6.3 Compaction: Principles, Calculations, and Equipment

Compaction: Principles, Calculations, and Equipment

Compaction is the process of mechanically densifying a soil by reducing the volume of air voids. Proper compaction increases the soil's shear strength, decreases its compressibility (reducing future settlement), and decreases its permeability. On the PE Construction exam, compaction questions often involve correlating laboratory test data with field density measurements and selecting the appropriate equipment for the job.

The Moisture-Density Relationship

The fundamental principle of soil compaction is that the achievable density is heavily dependent on the soil's moisture content during the compaction effort. Water acts as a lubricant between soil particles, allowing them to slide past one another and pack more densely.

However, this is only true up to a certain point:

• Dry of Optimum: When the soil is too dry, there is excessive friction between the particles, preventing them from achieving a tight configuration regardless of the compaction energy applied.
• Optimum Moisture Content (OMC): At this specific moisture level, the soil achieves its maximum possible dry density for a given compaction energy.
• Wet of Optimum: As more water is added beyond the OMC, water begins to replace soil particles in a given volume. Because water cannot be compressed, it prevents the soil from becoming denser. The density actually decreases as the soil becomes too wet and "pumping" (a jelly-like instability) occurs under heavy equipment.

Laboratory Proctor Tests

To determine the OMC and the Maximum Dry Density (MDD), a sample of the soil is taken to a laboratory and subjected to a Proctor test. The soil is compacted into a standard mold using a standard drop hammer at varying moisture contents to generate a moisture-density curve.

There are two primary standards:

• Standard Proctor (ASTM D698): Uses a 5.5 lb hammer dropping 12 inches. It represents lighter compaction equipment.
• Modified Proctor (ASTM D1557): Uses a 10 lb hammer dropping 18 inches. This test was developed to represent the higher compaction energies of modern, heavy construction equipment. The Modified Proctor curve will peak at a higher Maximum Dry Density and a lower Optimum Moisture Content compared to the Standard Proctor curve for the same soil.

Project specifications will explicitly state which Proctor test should be used as the baseline for the project.

Relative Compaction Calculation

Project specifications require the contractor to achieve a specific percentage of the laboratory maximum dry density, typically ranging from 90% for landscape areas to 95% or 98% for structural foundations and road bases.

Relative Compaction (%) = (Field Dry Density / Laboratory Maximum Dry Density) × 100

Worked Example:

An engineer specifies that a structural fill must be compacted to 95% of its Modified Proctor maximum dry density. Laboratory testing determines the Modified Proctor maximum dry density is 120.0 pcf at an OMC of 10%. In the field, a nuclear gauge reads a wet density of 126.5 pcf at a moisture content of 11.5%. Does this pass the specification?

1. Calculate Field Dry Density: Dry Density = Wet Density / (1 + w) Dry Density = 126.5 pcf / (1 + 0.115) = 126.5 / 1.115 = 113.45 pcf
2. Calculate Relative Compaction: Relative Compaction = (113.45 pcf / 120.0 pcf) × 100 = 94.5%
3. Conclusion: The field compaction is 94.5%. This is less than the required 95%. The test fails, and the contractor must apply more compaction effort.

Equipment Selection and Lift Thickness

Achieving the required density efficiently requires matching the compaction equipment to the soil type. Compaction is achieved through static weight, impact, vibration, or kneading.

Soil Types and Equipment

• Cohesive Soils (Clays and Silts): These soils require high contact pressure and a kneading action to break down the clods and shear the clay particles together. A sheepsfoot roller or padfoot roller is the ideal equipment. These rollers have studded drums that penetrate the lift and compact from the bottom up.
• Cohesionless Soils (Sands and Gravels): These soils lack internal cohesion and respond best to vibration, which causes the particles to temporarily lose contact and rearrange into a denser configuration. A smooth drum vibratory roller is the standard choice.
• Mixed Soils: For soils containing both fines and granular material, a pneumatic-tired roller (rubber-tired) is highly effective. The heavy rubber tires provide a combination of static pressure and a slight kneading action.

Lift Thickness

Soil must be placed and compacted in horizontal layers called lifts. If a lift is too thick, the compaction energy from the surface roller will not reach the bottom of the layer, leaving the lower portion uncompacted. Typical specifications limit loose lift thickness to 8 to 12 inches for heavy equipment, and 4 to 6 inches for hand-operated tampers (jumping jacks) used in tight trenches. Managing lift thickness is one of the most critical field QC tasks for an inspector.