Soil Investigation & Soil Exploration Planning of sub-soil explorationThe main purpose of soil exploration is to determine the stratification and engineering properties of soil, so that safe and economical foundation can be designed. Properties include strength, deformation and hydraulic characteristics of soil. Naturally, extent of investigation will depend on the structure, its weight, size and shape and also on the soil profile and its properties. The program is closely related to the relative cost of investigation and that of the project for which it is being undertaken. Extent of investigation also depends on the location of the project. New area without major structures requires more attention than the areas having important structures.Planning of sub-soil investigation requires not only thorough knowledge of soil engineering, but also experience and engineering judgement. During the progress of investigation, program is sometimes required to be changed depending upon the findings. If variation in soil condition is too much, extent of investigation is increased. On the other hand, if the sub-soil is fairly uniform, extent of investigation may be decreased.For soil investigation of any major project, planning of exploration includes the following three stages 1. Reconnaissance of the site2. Preliminary exploration3. Detailed exploration.

Methods of Exploration1. Open excavations or, open pits2. Boringa) Auger boringb) Shell & auger boringc) Wash boringd) Percussion boringe) Rotary boring.

Open Pits: are excavated at site for visual inspection of soil strata and for collecting disturbed/undisturbed soil samples. It is the cheapest method of boring. However, it can be excavated only up to a limited depth of 3 to 4 m. Open pits below water table is not feasible.Boring: Making and advancing boreholes is called boring. Diameter of boring varies from 50 to 200 mm. In case sides of boreholes cannot be left unsupported, sides are prevented from collapsing by means of casing pipes or using drilling mud (bentonite slurry). Different methods are used for boring, like auger boring, shell & auger boring, wash boring, percussion boring and rotary boring.Note: Please read the descriptions of these methods from any text book.

Soil Sampling:Soil samples are of two types a) Disturbed samples b) Undisturbed samples.Disturbed Sample: is that in which the natural structure of soil gets partly or fully disturbed, although natural moisture content may be preserved. These samples are however representative of the natural soil by maintaining the original properties of various soil particles. Samples obtained from direct excavation, augers, thick-wall samplers are of this type. They are used for grain-size analysis, water content determination, index properties tests and compaction tests.Undisturbed samples: are those in which natural structure and properties remain preserved. They are obtained by specially designed samplers. These samples are used to determine engineering properties of soil, like compressibility, shear strength and permeability.

Design features of undisturbed samplersThe extent of disturbance to the sample due to sampler depends on three features of its designa) Area ratiob) Inside clearancec) Outside clearance.a) Area ratio (Ar)Area ratio = x 100%Or, Ar = x 100% where, D1 = inner diameter of cutting edge D2 = outer diameter of cutting edgeFor good undisturbed sample, Ar should be 10%.b) Inside clearance (Ci)Inside clearance Ci = x 100% where, D3 = inside diameter of sampling tubeCi should be between 0.5 to 3%.

c) Outside clearance (Co)Outside clearance Co = x 100% where, D4 = outside diameter of sampling tubeCo should be between 0 to 2%.

Samplers to obtain undisturbed samples (UDS)1. Thin-walled samplers (IS: 2132 1972)2. Shelby tubes3. Piston samplers4. Denison sampler or Rotary sampler.Note: See the details of these samplers in any text book.

Standard Penetration Test (SPT)This is the most extensively used penetrometer test and is conducted using a split-spoon-sampler. The sampler consists of three parts a) Driving shoe, 75 mm longb) Steel tube, 450 mm long. It can be split open longitudinally in two halvesc) Sampler head, 150 mm long.Inside diameter of sampler is 38 mm and outside diameter is 50 mm.To conduct the test, borehole is advanced up to the required depth. The split-spoon-sampler is attached to the drill rods and lowered into the borehole and rested at the bottom. The split-spoon-sampler is then driven into the soil for a distance of 450 mm by blows of a drop hammer 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 borehole. The number of blows required for the first 150 mm of penetration is called seating blows and is disregarded.The split-spoon-sampler is then withdrawn, detached from drill rods and finally spilt opened after removing the shoe and head. Sample is collected from the barrel and preserved for laboratory tests.N-values obtained from SPT test are extremely useful for determining relative density and angle of shearing resistance of cohesionless soil. It can also be used to correlate consistency and unconfined compressive strength of cohesive soil, though the correlation is not very dependable and may be used for cohesive soil with caution.N-values obtained in cohesionless soil are corrected for dilatancy and over-burden pressure, before they can be used in any correlation and design charts.a) Over-burden correctionIt has been found that the penetration resistance (N-value) in a granular soil is influenced by the overburden pressure. If the two soils having the same relative density but with different confining pressure are tested, the one with higher confining pressure gives a higher penetration number. As the confining pressure in cohesionless soil increases with depth, the penetration number for soil at shallow depth is under-estimated and that at greater depth is over-estimated. Hence, for uniformity correction is applied for a standard effective overburden pressure.As per Gibbs & Holtz (for dry & moist sand) NC = NR x where, NR = observed N-value NC = corrected N-value = effective overburden pressure (kN/m2) The above equation is valid for 280 kN/m2.If NC > 2 NR, NC should be divided by 2.As per Peck, Hansen & Thornburn, NC = 0.77 NR log for 24 kN/m2. IS code has adopted this formula.b) Correction for dilatancyDilatancy correction is to be applied when NC obtained after overburden correction exceeds 15 in silty sand or fine sand stratum below water table. This is because an apparent increase in penetration resistance (N-value) occurs due to rise in pore water pressure during impact of hammer blows. Terzaghi and Peck recommended the following correction, which has also been adopted by IS:2131-1981. N = 15 + (NC 15) where, NC = corrected N-value after applying overburden correctionN = corrected N-value to be used in design charts.

P1: The field N-value in a deposit of fully submerged fine sand was 40 at a depth of 6 m. The average saturated unit weight of soil is 19 kN/m3. Calculate the corrected N-value as per IS code and Gibbs & Holtz.Ans:sat = 19 kN/m3 = 19 - 9.81 = 9.19 kN/m3 = 9.19 x 6 = 55.2 kN/m2a) As per IS codeOver-burden correction: NC = 0.77 NR log = 0.77 x 40 x log = 47.2 = 47 Note: N-value is always an integer.Dilatancy correction N = 15 + (NC 15) = 15 + (47 15) = 31b) As per Gibbs & HoltzOverburden correction NC = NR x = 40 x = 112 > 2 NR NC = 112/2 = 56Dilatancy correction N = 15 + (56 15) = 36.

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