1.2 In Situ Testing & Field Exploration

Overview of In Situ Geotechnical Testing

In situ field testing is essential in modern geotechnical practice because extracting truly "undisturbed" soil samples for laboratory testing is extremely difficult, particularly in loose sands, soft sensitive clays, and fractured rock masses. In situ tests measure soil and rock responses directly under existing geostatic stress conditions, avoiding sampling disturbance, stress relief, and moisture changes.

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Cone Penetration Test (CPT and CPTU - ASTM D5778)

The Cone Penetration Test (CPT) and piezocone test (CPTU) involve pushing a standard $60^\circ$ apex angle, $10\text{ cm}^2$ base area cone ($35.7\text{ mm}$ diameter) into the ground at a constant rate of $20\text{ mm/s}$ ($2.0\text{ cm/s}$).

Measured Parameters

• Cone Tip Resistance ($q_c$): Total force on cone tip divided by tip area ($10\text{ cm}^2$ or $15\text{ cm}^2$).
• Sleeve Friction ($f_s$): Frictional force on the friction sleeve ($150\text{ cm}^2$ area) divided by sleeve surface area.
• Pore Water Pressure ($u_2$): Dynamic pore water pressure measured at the shoulder filter element (immediately behind the cone tip).

Key Calculated Derivatives

1. Friction Ratio ($R_f$): $R_f = \left( \frac{f_s}{q_c} \right) \times 100\%$ High $R_f$ ($> 3\text{--}5%$): Cohesive silts and clays. Low $R_f$ ($< 1%$): Cohesionless sands and gravels.

2. Total Corrected Cone Tip Resistance ($q_t$): Corrects $q_c$ for unequal pore pressure effects acting on the cone shoulder: $q_t = q_c + u_2 (1 - a)$ Where $a$ is the net cone area ratio (typically $0.70\text{--}0.85$, determined via laboratory calibration).

3. Undrained Shear Strength ($s_u$): $s_u = \frac{q_t - \sigma_{v0}}{N_{kt}}$ Where $\sigma_{v0}$ is total vertical overburden stress and $N_{kt}$ is the empirical cone factor (typically $N_{kt} = 14\text{--}18$ for NC to LOC clays).

4. Soil Behavior Type Index ($I_c$): Proposed by Robertson (1990): $I_c = \left[ (3.47 - \log Q_t)^2 + (1.22 + \log F_r)^2 \right]^{0.5}$ Where $Q_t = (q_t - \sigma_{v0})/\sigma'{v0}$ is normalized tip resistance and $F_r = [f_s / (q_t - \sigma{v0})] \times 100%$. Soils with $I_c > 2.60$ exhibit fine-grained clay-like behavior; soils with $I_c < 2.05$ exhibit sand-like behavior.

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Field Vane Shear Test (VST - ASTM D2573)

The Field Vane Shear Test provides in situ measurements of undrained shear strength ($s_u$) in soft to stiff saturated cohesive soils. A four-bladed rectangular vane ($H/D = 2.0$, typically $D = 65\text{ mm}$ or $75\text{ mm}$) is pushed into the clay and rotated at $0.1^\circ / \text{sec}$ until soil failure occurs.

Governing Strength Equations

For a standard rectangular vane with height $H$ equal to twice the diameter $D$ ($H = 2D$):

$s_{u,\text{field}} = \frac{6 T_{max}}{7 \pi D^3}$

Where $T_{max}$ is the peak torque applied at failure. After measuring peak strength, the vane is rotated rapidly $10$ full revolutions to completely remold the clay, and a second torque test measures remolded strength ($s_{u,\text{remolded}}$). Soil Sensitivity ($S_t$) is defined as:

$S_t = \frac{s_{u,\text{peak}}}{s_{u,\text{remolded}}}$

Bjerrum Bjerrum Correction Factor ($\mu$)

Field vane undrained shear strength overestimates actual field strength in design due to rate of loading and soil anisotropy. Bjerrum (1972) established that measured $s_{u,\text{field}}$ must be corrected using factor $\mu$, which depends on the Plasticity Index ($PI$):

$s_{u,\text{design}} = \mu \cdot s_{u,\text{field}}$ $\mu = 1.7 - 0.54 \log(PI) \quad \text{(or via Bjerrum chart; } \mu \approx 1.0 \text{ for } PI=20, \mu \approx 0.8 \text{ for } PI=50\text{)}$

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Flat Plate Dilatometer Test (DMT - ASTM D6635)

The Marchetti Flat Dilatometer consists of a stainless steel blade ($95\text{ mm}$ wide, $14\text{ mm}$ thick) with a expandable thin circular steel membrane ($60\text{ mm}$ diameter) mounted on one face. The blade is pushed vertically into the ground. At target test depths, gas pressure expands the membrane:

• $A$-pressure ($p_0$): Pressure required to expand membrane flush with blade face ($0.00\text{ mm}$ displacement).
• $B$-pressure ($p_1$): Pressure required to move membrane center $1.10\text{ mm}$ outward.
• $C$-pressure ($p_2$): Deflation pressure at which membrane returns to flush position (yields pore pressure $u_0$).

Key Index Parameters

• Material Index ($I_D$): $I_D = \frac{p_1 - p_0}{p_0 - u_0}$ (Classifies soil: $I_D < 0.6$ clay; $0.6 < I_D < 1.8$ silt; $I_D > 1.8$ sand).
• Horizontal Stress Index ($K_D$): $K_D = \frac{p_0 - u_0}{\sigma'_{v0}}$ (Correlated to earth pressure coefficient $K_0$ and Overconsolidation Ratio $OCR$).
• Dilatometer Modulus ($M_{\text{DMT}}$): $M_{\text{DMT}} = 34.7 (p_1 - p_0)$ (One-dimensional constrained modulus for settlement calculations).

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Pressuremeter Test (PMT - ASTM D4719)

The Pressuremeter Test (Menard type) involves placing a cylindrical flexible probe inside a pre-bored hole and expanding it radially using hydraulic or pneumatic pressure. The test measures volume change ($\Delta V$) versus applied pressure ($p$).

Derived Soil Properties

• Limit Pressure ($p_L$): Applied pressure at which the borehole volume doubles; used directly in Menard's ultimate bearing capacity equations.
• Pressuremeter Modulus ($E_{\text{PMT}}$): Derived from the linear pseudo-elastic phase of the pressure-volume curve: $E_{\text{PMT}} = 2 (1 + \nu) (V_0 + V_m) \frac{\Delta p}{\Delta V}$ Where $\nu$ is Poisson's ratio, $V_0$ is initial cavity volume, and $V_m$ is average cavity volume during the linear phase.

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Geophysical Exploration Methods

Geophysical surveys provide continuous non-destructive cross-sections of subsurface stratigraphy.

Seismic Refraction Method

Seismic refraction measures arrival times of compressional $P$-waves generated by dynamic impacts. By applying Snell's Law to a two-layer subsurface where wave velocity increases with depth ($V_2 > V_1$):

$\sin \theta_c = \frac{V_1}{V_2}$

The critical distance ($x_c$) on the travel-time plot—where direct wave and refracted wave arrivals coincide—yields layer depth $z_1$:

$z_1 = \frac{x_c}{2} \sqrt{\frac{V_2 - V_1}{V_2 + V_1}}$

Multichannel Analysis of Surface Waves (MASW)

MASW records Rayleigh surface waves to construct shear wave velocity ($V_s$) profiles down to $30\text{ m}$ depth. $V_{s,30}$ is used universally for seismic site classification (NEHRP Site Classes A through F). Small-strain dynamic shear modulus ($G_{max}$) is computed as:

$G_{max} = \rho \cdot V_s^2$

Where $\rho = \gamma / g$ is total soil mass density.

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Detailed Worked Numerical Example

Problem Statement

A piezocone test (CPTU) is performed in a soft saturated marine clay layer at a depth of $10.0\text{ m}$. The groundwater table is at the ground surface. The soil unit weight is $\gamma_{sat} = 18.5\text{ kN/m}^3$. The CPTU measurements at $10.0\text{ m}$ depth are:

• Cone tip resistance $q_c = 1.45\text{ MPa} = 1450\text{ kPa}$
• Sleeve friction $f_s = 38.0\text{ kPa}$
• Penetration pore pressure $u_2 = 320\text{ kPa}$
• Net cone area ratio $a = 0.75$
• Empirical cone factor $N_{kt} = 15.0$

Concurrently, a Field Vane Shear Test (VST) was conducted at the same depth using a standard rectangular vane ($D = 75\text{ mm}$, $H = 150\text{ mm}$). The maximum measured torque was $T_{max} = 42.5\text{ N}\cdot\text{m}$. The soil Plasticity Index is $PI = 40%$, corresponding to a Bjerrum correction factor $\mu = 0.82$.

Calculate:

1. Total corrected tip resistance $q_t$.
2. Total vertical overburden stress $\sigma_{v0}$ and effective stress $\sigma'_{v0}$.
3. Undrained shear strength $s_u$ from CPTU.
4. Field vane peak undrained shear strength $s_{u,\text{field}}$ and design strength $s_{u,\text{design}}$.

Solution

Step 1: Calculate $q_t$ from CPTU $q_t = q_c + u_2(1 - a) = 1450\text{ kPa} + 320\text{ kPa} \times (1 - 0.75)$ $q_t = 1450 + 320 \times 0.25 = 1450 + 80 = 1530\text{ kPa}$

Step 2: Calculate overburden stresses at $z = 10.0\text{ m}$ $\sigma_{v0} = \gamma_{sat} \times z = 18.5\text{ kN/m}^3 \times 10.0\text{ m} = 185.0\text{ kPa}$ $u_0 = \gamma_w \times z = 9.81\text{ kN/m}^3 \times 10.0\text{ m} = 98.1\text{ kPa}$ $\sigma'_{v0} = \sigma_{v0} - u_0 = 185.0 - 98.1 = 86.9\text{ kPa}$

Step 3: Calculate CPTU undrained shear strength $s_u$ $s_u = \frac{q_t - \sigma_{v0}}{N_{kt}} = \frac{1530\text{ kPa} - 185.0\text{ kPa}}{15.0} = \frac{1345}{15.0} = 89.67\text{ kPa}$

Step 4: Calculate VST strength Convert vane diameter $D = 75\text{ mm} = 0.075\text{ m}$. Torque $T_{max} = 42.5\text{ N}\cdot\text{m} = 0.0425\text{ kN}\cdot\text{m}$. $s_{u,\text{field}} = \frac{6 T_{max}}{7 \pi D^3} = \frac{6 \times 0.0425\text{ kN}\cdot\text{m}}{7 \pi \times (0.075\text{ m})^3}$ $s_{u,\text{field}} = \frac{0.255}{7 \times 3.14159 \times 0.000421875} = \frac{0.255}{0.009278} = 27.48\text{ kPa}$

Applying Bjerrum correction $\mu = 0.82$: $s_{u,\text{design}} = \mu \times s_{u,\text{field}} = 0.82 \times 27.48\text{ kPa} = 22.53\text{ kPa}$

Note: The field vane measures localized localized shear strength along vertical and horizontal failure planes, providing a conservative design strength when corrected for rate effects.