Subsurface Exploration and Site Characterization

Planning exploration programs, interpreting boring logs, and determining bearing capacity from field tests.

Overview

A comprehensive subsurface exploration program is the crucial first step in any foundation engineering project. It involves a systematic investigation to determine the sequence, thickness, and engineering properties of soil and rock strata beneath a proposed structure. The primary objectives are to obtain representative samples for laboratory testing and to perform in-situ tests to accurately characterize the site conditions.

Planning the Exploration Program

Before any drilling begins, a geotechnical engineer must carefully plan the exploration program. This phase is essential to ensure that sufficient and relevant data is collected while optimizing costs.

Procedure

  • Phase 1: Reconnaissance and Desk Study: Review existing geological maps, topographical maps, aerial photographs, and previous site investigation reports for the area or adjacent sites.
  • Phase 2: Preliminary Exploration: Conduct a limited number of borings to establish the general soil stratigraphy and identify any potential major geotechnical issues (e.g., soft clay layers, shallow bedrock).
  • Phase 3: Detailed Exploration: Execute a comprehensive drilling and testing program tailored to the specific structure's layout, loads, and the findings of the preliminary exploration. This includes undisturbed sampling and advanced in-situ testing.

Depth and Spacing of Borings

Exploration Depth and Spacing Criteria

The number, depth, and spacing of boreholes depend on the type of structure, its weight, and the complexity of the soil profile. Generally:
  • Depth: Borings must penetrate through all unsuitable soil layers and reach a stratum of adequate bearing capacity. For isolated footings, depth is typically to a point where the increase in vertical stress is less than 10% of the effective overburden stress (often 1.5B1.5B to 2B2B below the footing, where BB is footing width). For piles, borings should extend well below the anticipated tip elevation.
  • Spacing: Depends on the uniformity of the site. Highly variable soil (e.g., glacial till) requires closer spacing (e.g., 10-30 meters), while uniform deposits (e.g., deep uniform clays) can have wider spacing (e.g., 30-60+ meters). Minimum requirements usually dictate at least one boring per major structural component or a grid pattern for large developments.

Soil Sampling Techniques

Obtaining physical soil samples is critical for laboratory testing to determine index properties, shear strength, and compressibility. Samples are classified into two broad categories:

Disturbed vs. Undisturbed Samples

  • Disturbed Samples: The natural structure of the soil is altered or destroyed during sampling, although the moisture content and mineral composition may remain intact. Used for index testing (Atterberg limits, sieve analysis, specific gravity). Common tools include the Split-Spoon Sampler (used in SPT) and continuous flight augers.
  • Undisturbed Samples: Great care is taken to preserve the natural structure, void ratio, and moisture content of the soil. Essential for determining advanced engineering properties like shear strength (triaxial tests) and consolidation (oedometer tests). Common tools include thin-walled Shelby tubes (for soft clays) and stationary piston samplers.

Area Ratio (ARA_R)

The degree of disturbance caused by a sampler is often evaluated by its Area Ratio. For a sample to be considered "undisturbed," the area ratio should typically be ≤10%\le 10\%.
AR=Do2−Di2Di2×100% A_R = \frac{D_o^2 - D_i^2}{D_i^2} \times 100\%
Where DoD_o is the outside diameter of the sampler tube, and DiD_i is the inside diameter.

Recovery Ratio (LrL_r)

The Recovery Ratio indicates the proportion of the sample recovered compared to the distance the sampler was advanced. It is a critical metric for assessing sample quality and potential disturbance.
Lr=Length of recovered sampleLength of sampler advance L_r = \frac{\text{Length of recovered sample}}{\text{Length of sampler advance}}
A recovery ratio equal to 1.0 implies perfect recovery, but values slightly less than 1.0 are common. Values significantly greater than 1.0 indicate soil heave or expansion, while values much less than 1.0 indicate soil loss or compaction within the sampler.

Groundwater Table Measurement

Determining the Phreatic Surface

Locating the groundwater table is paramount, as water significantly affects soil unit weight, effective stress, and bearing capacity.
  • Observation Wells: A slotted PVC pipe installed in a borehole surrounded by sand/gravel filter pack. Water levels are measured directly using a tape or electronic water level indicator. Measurements must be taken at least 24 hours after drilling to allow the water level to stabilize, especially in fine-grained soils.
  • Piezometers: Specialized instruments (e.g., vibrating wire piezometers, Casagrande piezometers) installed at specific depths to measure pore water pressure, useful for detecting perched water tables or artesian conditions.

Time Lag in Groundwater Measurement

In low-permeability soils like clays and silts, water takes a considerable amount of time to flow into an observation well or a standpipe piezometer to reach the true groundwater level. This "time lag" can span days or even weeks. Engineers must account for this by taking multiple readings over time until the water level is proven to be stationary, or by using faster-responding instruments like vibrating wire piezometers, which require virtually no water volume to register pressure.

Boring Logs

Boring Log

A boring log is a fundamental document that provides a visual and written record of the subsurface conditions encountered during drilling. It includes soil descriptions, stratigraphy, in-situ test results (like SPT N-values), groundwater levels, and sampling details.

In-Situ Testing Methods

In-situ tests provide a direct measurement of soil properties in their natural environment, avoiding the disturbance associated with sampling and transportation to a laboratory.

Standard Penetration Test (SPT)

Standard Penetration Test (SPT)

The Standard Penetration Test (ASTM D1586) is the most common in-situ dynamic penetration test worldwide. A standard 63.5 kg (140 lb) hammer is dropped repeatedly from a height of 760 mm (30 in) to drive a standard split-spoon sampler into the bottom of a borehole. The number of blows required to drive the sampler the last 300 mm (12 in) of a 450 mm (18 in) drive is recorded as the SPT N-value.

SPT Corrections

The raw field N-value must be corrected to account for various factors that affect the energy delivered to the sampler. The standard corrected value is N60N_{60}, representing an average hammer efficiency of 60%.
N60=N⋅ηH⋅ηB⋅ηS⋅ηR60 N_{60} = \frac{N \cdot \eta_H \cdot \eta_B \cdot \eta_S \cdot \eta_R}{60}
Where:
  • NN = raw field N-value
  • ηH\eta_H = hammer efficiency (%)
  • ηB\eta_B = borehole diameter correction
  • ηS\eta_S = sampler correction
  • ηR\eta_R = rod length correction

Overburden Correction ($N_1)_{60}$

For granular soils, the SPT N-value is heavily influenced by the effective overburden stress (σv′\sigma_v'). Sand at a greater depth will yield a higher N-value than the exact same sand near the surface. To standardize, N-values are corrected to a reference stress of 1 atm1 \text{ atm} (100 kPa100 \text{ kPa} or 1 tsf1 \text{ tsf}), denoted as (N1)60(N_1)_{60}.
(N1)60=CN⋅N60 (N_1)_{60} = C_N \cdot N_{60}
Where CN=100/σv′C_N = \sqrt{100 / \sigma_v'} (with σv′\sigma_v' in kPa) and CN≤2.0C_N \le 2.0.

SPT Correlations

The corrected N-value is widely correlated with engineering properties.
  • Granular Soils (Sands): N-value correlates strongly with Relative Density (DrD_r) and the effective angle of internal friction (ϕ′\phi'). E.g., (N1)60<4(N_1)_{60} < 4 is very loose sand; >50> 50 is very dense sand.
  • Cohesive Soils (Clays): N-value provides a rough estimate of undrained shear strength (sus_u) and consistency. E.g., N60<2N_{60} < 2 is very soft clay; >30> 30 is hard clay.

Cone Penetration Test (CPT)

Cone Penetration Test (CPT)

The CPT involves pushing an instrumented cone tip into the ground at a constant rate. It continuously records the tip resistance (qcq_c) and sleeve friction (fsf_s). It provides a highly detailed, continuous profile of the soil stratigraphy and is particularly effective in soft clays and fine sands.

Rock Quality Designation (RQD)

Rock Quality Designation (RQD)

RQD is a rough measure of the degree of jointing or fracture in a rock mass, measured as a percentage of the drill core in lengths of 100 mm (4 in) or more. It is used as a standard parameter in rock core logging.
RQD=∑Length of intact pieces ≥100 mmTotal length of core run×100% RQD = \frac{\sum \text{Length of intact pieces } \ge 100 \text{ mm}}{\text{Total length of core run}} \times 100\%

Core Recovery (CR)

Alongside RQD, the Core Recovery (CR) is a fundamental metric. It is simply the total length of rock core recovered divided by the total length of the core run, expressed as a percentage. CR ≥\ge RQD always.

Other In-Situ Tests

Advanced Testing Methods

  • Field Vane Shear Test (VST): Used extensively to determine the in-situ undrained shear strength (sus_u) of soft to medium cohesive soils (clays). A four-bladed vane is pushed into the soil and rotated until failure occurs. The torque required to cause failure is measured.
  • Flat Dilatometer Test (DMT): A stainless steel blade with a flat, circular, expandable steel membrane is pushed into the soil. Gas pressure expands the membrane against the soil, providing data to estimate lateral earth pressure (K0K_0), overconsolidation ratio (OCR), and constrained modulus.
  • Pressuremeter Test (PMT): Involves inflating a cylindrical probe inside a pre-drilled borehole. It provides a stress-strain curve of the soil in situ, from which the Menard pressuremeter modulus and limit pressure can be derived, useful for settlement and bearing capacity calculations.

Geophysical Methods

Non-Destructive Subsurface Investigation

Geophysical methods offer a non-destructive, cost-effective way to infer subsurface conditions over large areas without drilling. Common methods include:
  • Seismic Refraction: Uses seismic waves to determine the depth to bedrock and the seismic velocity of soil layers, useful for estimating soil stiffness and rippability.
  • Ground Penetrating Radar (GPR): Uses high-frequency radar pulses to image the subsurface, effective for locating buried utilities, voids, and shallow stratigraphy.
  • Electrical Resistivity: Measures the earth's resistance to electrical current, helping to identify groundwater tables, clay lenses, and contaminant plumes.

Determination of Bearing Capacity from SPT

For shallow foundations resting on granular soils (sands and gravels), obtaining undisturbed samples is extremely difficult. Therefore, the allowable bearing capacity (qaq_a) is often estimated directly from empirical correlations with the SPT N-value.
Meyerhof's empirical formula for estimating the allowable bearing capacity (for a settlement of 25 mm or 1 inch) is given by:
For a footing width B≤1.22 mB \le 1.22 \text{ m} (4 ft4 \text{ ft}):
qa=11.98⋅N60⋅Kd q_a = 11.98 \cdot N_{60} \cdot K_d
For a footing width B>1.22 mB > 1.22 \text{ m} (4 ft4 \text{ ft}):
qa=7.99⋅N60⋅(B+0.305B)2⋅Kd q_a = 7.99 \cdot N_{60} \cdot \left(\frac{B + 0.305}{B}\right)^2 \cdot K_d
Where:
  • qaq_a = allowable bearing capacity in kPa
  • N60N_{60} = corrected standard penetration number
  • Kd=1+0.33(DfB)≤1.33K_d = 1 + 0.33 \left(\frac{D_f}{B}\right) \le 1.33
  • DfD_f = depth of foundation in meters
  • BB = width of foundation in meters
Key Takeaways
  • A subsurface exploration program is vital for understanding soil stratigraphy and properties prior to design.
  • Soil samples are classified as disturbed (for index testing) or undisturbed (for strength and consolidation testing), depending heavily on the sampler's Area Ratio.
  • Accurate groundwater table measurement via observation wells or piezometers is critical for effective stress calculations.
  • The Standard Penetration Test (SPT) provides an N-value that must be corrected for various factors to obtain N60N_{60}.
  • Rock Quality Designation (RQD) assesses rock mass quality based on intact core length.
  • Geophysical methods like seismic refraction and GPR provide non-destructive insights over large areas.
  • The Cone Penetration Test (CPT) offers continuous soil profiling and is excellent for fine-grained soils.
  • Allowable bearing capacity for shallow foundations on granular soils can be empirically estimated directly from corrected SPT N-values.

SPT N-Value Correction Calculator

Energy-Corrected Blow Count
N60N_{60} = 0.0
Accounts for hammer efficiency and rod length.
Overburden Correction Factor
CNC_N = 0.00
Normalizes to 100 kPa effective stress.
Fully Corrected Blow Count
(N1)60(N_1)_{60} = 0
Used for design correlations (e.g., relative density, bearing capacity).

The Standard Penetration Test (SPT) involves dropping a 140 lb hammer 30 inches. Different hammers transfer energy differently. The N60N_{60} normalizes this to 60% theoretical free-fall energy. CNC_N adjusts for depth, because soil at greater depths appears artificially stronger due to confinement.

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