Foundation Engineering: Soil Exploration (Site Investigation) – Comprehensive Notes

Purpose and scope of soil exploration (site investigation)

• Field and laboratory tests to obtain subsoil characteristics, including groundwater (GW) position, are termed soil exploration.
• Main purposes:
  • i) To decide the type of foundation (shallow vs deep).
  • ii) To obtain information about soil strength.
  • iii) To estimate probable settlements and differential settlements (consolidation properties).
  • iv) To determine the location/position of the groundwater table.
  • v) To estimate lateral earth pressures on retaining structures.
  • vi) To select suitable construction techniques.
  • vii) To predict possible foundation problems.
• Overall flow of soil exploration:
  • Planning of the exploration program
  • Collection of disturbed and undisturbed soil samples
  • In-situ field tests
  • Study of GW position and water chemistry
  • Geophysical exploration if required
  • Preparation of drawings, charts, and tables
  • Data analysis
  • Recommendations
  • Report writing

Planning of soil exploration program

• The exploration program depends on several factors:
  • Type of structure and project cost: tests incur costs; for small/low-load projects approximate data may suffice; for large/heavy structures, detailed testing guides accurate design; skipping tests can lead to over-design and higher costs.
  • Location of project: if a small existing building on a built-up area, soil exploration may not be required; for new sites with heavy loads, subsoil investigation is essential.
  • Experience of technicians: experienced teams yield sufficient, accurate data and lower overall cost; inexperience may increase cost due to data gaps.
  • Subsoil investigation does not reflect 100% subsoil condition; methods may need to adapt during progress due to unforeseen problems.

Stages of soil exploration

1) Reconnaissance

• Activities and objectives:
  • i) Site visit.
  • ii) Before visiting: study location maps, records, photographs, soil maps.
  • iii) Purpose:
  • a) Suggest suitable soil exploration methods.
  • b) Help plan the program.
  • c) Determine types of samples to collect.
• Observations during reconnaissance:
  • i) Topography (underground services, drainage, cables, electric lines).
  • ii) Evidence of landslides (shrinkage cracks, inclined trees).
  • iii) Settlement cracks in nearby structures.
  • iv) Location of high flood level and GW depth (observed in wells).
  • v) Drainage pattern.
  • vi) Existence of swamp, springs, etc.
  • vii) Type of vegetation.

2) Preliminary exploration

• Goals:
  • To determine depth, extent, thickness, and composition of soil.
  • To determine GW table.
• Methods:
  • Test pits and/or borings (or both).
  • Soil from varying depths examined for index properties.
• Possible tests and methods:
  • CPT, sounding rods (to obtain shear strength and compressibility), photography
  • Geophysical methods to delineate boundaries of strata

3) Detailed exploration

• When needed:
  • For small projects with uniform soils, detailed exploration may be unnecessary.
  • For heavy projects, it is essential.
• Aims: determine engineering properties (permeability, compressibility, bearing capacity, etc.).
• Requires extensive boring and several field tests (permeability, vane shear test, PLT, CPT, etc.).

Methods of site investigation (field exploration techniques)

a) Open excavation

• Types: Pit and trenches
• Characteristics:
  • Suitable for shallow depths.
  • Typical pit size: 1.2 m × 1.2 m.
  • Depth determines need for lateral support:
  • Up to 3 m: no lateral support required.
  • Greater than 3 m: lateral support required (also below GW) irrespective of depth.
  • Boring cheaper for depths > 6 m.
  • Trench: longer pits; provides more representative view of subsoil than a single pit.
  • Excavations may be manual or mechanical.
  • Precautions to prevent caving.

b) Drifts (Adits) and Shafts

• Usefulness:
  • Determine minimum depth to sound rock, locate faults, shear zones, buried channels under river beds.
• Drifts: horizontal tunnels; minimum size roughly 1.5 m wide × 1.0 m high in hard rock; arched roofs preferred in soft rock to reduce vertical load; supports needed where soils cannot stand on their own.
• Shafts: vertical holes (circular or rectangular) for deeper study; minimum rectangular: 2.4 m × 2.4 m? (rectangular minimum width); circular diameter minimum 2.4 m; used for depths ≥ 6 m; deep shafts require ventilation.
• Note: These methods are expensive.

c) Boring

• For greater depths where open excavation is costly, boring is preferred.
• Types of boring:
  • i) Auger boring
  • ii) Wash boring
  • iii) Rotary drilling
  • iv) Percussion drilling
  • v) Core boring

a) Auger boring

• Equipment: simple auger; typical diameter 15–20 cm; pipe diameter 18 mm; useful to ~6 m in soft soils.
• Sampling:
  • Post-hole auger used for samples in existing bore holes.
• Operation: manual or mechanical; mechanical augers can reach ~12 m in hard strata.
• Suitability: good for soils that can stand on their own (clay, silt, partially saturated sand).
• Limitations: not suitable for cobbles/gravels; samples obtained are disturbed and of limited use.

b) Wash boring

• Procedure: casing driven to 2–3 m with drop hammer; drill rod inserted; chopping bit at bottom; wash water pumped down under high pressure through hollow drill rod (wash pipe).
• Process: water jetting and chopping disintegrate soil; wash water and solids flow to surface; samples collected later; heavier particles settle lower.
• Applicability: usable in all soil types except rock/hard soils; undisturbed samples are hard to obtain; equipment is cheap and light.

c) Rotary Drilling (Mud Rotary)

• System: hollow drill rod; top rotary drive; downward pressure; bottom cutting bit; drilling mud circulates in annulus to bring cuttings to surface.
• Sampling: bit replaced by sampler when needed.
• Suitability: useful for sand, clay, and rocks; not suitable for soils with high gravel content.
• Diameter range: 50–200 mm.

d) Core Drilling

• Core barrel with diamond bit; expensive but yields good cores.
• Procedure: diamond bit cuts rock; core enters core barrel; to collect sample, swap to core lifter and rotate to extract core; continuous water cooling.

e) Percussion drilling

• Operation: heavy chisel dropped onto point to drive hole; water added above GW to dissolve and wash out material.
• Suitability: useful in rock, boulders, hard strata; can be used for tube wells.
• Limitation: does not provide good undisturbed samples and limited stratigraphic detail.

Geophysical methods (subsurface investigation without direct sampling)

Geophysical methods help determine subsurface strata; key methods are explained below.

1) Seismic method

• Principle: impact on a plate excites waves; waves reflect/refract at layer interfaces; measured by geophones.
• Observations:
  • Wave velocity increases with density/with depth.
  • Plot distance from impact (x-axis) vs arrival time (y-axis).
  • Reciprocal of slope gives seismic velocity in a layer.
• Key relations (empirical):
  • $V<em>1 = \tan \alpha</em>1, \quad V<em>2 = \tan \alpha</em>2, \quad V<em>3 = \tan \alpha</em>3$
  • $H<em>1 = 0.50 X</em>1 \left( \frac{V<em>2 - V</em>1}{V<em>2 + V</em>1} \right)^{0.5}$
  • $H<em>2 = 0.85 H</em>1 + 0.50 X<em>2 \left( \frac{V</em>3 - V<em>2}{V</em>3 + V_2} \right)^{0.5}$
• Practical note: These are empirical relations used to estimate layer thicknesses and velocities.

Velocities in different strata (typical ranges)

• Granite: $4000 \le V \le 6000\, \text{m/s}$
• Sanstone: $1500 \le V \le 3000\, \text{m/s}$
• Shale: $1300 \le V \le 3000\, \text{m/s}$
• Hard clay: $600 \le V \le 1500\, \text{m/s}$
• Loose gravel (wet): $500 \le V \le 1000\, \text{m/s}$
• Loose sand (wet): $500 \le V \le 1500\, \text{m/s}$
• Loose sand (dry): $250 \le V \le 600\, \text{m/s}$
• Limitations:
  • Not suitable if harder/denser overlie softer layers.
  • Subsurface conduits or pavements can distort results.
  • Costly.
  • Frozen surface layers can invalidate results.

2) Electrical Profiling Method (Resistivity)

• Principle: DC current through outer electrodes; potential drop measured between inner electrodes.
• Resistivity formula: $\rho = \frac{2 \pi a V}{I}$ where:
  • a = electrode spacing, V = measured voltage, I = current.
• Depth of investigation increases with spacing; resistivity values help identify soil/rock types.
• Typical resistivity (ohm-m) ranges by material:
  • Sound rock: > 5000
  • Weathered rock: 1500–2500
  • Gravel: 1500–4500
  • Sand: 500–1500
  • Clayey sand: 200–500
  • Saturated clay & silt: 2–100
• Limitations:
  • Only indicates strata changes, not precise layering.
  • Values influenced by moisture and ion concentration.
  • Requires expert interpretation.

Types of soil samples (sampling concepts)

1) Disturbed samples

• Represent composition, but natural structure may be altered.
• Suitable for: specific gravity, plasticity, grain size analysis.
• Subtypes:
  • a) Representative samples: water content and mineral proportions preserved.
  • b) Non-representative samples: mixing of layers or alteration of constituents.

2) Undisturbed samples

• Preserve natural structure and water content.
• Used for engineering properties such as shear strength, compressibility, permeability.

Design features related to sample disturbance and sampling quality

• Disturbance metrics:
  • Area ratio (Ar):
    $\text{Ar} = \frac{\text{max. c/s area of cutting edge}}{\text{c/s area of sample}} = \frac{(D<em>2^2 - D</em>1^2) \times 100}{D_1^2}$
  • Target: Ar ≤ 10% for good quality undisturbed samples.
• Inside clearance (Ci):
  $\text{Ci} = \frac{(D<em>3 - D</em>1) \times 100}{D_1}$
• Outside clearance (Co):
  $\text{Co} = \frac{(D<em>2 - D</em>4) \times 100}{D_4}$
• Inside wall friction should be minimized by lubrication to avoid disturbance.
• Force application: push the sampler down continuously with minimal disturbance; avoid hammering.
• Non-return valve: ensure an orifice large enough to vent air and water to obtain an undisturbed sample.
• Recovery ratio (Lr): $L_r = \frac{\text{recovered length}}{\text{penetration length}}$
  • Lr = 1: good recovery; Lr < 1: compressed sample; Lr > 1: swollen sample.

Types of samplers and penetration modes

• Sampler classes by wall thickness (area ratio):
  • Thick-walled samplers: Ar > 10% → typically yield disturbed/representative samples.
  • Thin-walled samplers: Ar < 10% → designed to obtain undisturbed samples.
• Modes of penetration:
  • a) Open drive sampler
  • b) Rotary sampler
  • c) Piston sampler

a) Rotary sampler

• Features:
  • Can be double-walled (undisturbed sample) or single-walled (disturbed sample).
  • Outer tube: rotary barrel with a cutting bit; inner tube is stationary and collects sample.
  • Used for cohesive soils and rock to obtain undisturbed samples.
  • Rock Quality Designation (RQD) is used to assess core quality.
  • Core diameter: not less than 54 mm; core length not less than 100 mm; drilling diameter not less than 50 mm.
• RQD is used to assess compressive strength and modulus of elasticity (E).

b) Piston sampler

• Mechanism: piston attached to a long rod; sample rod lowered; piston held stationary while sampler descends to collect sample; piston prevents water/soil entry.
• Outcome: undisturbed sample.
• Applications: samples in saturated sand and soft soils.

c) Open drive sampler

• Types: Seamless sampler and Split spoon sampler
• Seamless sampler:
  • Thin or thick-walled options; usually 40–125 mm diameter; common diameters: 50.8 mm (2 in) and 76.2 mm (3 in).
  • Area ratio < 10%; inside clearance 0.5–3%; beveled bottom edge to aid penetration.
  • Continuous downward push at constant rate; then 2 revolutions to shear the soil; ends sealed for transport.
• Split spoon sampler:
  • Three parts: Driving shoe (75 mm, sharp edge), Steel tube (450 mm, split longitudinally), Coupling (150 mm) with drill rod connection.
  • Used to obtain disturbed samples.
• SPT in split spoon:
  • Conducted in clean borehole of 55–150 mm diameter.
  • Dropped hammer weight: 65 kg; free fall: 750 mm.
  • Seating drive: penetration of first 150 mm; next two 150 mm penetrations used to compute N-value (sum of last two blows).
• Refusal criteria for SPT (for 150 mm and 300 mm penetrations):
  • 10 consecutive blows: no penetration; or
  • 50 blows for 150 mm penetration; or
  • 100 blows for 300 mm penetration.
• Precautions:
  • Correct hammer weight and vertical fall; 750 mm fall height; straight drill rod; undamaged cutting edge; borehole clean.
  • For sandy soils with potential wetting, keep water level slightly above GW; if casing is used, place slightly above the test level.

Standard Penetration Test (SPT) – key details

• Conducted in a clean borehole with split spoon sampler.
• Seating drive and total N-value: sum of blows for the last two 150 mm penetrations.
• N-value calibration and interpretation depend on corrections (see later sections).

In-situ tests: quick-reference overview

• Standard Penetration Test (SPT) – N-value from blows for last 300 mm penetration.
• Dynamic Cone Penetration Test (DCPT) – Ncbr (dynamic cone resistance): sum of blows for 100 mm penetrations (three 100 mm increments).
• Static Cone Penetration Test (CPT) – qc (cone resistance) and frictional resistance; more stable readings; not ideal for very gravelly or very hard soils. Dutch cone configuration (60° apex, 37.5 mm diameter).
• Vane shear test – measures in-situ shear strength of soft clays.
• Plate Load Test (PLT) – tests ultimate bearing capacity, settlement, deformability modulus, etc.
• Permeability tests – field tests (e.g., constant head or falling head) or in-situ pervious tests.

Dynamic vs static probing: Correlations and interpretations

• DCPT (Ncbr) vs SPT (N) correlations (illustrative):
  • For 50 mm cone (without slurry):
  • Ncbr ≈ 1.5 N for depth ≤ 3 m
  • Ncbr ≈ 1.75 N for 3 m < depth ≤ 6 m
  • Ncbr ≈ 2 N for depth > 6 m
  • For 65 mm cone (with slurry, as per Central Building Research Institute, Roorkee):
  • Ncbr ≈ 1.5 N for depth ≤ 4 m
  • Ncbr ≈ 1.75 N for 4 m < depth ≤ 9 m
  • Ncbr ≈ 2 N for depth > 9 m
• CPT correlation with SPT (typical ranges for qc):
  • Sandy/gravelly soils: qc ≈ 800–1000 KN/m^2
  • Coarse sand: qc ≈ 500–1000 KN/m^2
  • Clean to slightly silty sand: qc ≈ 300–400 KN/m^2
  • Silt/silty sands: qc ≈ 200 KN/m^2
• Note: These correlations are approximate and depend on soil type and conditions.

Plate Load Test (PLT)

• Uses:
  • Determine ultimate and safe bearing capacity of soil.
  • Estimate probable settlement.
  • Determine bearing pressure for allowable settlement.
  • Determine subgrade strength and pavement component behavior.
  • Determine deformation modulus and undrained shear strength.

Plates and test setup

• Plates: minimum thickness 25 mm; circular or square.
• Plate sizes (typical):
  • Circular plates: 226 mm dia (area 0.04 m^2); 253 mm dia (0.05 m^2); 277 mm dia (0.06 m^2); 360 mm dia (0.10 m^2); 505 mm dia (0.20 m^2).
  • Square plates: 316 or 320 mm wide (area 0.10 m^2); 300 mm (0.09 m^2); 600 mm (0.36 m^2); 750 mm (0.56 m^2); 798 mm (0.50 m^2).
• For borehole testing, smaller plates used; for test pits, larger plates may be required.
• Placement: plates placed under the foundation, precast blocks, or concrete blocks; central pit excavation is used for seating.

Test pit and seating pressure

• Test pit: minimum size 5 × plate width, dug to the depth of proposed foundation.
• Central hole (BP × BP) is excavated with depth DP, where DP = BP × Df / Bf, with Bf being width of pit and Df depth of footing.
• Test should maintain moisture content, avoid disturbance, and account for potential wetting effects.
• If GW is present, water table management used (maintain minimum water conditions as required).
• Level the test surface; place a 5 mm layer of fine sand as bedding.
• Seating pressure: apply about 5–7 kN/m^2 before starting.

Loading protocol

• Loading types: gravity or reaction loading.
• Gravity: sandbags or similar weights arranged on a support frame.
• Reaction: hydraulic jack, with supports fixed.
• Settlement monitoring: at least two dial gauges (often four).
• Loading increments: about 1/5 of the estimated safe load or until total settlement reaches 25 mm, whichever occurs first.
• At each increment, settlements are recorded at times: 1, 2.25, 4, 6.25, 9, 16, 25, 36, 49, 60 minutes, then every 1 hour.
• For clayey soils, increase load when settlement rate < 0.2 mm/h; for sandy soils, until settlement rate < 0.02 mm/min.
• Minimum duration of each load step: 1 hour, regardless of rate.
• Stop criteria: system overload, total settlement > 25 mm, or applied pressure > 3 × allowable pressure.

Test results interpretation

• Plate size: the plate should be not less than 30 cm in diameter to avoid size effects.
• Settlement estimates:
  • For sandy/granular soils (Sf): foundation settlement; Sp: plate settlement.
• Bearing capacity: derived differently for clayey vs sandy soils (formulae depend on soil behavior).
• If test is conducted above GW and GW is expected to rise, settlements are adjusted using a specified correction factor.
• Actual settlement = settlement from PLT ÷ correction factor.

Limitations of PLT

• Time effect: short duration vs long-term consolidation (especially in clays).
• Interpretation error: misreading the load–settlement curve can misestimate ultimate bearing capacity.
• Size effect: test plate is smaller than actual foundation; not fully representative of field behavior.
• Scale effect: clayey soils: ultimate bearing capacity relatively plate-size independent; sandy soils show size dependence; recommend multiple plate sizes.
• Water table: fluctuations influence results; conduct tests with water table management.

Depth of exploration (how deep to investigate)

• Depth depends on:
  • Load intensity, loaded area, shape, structure type, soil characteristics, and soil profile.
• Significant depth: depth to which stress and shear from the superimposed load cause meaningful settlements.
• Empirical guidance for required exploration depth (minimum values):
  • i) Square footing: 1.5 × footing width
  • ii) Strip footing: 3 × footing width
  • iii) Closely spaced footing: 1.5 × width of entire loaded area
  • iv) Pile foundation:
  • a) End-bearing pile: 1.5 × pile width measured from pile tip
  • b) Friction piles: 1.5 × pile width measured from the lower third of the pile
  • v) Multistoried building: D = C S^0.7; C = 3 for light, 6 for heavy structures; S = number of storeys
  • vi) Base of retaining wall: 1.5 × base width or 1.5 × exposed height, whichever is greater
  • vii) Dams: 1.5 × bottom width for earthen dams; 2 × height for concrete dams (less than 30 m high)
  • viii) Roads: 1 m below formation level in cut; otherwise, equal to bottom width/depth of cut as applicable

Pressure bulb (isobar) concept

• Isobar: a curve of equal pressure intensity; 10% isobar is the pressure bulb.
• Beyond the pressure bulb, stresses are assumed negligible.
• 10% isobar construction (illustrative procedure):
  • Use Boussinesq equation: $\sigma_z = \frac{IB \cdot Q}{Z^2}$ or equivalently $0.1 Q = \frac{IB \cdot Q}{Z^2} \Rightarrow IB = 0.1 Z^2$
  • For various Z, determine IB; plot r vs Z to obtain the 10% isobar.

Lateral extent and borehole spacing of exploration

• The exploration extent and borehole spacing reflect expected soil variation:

For buildings

• Small building: one borehole or test pit near plot center.
• Compact building: one borehole at center and one at each corner (0.4 ha area).
• Multistoried building: one at each corner and at other major locations; typical spacing: 10–30 m.

For highways

• Along the centerline, spacing: 150–300 m.

For concrete dams

• Spacing: 40–80 m.

Borehole logs and ground-water observations

• Borehole log contents:
  • Soil profile with elevations
  • Groundwater level
  • Depths of sampled intervals
  • Termination level of borehole
  • Sample types and diameters
  • SPT values
  • Layer sequence and soil strata
  • Project name and location
  • Boring method used
• Ground-water observations:
  • Groundwater (pore water pressure) is influenced by fluctuating GW levels.
  • In highly permeable soils, GW level stabilizes within ~24 hours.

Ground-water measurement and dewatering methods (silty soils and others)

• Methods to measure groundwater depth:
  • Chalk-coated tape down the borehole
  • Electrical leads with a circuit that lights a lamp when ends touch water (indicating water depth)
• For soils with very low permeability: Casagrande piezometer is used
  • Structure: Norton porous tube surrounded by sand; connected to a plastic tube with top above ground.
  • Water level in the plastic tube reflects GW table.
• Hvorslev method for silty soils: temporary dewatering and observing rise in water level in the borehole; time intervals: not less than 5 minutes; for freely drained materials, intervals of 1–2 hours; for low-permeability soils, intervals up to 24 hours.

Report writing (content outline)

• Required sections:
  • Introduction
  • Details of site investigation
  • Exploration planning
  • Exploration methods
  • Field tests conducted
  • Laboratory tests conducted
  • Groundwater position and properties
  • Data analysis
  • Recommendations
  • Limitations (if any)

Miscellaneous: key numeric concepts and relationships from the notes

• Correlation and correction concepts for SPT (N-value) and related adjustments:
  • Overburden correction for N (N'):
    $N' = C<em>N N,\quad C</em>N = 0.77 \log_{10}\left(\frac{20}{p}\right)$
  • Here, p is the effective overburden pressure (kg/cm^2).
  • The relation above applies when p > 0.25 kg/cm^2; for p ≤ 0.25 kg/cm^2, CN is obtained from a graph.
  • Dilatancy correction for fine sands (to account for pore-water pressure during penetration):
    $N'' = \begin{cases} 15 + 0.5 (N' - 15), &amp; N' &gt; 15 \ N', &amp; N' \le 15 \end{cases}$
• Recovery ratio concept (Lr) defined above.
• N–φ and qc correlations (illustrative ranges):
  • For corrected N value and corresponding relative density (φ) ranges:
  • Very loose: φ ≈ 0–4
  • Loose: φ ≈ 4–10
  • Medium: φ ≈ 10–30
  • Dense: φ ≈ 30–50
  • Very dense: φ > 50
• Correlation between corrected N and qu (undrained shear strength proxy):
  • qu ≈ 12.5 N (for N-values applying to certain soil types) with qu in KN/m^2
• Dynamic cone penetration (DCPT) practical stop rules for 50 mm cone (without slurry):
  • Stop if more than 20 blows are required for 100 mm penetration.
• DCPT correlation rules vary with cone size and slurry use; recommended to use the central building research institute (CBRI) Roorkee correlations for 65 mm cones with slurry, with depth cutoffs as noted above.

Summary: practical takeaways for exam preparation

• Soil exploration is a planned, staged process to gather subsoil data, groundwater information, and foresee foundation performance.
• The planning stage weighs project size, location, technician experience, and the likelihood of data gaps.
• The three stages (Reconnaissance, Preliminary, Detailed) progressively refine the understanding of soil conditions and guide the testing program.
• In-situ tests (SPT, DCPT, CPT, Vane, PLT) provide crucial parameters for bearing capacity, settlement, and soil strength; SPT remains a widely used, simple, qualitative tool, though corrections are essential.
• Geophysical methods (Seismic and Electrical Resistivity) supplement boreholes to map subsurface stratigraphy, especially when drilling is difficult or invasive.
• Correct handling of samples (disturbed vs undisturbed) and appropriate sampler selection are critical for reliable laboratory tests.
• Plate Load Test, while informative, has limitations—chiefly time effects, scale effects, and groundwater considerations—and corrections may be needed to relate test results to actual foundations.
• Groundwater measurement and dewatering strategies (Casagrande piezometer, Hvorslev) are essential, especially in silty or low-permeability soils.
• The depth of exploration should be sufficient to capture significant bearing and settlement behavior, guided by empirical rules for different foundation types.
• Reporting should comprehensively document all planning, methods, tests, groundwater data, data analysis, and limitations to enable reliable design decisions.