Soil Exploration and Sampling - Comprehensive Study Notes

Introduction

• Site investigations and sub-surface explorations are conducted to obtain information about sub-surface conditions at the site of proposed construction.

• Information about surface and subsurface features is essential for the design of structures and for planning construction techniques.

• Site investigations consist of:

  • Determining the profile of natural soil deposits at the site

  • Taking soil samples and determining the engineering properties of the soil

  • In-situ testing of the soils

Objectives of Site Investigations

• To select the type and depth of foundation for a given structure.

• To determine the bearing capacity of the soil.

• To determine the probable maximum and differential settlements.

• To establish the ground water table and determine properties of water.

• To predict the lateral earth pressure against retaining walls and abutments.

• To select suitable construction techniques.

• To predict and solve potential foundation problems.

• To ascertain the suitability of the soil as construction material.

• To investigate the safety of existing structures and suggest remedial measures.

Planning a Subsurface Exploration Programme

• A subsurface exploration programme depends on:

  • The type of structure to be built

  • The variability of the strata at the proposed site

• Extent of subsurface exploration is related to the relative cost of the investigation and the overall project cost.

• More detailed investigations reduce uncertainty and can lead to economic construction.

• Location considerations:

  • A small house in a built-up area requires less exploration.

  • A house in a newly developed area requires detailed investigation.

  • Multi-storey buildings require extensive sub-surface exploration due to heavy loads and deep influence zones.

• Rationale: investing in sub-surface exploration can be more economical than over-designing structures.

Stages in Subsurface Exploration

1) Reconnaissance

• First step: site visit and study of maps and records to plan the investigation.

• Information obtained during reconnaissance includes:

  • General topography, drainage ditches, dumps of debris and sanitary fills

  • Existence of settlement cracks in nearby structures

  • Evidence of landslides, slope creep, shrinkage cracks

  • Stratification from deep cuts near the site

  • Location of high water marks on nearby buildings and bridges

  • Depth of ground water table as observed in wells

  • Existence of springs, swamps, etc.

  • Drainage pattern and vegetation at the site

  • Existence of underground services (water mains, conduits, etc.)

• Outcome: helps in evolving a suitable subsurface investigation programme.

• In addition to site visits, the geotechnical engineer should study:

  • Geological maps, aerial photographs, toposheets, soil maps, blueprints of existing buildings

• Study of maps may include:

  • Topographical maps (Survey of India toposheets) and geological maps (GSI)

  • Soil conservation maps

  • Indications of faults, folds, fissures, dikes, caves

  • Earthquake zone maps and seismic potential considerations (major factor in design for dams and nuclear plants)

2) Preliminary exploration

• Aim: determine depth, thickness, extent, and composition of each soil stratum; determine bed rock depth and ground water table.

• Typically involves a few borings or test pits.

• Tests used:

  • Cone penetrometers and sounding rods to assess strength and compressibility

  • Geophysical methods to locate boundaries between strata

3) Detailed exploration

• Purpose: determine engineering properties of soils in different strata.

• Activities: extensive boring programme, sampling, and laboratory testing.

• Field tests to determine in-situ properties: vane shear tests, plate load tests, permeability tests.

• Dynamic properties tests may be carried out if required.

• For complex projects (bridges, dams, multi-storey buildings) detailed exploration is essential.

• For smaller projects in uniform strata, reconnaissance and preliminary investigations may suffice; design relies on data from those phases.

Depth of Exploration

• Depth depends on:

  • Type of proposed structure

  • Total weight, loaded area, shape, and disposition

  • Physical properties of soils constituting the strata

• Exploration should extend to a depth where the increase in vertical pressure due to loading is likely to cause foundation failure. This is the meaningful or significant depth.

• Net loading intensity at depth is approximated by assuming load spread of 2 vertical to 1 horizontal from all sides of the foundation (2:1 spread).

• Allowance for overlap of loads from closely spaced footings.

• Practical criterion (as given): significant depth is the depth at which the net increase in vertical pressure becomes less than 10% of the initial overburden pressure.

Guide rules to decide depth of exploration

• Type of foundation and suggested depths:

  • Isolated spread footing or raft: depth ≈ 1.5 × width

  • Adjacent footings with clear spacing less than 2 × width: depth ≈ 1.5 × width (or as implied by design guidance)

  • Pile foundations: depth ≈ 10–30 m or at least 1.5 × width

  • Base of retaining walls: depth ≈ 1.5 × base width or the exposed height, whichever is greater

• For black cotton soils (weathering considerations), exploration should extend to a minimum depth of ≈ 4 m

Methods of Soil Exploration

• Three broad categories:
  1) Direct methods
  2) Semi-direct methods
  3) Indirect methods

1) Direct methods

• Open excavation methods of exploration:
  a) Pits and Trenches

• Pits and trenches are excavated to inspect strata. IS:4453-1967 recommends a clear working space of 1.2 m × 1.0 m at the pit bottom.

• Pit depth depends on investigation needs. Shallow pits up to 3 m can be dug without lateral support; deeper pits require sheeting and bracing.

• For depths > 6 m, bore holes are more economical than pits. Deep pits need proper ventilation and dewatering if water is encountered.

• Trenches are long shallow pits exposing a line along a slope and are often more suitable than pits for exploration on slopes.
  b) Drifts and Shafts

• Drifts: horizontal tunnels in hillside to determine geological formation

• Minimum clear dimensions in hard rock: 1.5 m width × 2 m height

• Shafts: vertical holes; rectangular or circular; minimum dimensions: rectangular width ≈ 2.0 m, circular diameter ≈ 2.0 m

2) Semi-direct methods

• When depth is large, borings are used.

• Process: drill a vertical bore hole; collect soil samples from the bore and test in the laboratory.

Boring Methods (Types)

1) Auger Boring

• Types: Helical auger and post-hole auger.

• Augers are used in cohesive and other soft soils above water table.

• Hand augers for depths up to ~6 m; mechanical augers for greater depths; in gravelly soils, samples may be disturbed.

• Disadvantages: sandy soils below water table require casing; slow in hard soils; unsuitable for large cobbles/boulders.

• Applications: rapid and economical for highways, railways, airfields; shallow explorations.

2) Auger and Shell Boring

• Cylindrical augers and shell with cutting teeth; used for deeper borings.

• Hand rings for up to ~2 m; mechanical rings up to ~50 m.

• Suitable for soft to stiff clays; shells for very stiff/hard clays; casing often required.

3) Wash Boring

• Hole drilled by driving casing (~2–3 m) and using a hollow drill rod with a chisel bit.

• Water is pumped down (wash pipe) to jet and cut soil; soil and water rise through annulus to surface.

• Lowering and lifting continue until hole caves in; casing extended as needed.

• Drilling fluids (bentonite suspensions) may be used for wall support.

• Disadvantages: slow in stiff soils; not good for hard soils or samples above groundwater; wash water can alter soil water content.

4) Percussion Drilling

• Used for rocks, boulders, hard strata.

• Heavy chisel is lifted and dropped; water added to keep slurry below water table.

• May require casing; useful for drilling in glacial tills with boulders.

• Disadvantages: bottom material disturbed by blows; samples may be disturbed; generally more expensive.

5) Rotary Drilling

• Very fast for rocks and soils.

• Rotating drill bit at bottom; drilling mud (bentonite solution) circulated down and back up to carry cuttings.

• Usually requires no casing; can obtain samples by raising drill rod and replacing bit with sampler.

• Sizes: 50–200 mm bore holes; suitable for clays, sands, rocks.

6) Core Drilling

• Used for rock cores.

• Core barrel with a hollow drill rod; core is retained and brought to surface.

• Bit options: diamond-studded or cutting-edge (e.g., chilled shot).

• Water is pumped to keep bit cool and carry material to surface.

Types of Soil Samples

• Two main types: Disturbed and Undisturbed samples

1) Disturbed sample

• Natural structures partially or fully modified; natural moisture content may be preserved.

• Obtained by direct excavations, auger, thick-walled samplers.

• Disturbances include: change in stress conditions, change in water content/void ratio, disturbance of structure, chemical changes, mixing/segregation.

• A representative sample contains all constituents of the same soil type; remolded samples have altered properties.

2) Undisturbed sample

• Natural structure and properties preserved; used for shear, consolidation, and permeability testing.

• Stress changes cannot be avoided.

Sample Disturbances and Sampler Design Factors

• Disturbances depend on sampler design and sampling method.

• Design factors governing disturbance include cutting edge geometry, inside/outside clearances, wall friction, non-return valve, and recovery ratio.

Cutting edge design features (typical cutting edge diagram shows)

• Area ratio: $A<em>1 = rac{D</em>2^2 - D<em>1^2}{D</em>1^2} imes 100$

  • D1: internal diameter of cutting edge; D2: external diameter of cutting edge

  • Area ratio should be ≤ 25% for good quality samples; ideally ≤ 10%.

• Inside clearance: $ext{Inside Clearance} = rac{D<em>3 - D</em>1}{D_1} imes 100$

  • D3: inside diameter of sample tube

  • Desired: 1–10% (0.5–3% for undisturbed samples)

• Outside clearance: $ext{Outside Clearance} = rac{D<em>2 - D</em>4}{D_4} imes 100$

  • D4: outside diameter of sample tube

  • Typically 0–2%; helps reduce withdrawal forces

• Inside wall friction: ensure smooth, properly oiled walls

• Non-return valve: should permit easy escape of water/air during driving

• Recovery ratio: $R = rac{L}{H} imes 100$

  • L: length of sample within the tube; H: depth of penetration of the sampling tube

  • Recovery ratio ideally 96–98%

Soil Samples and Samplers (Detailed Types)

1). Open Driver Sampler

• Thick-wall or thin-wall type; head includes vent valves to permit water/air escape; check valve retains the sample; tube may be seamless or split (split spoon sampler).
  2). Split Spoon Sampler

• Bottom driving shoe; a split steel tube (two halves) with a top coupling; internal diameter details given in standard descriptions.

• Used with a hammer to drive into soil; standard blows recorded to compute the Standard Penetration Number N; usually performed at 1.5 m intervals after advancing the bore.
  3). Stationary Piston Sampler

• Consists of a piston at the bottom with a long piston rod passing to surface; piston can be clamped to stop water/soil entry; suited for soft soils and saturated sands.
  4). Rotary Sampler

• Core-barrel type with outer tube and cutting teeth; used for sampling in stiff cohesive soils.
  5). Scraper Bucket Sampler

• Used when deposits contain pebbles where standard split spoon cannot obtain representative samples; also used below water table in cohesionless soils.
  6). Shelby Tubes and Thin-Walled Sampler

• Shelby tubes: thin-walled seamless steel tubes; outer diameter typically 40–125 mm; length 5–15×diameter depending on soil type; area ratio < 15%; inside clearance 0.5–3%; push into soil by rapid, continuous motion without impact; cores are obtained by shearing at the bottom after insertion; ends sealed after extraction; used for undisturbed clay samples.
  7). Denison Sampler

• Double-walled sampler; outer barrel rotates, inner barrel contains liner; may include a basket-type core retainer.

• Used for stiff to hard cohesive soils and slightly cohesive sands; not suitable for gravels, loose sands, silt below water table, or very soft clays.

8) Hand-Curved Samples

• Also called chunk samples; obtained when soil is exposed in pits, shafts, or tunnels.

• Requires cohesion in soil for support; samples are cut from a soil column, trimmed, and preserved with paraffin to prevent moisture loss.

• Hand-curved samples are undisturbed.

Preservation of Samples

• Undisturbed samples require moisture content preservation and avoidance of evaporation.

• Common preservation methods include coating ends with paraffin and petroleum jelly (two coats, 12 mm thick) for humidity-controlled storage; or Hessian bags with water sprinkling; block samples may be wax-coated and stored in airtight boxes with sawdust to fill annular space.

Field Tests

• Field tests test soils in their natural, undisturbed condition, though undisturbed samples are often difficult to obtain for non-cohesive soils; laboratory tests are commonly relied upon for such soils.

• Field tests provide data used in empirical correlations for design; for cohesive soils, laboratory tests are often preferred.

• Common field tests in subsurface investigations:

  • Penetration Test (SPT)

  • Standard Cone Penetration Test (CPT) and Dynamic Cone Penetration Test (DCPT/SCPT)

  • Plate Load Test

Standard Penetration Test (SPT)

• Carried out in a borehole; uses a split-spoon sampler attached to standard drill rods.

• Procedure:

  • Borehole depth reached; bottom cleaned.

  • Split-spoon lowered to bottom and driven by 65 kg hammer from a 750 mm height for 450 mm penetration per drive.

  • The number of blows for every 150 mm penetration is recorded; the last 300 mm’s blows are summed as N (the blow count).

  • If a liner is used, it is sealed after sampling.

  • SPT is conducted at every 0.75 m depth intervals (can be up to 1.5 m for deep holes).

• Refusal criteria:

  • 50 blows for any 150 mm penetration

  • 100 blows for 300 mm penetration

  • 10 consecutive blows produce no advance

• Precautions:

  • Drill rods must be standard and straightened; split spoon should be in good condition; drop hammer weight and fall height must be exact (750 mm).

  • Borehole bottom must be cleaned prior to SPT; casing should not disturb the test; if used, it should extend just short of the SPT depth to avoid soil plugs.

  • If testing in sandy soil below groundwater, water level must be maintained slightly above groundwater to avoid dry-out and anomalously low N values.

• Rationale for continued use:

  • Simple, economical, and the only test providing representative samples for both field inspection and lab moisture-content/tests.

• Corrections to SPT N values (IS: 2131-1981)

  • Corrections adjust for the effect of overburden pressure and dilatancy.

  • Overburden correction: N' = CN × N, where CN is the correction factor for overburden pressure.

  • If N' > 15, a dilatancy correction is applied: N" = 15 + 0.5 (N' - 15). If N' ≤ 15, N" = N'.

• Notes:

  • Corrected N values are used in empirical correlations with soil properties.

  • In sands, N values must be corrected for overburden and dilatancy effects before use in design correlations.

Cone Penetration Test (CPT)

• Developed internationally (Dutch cone test) and can be performed via static or dynamic methods.

• Cone geometry: apex angle 60°, base diameter 35.7 mm; end area ≈ 10 cm^2.

• Static CPT: cone is pushed down at a steady rate (about 10 mm/s) to a certain depth, measuring cone resistance qc. After penetrating, a sleeve resistance is also measured by pushing the sleeve together with the cone; sleeve resistance is qcsleeve = qctotal − qc_cone.

• Refined Dutch cone has a friction sleeve above the cone point to measure point resistance and frictional resistance separately.

• Calibration: CPT results are correlated with lab undisturbed test data and with SPT N-values to establish engineering correlations.

• Dynamic CPT (DCPT): cone is driven by a hammer; the number of blows for a specified penetration distance gives the dynamic cone resistance Ncbr. If skin friction is removed, tests are conducted in casings; with bentonite slurry, friction is reduced.

• Correlations:

  • For a 50 mm diameter cone: N_corr ≈ a × N for various depths (example correlations given in the text):

  • Nebr ≈ 1.5 N for depth < 3 m

  • Nebr ≈ 1.75 N for 3–6 m

  • Nebr ≈ 2.0 N for > 6 m

  • For central building research Institute correlations (Roorkee) with Ncbr and N:

  • Nebr ≈ 1.5 N for depth < 4 m

  • Nebr ≈ 1.75 N for 4–9 m

  • Nebr ≈ 2.0 N for > 9 m

Plate Load Test

• A semi-direct test to measure allowable soil pressure to induce a given settlement.

• Equipment: square/round plates (30–60 cm diameter, ~2.5 cm thick); hydraulic jack; reaction system (cross-beam/truss); three dial gauges (0.02 mm sensitivity) spaced 1200 mm apart.

• Procedure:

  • Excavate a pit at least 5 times the plate size; bottom at foundation level; pump out water to keep water level at the foundation level if needed.

  • Place a seating load (~70 g/cm^2) and release; incrementally apply higher loads and record settlements.

  • Continue until settlement rate is < 0.25 mm/h or until planned maximum settlement reached; record elastic rebound after unloading.

• Interpretation (Terzaghi and Peck): determine allowable foundation settlement Sf for a given prototype foundation settlement Sp and plate size B.

  • For granular soils: Sf = Sp rac{B (eta2 + 0.3)}{eta1 (B + 0.3)} ext{ (per the document)}

  • For clay soils: $S<em>f = rac{S</em>p}{B}$

  • The final bearing pressure is obtained from the plate-load settlement curve by finding the pressure corresponding to Sp; this pressure is the safe bearing pressure for the given permissible settlement Sf.

• Limitations:

  • Plate load tests reflect short-term (immediate) settlements; consolidation settlements are not predicted.

  • In sandy soils, immediate settlement may approximate total settlement; in clays, immediate settlement is only a portion of total settlement.

  • Results can be misleading in non-homogeneous soils or where scale effects are significant.

Geophysical Methods of Subsurface Exploration

Seismic Refraction Method

• Based on velocity differences of seismic waves in different soils and refraction at boundaries between media with different velocities.

• Source of impulse: explosive detonation or mechanical hammer blow.

• Waves: Longitudinal (P-waves), Transverse (S-waves), and surface waves; this method mainly uses P-waves (Vp) and relationships with material properties.

• Governing equation (for P-waves Vp):
  $V_p = <br>rac{E(1-<br>u)}{(1+<br>u)(1-2</p></li></ul><p>u)<br>ho} \,?$
  (Note: The document provides a form involving E, μ, p, etc.; the important point is Vp depends on dynamic modulus E, Poisson’s ratio μ, and density p.)

  • Waves can be direct, reflected, or refracted; refracted waves emerge at boundaries and travel along layers with higher velocity.

  • Data collection uses geophones and surface-recording equipment; timing of arrivals yields layer velocities and depths.

  • Assumptions:

    • All layers are horizontal

    • Layers are thick enough to yield a response

    • Layers are homogeneous and isotropic

    • Velocity increases with depth (Snell’s law) when transitioning to higher-velocity materials

  • Procedure: geophones placed along a line from shear source; arrival times of first impulses at geophones are used to infer subsurface layering.

  • Advantages:

    • Provides a complete picture of stratification to about 10 m depth

    • Relatively cheap due to fewer source/receiver requirements

    • Simple processing

    • Yields seismic velocity information for material properties

    • High vertical resolution; non-destructive

  • Disadvantages:

    • Blind zone if V2 < V1 (thickness may be underestimated)

    • Errors due to velocity dissipation with depth

  • Applications:

    • Bed rock depth and surfaces

    • Buried channels

    • Water table depth

    • Stratigraphy interfaces and faults

  Electrical Resistivity Method

  • Based on differences in electrical conductivity/resistivity of soils.

  • Resistivity definition: $ho = rac{R A}{L}$

    • ρ: resistivity (ohm-cm)

    • R: resistance (ohms)

    • A: cross-sectional area (cm^2)

    • L: distance between electrodes (cm)

  • Procedure: electrodes driven ~20 cm into ground; a DC or low-frequency AC current I is passed; potentials Vc and Va are measured by inner electrodes; resistivity is inferred from measurements.

  • Apparent resistivity: the measured resistivity that reflects a layered medium; analyzed via plotting apparent resistivity vs electrode spacing to infer subsurface conditions.

  • Arrays:

    • Wenner array: ρ_a = 2π a (V/I), where a is electrode spacing

    • Schlumberger array: ρ_a = π (s^2 − a^2)/a × (V/I), where s and a define electrode spacings in the Schlumberger layout

  • Advantages:

    • Very rapid, economical, and non-destructive

    • Useful up to about 30 m depth

  • Disadvantages:

    • Only detects contrasting strata; provides no direct sample information

    • Cultural interference (e.g., pipelines, metallic objects) can distort readings

    • Data acquisition can be slow with some modern techniques improving speed

  • Applications:

    • Resistivity mapping and profiling to locate voids, contaminant plumes, heavy metals, paleochannels, archaeological sites

  Notes on Formulas and Key Terms

  • Significant depth criterion for exploration:
    ext{If } rac{ ext{net increase in vertical pressure}}{ ext{initial overburden pressure}} < 0.10, ext{ then that depth is not critical.}

  • Load-spread assumption for depth estimation:

    • Load spread is taken as 2 vertical to 1 horizontal (2:1 spread).

  • Plate load interpretation (Terzaghi & Peck):

    • Granular soils: $S<em>f = S</em>p rac{B (b<em>2 + 0.3)}{b</em>1 (B + 0.3)}$

    • Clay soils: $S<em>f = rac{S</em>p}{B}$

    • Sf: permissible settlement of the foundation; Sp: settlement of the plate; B: plate size; bp: plate size parameter (as used in the original notes).

  • Standard Penetration Test (SPT) corrections (IS: 2131-1981):

    • N' = C_N × N (overburden correction)

    • If N' > 15, apply dilatancy correction: N" = 15 + 0.5 (N' − 15)

    • If N' ≤ 15, N" = N'

  • Cone Penetration Test: qc is cone resistance; fsleeve is sleeve resistance; dynamic CPT (DCPT) uses Ncbr (blows per penetration distance) with correlations to N values from SPT for linking CPT and SPT data.

  • Key relations for CPT correlations (typical forms):
    -qc (cone resistance in KN/m^2 or MPa) can be correlated to N values depending on soil type (gravels, sands, silty sands, silts/clays).

  • Resistivity arrays: ρ_a values depend on geometry; Wenner and Schlumberger formulas provide direct ways to estimate apparent resistivity from measured responses.

  Practical and Real-World Context

  • Planning deeper exploration reduces uncertainty and can lead to more economical designs by avoiding overdesign.

  • Field tests like SPT and CPT provide quick, cost-effective data that can be correlated with laboratory tests to estimate shear strength, compressibility, and permeability.

  • Geo-structural interpretations rely on multi-method integration (drilling data, field tests, and geophysical surveys) to produce a reliable subsurface model.

  • Ethical/practical notes: ensure safety during excavations, accurate sampling to avoid misleading conclusions, and acknowledge limitations of each method (disturbance in disturbed samples, short-term vs. long-term settlements, and non-destructive vs. destructive nature of tests).