1. SITE INVESTIGATION AND SELECTION OF FOUNDATION

2. SITE INVESTIGATION AND SELECTION OF FOUNDATION Scope and objectives – Methods of exploration – auguring and boring – Wash boring and rotary drilling – Depth of boring – Spacing of bore hole – Sampling techniques – Representative and undisturbed sampling – methods - Split spoon sampler, Thin wall sampler, Stationery piston sampler – Penetration tests (SPT and SCPT) - Bore log report – Data interpretation - strength parameters and Liquefaction potential - Selection of foundation based on soil condition.

3. Definition
The process of determining the layers of natural soil deposits that will underlie a proposed structure and their physical properties is generally referred to as site investigation.
The field and laboratory studies carried out for obtaining the necessary information about the sub soil characteristics including the position of ground water table are termed as soil exploration

4. METHODS OF EXPLORATION
Preliminary exploration
Local topography, excavations, cuttings, drainage pattern and other natural features like streams, flood marks etc.
Geophysical methods
Detailed investigation
Nature, sequence and thickness of layers. Borings and detailed sampling, insitu test.
Limited: erratic soil, light loads, inexpensive and un important structure, good record, sound rock at shallow depth

5. OBJECTIVES OF SITE INVESTIGATION
1. Site selection. 2. Foundation and earthworks design. 3. Temporary works design. 4. The effects of the proposed project on its environment. 5. Investigation of existing construction. 6. The design of remedial works. 7. Safety checks.

6. EXPLORATION PROGRAM
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.

7. The general objective of an exploration program is to identify all of the significant features of the geologic environment that may impact on the proposed construction. Specific objectives are to: 1. Define the lateral distribution and thickness of soil and rock strata within the zone of influence of the proposed construction. 2. Define groundwater conditions considering seasonal changes and the effects of construction or development extraction. 3. Identify geologic hazards, such as unstable slopes, faults, ground subsidence and collapse, floodplains, regional seismicity, and lahars.

8. 4. Procure samples of geologic materials for the identification, classification, and measurement of engineering properties. 5. Perform in situ testing to measure the engineering properties of the geologic materials

9. 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, 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.
7. Establishment of construction methods for changing subsoil conditions.

10. If only they had proper site investigation…
…Tower of Pisa will not be leaning today!

11. Depth of Boring
1. Determine the net increase of stress, under a foundation with depth as shown in the Figure.
2. Estimate the variation of the vertical effective stress, ', with depth.
3. Determine the depth, D = D1, at which the stress increase  is equal to (1/10) q (q = estimated net stress on the foundation).
4. Determine the depth, D = D2, at which /' = 0.05.
5. Unless bedrock is encountered, the smaller of the two depths, D1 and D2, just determined is the approximate minimum depth of boring required. Table shows the minimum depths of borings for buildings based on the preceding rule.

12. Depth of Boring
Determination of the minimum depth of boring

13. Boring Depth Building width Number of Storeys 1 2 4 8 16 30.5 3.4 6.1
10.1
16.2
24.1 61
3.7
6.7
12.5
20.7
32.9
122 7
13.7
24.7
41.5

14. Depth of Boring
When deep excavations are anticipated, the depth of boring should be at, least 1.5 times the depth of excavation. Sometimes subsoil conditions are such that the foundation load may have to be transmitted to the bedrock. The minimum depth of core boring into the bedrock is about 3m. If the bedrock is irregular or weathered, the core borings may have to be extended to greater depths.

21. 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. If various soil strata are more or less uniform and predictable, the number of boreholes can be reduced.

23. SOIL BORING
The earliest method of obtaining a test hole was to excavate a test pit using a pick and shovel. Because of economics, the current procedure is to use power-excavation equipment such as a backhoe to excavate the pit and then to use hand tools to remove a block sample or shape the site for in situ testing. This is the best method at present for obtaining quality undisturbed samples or samples for testing at other than vertical orientation.

24. SOIL BORING

26. Boring tools
Auger boring

27. Diamond drill bit

28. Holding verically and pressing it down while the auger is rotated
Fills annular space
Upto depth of 6m in soft soils with or with out casing
Samples are highly disturbed
For shallow foundaion, highways .etc
SHELL and AUGER method is widely used in india.

29. / Wash boring
Water is forced under pressure through a hollow drill which may be rotated or moved up and down inside casing
Lower end has chopping bit
Only boring can be done and sample of no use

30. Breaking up the formation by repeated blows of heavy bit or chisel inside casing pipe
Limited water forming slurry of pulverized material and removed using bailer
Cables in place of drill rods

31. Cutting action of rotating bit –kept in contact with the bottom of hole
(Bentonite) Drilling mud is used
Core-barrel with diamond bits for rock cores

42. Boring tools

44. Preparation of Boring Logs
1. Name and address of the drilling company
2. Driller’s name
3. Job description and number
4. Number, type, and location of boring
5. Date of boring
6. Subsurface stratification, which can he obtained by visual observation of the soil brought out by auger, split-spoon sampler, and thin-walled Shelby tube sampler
7. Elevation of water table and date observed, use of casing and mud losses, and so on
8. Standard penetration resistance and the depth of SPT
9. Number, type, and depth of soil sample collected
10. In case of rock coring, type of core barrel used and, for each run, the actual length of coring, length of core recovery, and RQD

46. SOIL SAMPLING
Two types of soil samples can be obtained during sampling disturbed and undisturbed.
Reasonably good estimates of 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.

47. Disturbed vs Undisturbed
Disturbed samples are those where the natural soil structure gets modified or destroyed during the sampling operation.
Natural moisture content and proportion of mineral constituents are preserved
“Representative samples”-useful in identification and are partially deformed. The engineering properties are changed, but the original fabric and structure vary from unchanged to distorted, and are still apparent. Such distortion occurs with split-barrel samples.

48. “Non-representative samples”- Alteration in original soil structure,soils from other layers gets mixed up or the mineral constituents gets altered
Extent of sample disturbance
Cutting edge
Inside wall friction
Non-return valve
Inside clearance Ci=((D3-D1)/D1)x100
Outside clearance Co= ((D2-D4/D4)x100
Area ratio Ar= ((D22-D12)/D12)x100

49. Inside clearance
Reduce friction between the soil sample and the sampler when the soil enters, by allowing for elastic expansion.If too large, there will be too much lateral expansion
Should be between 1 to 3 percent
Outside clearance
Reduce friction while sampler is being driven and when it is being withdrawn after sample is collected
Not greater than inside clearance
Lies between 0 and 2 percent

50. Area ratio
Kept as low as possible
Not greater than 20 percent for stiff formations and 10 percent for soft sensitive clays
Index of sample disturbance
Recovery ratio ,Lr= recovered length of the sample/penetration length of the sampler
Lr=1
Lr<1
Lr>1

51. Disturbed vs Undisturbed
UNDisturbed samples-Original soil structure is preserved and the material properties have not undergone any alteration or modification.
Undisturbed samples may display slight deformations around their perimeter, but for the most part, the engineering properties are unchanged. Such results are obtained with tube or block samples.
However an undisturbed sample may be considered as one of in which the material has been subjected to such a small disturbance that it is still suitable for all lab test like shear strength and consolidation

52. Disturbed vs Undisturbed
Good quality samples necessary.
AR<10%
sampling tube
soil
area ratio
Thicker the wall, greater the disturbance.

53. Disturbed vs Undisturbed

54. SPLIT SPOON SAMPLER Split-Barrel Sampler (Split Spoon) (ASTM D1586-99)
Purpose
Split-barrel samplers are used to obtain representative samples suitable for field examination of soil texture and fabric and for laboratory tests, including measurements of grainsize distribution, specific gravity, and plasticity index, which require retaining the entire sample in a large jar.

57. Sampler Description
Split-barrel samplers are available with and without liners; A common O.D. is 51mm and I.D 35mm. with ¼ in. wall thickness (1½ in. sample). Larger diameters are used for sampling gravelly soils. Lengths are either 18 or 24 in.
A ball check valve prevents drill pipe fluid from pushing the sample out during retrieval. To prevent sample spillage during retrieval, flap valves can be installed in the shoe for loose sands, or a leaf-spring core retainer (basket) can be installed for very soft clays and fine cohesionless soils. Upon retrieval, the barrel between the head and the shoe is split open, the sample is examined and described, removed, and stored.

58. In some sampler types, brass liners are used for procuring drive samples of strong cohesive soils for laboratory direct-shear testing.
Sampling Procedure
The sampler is installed on the hole bottom, then driven into the soil with a hammer falling on the drill rods. The number of blows required for a given weight and drop height, and a given penetration, are recorded to provide a measure of soil compactness or consistency as in Standard Penetration Test.

59. Thin-Wall Tube Samplers
Purpose
Thin-wall tube samplers are used to obtain UD of soft to stiff cohesive soils for laboratory testing of strength, compressibility, and permeability.
Tube Materials
Cold-drawn, seamless steel tubing (trade name “Shelby tube”) is used for most soil materials; brass tubes are used for organic soils where corrosion resistance is required. Lacquer coating can provide corrosion protection and reduce internal frictional resistance and sample disturbance. and sample disturbance.

61. Tube diameters and lengths range from 2 to 6 in
Tube diameters and lengths range from 2 to 6 in. in diameter, 24 to 30 in. in length. 2 in. diameter samples have a large ratio of perimeter disturbance to area and are considered too small for reliable laboratory engineering-property testing.
The tube should be provided with a cutting edge drawn in to provide inside clearance (or 0.5 to 3% less than the tube I.D.), which permits the sample to expand slightly upon entering the tube, thereby relieving sample friction along the walls and reducing disturbance.
Tubes 4 to 6 in. in diameter reduce disturbance but require more costly borings.
Outside dia- 40 to 125mm
Tickness 1.25 mm to 3.15mm
Length- 5 to 10 times dia for sandy soil and 10 to 15 times dai for clay

62. Operations
Thin-wall tubes are normally pressed into the soil by hydraulically applied force. After pressing, the sample is left to rest in the ground for 2 to 3 min to permit slight expansion and an increase in wall friction to aid in retrieval. The rods and sampler are rotated clockwise about two revolutions to free the sampler by shearing the soil at the sampler bottom.
The sample is withdrawn slowly from the hole with an even pull and no jerking. In soft soils and loose granular soils, the sampler bottom is capped just before it emerges from the casing fluid to prevent the soil from falling from the tube.

63. Shellby Tube Sampling
A thin-wall tube is fitted to a head assembly that is attached to drill rod. An“O ring” provides a seal between the head and the tube, and a ball check valve prevents water in the rods from flushing the sample out during retrieval. Application is most satisfactory in firm to hard cohesive soils.
Care is required that the sampler is not pressed to a distance greater than its length.
Soft soils are difficult to sample and retain because they have insufficient strength to push the column of fluid in the tube past the ball check valve. In stiff to hard cohesive soils, samples are often taken by driving heavy-gage tubes.

65. Standard Stationary Piston Sampler
A thin-wall tube is attached to a head assembly. The tube contains a piston which is connected to a rod passing through the drill rod to the surface.
When at the bottom of the tube, the piston prevents soil from entering the tube as it is lowered into the hole and permits seating through soft cuttings. The rod connected to the piston is held fixed at the surface, while the hydraulic system on the drilling machine presses the tube past the piston into the soil.
With light rigs, the reaction can be increased by using Earth anchors. In properly fitted piston samplers, a strong vacuum is created to hold the sample in the tubes during withdrawal from the hole. The stationary piston sampler is used to retrieve very soft to firm cohesive soils.

68. 1. Determine the area ratio of the samplers and comment the results 2
1.Determine the area ratio of the samplers and comment the results 2.The inner diameter of the sampling tube and that of cutting edge of sampler are 70mm and 68mm respectively, their outer diameters are 72mm and 74 mm respectively. Det inside clearance, outside clearance and the area ratio of the sampler. Comment its suitability for collecting undisturbed sample
Do (mm)
Di (mm)
Split spoon sampler 50 35
Drive tube
100 90
Shelby 47

70. Factors Affecting Sample Quality Sampler Wall Thickness A large outside diameter relative to the inside diameter causes deformation by material displacement. Sampler Conditions ● Dull, bent, or otherwise deformed cutting edges on the sampler cause sample deformation. ● Inside friction, increased by rust, dirt, or, in the case of tubes, omission of lacquer, causes distortions which are evidenced by a turning downward of layers, resulting in conical shapes under extreme cases. Boring Operations ● Dynamic forces caused by driving casing can loosen dense granular soils or densify loose granular soils.

71. Sands may rise in the casing when below the GWL .
● Overwashing, jetting, and high fluid pressures also loosen granular soils or soften cohesive materials .
● Coarse materials often remain in the hole after washing, particularly in cased borings . These “cuttings” should be removed by driving a split barrel sampler, by pushing a Shelby tube, or with a cleanout auger. Contamination is common after boring through gravel layers or miscellaneous fills containing cinders, etc.
Hole squeezing may occur in soft clays if the drilling mud is too thin.
● Plastic clays may remain along the casing walls if cleaning is not thorough.
● Hollow stem augers can cause severe disturbances depending on the rate of advance and rotation, and the choice of teeth on the bit. It must be advanced with the plug in the auger prior to sampling to prevent soil from entering the auger.

72. Soil Factors ● Soft to firm clays generally provide the best “undisturbed” samples, except for “quick” clays, which are easily disturbed. ● Air or gas dissolved in pore water and released during sampling and storage can reduce shear strength. ● Heavily overconsolidated clays may be subject to the opening of fissures from stress release during boring and sampling, thereby substantially reducing strength. ● Gravel particles in a clay matrix will cause disturbance. ● Cohesionless granular soils cannot be sampled “undisturbed” in the present state of the art. ● Disturbance in cohesive materials usually results in a decrease in shear strength and an increase in compressibility.

73. ROCK SAMPLING - Definition

74. Rock Quality Designation
RQD
Rock Quality Designation (RQD) is defined as the percentage of rock cores that have length equal or greater than 10 cm over the total drill length.

75. FIELD STRENGTH TESTS
The following are the major field tests for determining the soil strength:
1. Vane shear test (VST).
2. Standard Penetration Test (SPT).
3. Cone Penetration Test (CPT).
4. The Borehole Shear Test (BST).
5. The Flat Dilatometer Test (DMT).
6. The Pressure-meter Test (PMT).
7. The Plate Load Test (PLT).

76. FIELD STRENGTH TESTS

77. Standard Penetration Test (SPT)

78. Standard Penetration Test (SPT)

79. Standard penetration test IS 2131-1963
Useful in determining the relative density and the angle of shearing resistance of cohesionless soils
Also can find the compressive strength of cohesive soils
Test is conducted in BH using split spoon sampler
Drop hammer of weight 63.5 kg falling from a height of 75cm
First 15 cm seating drive
Number of blows required for next 30 cm penetration was noted-SPT number
Test is discontinued if the number of blows exceed 50

80. Standard Penetration Test (SPT)
Corrections are normally applied to the SPT blow count to account for differences in:
• energy imparted during the test (60%
hammer efficiency)
• the stress level at the test depth
The following equation is used to compensate for the testing factors (Skempton, 1986):

81. Standard Penetration Test (SPT)

82. SPT Correlations in Granular Soils
not corrected for overburden
(N)60
Dr (%)
consistency
0-4
0-15
very loose
4-10
15-35
loose
10-30
35-65
medium
30-50
65-85
dense
>50
85-100
very dense
These are for granular soils, and can be used as a rough guide.

83.  = RD (degrees)
it can vary 2

84. SPT Correlations in Clays
not corrected for overburden
N60
cu (kPa)
consistency
visual identification
0-2
0 - 12
very soft
Thumb can penetrate > 25 mm
2-4
12-25
soft
Thumb can penetrate 25 mm
4-8
25-50
medium
Thumb penetrates with moderate effort
8-15
50-100
stiff
Thumb will indent 8 mm
15-30
very stiff
Can indent with thumb nail; not thumb
>30
>200
hard
Cannot indent even with thumb nail
Although SPT is not recommended for clays, there is still some value. This table relates N60 values in clays to the undrained shear strength and consistency.
Use with caution; unreliable.

85. Peck, hansen and thornburn correction for N value
Overburden pressure correction
1.High confining pressure gives high penetration number.
2.As the confining pressure in cohesionless soils increases with depth, the penetration number at shallow depth are under estimated and at deeper depth are over estimated.
Peck, hansen and thornburn correction for N value
N=0.77 Nr log (2000/p’o) Eq 3.12
for p’o> 25 kN/m2

86. Dilatancy correction
Silty and fine sands below water table develop PWP which is not easily dissipated. This will increase the penetration number.
Nc= (Nr-15)

89. A SPT test was conducted on saturated fine sand below the ground water table at 10m depth. The SPT value was 32. Assume γ = 18.2 kN/m3, find the corrected SPT N value

90. Cone penetration test
Cone test was developed by Dutch government. Hence known as Dutch cone test
“IS: 4968 (Part-III)-1976—Method for subsurface sounding for soils—Part III Static cone penetration test”.
Static method or dynamic method
STATIC METHOD
Cone is pushed downward by applying thrust at a steady rate of 10mm/sec through a depth of 35mm each time-Cone resistance
Cone is withdrawn and the sleeve is pushed on to the cone and both are pushed together.-Combined resistance
Sleeve resistance-Combined resistance-Cone resistance

91. Results of SCPT is compared with SPT
Gravels- qc= 800N to 1000N
Sands-qc= 500N to 600 N
Silty sands qc= 300 N to 400 N
Silts and Clayey silts qc- 200N
N is the SPT number

92. Among the field sounding tests the static cone tests in a valuable method of recording variation in the in-situ penetration resistance of soils, in cases where the in-situ density is disturbed by boring operations, thus making the standard penetration test unreliable especially under water. The results of the test are also useful in determining the bearing capacity of the soil at various depths below the ground level.
In addition to bearing capacity values, it is also possible to determine by this test the skin friction values used for the determination of the required lengths of piles in a given situation. The static cone test is most successful in soft or loose soils like silty sands, loose sands, layered deposits of sands, silts and clays as well as in clayey deposits.

93. In areas where some information regarding the foundation strata is already available, the use of test piles and loading tests there of can be avoided by conducting static cone penetration tests.
The cone resistance shall be corrected for the dead weight of the cone and sounding rods in use. The combined cone and friction resistance shall be corrected for the dead weight of the cone, friction jacket and sounding rods. These values shall also be corrected for the ratio of the ram area to the base area of the cone.

94. Cone Penetration Test (CPT)
Diameter 35.7 mm
Apex angle 60o

95. Cone Penetration Test (CPT)

96. Electrical Cone

97. Cone Penetrometer (CPT)

99. CPT Truck

100. Crawler Type CPT Truck

101. Typical CPT Data

102. Cone Penetration Test (CPT)

103. Cone Penetration Test (CPT)

104. Type of clay
Cone factor
Normally consolidted
11 – 19
Over consolidated
At shallow depth
15 to 20
At large depth
12 to 18

105. What is liquefaction?
Liquefaction is a process by which sediments below the water table temporarily lose strength and behave as a viscous liquid rather than a solid.
The types of sediments most susceptible are clay-free deposits of sand and silts; occasionally, gravel liquefies.
seismic waves, primarily shear waves, passing through saturated granular layers, distort the granular structure, and cause loosely packed groups of particles to collapse. These collapses increase the pore-water pressure between the grains if drainage cannot occur. If the pore-water pressure rises to a level approaching the weight of the overlying soil, the granular layer temporarily behaves as a viscous liquid rather than a solid. This phenomenon is called Liquefaction.

107. What is liquefaction?
In the liquefied condition, soil may deform with little shear resistance; deformations large enough to cause damage to buildings and other structures are called ground failures.
The ease with which a soil can be liquefied depends primarily on the looseness of the soil, the amount of cementing or clay between particles, and the amount of drainage restriction.

109. What is liquefaction?
Liquefaction does not occur at random, but is restricted to certain geologic and hydrologic environments, primarily recently deposited sands and silts in areas with high ground water levels. Generally, the younger and looser the sediment, and the higher the water table, the more susceptible the soil is to liquefaction.
Liquefaction has been most abundant in areas where ground water lies within 10 m of the ground surface; few instances of liquefaction have occurred in areas with ground water deeper than 20 m. Dense soils, including well-compacted fills, have low susceptibility to liquefaction.

110. Hole Stabilization
Some form of stabilization is often needed to prevent hole collapse. None is required in strong cohesive soils above GWL.
Casing, used in sands and gravels above the water table, in most soils below GWL, and normally in very soft soils, . The hollow-stem auger serves as casing
Driven casing has a number of disadvantages:
Installation is slow in strong soils and casing recovery is often difficult.
Sampling at stratum changes is prevented unless they occur at the end of a driven section, and in situ testing is limited.

111. Obstacles such as boulders cannot be penetrated and require removal by blasting or drilling. The latter results in a reduced hole diameter which restricts samling methods unless larger-diameter casing is installed before drilling.
Loose granular soils below GWL tend to rise in casing during soil removal,resulting in plugged casing and loosened soils below.
Removal of gravel particles is difficult and requires chopping to reduce particle sizes.
Casing plugged with sand or gravel prevents sampler penetration adequate for recovery of undisturbed samples and representative SPT values.  

112. Mud slurry, formed naturally by the mixing of clayey soils during drilling, or by the addition of bentonite, is a fast and efficient method suitable for most forms of sampling.There are several disadvantages to mud slurry: ● Hole closure may occur in soft soils or crushed, porous materials such as shell beds. ● Relatively large pumps are required to circulate the slurry, particularly when boring depths exceed 30 ft (10 m). ● Mud-cased holes do not permit accurate water level readings, unless environmentally friendly biodegradable muds are used. Such muds incorporate an organic substance that degrades in a period of 24 to 48 h, allowing GWL measurements. ● Excessive wear on pumps and other circulating equipment occurs unless sand particles are removed in settling pits. Mud may penetrate some soils and contaminate samples. Mud loss is high in cavity-prone and highly fractured rock.

113. Reference Geotechnical investigation methods – Roy E Hunt
Venkatramaiah, C. “Geotechnical Engineering”, New Age International Publishers, New Delhi, 2007 (Reprint)
Arora K.R. “Soil Mechanics and Foundation Engineering”, Standard Publishers and Distributors, New Delhi, 2005.