BEARYS INSTITUTE OF TECHNOLOGY

DEPARTMENT OF CIVIL EGINEERING

Subject Handled by,

Dr. Pooja Kumble

Assistant Professor

Civil Engineering Department

BIT, Mangalore

MODULE -1

SOIL EXPLORATION

Subject : Applied Geotechnical Engineering (BCV613C)

MODULE -1

SOIL EXPLORATION

SOIL EXPLORATION

• Soil exploration may be needed not only for the design and construction of new structures, but also for deciding upon remedial measures if a structure shows signs of distress after construction. The design and construction of highway and airport pavements will also depend upon the characteristics of the soil strata upon which they are to be aligned.

OBJECTIVES OR IMPORTANCE OF SOIL EXPLORATION

1. Determination of the nature of the deposits of soil,
2. Determination of the depth and thickness of the various soil strata and their extent in horizontal direction,
3. The location of groundwater and fluctuations in Ground Water Table,
4. Obtaining soil and rock samples from the various strata,
5. The determination of the engineering properties of the soil and rock strata that affect the Performance of the structure, and
6. Determination of the in-situ properties by performing field tests.
7. The different methods to know the different strata of the soil is called as methods of exploration

STAGES IN SOIL EXPLORATION

• STAGE 1: RECONNAISSANCE
• This may be in the form of a field trip to the site which can reveal information on the type and behavior of adjacent sites and structures such as cracks, noticeable sags, and possibly sticking doors and windows. The type of local existing structure may influence, to a considerable extent, the exploration program and the best foundation type for the proposed adjacent structure. Since nearby existing structures must be maintained, excavations or vibrations will have to be carefully controlled. Erosion in existing cuts (or ditches) may also be observed. For highways, run off patterns, as well as soil stratification to the depth of the erosion cut, may be observed. Rocky outcrops may give an indication of the presence or the depth of bedrock.

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• STAGE 2: PRELIMINARY EXPLORATION
• In this phase a few borings are made or a test pit is opened to establish in a general manner the stratification, types of soil to be expected, and possibly the location of the groundwater table. One or more borings should be taken to rock, or competent strata, if the initial borings indicate the upper soil is loose or highly compressible. This amount of soil exploration is usually the extent of the site investigation for small structures. A feasibility exploration program should include enough site data and sample recovery to approximately establish the foundation design and identify the construction procedures. It is common at this stage to limit the number of good quality samples recovered and rely heavily on strength and settlement correlations using index properties such as liquid limit, plasticity index, and penetration data together with unconfined compression tests on samples recovered during penetration testing.

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• STAGE 3: DETAILED EXPLORATION
• Where the preliminary site investigation has established the feasibility of the project, a more detailed exploration program is undertaken. The preliminary borings and data are used as a basis for locating additional borings, which should be confirmatory in nature, and determining the additional samples required. If the soil is relatively uniform in stratification, a rather orderly spacing of borings at locations close to critical superstructure elements should be made. On occasion additional borings will be required to delineate zones of poor soil, rocky outcrops, fills, and other areas which can influence the design and construction of the foundation. Sufficient additional samples should be recovered to redefine the design and for any construction procedure required by the contractor to install the foundation. This should avoid an excessive bid for the foundation work, cost overruns, and damage to adjacent property owners from unanticipated soil conditions discovered when the excavation is opened.

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METHODS OF EXPLORATIONS�

1. Direct method
2. Semi direct method
3. Indirect method

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DIRECT METHOD

• TRIAL PITS
• Applicable to all types of soils Provide for visual examination in their natural condition. Disturbed and undisturbed soil samples can be conveniently obtained at different depths. Depth of investigation is limited to 3 to 3.5 m.

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SEMI DIRECT METHOD

• BORING TECHNIQUES
• Making or drilling bore holes into the ground with a view to obtaining soil or rock samples from specified or known depths is called ‘boring’.
• The common methods of advancing bore holes are:
• Auger boring
• Auger and shell boring
• Wash boring
• Percussion drilling
• Rotary drilling

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1) Auger boring

2) Auger and shell boring�

• If the sides of the hole cannot remain unsupported (filled soils), then the soil presented besides should be prevented from sliding in by means of a pipe known as ‘shell’ or ‘casing’. The casing is to be driven first and then the auger; whenever the casing is to be extended, the auger has to be withdrawn, this being an impediment to quick progress of the work. An equipment called a ‘boring rig’ is employed for power-driven augers, which may be used up to 50 m depth (A hand rig may be sufficient for borings up to 25 m in depth). Casings may be used for sands or stiff clays. Soft rock or gravel can be broken by chisel bits attached to drill rods. Sand pumps are used in the case of sandy soils.

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3) Wash boring�

4) Percussion drilling�

5) Rotary drilling�

INDIRECT METHODS

• 1) PENETRATION TEST

2) GEOPHYSICAL METHOD

3) ELECTRICAL RESISTIVITY METHOD

SAMPLING TECHNIQUES

1. Open drive sampler
2. Stationary piston sampler
3. Rotary samplers
• Split-Spoon Sampler
• Thin-Walled Samplers

• Open drive sampler �Open drive sampler can be of the thick wall type as well as of the thin wall type. The head of the sampler is provided with valves to permit water and air to escape during driving. The check valve helps to retain the sample when the sampler is lifted. The tube may be seamless or may be split in two parts; in the latter case it is known as the split tube or split spoon sampler.
• Stationary piston sampler consists of a sampler with a piston attached to a long piston rod extending up to the ground surface through drill rods. The lower end of the sampler is kept closed with the piston while the sampler is lowered through the bore hole. When the desired elevation is reached, the piston rod is clamped, thereby keeping the piston stationary, and the sampler tube is advanced further into the soil. The sampler is then lifted and the piston rod clamped in position. The piston prevents the entry of water and soil into the tube when it is being lowered, and also helps to retain the sample during the process of lifting the tube. The sampler is, therefore, very much suited for sampling in soft soils and saturated sands.

• Rotary samplers
• These are of the core barrel type (USBR, 1960) with an outer tube provided with cutting teeth and a removable thin liner inside. It is used for sampling in stiff cohesive soils.
• Split-Spoon Sampler
• The split spoon sampler is basically a thick-walled steel tube, split length wise. The sampler as per BIS (IS: 2131-1986—Standard Penetration Test for soils) is shown in Fig.

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• Thin-Walled Samplers

Thin-walled sampler, as per BIS (I.S.: 2132-1986 Code of Practice) for Thin walled Tube Sampling of Soils), is shown in Fig.

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STABILIZATION OF BORE HOLE

• For geotechnical engineering purposes the borehole is not drilled to its maximum depth in a single operation. The drilling operation is to be stopped at regular intervals for in-situ testing and sampling. At all time, the boreholes once drilled must remain as a borehole i.e. the soils on the sites of the borehole must not cave in and fill up the borehole. Maintaining the integrity of the borehole is known as stabilization of borehole.
• The following methods are commonly employed in practice to stabilize the borehole:
• Self supportive.
• Stabilizing by filling with water.
• Stabilizing by filling with drilling mud.
• Stabilizing by casing.

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• SELF SUPPORTIVE
• Borehole in clay are usually self supportive. Above the water table such soil has high apparent cohesion and below the water table enough undrained shear strength to prevent the soil caving in the borehole.
• Silty soil above the water table are also self supportive because of apparent cohesion due to negative pore water pressure. Below the water table, negative pore water pressure gets eliminated and borehole needs suitable support.
• STABILIZING BY FILLING WITH WATER
• When the GWT is at a higher elevation than that of water in the borehole, water flows into the borehole and seepage forces tends to push the soil into the borehole.
• Seepage forces can be used to keep the soil particles in their original position if the direction of flow is reversed. This can be achieved by filling the boreholes with water to a level above that of GWT. Boreholes in sites and sandy silts can be stabilized by this method.

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• STABILIZING BY FILLING WITH DRILLING MUD
• Drilling mud is water with bentonite clay. The stabilizing capacity of a drilling mud lies in the fact that it provides a coating of bentonite on the walls of borehole. This coating of high plastic material helps coarse grained particle to stick with each other and prevents falling into the borehole. Since, the level of drilling mud in the borehole is kept higher than GWT, no flow occurs into the borehole. The disadvantage of using drilling mud is that it is messy.
• STABILIZING BY CASING
• Casing pipe method of stabilizing borehole is adopted in medium and coarse sand, soft clays and whenever the other methods do not work. The hole is drilled for a short distance, the drilling rod is withdrawn and the casing pipe having an outside diameter equal to the diameter of borehole is pushed into the borehole. Drilling the borehole and penetrating the casing pipe is to be continued upto the desired depth. The water level in the pipe is to be maintained at a level higher than GWT.

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BORING LOG

• Information on subsurface conditions obtained from the boring operation is typically presented in the form of a boring record, commonly known as “boring log”. A continuous record of the various strata identified at various depths of the boring is presented. Description or classification of the various soil and rock types encountered, and data regarding ground water level have to be necessarily given in a pictorial manner on the log. A “field” log will consist of this minimum information, while a “lab” log might include test data presented alongside the boring sample actually tested.
• Sometimes a subsurface profile indicating the conditions and strata in all borings in series is made. This provides valuable information regarding the nature of variation or degree of uniformity of strata at the site. This helps in delineating between “good” and “poor” area.

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BOREHOLE SPACING- GUIDELINES

The following table gives the general guidelines for the spacing of boreholes:

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THE END