4 Sandy gravel & gravel = 0.8 to 1.0

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Ce2305 Foundation Engg Unit 1