Soil Compaction and Relative Density

Soil Compaction

• Soil compaction is a method used to mechanically increase the density of soil.
• It's an important part of the building process in construction.
• Settlement of the soil could occur if not performed properly, resulting in structure failure and costly maintenance.

Purpose of Soil Compaction

• Improves soil quality by increasing shear strength and bearing capacity.
• Decreases future settlements and permeability.
• Increases the stability of slopes for embankments.

Relative Density

• Relative density indicates the degree of compactness of in-situ soils and compacted fills.
• Loose soil is indicated by a lower relative density value and is relatively unstable.
• More stable soils have a higher relative density value.
• Optimum Moisture Content is the moisture content at which the maximum dry unit weight is obtained.

Equations for Relative Density

• Dr = (emax - e0) / (emax - emin) x 100%
  • emax = void ratio of the soil in its loosest condition
  • emin = void ratio of the soil in its densest condition
  • e0 = in-situ void ratio
• Dr = (γmax(γ - γmin)) / (γ(γmax - γmin)) x 100%
  • γ = in-situ dry unit weight
  • γmax = maximum dry unit weight
  • γmin = minimum dry unit weight
  • Dr = relative density

Soil Compaction Tests

• Laboratory tests include the Standard Proctor Test and the Modified Proctor Test.
• Field tests include the Sand Cone Method, Rubber Balloon Method, and Nuclear Method.

Standard Proctor Test

• The soil is compacted within a cylindrical tube.
• Equipment specifications include:
  • Hammer weight = 2.5 kg (5.5 lb)
  • Falling height = 1 ft
  • Amount of layers = 3
  • Number of blows per layer = 25
  • Compaction effort = 592.5 kJ/m³
  • Soil type = passes through sieve no. 4
• The test is performed several times with different water contents.
• After compaction, the weight, moisture content, and unit weight of samples are measured.
• Test standards are AASHTO T 99 and ASTM D698.

Modified Proctor Test

• The soil is compacted within a cylindrical tube.
• Specifications include:
  • Hammer weight = 4.5 kg (10 lb)
  • Falling height = 1.5 ft
  • Amount of layers = 5
  • Number of blows per layer = 25, 56
  • Compaction effort = 2693 kJ/m³
  • Soil type = passes sieve no. 4
• The test is performed multiple times with varying water content.
• Weight, moisture content, and unit weight of samples are measured after compaction.
• Test standards include AASHTO T 180 and ASTM D1557.

Parameters for Soil Compaction

• Moist Unit Weight of Compaction is calculated as γ = W / V(m)
  • γ = moist unit weight
  • W = weight of compacted soil in the mold
  • V(m) = volume of mold (1/30 cu.ft)
• Dry Unit Weight of Compaction
  • γdry = γ / (1 + (ω(%)/100))
  • γdry = (Gsγw) / (1 + (ω/Gs))
    • γ = moist unit weight
    • γdry = dry unit weight
    • ω = moisture content in percent
    • Gs = specific gravity
    • S = degrees of saturation
• Zero-Air-Void Unit Weight is calculated as γzav = (Gsγw) / (1 + ωGs) or γzav = γw / (ω + (1/Gs))
  • γzav = zero air void unit weight
  • γw = unit weight if water
  • ω = moisture content in percent
  • Gs = specific gravity

Sand Cone Method (Field Test)

• ASTM Designation D – 1556 dictates the method.
• A device consisting of a glass or plastic jar with a metal cone is used. The jar is filled with Ottawa sand.

Sand Cone Method Procedure

• W1 = the weight of the jar, cone, and sand filling
• W2 = weight of the moist soil excavated from the hole
• W3 = dry unit weight of the soil
  • calculated as W3 = W2 / (1 + (ω(%)/100))
• W4 = combined weight of the jar, cone, and remaining sand in the jar.
• W5 = weight of sand to fill the hole and cone
  • calculated as W5 = W1-W4
• V = volume of the hole
  • calculated as V = (W5 – Wc) / γd (sand)
• γd = Dry weight of the soil excavated from the soil / Volume of the hole
  • calculated as γd = W3 / V

Rubber Balloon Method (Field Test)

• ASTM Designation D – 2167 is the standard.
• The moist weight of soil removed from a test hole and its moisture content are determined.
• Volume of the hole is determined by introducing a rubber balloon filled with water from a calibrated vessel.

Nuclear Method (Field Test)

• Density meters are used to determine the compacted dry unit weight of soil.
• Density meters operate in drilled holes or on the ground surface.
• The instrument measures the weight of wet soil per unit volume as well as the weight of water.

Specification of Compaction: Relative Compaction

• Relative Compaction is an end product specification calculated as RC = (γd(field) / γd(max)-lab) x 100%

Types of Field Compaction Equipment

• Smooth Wheel Roller: supplies 100% coverage, ground contact pressures up to 400 kPa, used on all soil types except rocky soils, and mainly for proof rolling subgrades and compacting asphalt pavements.
• Rubber Tire Roller: heavily loaded wagon with rows of tires (3-6), tire pressure up to 700 kPa, 80% coverage, and used for granular and cohesive highway fills.
• Sheepsfoot Roller: round or rectangular protrusions ("feet") attached to a steel drum, protusion area ranges from 30 - 80 cm2, 8-12% coverage, very high contact pressures from 1400 to 7000 kPa, best suited for cohesive soils.
• Tamping Foot Roller: similar to sheepsfoot roller, approximately 40% coverage, pressures between 1400-8400 kPa, and best for compacting fine-grained soils.
• Grid Roller: about 50% coverage, pressures between 1400-6200 kPa, best suited for compacting rocky soils, gravels, and sand.
• Baby Roller: small type of smooth wheel roller with pressures from 10 to 30 kPa with performance based on weight and effect from vibration.
• Vibrating Plate: plate shaped compaction equipment sometimes called a “stamper”, used in narrow and high-risk areas where use is needed.

Conditioner Factors

• Characteristics of compaction equipment such as weight, size, operation frequency, and its proper range.
• Soil characteristics like initial density, soil type, size and shape of soil particles, and moisture content.
• Compaction procedure: number of roller passes, layer thickness, frequency of operation of vibrator, and towing speed.

Special Compaction Techniques

• Vibration: in-situ densification of loose granular soil deposits using a vibroflot (vibrating unit) with an eccentric weight inside for centrifugal force.
• Dynamic Compaction: repeated dropping of heavy weight objects on the ground at regular intervals; hammer weight varies from 80 to 356 kN, hammer drop height varies from 7.5 to 30.5m; the degree of compaction achieved depends on the weight of the hammer, height of hammer drop, and spacing of locations at which the hammer is dropped.
• Blasting: detonation of explosive charges (dynamite) at a certain depth, lateral spacing of charges ranges from 3 to 9m, charges are placed at 2/3 depth of soil layer for compaction, and three to five successful detonations are necessary.