360R-06 Design of Slabs-on-Ground
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| 3.6—Site preparation
3.6.1 Introduction—Initially, the top layer of soil should be stripped of all organic material, debris, and frozen material. Normally, to produce a uniform support, the surface is stripped, tilled, and recompacted before the subbase is placed. Both hard and soft pockets of soil should be located by proof rolling or other means, removed, and replaced by compacted soil to provide a uniform subgrade for the base, subbase, or concrete slab. Refer to ACI 302.1R for additional information. The site should be graded to provide good surface drainage throughout the construction period and for the lifetime of the structure. Groundwater may have to be intercepted and routed around the site. Combinations of base and subbase materials and thicknesses can be used to increase the subgrade capacity. Sinkholes, expansive soils, highly compressible materials, or other subgrade problems, however, can influence the performance of the slab and should be examined in detail. 3.6.2 Proof rolling—As is discussed in ACI 302.1R, proof rolling usually refers to the use of a loaded vehicle driven in a grid pattern over the subgrade in an effort to locate soft and compressible areas at or close to the surface. This should be a part of the process for quality assurance for the soil-support system, and should be set forth in the project specifications. The wheel load should be sized to avoid bearing failure, but be large enough to stress at least the upper foot of subgrade. Three cycles of the wheel load over the same track are usually specified. These repeated applications may expose weak areas by rutting or pumping of the surface. Rutting normally indicates excess moisture at the surface. Pumping indicates subgrade soils wet of the optimum moisture to achieve and maintain compaction. Areas of poor support should be removed and replaced with compacted material to provide a more uniform subgrade. After repairs, proof rolling can be repeated. There are no standards for proof rolling, and quantitative assessment cannot be made from its use; however, guidelines are given in ACI 302.1R. If a thick layer of dry and dense material, such as a base or subbase course, exists over the surface or the subgrade surface has become hard due to drying and construction traffic, then proof rolling may not be able to detect any soft or compressible areas under the surface. On some projects, proof rolling is employed three times: after stripping (before any fill is placed); after the fill has been installed; and after the base course is placed. To locate suspected deeper soft areas or buried debris, borings, test pits, resistivity, or other procedures may be needed. Proof rolling should be scheduled to permit remedial work to be performed without interfering with the construction schedule. 3.6.3 Subgrade stabilization—There are a number of methods that can improve the performance of a soil subgrade. Generally, for slabs-on-ground, the soil is densified by using compaction equipment such as sheepsfoot, rubber tire, or vibra- tory rollers. Chemical stabilization may also be appropriate. Weak subgrade material can be stabilized by the addition of chemicals that are combined with the soil, as shown in Table 3.3 . Generally, portland cement, lime, or fly ash is mixed into the soil substrata with water, and the mixture is recompacted. Lime and fly ash are also used to lower the plasticity index of subgrade and subbase materials. For silty soils, portland cement may be effective. A geotechnical engineer should plan, supervise, and analyze the soil conditions before chemical stabilization is used. Depending on the situation and soil conditions, certain compactors are more effective than others. Generally, granular soils are most responsive to vibratory equipment, and cohesive soils respond best to sheepsfoot and rubber-tired rollers, but there are exceptions. The depth of compacted lifts varies with soil type and compaction equipment, but in most cases, the depth of compacted lifts should be 6 to 9 in. (150 to 230 mm). The dry density achieved after compaction is normally measured and compared with maximum dry density values obtained from laboratory compaction tests. The maximum dry density and optimum moisture content values vary with texture and plasticity. This is illustrated by Fig. 3.4 for standard Proctor tests (ASTM D 698) on eight different soils. Because the modified Proctor test (ASTM D 1557) uses a higher level of energy, the maximum dry density will be higher and the optimum moisture content will be lower than that of the standard Proctor test. Furthermore, the difference will vary with the texture and plasticity of the soil. This is shown in Fig. 3.5 . |