10.1 Sampling, Testing, and Soil Stabilization

10.1 Sampling, Testing, and Soil Stabilization

Characterizing and improving the subgrade is the foundation of transportation engineering. Subgrade soils must support pavement layers under dynamic traffic loads without excessive deformation.

Soil Classification Systems

Engineers classify soils to predict engineering behavior using two primary systems: the Unified Soil Classification System (USCS) and the AASHTO classification system.

Unified Soil Classification System (USCS)

USCS (ASTM D2487) categorizes soils based on grain size and plasticity. Soils are coarse-grained if $>50%$ is retained on the No. 200 sieve ($0.075\text{ mm}$), and fine-grained if $\ge 50%$ passes.

• Coarse fraction division: Gravels (G) if $>50%$ of the coarse fraction is retained on the No. 4 sieve ($4.75\text{ mm}$); Sands (S) if $\ge 50%$ passes.
• Grading Coefficients: Well-graded soils have smooth grain size distributions. Well-graded gravel (GW) requires $C_u \ge 4$ and $1 \le C_c \le 3$. Well-graded sand (SW) requires $C_u \ge 6$ and $1 \le C_c \le 3$. Otherwise, they are poorly graded (GP or SP). $C_u = \frac{D_{60}}{D_{10}}, \quad C_c = \frac{(D_{30})^2}{D_{10} \times D_{60}}$
• Plasticity Chart: If fines exceed $12%$, classification uses the A-line: $PI = 0.73(LL - 20)$. Clays (C) plot above the A-line with $PI > 7$; silts (M) plot below or have $PI < 4$.

AASHTO Soil Classification System

AASHTO M 145 is tailored for highway subgrades, classifying soils into groups A-1 (best) through A-7 (worst). Granular materials have $\le 35%$ passing the No. 200 sieve; silt-clay materials have $>35%$.

• Group Index (GI): Used to evaluate soils within a group: $GI = (F - 35)[0.2 + 0.005(LL - 40)] + 0.01(F - 15)(PI - 10)$ where $F$ is percent passing the No. 200 sieve. If $GI < 0$, report $0$. Round to the nearest integer. For A-1-a, A-1-b, A-3, A-2-4, and A-2-5, $GI = 0$. For A-2-6 and A-2-7, use only the partial formula: $GI = 0.01(F - 15)(PI - 10)$.

Laboratory Compaction and Strength Testing

Proctor Compaction Test

Determines the relationship between moisture content ($w$) and compacted dry unit weight ($\gamma_d$) to identify the Optimum Moisture Content (OMC) and Maximum Dry Density (MDD).

• Standard Proctor (ASTM D698): $12,400\text{ ft-lb/ft}^3$ energy ($5.5\text{-lb}$ hammer, $12\text{-in}$ drop, $3$ layers, $25$ blows/layer).
• Modified Proctor (ASTM D1557): $56,250\text{ ft-lb/ft}^3$ energy ($10\text{-lb}$ hammer, $18\text{-in}$ drop, $5$ layers, $25$ blows/layer).
• Zero Air Voids (ZAV) Curve: The theoretical dry density at $100%$ saturation ($S=1$): $\gamma_{zav} = \frac{G_s \gamma_w}{1 + w G_s}$

California Bearing Ratio (CBR)

Measures soil penetration resistance compared to standard crushed rock plunger tests at a rate of $0.05\text{ in/min}$. $CBR (\%) = \left( \frac{\text{Stress at } 0.1\text{ in}}{1,000\text{ psi}} \right) \times 100 \quad \text{or} \quad \left( \frac{\text{Stress at } 0.2\text{ in}}{1,500\text{ psi}} \right) \times 100$

Hveem R-Value

Measures lateral pressure transmitted through compacted soil under vertical load in a Hveem stabilometer. R-values range from $0$ (liquid) to $100$ (rigid solid).

Soil Resilient Modulus ($M_r$)

The resilient modulus ($M_r$) measures soil elastic response under cyclic traffic loading: $M_r = \frac{\sigma_d}{\epsilon_r}$ where $\sigma_d$ is cyclic deviator stress and $\epsilon_r$ is recoverable axial strain. Standard correlations for subgrade design include:

• For fine-grained soils ($CBR \le 10$): $M_r\text{ (psi)} = 1,500 \times CBR$
• From R-value: $M_r\text{ (psi)} = 1,200 \times R + 448$

Chemical Soil Stabilization

When subgrades are unstable, chemical stabilizers are mixed into the soil.

• Lime Stabilization: Adding quicklime (CaO) or hydrated lime ($Ca(OH)_2$) to clay soils ($PI > 10$, clay content $>25%$). Cation exchange and flocculation occur immediately, reducing plasticity. Long-term pozzolanic reactions form cementing calcium silicates (C-S-H) and aluminates (C-A-H).
• Cement Stabilization: Adding Portland cement to granular or low-plasticity silty soils. Hydration forms C-S-H gels independent of clay minerals, binding soil grains.
• Fly Ash Stabilization: Class C fly ash (self-cementing, $>20%$ CaO) is used for silts and sands. Class F fly ash requires lime or cement as an activator.

Soil Classification Example

Scenario: A subgrade soil has $55%$ passing the No. 200 sieve, $LL = 48$, and $PL = 24$. Find the AASHTO and USCS classification.

Solution:

1. AASHTO: Silt-clay material ($>35%$ passing No. 200). $PI = LL - PL = 48 - 24 = 24$. Since $LL = 48 > 40$ and $PI = 24 > 10$, the group is A-7. Since $PI = 24 > LL - 30$ ($18$), the soil is A-7-6. Calculate Group Index (GI): $GI = (55 - 35)[0.2 + 0.005(48 - 40)] + 0.01(55 - 15)(24 - 10) = 20(0.24) + 0.01(40)(14) = 4.8 + 5.6 = 10.4$ Rounding to the nearest integer yields GI = 10. Classification is A-7-6 (10).
2. USCS: Fine soil ($>50%$ passing No. 200). $LL = 48 < 50$. Check A-line limit: $0.73(LL - 20) = 0.73(48 - 20) = 20.44$. Since $PI = 24 > 20.44$, the soil plots above the A-line. Classification is CL (Lean Clay).