Soil Compaction Principles, Testing, and Methods
Understanding Soil Compaction
Compaction decreases the void content of a soil mass, forcing solid particles into more intimate contact with each other.
Factors Influencing Soil Compaction
Several factors affect soil compaction:
- Water content (moisture)
- Soil type
- Shape and magnitude of the energy applied
- Compaction equipment
- Layer thickness
Optimum Moisture Content
The optimum moisture content is the moisture level at which the soil achieves its maximum density. It is a characteristic value for each soil and is generally higher for soils with a higher fines content.
Soil Characteristics Affecting Compaction
Soil type influences compaction based on:
- Specific gravity
- Particle shape
- Texture
- Plasticity
Saturation represents the theoretical upper limit of compaction for a given moisture content.
Benefits of Soil Compaction
Proper soil compaction offers several advantages:
- Increases the load-bearing capacity of the soil.
- Prevents or minimizes soil settlement.
- Reduces soil permeability.
- Improves the mechanical properties of the soil.
Laboratory Soil Density Testing
To determine the maximum density of soil in the laboratory, standardized tests are employed.
Modified Proctor Test Details
The Department of Roads (or relevant authority) often uses the Modified Proctor Test (e.g., ASTM D1557 Method D). This test involves compacting a soil sample in layers within a cylindrical mold. A standardized rammer is dropped a specific number of times per layer from a set height. Data obtained are used to construct a Density-Moisture curve, determining the Maximum Dry Density (MDD) and Optimum Moisture Content (OMC) for the material.
Typical Modified Proctor (Method D) parameters:
- Number of determinations: Typically 5
- Rammer weight: 4.5 kg (10 lb)
- Drop height: 457 mm (18 in)
- Material used: Passing 19 mm (3/4 in) sieve
- Mold diameter: 152.4 mm (6 in)
- Number of layers: 5
- Blows per layer: 56
- Compaction energy: Approx. 2,700 kN-m/m³ (56,000 ft-lbf/ft³)
Relative Density Test (Vibration Method)
When testing cohesionless soils (typically containing less than 12% fines passing the 0.075 mm sieve), the Relative Density test (e.g., ASTM D4253 and D4254) should be performed instead of the Proctor test. This method determines the minimum and maximum dry densities of free-draining soils containing particles up to 80mm, often using vibration.
Field Compaction Methods and Equipment
During construction, soil is compacted using specific energy application methods:
- Pressure: Applying static weight.
- Impact: Delivering blows to the surface.
- Vibration: Using high-frequency oscillations.
Pressure Compaction Equipment
Examples include smooth drum rollers, pneumatic-tired rollers, and sheepsfoot rollers.
Impact Compaction Equipment
This equipment, often pneumatic tampers (rammers), typically weighs less than 300 kg. The greatest density increase usually occurs within the first 5 passes. Granular (non-cohesive) soils can often be compacted in layers up to 30 cm thick. For fine-grained cohesive soils, layer thickness should generally not exceed 20 cm, and moisture content should ideally be kept slightly below optimum.
Vibration Compaction Equipment
Key characteristics include frequency, amplitude, static weight, and dynamic force. Examples include vibro-tampers (rammers), vibrating plates, and vibratory rollers.
Selecting Compaction Equipment
Equipment choice depends on the site and soil conditions:
- For restricted or small areas, vibrating plates are often recommended.
- For large surfaces like roads, vibratory rollers are most suitable.
- Vibro-tampers are highly effective for compacting cohesive soils.
- For trenching, trench rollers (drum vibratory rollers, often padfoot type) are recommended if width permits. In narrow trenches, appropriately sized vibro-tampers are used.
Site Investigation and Sampling
Site investigation planning should aim to reveal the site’s stratigraphy through profiles showing the different soil layers or strata.
Investigation Methods
The most common methods for examining the subsurface profile are:
- Test pits
- Borings (Soundings)
Seismic refraction profiles are also occasionally used.
Types of Soil Samples
Two main types of samples can be obtained:
- Disturbed Samples: Soil structure is significantly altered during collection, but suitable for classification tests.
- Undisturbed Samples: Efforts are made to preserve the natural structure and moisture content, used for strength and compressibility tests.
Rock Quality Designation (RQD)
The RQD is a measure derived from rock core samples obtained during drilling, used to assess the quality and properties of the rock mass.