Soil Compaction and Properties for Construction Planning

Fundamentals of Soil Phase Diagrams and Properties

• Definition of Phase Diagram: A phase diagram is a schematic and simplified diagram showing the components of a specific material (e.g., concrete consists of cement, water, sand, gravel, and additives). It is used to analyze the properties and volumetrics of a material.

• Soil Components: A typical soil sample collected from a site consists of three key components:     * Solids: The actual soil particles.     * Water: Moisture contained between the particles.     * Air: Gas occupying the voids between particles.

• Phase Diagram Notation (Weights): Typically represented on the right-hand side of the diagram.     * $W_s$: Weight of solids. Also known as the "oven dry weight."     * $W_w$: Weight of water.     * $W_a$: Weight of air, which is mathematically treated as $0$.     * $W_{total}$ (or $W$): Total weight or "in situ weight" ($W = W_s + W_w$).

• Phase Diagram Notation (Volumes): Typically represented on the left-hand side of the diagram.     * $V_s$: Volume of solids.     * $V_w$: Volume of water.     * $V_a$: Volume of air.     * $V_{total}$ (or $V$): Total volume of the sample ($V = V_s + V_w + V_a$).     * $V_v$: Volume of voids ($V_v = V_w + V_a$), though this is used less frequently in primary compaction calculations.

• Determining Weights:     * To get the Total Weight ($W$): Take the in situ sample and weigh it immediately on a balance.     * To get the Weight of Solids ($W_s$): Place the sample in an oven at a specific temperature for eight hours. Weigh the sample afterward; the result is the oven dry weight.

Key Soil Density and Moisture Equations

• Unit Weight/Density ($\gamma$): Generic term for mass over volume ($\gamma = \frac{W}{V}$). In soil mechanics, there are two distinct types of density used to evaluate compaction.

• Equation 1: Moisture Content ($MC$ or $w$):     * Formula: $MC = \frac{W_w}{W_s}$     * Definition: The weight of water divided by the weight of solids.     * Warning: It is not water weight divided by total weight.     * Representation: Always expressed as a decimal between $0$ and $1$ in calculations.

• Equation 2: Wet Density ($\gamma_{wet}$):     * Formula: $\gamma_{wet} = \frac{W_{total}}{V_{total}}$     * Definition: Includes everything—solids, water, and air—using the total weight in its natural state.

• Equation 3: Dry Density ($\gamma_{dry}$):     * Formula: $\gamma_{dry} = \frac{W_s}{V_{total}}$     * Definition: Includes ONLY the weight of the dry soil particles divided by the total volume of the sample. This is the primary indicator of soil compaction.

• Equation 4: Relationship between Wet and Dry Density:     * Formula: $\gamma_{dry} = \frac{\gamma_{wet}}{1 + MC}$     * Utility: This allows for the calculation of dry density without an oven-drying process, provided the moisture content is known.

Illustrative Density Examples

• Example 1: Moisture Content Calculation     * Data: In situ weight = $12.0\,lbs$, Oven dry weight = $10.0\,lbs$.     * Step 1: Identify $W = 12.0\,lbs$ and $W_s = 10.0\,lbs$.     * Step 2: Calculate water weight: $W_w = 12.0\,lbs - 10.0\,lbs = 2.0\,lbs$.     * Step 3: Moisture content: $MC = \frac{2.0\,lbs}{10.0\,lbs} = 0.20$.

• Example 2: Density and Dry Density Calculation     * Data: In situ weight ($W$) = $5.0\,lbs$, Moisture content ($MC$) = $0.20$, Total Volume ($V$) = $0.03\,ft^3$.     * Wet Density: $\gamma_{wet} = \frac{5.0\,lbs}{0.03\,ft^3} = 166.0\,lbs/ft^3$.     * Dry Density: $\gamma_{dry} = \frac{166.0\,lbs/ft^3}{1 + 0.20} = 138.0\,lbs/ft^3$.

Principles of Soil Compaction

• Definition: Compaction is the application of mechanical energy to soil to remove air between soil particles, resulting in a more stable foundation.

• Phase Diagram Transformation: During compaction, mechanical energy reduces the volume of air ($V_a$). Because air has no weight, the total weight remains the same, but the total volume ($V$) decreases.

• Impact on Dry Density: Since $\gamma_{dry} = \frac{W_s}{V}$, a decrease in volume due to compaction causes an increase in dry density.

• Ultimate Objective: In construction, the goal is to maximize the dry density to ensure adequate compaction and minimize future settlement.

• The Optimization Problem:     * If soil is too dry (minimize moisture), it is difficult for particles to move and compact.     * If soil is too saturated (maximize moisture), it becomes soapy, loose, and uncompactable.     * The solution is to determine the Optimum Moisture Content (OMC) which allows for the Maximum Dry Density (MDD).

Laboratory Testing: The Proctor Test

• Process Overview: A sample is collected from the field, tested in a lab to find the OMC and MDD, and these values serve as the baseline for field work.

• Soil Sample Preparation:     * Extraction: Use a hand auger to collect about $4.0\,lbs$ of soil.     * Air Drying: The sample is left to dry naturally to remove excess water. This assumes the weight of the air-dried sample is approximately equal to the weight of solids ($W_s$).     * Sieving: The sample must pass through Sieve #4 (opening size of $4.75\,mm$ or $0.2\,inch$). This separates fine aggregate (sand) from coarse aggregate (gravel). Only fine aggregate is used for the test.

• Standard vs. Modified Proctor Test: Both use the same concepts. The difference lies in the weight of the hammer, height of the drop, and Number of blows.

• The Standard Proctor Apparatus:     * Mold Volume: Fixed at $\frac{1}{30}\,ft^3$.     * Components: Detachable base plate, mold, and a removable collar.

• Testing Procedure (Step-by-Step):     * 1. Select an initial moisture content (e.g., $8\%$ or $0.08$).     * 2. Add the corresponding water weight to the dry soil ($W_w = MC \times W_s$).     * 3. Fill the mold in three layers (lifts).     * 4. Compact each lift with a $5.5\,lb$ hammer falling from a height of $12.0\,inches$ for $25$ blows.     * 5. For the final lift, the soil should project slightly above the mold into the collar.     * 6. Remove the collar and trim the excess soil flush with the top of the mold to ensure a total volume of exactly $\frac{1}{30}\,ft^3$.     * 7. Weigh the mold with soil, subtract the mold weight to get the total soil weight ($W_{total}$).     * 8. Repeated the process for 5 or 6 samples, increasing moisture content by $2\%$ increments (e.g., $8\%$, $10\%$, $12\%$).

• Data Analysis: Calculate $\gamma_{wet}$ for each sample, then convert to $\gamma_{dry}$ using equation 4.

The Compaction Curve and Optimization

• Plotting the Results: Plot Moisture Content ($x$-axis) versus Dry Unit Weight ($y$-axis). The resulting trend is a parabola.

• Identifying Key Lab Values:     * Maximum Dry Density (MDD): The peak of the parabola.     * Optimum Moisture Content (OMC): The moisture content value on the $x$-axis that corresponds to the peak dry density.

• Reading Blueprints (Specifications):     * Engineered fill typically requires "Relative Compaction," such as $98\%$ of MDD.     * The field target dry density is calculated as: $\gamma_{target} = \text{Relative Compaction Percentage} \times MDD_{lab}$.     * Example: If $MDD_{lab} = 120.0\,lbs/ft^3$ and the spec is $98\%$, the field target is $117.6\,lbs/ft^3$.

• Field Moisture Range: Using the graph, draw a horizontal line at the target density (e.g., $117.6$). The two intersection points on the curve indicate the acceptable moisture range for the field (e.g., $4.5\%$ to $7.5\%$\n).

Field Implementation and Verification

• Field Action: Contractors spray water to bring soil within the specified moisture range and use rollers for compaction.

• Verification: After compaction, the dry density must be measured in the field using a Nuclear Gauge.     * Nuclear Gauge: A tool placed on the ground that provides instant readings of actual field moisture content and actual field dry density.

• Final Verification Rule:     * The actual dry density measured by the nuclear gauge ($\gamma_{field}$) must be greater than or equal to the target relative compaction density ($\text{Relative Compaction} \times MDD_{lab}$).

• Real-World Application: Aviation projects (taxi lanes) may specify different requirements, such as $95\%$ compaction and moisture within $\pm 2\%$ of OMC.

Questions & Discussion

• The lecturer emphasizes that the lab dry density ($120.0\,lbs/ft^3$) is always significantly higher than field density results because the lab environment is standardized and controlled. Achieving more than $100\%$ of the lab value in the field is nearly impossible.

• Students with questions are encouraged to contact the instructor via Microsoft Teams or email.