SOIL STABILIZATION

There are three purposes for soil stabilization. The first one is strength improvement. This increases the strength of the existing soil to enhance its load-bearing capacity. The second purpose is for dust control. This is done to eliminate or alleviate dust, generated by the operation of equipment and aircraft during dry weather or in arid climates. The third purpose is soil waterproofing, which is done to preserve the natural or constructed strength of a soil by preventing the entry of surface water.

There are two methods used to apply soil stabilization materials. The first is the admix way. This is used where it is necessary to combine two different soils together for stabilization. This can be done as follows: 

. In-place mixing: accomplished by blending of soil and stabilization materials on the jobsite.

. Off-site mixing: accomplished by using stationary mixing plants.

. Windrow mixing: accomplished by mixing the materials using a grader.

The second way is the surface penetration application, which is accomplished by placing a soil treatment material directly to the existing ground surface by spraying or other means of distribution. Some of the additives used in soil stabilization are cement, lime, bituminous products, and calcium chloride. Cement-treated bases are the most commonly used for the purpose of upgrading a poor quality soil. Soil-cement is a mixture of pulverized soil and measured amounts of portland cement and water, compacted to a high density.

There are three types of soil-cement. The first type is compacted soil-cement that contains sufficient amounts of cement to harden the soil and enough moisture for both compaction and hydration of the cement. The second type is cement modified soil which is an unhardened or semihardened mixture of soil and cement. Only enough cement is used to change the physical properties of the soil. The third is plastic soil-cement. It is a hardened mixture of soil and cement that contains at the time of placing, enough water to produce a consistency similar to that of plastering mortar. The three basic materials needed when working with soil-cement are soil, portland cement, and water. The soil can almost be any combination of gravel, sand, silt, or clay.

Three major control factors when working with soil-cement are as follows:

1. The proper cement content is needed. A rule of thumb: use one 50-pound bag per square yard.

2. Proper moisture content. On a soil sample, a firm cast should be made when squeezed in your hand without squeezing out any water.

3. Adequate compaction. The principles of compacting soil-cement are the same for compacting the same soils without cement treatment. The soil-cement mixture at optimum moisture content should be compacted to maximum density and finished immediately. Moisture loss by evaporation during compaction, as indicated by the graying of the surface, should be replaced with light applications of water.

Occasionally during compaction, the treated area may yield under the compaction equipment. This may be due to one or more of the following causes: (1) the soil-cement mix is much wetter than optimum moisture content, (2) the soil may be too wet and unstable, and (3) the roller may be too heavy for the soil. If the soil-cement mix is too damp, it should be aerated by using the scarifier on the grader. After it has dried to near optimum moisture content, then it is compacted.

TECHNIQUES OF EARTHWORK OPERATIONS

Techniques of earthwork operations consist of knowing the equipment needed and the operations of pioneering, clearing, grubbing, stripping, draining, and grading and excavating. These operations are done primarily with heavy construction equipment, such as bulldozers and graders. Hand- or powerfelling equipment, explosives, and fire are used when they make the completion of these operations easier.

CAUTION

Large-scale clearing and grubbing ^operations often produce damaging environmental effects, such as increased soil erosion, reduction of atmospheric oxygen, and destruction of wildlife habitat. Additionally, introduction of particulate matter into streams and riverbeds causes increased siltation and algae growth. Federal regulations may require an environmental impact statement or assessment prior to beginning clearing operations.

To prevent these damaging effects, save as much vegetation as possible, such as trees, grass, and other plants, to hold the soil in place. Constructing a shallow trench or application of plastic barriers or hay bales around the perimeter of a project will help to contain water runoff into streams and rivers, preventing siltation. Burning of scrubs and stumps should be done only when atmospheric conditions are favorable and the material to be burned is dry, However, do NOT use petroleum base fuels to start fires, as fuels do not burn completely and seep into the underground water table.

NOTE: A burn permit is required in all burning operations on NCF projects to prevent wild fires and production of smog.

When determining the methods of earthwork operations needed, consider the following factors:

. The acreage to be cleared

. The type and density of vegetation

. The physical features of the land

. The expected weather conditions

. The time available for completion of the job

For best results, a combination of methods should be used in a sequence of operations. Use the method most suitable and effective for the job.