1. Mr. Vedprakash Maralapalle, Asst. Professor
Soil Exploration
Mr. Vedprakash Maralapalle, Asst. Professor
Department: B.E. Civil Engineering
Subject: Geotechnical Engineering - I
Semester: V
Teaching Aids Service by KRRC Information Section

2. Engineering Failure E

3. The purpose of a soil investigation program
1. Selection of the type and the depth of foundation suitable for a given structure.
2. Evaluation of the load-bearing capacity of the foundation.
3. Estimation of the probable settlement of a structure.
4. Determination of potential foundation problems (for example, expansive soil, collapsible soil, sanitary landfill, and so on).
5. Establishment of ground water table.
6. Prediction of lateral earth pressure for structures like retaining walls, sheet pile bulkheads, and braced cuts.
Establishment of construction methods for changing subsoil conditions.
To investigate the safety of the existing structures and to suggest the remedial measure.

4. Stages of subsurface exploration program
Reconnaissance : it is the first step of the of the surface exploration of the programme. It includes visit to the site , study the maps and relevant record. It helps in deciding future programme of site investigation, scope of work , method of exploration to be adopted.
Study site history – if previously used as quarry, agricultural land, industrial unit, etc.
2. Preliminary exploration: aim of preliminary exploration is to determine the depth, thickness, extend and composition of each soil stratum at the site. The depth o the hard rock and ground water table is also determined.

5. 3. Detailed Explorations : it includes an extensive boring programme, sampling and testing of sample in a laboratory. Filed test such as vane shear test, plate load test and permeability test are conducted to determine the properties of the soils in natural state.

6. Depth of Exploration
Depth of exploration required at a particular site depends upon the degree of variation of the subsurface data in the horizontal and vertical directions. It is not possible to fix the number , disposition and depth of borings without making a few preliminary borings at the site.
The depth of exploration is governed by the depth of the influence zone. The depth upto which the stress increment due to superimposed loads can produce significant settlement and shear stresses is knows as the significant depth. The depth of exploration should be at least equal to the significant depth.

7. The significant depth is generally taken as the depth at which the vertical stress is 20% of the load intensity.
Depth of Exploration
Square footing= 1.5 times the width of the square
Strip footing = 1.5 times the width of the strip footing

8. In case of pile foundation the depth o exploration below the tip of bearing piles is kept at least 1.5 times the width of the pile group. However, in the case of friction piles the depth of the exploration is taken 1.5 times the width of the pile group measured from the lower third point.
When foundation are taken up to rock , it should be ensure that large boulders are not mistaken as bed rock.

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

10. Spacing Boring
There are no hard and fast rules for the spacing of the boreholes. The following table gives some general guidelines for borehole spacing. These spacing can be increased or decreased, depending on the subsoil condition.

11. Approximate Spacing of Boreholes

12. Methods of soil Exploration
Exploration methods
In Direct Methods
Direct Methods
Semi Direct
Test pits, Trial pits, Trenches
Borings
Auger
Auger and shell
Wash Boring
Percussion drilling
Rotary Drilling
Sounding or penetration Tests and Geophysical methods

13. Test pits Depth upto 3m Uneconomical at greater depths.
Supports are required at greater depths. Especially in case of weak strata
Problems with GWT and the same should be lowered
Open type Exploration
Soils are investigated in natural condition
Soil samples are collected for determining strength and Engineering properties

14. Excavated test pit

15. Boring
Drilling a hole into the soil strata upto specified depth is known as boring 1. Auger boring 2. Core boring 3. Wash boring 4. Percussion drilling 5. Rotary drilling

16. Auger Boring Drilling is made using a device called Soil Auger
Power driven (upto 3 to 5m) and Hand operated.
Advancement is made by drilling the auger by simultaneous rotating and pressing it into the soil
Dry and unsupported bore holes
When the auger gets filled with soil same, it is taken out and the soil sample collected.
It is particularly useful for subsurface investigations of highway, railway and airfield where the depth of exploration is small.
Auger boring can not be used when there are large boulder present in the soil profile.

17. Soil augers

18. Core Boring
Core boring is method is used for drilling holes and obtaining rock cores.
As the drilling is rotated , the bit cut an annular hole around an intact core.
The core is then removed from its bottom and is retained by a core lifter and brought to the ground surface.
Water is pumped continuously in to the drilling rod to keep the drilling bit cool and to carry the disintegrated material to the ground surface.
The core drilling ma be done using either a diamond studded bit or a cutting edge. The diamond drilling is superior to the other type o drilling , but it costlier.

20. RQD

21. Wash Boring
Initially, the hole is advanced for a short depth by using an auger.
Then a casing pipe is pushed in and driven with a drop weight. The driving may be with the aid of power.
Water jet under pressure is forced through the rod and the bit into the hole.
This loosens the soil at the lower end and forces the soil-water suspension upwards along the annular surface between the rod and the side of the hole.
This suspension is collected in a settling tank.
Soil particles are allowed to settle down and water is allowed to overflow into a sump which is then recalculated

22. Very disturbed sample is obtained.
whenever a soil sample is required, the chopping bit is to be replaced by a sampler.
The change of the rate of progress and change of colour of wash water indicate changes in soil strata.

23. Wash boring

25. Percussion Drilling
This method is used for making holes in rocks, boulders and other hard strata.
In this method heavy chisel is lifted & dropped. Sample will be collected with the help of bailer at regular interval.
This drilling may required casing.
This is particularly used in drilling holes in boulder.
Disadvantages
The formation gets badly disturbed by impact at the bottom.

26. i

27. Rotary Drilling
A drill bit, fixed to the lower end of a drill rod, is rotated by power while being kept in firm contact with the hole.
Drilling rod is rotated , the cutting bit shears off chips of the material penetrated.
Drilling fluid or bentonite slurry is used under pressure which brings up the cuttings to the surface.
Drilling fluid also cool down the drilling bit, in case of an uncased hole drilling fluid also support the walls of the hole.
When soil sample is required to be taken the drilling rod is raised and the drilling bit is replaced by a sampler.

29. SOIL SAMPLING
Two types of soil samples can be obtained during sampling disturbed and undisturbed.
The most important engineering properties required for foundation design are strength, compressibility, and permeability. Reasonably good estimates of these properties for cohesive soils can be made by laboratory tests on undisturbed samples which can be obtained with moderate difficulty.
It is nearly impossible to obtain a truly undisturbed sample of soil; so in general usage the term "undisturbed" means a sample where some precautions have been taken to minimize disturbance or remolding effects. In this context, the quality of an "undisturbed" sample varies widely between soil laboratories.

32. STANDARD PENETRATION TESTING
This test extremely useful for determine the relative density , angle of shearing resistance and unconfined compressive strength of cohesive soil.
this test is conducted with the help of split-spoon sampler.
When bore hole has been drilled to the desired depth, the sampler is lowered at the bottom. Drop hammer of 63.5 kg mass falling through a height of 750mm at the rate of 30 blows per minute.
The number of hammer blow required to drive 150mm of the sample is counted. The sample is further driven by 150mm and number of blow recorded.
The number of blow recorded for first 150mm is discarded.
If the number of blow for 150mm drive exceeds 50, it is taken as refusal and test is discontinued.

33. DILATANCY CORRECTION
water table develop pore pressure which is not easily dissipated. Pore pressure increases the resistance of the soil thus, Penetration Number (N) also increases
This correction is applied when observed value of N exceeds 15

34. Terzaghi and Peck (1967) recommended the following correction-

35. OVERBURDEN PRESSURE CORRECTION
In granular soils, overburden pressure affects the penetration resistance
If two soils, having same relative density but different confining pressures are tested, the one with a higher confining pressure gives a higher penetration number as the confining pressure in cohesion less soils increases with the depth, the penetration number for soils at shallow depths is underestimated and that at greater depths is overestimated.

37. Relative Density SPT N-Value Relative Density 0-4 Very loose 4-10
10-30
Medium
30-50
Dense
Over 50
Very dense

38. Unconfined Compressive Strength Of Cohesive Soils
Consistency
qu (kN/m2)
Very Soft
Soft
Medium
Stiff
Very Stiff
Hard
SPT N-value
<2
2-4
4-8
8-15
15-30
>30 qu
<25
25-50
50-100
>400

40. This method is also used to skin friction values which is used to determine the length of the piles
The cone is pushed only by thrust and not by driving
In order to find out the cone resistance , the cone alone is pushed
Later the cone and sleeve is pushed together to find out the combined frictional and point resistance of the cone.
Hydraulic gauges are used for measuring pressure developed

41. Cone used for SCPT

42. Typical Test Set up for SCPT

43. Boring Logs

45. Reference
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Dr. K.R. Arora Soil Mechanics & Foundation Engineering
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