4.1 Subsurface Exploration and Laboratory Testing

3.1 Subsurface Exploration and Laboratory Testing

Introduction to Subsurface Exploration

Subsurface exploration is the foundational phase of geotechnical engineering. It involves identifying the stratigraphic sequence, groundwater table elevations, and mechanical characteristics of the soil and rock layers beneath a proposed structure. Without an accurate representation of the subsurface profile, subsequent foundation design, slope stability analysis, and earthwork estimations are prone to failure. Geotechnical site investigations must adhere to strict regulatory standards, such as those set by the American Society for Testing and Materials (ASTM) and the American Association of State Highway and Transportation Officials (AASHTO).

Boring Logs and Drilling Records

The primary field record of a site investigation is the boring log. This document acts as a continuous stratigraphic chart of the borehole. A standard boring log includes:

• Borehole Identification and Location Data: Coordinates, surface elevation, and borehole ID.
• Drilling Parameters: Type of drill rig (e.g., hollow-stem auger, rotary wash, or cable tool), drill casing size, drilling fluid type, and casing depth.
• Soil Descriptions: Classified in accordance with ASTM D2488 (visual-manual procedure) and ASTM D2487 (laboratory classification).
• Sampling Information: Depths of samples, recovery lengths, and sample types (e.g., split-spoon or thin-walled Shelby tubes).
• In-Situ Test Data: Standard Penetration Test blow counts and pocket penetrometer readings.
• Groundwater Data: The depth at which groundwater was first encountered during drilling, the water level at the completion of drilling, and the stabilized groundwater level (measured typically 24 to 72 hours later to allow pore pressure equilibrium).

Rock Coring Recovery Metrics

When drilling encounters rock, rotary core drilling is employed using a diamond-tipped core bit. To quantify the quality of the rock mass, engineers calculate two key percentages from each core run:

1. Recovery Ratio (RR): The ratio of the length of rock core recovered to the total length of the core run, expressed as a percentage: RR = (Σ L_recovered / L_run) × 100%
2. Rock Quality Designation (RQD): Developed by Deere in 1964, RQD is a index of rock mass quality. It only sums intact pieces of rock core that are equal to or greater than 100 mm (4 inches) in length: RQD = (Σ L_pieces_10cm / L_run) × 100%

Core pieces broken by the drilling process (characterized by fresh, clean fractures) should be fitted back together and counted as single intact pieces. Highly fractured, weathered rock yields a low RQD, indicating poor engineering behavior.

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Standard Penetration Test (SPT)

The Standard Penetration Test (SPT), governed by ASTM D1586, is the most common in-situ test used in geotechnical engineering. The test provides both an index of soil density (N-value) and a disturbed physical sample for laboratory index testing.

Equipment and Procedure

The test setup utilizes a standard split-spoon sampler with an outer diameter of 50.8 mm (2.0 inches) and an inner diameter of 34.9 mm (1.375 inches). The sampler is driven into the bottom of a borehole by a slide hammer weighing 622.7 N (140 lbs) dropping from a free-fall height of 762 mm (30 inches). The driving is performed in three consecutive 152.4 mm (6 inches) increments for a total penetration of 457.2 mm (18 inches).

• The blow counts for each of the three increments are recorded.
• The blow count for the first 152.4 mm (6 inches) is the seating drive and is discarded because it represents soil disturbed by the drilling action.
• The sum of the blow counts for the second and third increments is the standard penetration number, commonly designated as the N-value (in blows/foot or blows/300 mm). If more than 50 blows are required for any single 6-inch increment, or if 100 total blows are reached, the test is terminated, and a refusal condition is noted.

N-Value Corrections

Field conditions vary, meaning the raw N-value (N_raw) must be corrected to account for differences in hammer energy efficiency and overburden pressure. The corrected SPT blow count at 60% hammer energy efficiency (N60) is given by: N60 = (N_raw × η_H × η_B × η_S × η_R) / 60 Where:

• η_H = Hammer energy efficiency (%) (e.g., 60% for safety hammers, 80% for automatic hammers).
• η_B = Borehole diameter factor (1.0 for 65–115 mm, 1.05 for 150 mm, 1.15 for 200 mm).
• η_S = Sampler correction factor (1.0 for standard sampler without liner, 1.2 with liner).
• η_R = Drill rod length correction factor (0.75 for 3–4 m, 0.85 for 4–6 m, 0.95 for 6–10 m, 1.0 for >10 m).

In coarse-grained soils, overburden stress affects the confining pressure and thus the measured N-value. To normalize this effect to a standard effective overburden pressure of 100 kPa (1 tsf), the overburden-corrected value (N1)60 is used: (N1)60 = CN × N60 Where CN is the overburden correction factor. A common equation for CN is: CN = √(p_a / σ'v0) Where p_a is the reference atmospheric pressure (100 kPa or 2000 psf) and σ'v0 is the effective vertical stress at the sample depth. CN is typically capped at a maximum value of 2.0.

Engineering Correlations

The (N1)60 value is correlated to several design parameters:

• For Granular Soils: Relates to relative density (Dr) and angle of internal friction (φ'). An (N1)60 of 4 to 10 indicates loose sand, 10 to 30 indicates medium dense sand, and >30 indicates dense to very dense sand.
• For Cohesive Soils: Relates to consistency and undrained shear strength (su). An N60 of <2 indicates very soft clay, 4 to 8 indicates medium stiff clay, and >15 indicates stiff to hard clay. Note that SPT is less reliable in clays than in sands due to pore pressure effects.

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Cone Penetration Test (CPT)

The Cone Penetration Test (CPT), governed by ASTM D5778, is an alternative to the SPT that provides a continuous profile of subsurface strata without generating soil cuttings or requiring a borehole.

Test Mechanics and Parameters

The test involves pushing an instrumented steel cone with a 60-degree apex and a projected tip area of 10 cm² (or 15 cm²) vertically into the soil at a constant rate of 20 mm/sec. The cone assembly measures:

• Cone Tip Resistance (qc): The total force on the tip divided by the projected tip area. It reflects the bearing capacity of the soil.
• Sleeve Friction (fs): The frictional force acting on a cylindrical sleeve situated above the cone tip, divided by the sleeve surface area (150 cm²).
• Friction Ratio (Rf): Expressed as a percentage: Rf = (fs / qc) × 100%
• Pore Water Pressure (u): Measured using a pressure transducer embedded behind the cone tip (referred to as piezocone test, CPTu). This provides data on excess pore pressures generated during insertion.

Interpretation and Application

The friction ratio Rf is a diagnostic tool for soil type identification:

• Clean Sands: High tip resistance (qc > 10 MPa) and low friction ratio (Rf < 1%).
• Plastic Clays: Low tip resistance (qc < 2 MPa) and high friction ratio (Rf > 3%). The advantages of CPT include high speed, a continuous stratigraphy record, and the elimination of human error in blow-counting. The main disadvantage is that it retrieves no physical soil samples, making laboratory verification impossible without adjacent borings.

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Soil Index Properties and Atterberg Limits

Laboratory testing begins with defining index properties, which indicate the soil's state and its susceptibility to volume change.

Moisture Content and Atterberg Limits

The moisture content (w) represents the mass ratio of water to solid mineral grains in a soil sample: w = (M_w / M_s) × 100% For fine-grained soils, consistency changes as water is added or removed. Albert Atterberg defined the boundary water contents separating these consistency states:

1. Liquid Limit (LL): The water content at which a soil paste transitions from a plastic state to a liquid state. It is determined in the laboratory using the Casagrande liquid limit device (ASTM D4318) or the fall cone method.
2. Plastic Limit (PL): The water content at which a soil thread begins to crumble when rolled to a diameter of 3.2 mm (1/8 inch).
3. Shrinkage Limit (SL): The water content below which further drying does not cause a decrease in the soil's volume.

Consistency Indices

• Plasticity Index (PI): The range of water content over which the soil remains plastic: PI = LL − PL
• Liquidity Index (LI): Quantifies the natural water content (w) relative to the Liquid and Plastic limits: LI = (w − PL) / PI If LI ≥ 1.0, the natural soil is at or above its liquid limit, presenting a high risk of liquefaction or flow upon disturbance.
• Activity (A): Correlates the plasticity of a clay to its clay-size fraction (percentage of particles by weight finer than 2 μm): A = PI / (% Clay Fraction) High activity (A > 1.25) is characteristic of swelling clays, such as sodium montmorillonite.

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Soil Classification Systems

Engineering standards categorize soils based on grain-size distribution and plasticity.

Unified Soil Classification System (USCS)

The Unified Soil Classification System (ASTM D2487) is the standard system for civil engineering design. The classification process utilizes the following symbols:

• First Letter (Soil Type): G (Gravel), S (Sand), M (Silt), C (Clay), O (Organic).
• Second Letter (Gradation/Plasticity): W (Well-graded), P (Poorly graded), M (Silty), C (Clayey), L (Low plasticity, LL < 50), H (High plasticity, LL ≥ 50).

The classification protocol is structured as follows:

1. Coarse vs. Fine: Determine the percentage passing the No. 200 sieve (0.075 mm).
   • If less than 50% passes the No. 200 sieve, the soil is coarse-grained.
   • If 50% or more passes the No. 200 sieve, the soil is fine-grained.
2. Coarse-Grained Classification: Determine the percentage of the coarse fraction retained on the No. 4 sieve (4.75 mm).
   • If 50% or more of the coarse fraction is retained on the No. 4 sieve, the soil is Gravel (G).
   • If more than 50% of the coarse fraction passes the No. 4 sieve, the soil is Sand (S).
   • Evaluate the percentage passing the No. 200 sieve (F200):
     • Clean (F200 < 5%): Classify as GW, GP, SW, or SP depending on:
       • Coefficient of Uniformity Cu = D_60 / D_10
       • Coefficient of Curvature Cc = (D_30)² / (D_10 × D_60)
       • For Gravel to be well-graded (GW): Cu ≥ 4 and 1 ≤ Cc ≤ 3.
       • For Sand to be well-graded (SW): Cu ≥ 6 and 1 ≤ Cc ≤ 3.
       • Otherwise, the soil is poorly graded (GP or SP).
     • Dirty (F200 > 12%): Classify as GM, GC, SM, or SC using the plasticity chart. Silts (M) plot below the A-line PI = 0.73(LL − 20) or PI < 4. Clays (C) plot above the A-line and PI > 7.
     • Borderline (5% ≤ F200 ≤ 12%): Dual symbols are assigned (e.g., GW-GM, SP-SC).
3. Fine-Grained Classification: Plot the LL and PI on the plasticity chart:
   • Clays with LL < 50 are CL. Clays with LL ≥ 50 are CH.
   • Silts with LL < 50 are ML. Silts with LL ≥ 50 are MH.
   • Dual symbol CL-ML is used if 4 ≤ PI ≤ 7 and plots above the A-line.

AASHTO Soil Classification

The AASHTO classification system is used for highway pavements. Soils are categorized into groups A-1 through A-7 based on their performance as a subgrade.

• Granular Materials: ≤ 35% passing the No. 200 sieve.
• Silt-Clay Materials: > 35% passing the No. 200 sieve.

The quality of the subgrade is evaluated using the Group Index (GI): GI = (F_200 − 35)[0.2 + 0.005(LL − 40)] + 0.01(F_200 − 15)(PI − 10) Where:

• F_200 = Percent passing No. 200 sieve as a whole number.
• LL = Liquid Limit.
• PI = Plasticity Index. The calculated GI is rounded to the nearest integer. If the value is negative, it is set to zero. A low GI (near 0) indicates a good subgrade, whereas a high GI (20 or more) indicates a very poor subgrade.

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Soil Phase Relationships

Soil is a multi-phase system. In calculations, we model the soil mass using a three-phase diagram representing the volumes and weights of air, water, and solid particles.

Volumetric Ratios

• Void Ratio (e): The volume of voids (V_v) relative to the volume of solid particles (V_s): e = V_v / V_s
• Porosity (n): The volume of voids (V_v) relative to the total volume (V_t): n = V_v / V_t Porosity and void ratio are interrelated: n = e / (1 + e) and e = n / (1 − n)
• Degree of Saturation (S): The volume of water (V_w) relative to the volume of voids (V_v): S = V_w / V_v When S = 0, the soil is dry. When S = 1.0 (or 100%), the soil is fully saturated.

Mass-Volume Bridging Formulas

The fundamental equation that links weight and volume phases is: S × e = w × G_s Where:

• w = Moisture content (W_w / W_s).
• G_s = Specific gravity of soil solids (γ_s / γ_w).

Unit weights (γ) are calculated by dividing the weight of a phase by the total volume.

• Dry Unit Weight (γ_d): γ_d = W_s / V = (G_s × γ_w) / (1 + e)
• Moist (Total) Unit Weight (γ): γ = W_t / V = [(G_s + S × e) × γ_w] / (1 + e) = γ_d × (1 + w)
• Saturated Unit Weight (γ_sat): γ_sat = [(G_s + e) × γ_w] / (1 + e)
• Buoyant (Submerged) Unit Weight (γ'): γ' = γ_sat − γ_w = [(G_s − 1) × γ_w] / (1 + e) Where the unit weight of water (γ_w) is 9.81 kN/m³ (SI) or 62.4 pcf (US Customary).

Worked Phase Relationship Example

A soil sample has a total volume of 0.015 m³ and a moist mass of 28.5 kg. The moisture content is 12.5%, and the specific gravity of solids is 2.68. Let's calculate the dry density, void ratio, porosity, and degree of saturation.

1. Moist Density (ρ): ρ = M / V = 28.5 / 0.015 = 1900 kg/m³
2. Dry Density (ρ_d): ρ_d = ρ / (1 + w) = 1900 / (1 + 0.125) = 1688.9 kg/m³
3. Void Ratio (e): Using ρ_d = (G_s × ρ_w) / (1 + e) (with ρ_w = 1000 kg/m³): 1 + e = (2.68 × 1000) / 1688.9 = 1.587 e = 0.587
4. Porosity (n): n = e / (1 + e) = 0.587 / 1.587 = 0.370 (or 37.0%)
5. Degree of Saturation (S): S = (w × G_s) / e = (0.125 × 2.68) / 0.587 = 0.571 (or 57.1%)