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136 RESII)ENTIAL SLABS ON GROUND Concrete that is to serve as a wearing surface should be cured for at least 7 days. When a wearing surface is not required, at least 3 days' curing should be provided. For temperatures below 40° F. 2 additional days of curing should be provided in either case. PART C: Soils 1.0 GENERAL While there is general appreciation of the importance of soil per- formance in the design and construction of large and/or heavy buildings, relatively little importance has been attached to it in light construction. Some justification for this attitude is that many soils can, in fact, be used as a base for slab construction with little or no modification; others, however, must be handled with extreme caution, and many will require competent engineering analysis. The importance attached to soil in the design of residential slabs needs to be understood. Since it is assumed that the soil will remain in contact with the slab and thus provide support, it is im- portant that the designer recognize the possibility of soil activity. Many factors influence the behavior of soils beneath structures, and key factors among these must be taken into consideration in the design of slabs for residential construction. These include the type of soil; its composition with respect to grain size, moisture content, density, drainage characteristics, stratification, and con- sistency; and climatic variations. The discussion which follows will cover the most important factors involved in evaluating soils for use with residential slabs- on-ground. Using the Unified Soils Classification System1 as a basis, soils identification and classification are considered. A method for rating the effect on soils of climatic variations is pro- vided. For discussion purposes, distinction is drawn between soils 1Appendix C, Reference 4; this publication discusses soils in detail. See also Appendix D.
SUPPLEMENTARY INFORMATION 137 which present problems and those which do not. Heated-slab effects on soils, natural moisture conditions, site investigations, and the various types of fill are treated. Site preparation is discussed, and a scale of suggested compaction densities based on climatic rating and soil plasticity index is provided. 2 . O UNI TIED SOIL CLASSI LOCATION SYSTEM The Unified Soil Classification System is based on the system developed during World War II for the Corps of Engineers by Dr. Arthur Cassagrande of Harvard University. It identifies soils ac- cording to textural qualities and plasticity, and sets up groupings with respect to performance as engineering construction materials. The following properties form the basis of this system: a. Percentages of gravel, sand, and fine-textured materials b. Shape of the grain-size distribution curve c. Plasticity and compressibility characteristics. Each soil is given a descriptive name and a letter symbol indi- cating its principal characteristics, as shown in Appendix D, p. 289. It is felt that this system more adequately meets the needs of the designer of residential slabs than any other currently in use, since it provides a means of identifying a soil accurately and placing it in a class with well-established engineering properties. This clas- sification, together with density, consistency, and moisture content, provides adequate means of describing and/or distinguishing among foundation soils for slab-on-ground construction. 2.1 Identification and Classification The designer must know the engineering characteristics of soil, including grain size, gradation, and plasticity. As these charac- teristics vary widely and descriptions of essentially similar soils often differ substantially from one locality to another, adequate evaluation is not always easy. An important factor in soil classification is that rarely will only
138 RESIDENTIA L SLABS ON GROUND a single soil type be found on a residential building site. In fact, soils generally vary in composition and characteristics, vertically and horizontally, sometimes within inches. Thus it is important to know what types of soil are present, and the amount and distri button of each. 2.2 Aids to Soil Identification Before any conclusions can be reached concerning the suitability of a particular site for a particular slab design, the basic charac- teristics of the soil must be determined. Agricultural soil surveys, geological maps, and similar items are among the many aids to the identification of soils. These gen- erally can be helpful if properly interpreted, but caution must be exercised in their use. The procedures outlined in the Unified Soil Classification System are sufficient for the field identification of most soils; for complete classification, there must be information regarding natural mois- ture content, density, and consistency. If there is still any question as to soil type, more extensive and exacting tests should be con- ducted by a soils laboratory. 2 .3 Major Soil Classes Broadly speaking, soils are either granular or cohesive; in many areas, the soil consists of a combination of these broad classes, in varying proportions. Granular soils generally have a grain size too large for a No. 22 sieve, though so-called "rock flour" soils are considered granular in spite of being finer than No. 200.2 Co 1In this connection, the Federal Housing Administration has been re- sponsible for the development of two special aids which are described in the following: Federal Housing Administration, Engineering Soil Classification for Res- idential Developments, FHA No. 373. From data prepared principally by the Virginia Polytechnic Institute and the United States Bureau of Public Roads. Washington: U.S. Government Printing Office, revised November 1961. Federal Housing Administration, Soil PVC Meter, FHA 701, December 1960. 2ASTM Designation E-11, Sieves for Testing Purposes (Philadelphia: American Society for Testing and Materials), contains detailed specifica- tions for sieves. .
SUPPLEMENTARY INFORMATION 139 hesive soils are fine-textured soils that have plasticity. Such soils, when mixed with water, can be molded without rupture. When dried, they become hard and brittle. Organic soils include peat, topsoil, and organic silt containing decayed vegetable and animal matter in various percentages. A natural soil consisting primarily of granular particles greater than No. 200 sieve, but containing perhaps 20% of clay particles, would probably behave as a cohesive soil even though the dominant material is not clay. Thus, for purposes of this report, a soil can- not be considered granular unless it is nonplastic (as determined by Atterberg Limit Test). Lack of strength when the soil is dry is indicative of granular soils. There is a wide variation in the physical properties of soils even for almost identical grain-size curves, and neither grain size nor plasticity index alone can be fully relied upon to define expansive or compressible soils. Yet, the potential for volume change in a soil due to fluctuations in the moisture content is usu- ally associated with the clay fraction and is indicated by PI. Vol- ume changes, however, can be better predicted from the results of consolidation tests on undisturbed samples. 3.0 PROBLEM SOILS A number of soils may be considered problem soils-if only because of climatic or moisture conditions-and will, therefore, require special consideration. 3.1 Loess Loess soils will fall in the ML or MH groups of the Unified Soil Classification System and should usually be treated in the same manner as other soils in this group. It is important, however, to recognize that heavily loaded slabs may settle unduly when loess becomes saturated. Proper drainage is extremely important where these soils are encountered. In residential construction the loads are generally not sufficient to cause trouble with loess-type soils except where slabs support chimneys or other heavy, concentrated loads.
140 RESO)ENTIAL SLABS ON GROUND 3.2 Highly Compressible Clays Highly compressible clays are those with sufficient moisture con- tent to have a soft or very soft consistency. Every effort must be made to avoid overloading these clays, as they may settle unduly even under residential loads. 3.3 Expansive Clays Expansive clays are problem soils because of the extent to which their activity is affected by climatic conditions. Any clay with PI greater than 10 may cause trouble by expansion under exposure to severe climatic conditions; yet this same material, under fa- vorable or intermediate conditions, may be considered satisfactory for foundation purposes. Under severe climatic conditions, clays will generate very high swelling pressures which may reach many tons per square foot. It is, therefore, impossible to control the swelling by loading with a residential slab. Where a framed slab (Type IV) is used, care is necessary to ensure that the slab and beams cannot possibly be reached by the swelling clay and that the supporting portions are anchored below the line of seasonal moisture change. In the absence of such precautions, damage can be expected whenever the soil moisture content varies appreciably. 3.4 Highly Plastic Soils It is with highly plastic soils that the designer is most likely to experience difficulty. The potential pressures generated from expansion caused by an increase in moisture are difficult to estimate The fact remains? however, that they may reach the magnitude of many tons per square foot. This is indicated by separation from bell-bottomed piles be- cause of soil expansion. Since the loads associated with residential construction are not sufficient to control expansion by loading, ex- treme care must be exercised in placing slabs on highly plastic soils. (Although there is not yet full substantiation for this view, it is believed that removal of the highly plastic soils and replacement with a coarse granular mat may equalize settlement; it is further believed that this practice would result in maintenance of more stable moisture content in the soil beneath, reducing the possibility of expansion and the consequent generation of excessive pressures.)
SUPPLEMENTARY INFORMATION 141 3.5 Sands and Silts A very loose sand or silt may cause serious trouble beneath a foundation or slab. Fine sands and silts can be especially trouble- some when loose. A damp sand may be "bulked" to such an extent that drying out or flooding can produce a decrease in volume that will cause excessive and unequal settlements. Vibration of loose sands and silts is sure to cause a decrease in volume, with result- ant rapid settlement. Gravels usually occur in a dense state and are unlikely to cause problems. If a sand or silt is encountered in the site investigation, an attempt should be made to determine whether it is dense or loose. If loose, either the sand or silt should be compacted to a dense state before building on the site, or the building should be supported on soil beneath the loose material in such a manner that volume change of the sand or silt will produce no effect on the building. 4.0 UNDISTURBED CONDITION OF SOILS 4.1 Natural Density of Sands and Silts The natural density of sands and silts is of importance. A reason- ably accurate method of determining the relative density of a cohe- sionless soil at the bottom of a bore hole has been adopted by ASTM. This test requires power-driven equipment. Its use could not nor- mally be justified for investigation of a site for a one- or two-family residence, where the test holes usually need to be only 15 feet deep, but for larger residential developments it would be justified. The resistance to penetration varies somewhat with the grain size of We cohesionless material and depends upon whether the soil lies above or below the water table. Table VI, p. 142, shows rela- tive densities of sands. 1ASTM Designation D1586-64T (or most recent edition), Penetration Test and Solit-Barrel Sampling of Soils. Philadelohia: American Societv for Testing and Materials.
142 RESIr)ENTLAL SLABS ON GROUND TABLE VI Relative Density of Sands1 Relative ASTM Method Density (No. of Blows) (%) Classification o- 4 0- 20 Very Loose 4- 10 20- 40 Loose 10- 30 40- 60 Medium 30-50 60-80 Dense Over 50 80 - 100 Very Dense 1Penetration resistance values of sands determined in accordance with ASTM Designation D158~64T. The number of blows causing the split- barrel sampler to penetrate to a depth of one foot is recorded; the sampler ::'nd driving apparatus must be in accordance with specifications. TABLE VII Consistency of Undisturbed Clay Soils Undisturbed qu Consistency (ts01 Rule-of-thumb Test Very soft 0.25 Core~eight = twice diameter) sags under own weight Soft 0.25 - 0.50 Can be pinched in two between thumb and forefinger Medium 0.50 - 1.00 Can be imprinted easily with fingers Stiff 1.00 - 2.00 Can be imprinted with consid erable pressure from fingers Very stiff 2.00 - 4.00 Can barely be imprinted by pressure from fingers Hard 4.00+ Cannot be imprinted by fingers lqu = unconfined compressive strength in tons per sq It (not the bearing capacity).
SUPPLEMENTARY INFORMATION 143 4.2 Consistency of Cohesive Soils It is also important to determine the consistency of the soil in its undisturbed stage; otherwise, the assumed behavior character- istics may not be valid. Table VII, p. 142, can be used as a guide to the consistency of clay-type soils. 5.0 NONPROBLEM SOILS Dense, coarse-grained soils which are properly drained offer few problems with respect to slabs-on-ground. By their very nature, these soils possess excellent bearing capacity, since they are not subject to volume change. The only consideration required is that of ensuring that the natural confinement of the soil is not disturbed by trenching or grading operations.
