CE 240 Soil Mechanics & Foundations Lecture 4.1 Soil Compaction: Field work (Das, Ch. 5)
Outline of this Lecture 1. Factors controlling degree of compaction 2. Compaction field work 3. Compaction quality control on site
Objectives for Compaction Increasing the bearing capacity of foundations; Decreasing the undesirable settlement of structures; Control undesirable volume changes; Reduction in hydraulic conductivity; Increasing the stability of slopes.
There are 4 control factors affecting the extent of compaction: 1. Compaction effort; 2. soil type and gradation; 3. moisture content; and 4. dry unit weight (dry density).
Compaction with the engineering sense: Compaction is for making the optimum density as close as the zero-air-void as possible. More compaction effort make the optimum density closed to ρ(zav) by the fact of the 2 linear lines have different slopes.
After passing the point of optimum water content, increasing water content will not decrease the hydraulic conductivity any more.
Compaction Specifications Work-Type specification contractor what to do and how to do it Lowest bid Performance based specification contractor must achieve a certain degree of compaction based on lab data. Most common
Field Compaction Equipment Smooth wheel roller Sheepsfoot roller Pneumatic tired roller Vibratory roller Dynamic Compaction ramming
Types of Rollers 1. Smooth Wheel Rollers or Smooth Drum Rollers are suitable for thin layers, and for finishing operations. 2. Pneumatic Rubber-Tired Rollers have a series of rubber tires side-by-side instead of a smooth drum. Compaction is achieved by a combination of weight and kneading action. 3. Sheepsfoot Rollers are drums with a large number of "sheepsfoot" projections. They are most effective in compacting clayey soils. The initial pass in the field compacts the lower portion of the lift, with subsequent passes compacting the middle and upper portions. The roller is said to "walk out" as the soil is compacted and the feet do not penetrate as deeply as the initial pass. 4. Vibratory Rollers work well in compacting granular soils. Vibratory action can be used with smooth drum, rubber tired, or sheepsfoot rollers.
Compaction
Compaction
Inflation pressure-slight tire bulge with the proper pressure on radial ply tires 18 psi 6 psi Bulge
Impact of Tire Pressure 18 psi 6 psi
Top View: Measured soil-tire interface stress (psi) distribution of an 18.4R38 tire
At Proper Inflation Pressure, Radials have greater Contact Area
Impact of soil moisture on compaction (depth and sinkage) Depth, In. 0 4 8 12 16 20 24 28 Hard dry soil Normal Wet soil
B. Ramming Rammers have a low frequency (800 blows/minute), and a higher stroke (1.5" to 3.5 height). As the machine operates the ramming action breaks down the soil, pushes the particles closer together, and forces air out of the voids.
C. Vibration Vibratory machines are distinguished by their high frequency (2,000 to 6,000 blows/minute) and a low amplitude. The vibratory action sets the soil particles in motion, which rearranges them into a denser packing. (Das, 2006)
Field Compaction A. Relative Compaction Relative compaction is a way of comparing compaction on the construction site with the laboratory compaction results. It is defined as:
Specification for Field Compaction Specifications will refer to % Relative Compaction Relative to what? Proctor Test standard or modified % Relative Compaction R ( ) d ( field ) % = γ γ d max ( lab ) 100 R ~ (90 100%) If R > 100 % use Modified Proctor Test
A. Water Content the experience In the field it has been found that soils compact better at water contents on the dry side of the optimum water content determined in the lab. B. Why? Lab and field have used different compaction pressure.
The clayey soil compacted at the dry side of the optimum water content will possess greater strength.
A number of 10-15 roller passes are the most efficient and economical way to yield the maximum dry unit weight.
Specification for Field Compaction 98% means the soil in the field should be 98% of the lab result For example, if the peak of the curve is at 100 pcf and 22% moisture The field compaction must be at least 98 pcf and within the stated moisture range (20 ~24%)
Specification for Field Compaction (cont.) Soil will be compacted to 98% relative compaction as compared to a standard proctor test, ASTM D-698 The soil moisture content will be ± 2% of optimum.
Measurement of Field Compaction Most common methods are Nuclear Method Sand Cone method Rubber Balloon method
In-Place Soil Unit Weight Test Density of soil in-place by drive cylinder method ASTM D2937 or AASHTO T 204 Unit weight of soil in-place by the sandcone method (Fig. 4-12) ASTM D 1556 or AASHTO T 191
In-Place Soil Unit Weight Test (cont.) Unit weight of soil in-place by the rubberballoon method ASTM 2167 or AASHTO T 205 Unit weight of soil and soil-aggregate inplace by nuclear methods ASTM D 2922
Figure 5.28 (Das, 2006) Calibrated vessel used with rubber balloon
Nuclear density gauges consist of a sealed source in a source holder and a scintillation detector. The source holder is mounted on the end of a pipe or chute with the detector in the instrument console. The nuclear source radiation transmits from the source through the material to the detector. As the density of the material changes the amount of radiation reaching the detector changes. The greater the density of the material, the lower the radiation field at the detector; the lower the density of the material, the higher the radiation field at the detector. A big disadvantage of this instrument is the safety precautions that has to be taken into account. It is even necessary to have the operators attend a special training just to work with the gauge.
Non-nuclear density gauge To avoid the hassle of the existing nuclear method, non-nuclear density gauge has also been developing and remaining as an active research field in geotechnical engineering. One of such kind f density meter called Pavement Quality Indicator (PQI). A measuring plate underneath the gauge generates an altering electric field through the asphalt. This way it measures the dielectric properties of the material. This property is tells how much electrical energy the material can hold. It is obvious that air has a complete other properties than solid material. Because more dense material holds less air, it is possible to calculate the density based on the electrical properties of that material. Because it is 'just' an electrical device anyone is allowed to work with it. This makes it possible that staff working with the compactor or near the paving process can measure the compaction during the day as often as necessary.
Reading assignment: Ch. 5 Homework: Problems 5.9, 5.10, 5.11, 5.12