Construction Methods Lecture 9 Compacting Lecture 9

Compacting Principles of compaction Compaction is the process of increasing the density of a soil by mechanically forcing the soil particles closer together, thereby expelling air from the void spaces in the soil. Compaction The reason for compaction is to improve soil properties to Reduce or prevent settlements. Increase strength Improve bearing capacity Control volume changes Lower permeability Density, however, is the most commonly used parameter for specifying construction operations because there is a direct correlation between these desired properties and a soil’s density. Lecture 9

Compacting The degree of compaction that may be achieved depends on The soil’s physical and chemical properties The soil’s moisture content The compaction method employed The amount of compactive effort The thickness of the soil layer being compacted Lecture 9

Compacting Maximum Dry Density/Optimum Moisture Content Soil moisture content is a main factor influencing compaction results. The maximum dry unit weight (density) and the percentage of water required to achieve maximum density can be determined from the compaction curve Compaction curve Standard laboratory test called a Proctor test has been developed to evaluate a soil's moisture-density relationship under a specified compaction effort. There are two Proctor tests which have been standardized by the AASHTO and ASTM; Standard Proctor Test Modified Proctor Test Lecture 9

Compacting Lecture 9

Compacting The percentage of water, which corresponds to the maximum dry density is known as the optimum water content. It is the amount of water required for a given soil to reach maximum density. Lecture 9

Compacting Modified Proctor Test results for various soils Lecture 9

Compacting Compaction Specifications Typical density requirements range from 90% of Standard Proctor to 100% of Modified Proctor. For example, 95% of Standard Proctor is often specified for embankments, dams, and backfills. A requirement of 90% of Modified Proctor might be used for the support of floor slabs. For the support of structures and for pavement base courses where high wheel loads are expected, requirements of 95 to 100% of Modified Proctor are commonly used. A lack of uniformity in compaction may result in differential settlement of structures or may produce a bump or depression in pavements. Therefore, it is important that uniform compaction be obtained. Lecture 9

Compacting Measuring Field Density Traditional test Traditional test (sand tests and liquid tests) Nuclear density gauges nonnuclear density gauges equipment-mounted compaction measurement systems Traditional test All of the traditional test methods involve removing a material sample, measuring the volume of the hole produced, and determining the dry weight of the material removed. Compacted density is then found as the dry weight of material removed divided by the volume of the hole. Lecture 9

Compacting Nuclear density devices Nonnuclear density gauges Nuclear methods are used extensively to determine the water content and density of soils. It is measure the amount of radioactivity from a calibrated source that is ret1ected back from the compacted material to determine both material density and moisture content. The result can be read directly from the digital display within a few minutes. It is nondestructive test, does not require the removal of soil samples Nonnuclear density gauges Nonnuclear gauges that measure asphalt density, temperature, and moisture content are also available. Nonnuclear devices are safer, lighter, and easier to transport than nuclear devices. When properly calibrated, such devices claim to be as accurate as nuclear devices. Lecture 9