Custom Search
|
|
 
|
||
|
SOIL-CEMENT STABILIZATION In general, there are three types of soil-and-cement mixtures as follows:l Plastic soil-cement is a hardened mixture of soil and cement that contains, at the time of placing, enough water to produce a consistency similar to plastering mortar. It is used to line or pave ditches, slopes, and other areas that are subject to erosion. It also maybe used for emergency road repair by mixing high-early-strength cement into the natural material in mudholes.l Cement-modified soil is an unhardened or semihardened mixture of soil and cement. When relatively small quantities of portland cement are added to granular soil or silt-clay soil, the chemical and physical properties of that soil are changed. Cement reduces the plasticity and water-holding capacity of the soil and increases its bearing value. The degree of improvement depends upon the quantity of the cement used and the type of soil. In cement-modified soil, only enough cement is used to change the physical properties of the soil to the degree desired. Cement-modified soils may be used for base courses, subbases, treated sub-grades, highway fills, and as trench backfill material.. Compacted soil-cement, often referred to as simply soil-cement, is a mixture of pulverized soil and calculated amounts of portland cement and water that is compacted to a high density. The result is a rigid slab having moderate compressive strength and resistance to the disintegrating effects of wetting and drying and freezing and thawing. The remainder of our discussion of soil-cement is directed towards this type of soil-and-cement mixture. MATERIALS FOR SOIL-CEMENT Soil, portland cement, and water are the three basic materials needed to produce soil-cement. Low cost is achieved mainly by using inexpensive local materials. The soil that makes up the bulk of soil-cement is either in place or obtained nearby, and the water is usually hauled only short distances.The word soil, as used in soil-cement, means almost any combination of gravel, sand, silt, and clay, and includes such materials as cinder, caliche, shale, laterite, and many waste materials including dirty and poorly graded sands from gravel pits.The quantities of Portland cement and water to be added and the density to which the mixture must be compacted are determined from tests. The water serves two purposes: it helps to obtain maximum compaction (density) by lubricating the soil grains and it is necessary for hydration of the cement that hardens and binds the soil into a solid mass. Properly produced soil-cement contains enough water for both purposes. The cement could be almost any type of portland cement that complies with the requirements of the latest ASTM (American Safety for testing and Materials), AASHTO(American Association of State Highway and Transportation Officials), or federal specifications. Types I (normal) and IA (air entrained) portland cements are the most commonly used.The water used in soil-cement should be relatively clean and free from harmful amounts of alkalies, acid, or organic matter. Water fit to drink is satisfactory. Sometimes seawater has been used satisfactorily when fresh water has been unobtainable. Practically all soils and soil combinations can be hardened with portland cement. They do not need to be well-graded aggregates since stability is attained primarily through hydration of cement and not by cohesion and internal friction of the materials. The general suitability of soils for soil-cement can be judged before they are tested on the basis of their gradation and their position in the soil profile. On the basis of gradation, soils for soil-cement construction can be divided into three broad groups as follows:1. Sandy and gravelly soils with about 10- to 35-percent silt and clay combined have the most favorable characteristics and generally require the least amount of cement for adequate hardening. Glacial-and water-deposited sands and gravels, crusher-run limestone, caliche, lime rock and almost all granular materials work well if they contain 55 percent or more material passing the No. 4 sieve and 37 percent passing the No. 10 sieve. Stones over an inch or two in diameter are undesirable. Exceptionally well-graded materials may contain up to 65-percent gravel retained on the No. 4 sieve and have sufficient fine material for adequate binding. These soils are readily pulverized, easily mixed and can be used under a wide range of weather conditions.2 Sandy soils deficient in fines, such as some beach sands, glacial sands, and windblown sands, make good soil-cement although the amount of cement needed for adequate hardening is usually slightly greater than with the soil in Group 1 above. Because of poor gradation and absence of fines in these sands, construction equipment may have difficulty in obtaining traction. Traction can be vastly improved by keeping the sand wet and by using track-type equipment. These soils are likely to be "tender" and to require care during final packing and finishing so that a smooth, dense surface may be obtained. 3. Silty and clayey soils make satisfactory soil-cement but those containing high clay contents are harder to pulverize. Generally the more clayey the soil, the higher the cement content required to harden it adequately. Construction with these soils is more dependent on weather conditions. If the soil can be pulverized it is not too heavy textured for use in soil-cement. |
 
|
|
|
||
