19-05-2012, 05:28 PM
Soil Sub-Grade Modulus
Soil Sub-Grade Modulus.pdf (Size: 19.01 KB / Downloads: 283)
Subgrade-Subbase Strength
Soil bearing capacity, soil compressibility, and soil modulus of subgrade reaction arevarious measures of strength-deformation properties of soil. It is important to considerhow these parameters apply to the design of floor slabs.The allowable soil bearing capacity is the maximum pressure that can be permitted onfoundation soil with adequate safety against soil rupture or excessive settlement.
Allowable soil pressure may be based on:
Laboratory shear strength tests (of soil samples) such as the direct shear test,
triaxial compression test, or unconfined compression test
Field tests such as the standard penetration test or cone penetrometer test
Soil classification
Moisture-density-strength relationships (established by conducting strength testson soil specimens prepared for moisture-density testing)
Beyond the allowable soil pressure is the ultimate bearing capacity, the load per unitarea (soil pressure) that will reduce failure by rupture of a supporting soil.Another soil characteristic, compressibility of cohesive soils, determines the amount oflong-term settlement under load. The usual method for predicting settlement is basedon conducting soil consolidation tests and determining the compression index for use in
the settlement computations. The compression index may be estimated by correlation tothe liquid limit of the soil.A third measure of soil strength, Westergaard’s modulus of subgrade reaction, k, iscommonly used in design procedures for concrete pavements and floors-on-grade thatare not structural elements in the building (floors not supporting columns and loadbearingwalls).There is no reliable correlation between the three measures of soil properties-modulusof subgrade reaction, soil bearing capacity, and soil compressibility-because they are
measurements of entirely different characteristics of a soil. The k-value used for floorslabdesign reflects the response of the subgrade under temporary (elastic) conditionsand small deflections, usually 1.25 mm (0.05 in.) or less. Soil compressibility andbearing capacity values (normally used to predict and limit differential settlements
between footings or parts of a foundation) reflect total permanent (inelastic) subgradedeformations that may be 20 to 40 (or more) times greater than the small deflections onwhich k-valuesarebased.Substantial pavement research has shown that elastic deflections and stresses of theslab can be predicted reasonably well when using k-value to represent the subgraderesponse. Consequently, the control of slab stresses based on the subgrade k-value isa valid design procedure.
Although the k-value does not reflect the effect of compressible soil layers at somedepth in the subgrade, it is the correct factor to use in design for wheel loads and otherconcentrated loads because soil pressures under a slab of adequate thickness are not
excessive. However, if heavy distributed loads will be applied to the floor, the allowablesoil pressure and the amount of settlement should be estimated to determine if shearfailure or excessive settlement might occur.If there are no unusually adverse soil conditions, the design analysis requires only thedetermination of the strength of the subgrade in terms of k. The k-value is measured byplate-loading tests taken on top of the compacted subgrade (or subbase, if used). Ageneral procedure for load testing is given in ASTM D 1196, Standard Test Method forNonrepetitive Static Plate Load Tests of Soils and Flexible Pavement Components, forUse in Evaluation and Design of Airport and Highway Pavements. This method providesguidance in the field determination of subgrade modulus with various plate diameters.
Design of Slabs on Grade (ACI 360R) is specifically oriented to the determination ofmodulus of subgrade reaction using a 760-mm (30-in.) diameter plate and gives moredetailed information on test methods using this size plate. This plate is loaded to adeflection not greater than 1.25 mm (0.05 in.), and the k-value is computed by dividingthe unit load by the deflection obtained. A more economical test using smaller plates(300 mm [12 in.]) that determines a modified subgrade reaction modulus is mentioned inACI 360R. In each case, the units of k are given in pressure per length: MPa/m in themetric system, or in in.-lb units, pounds per cubic square inch, or psi per in. or, ascommonly expressed, pounds per cubic inch (pci). The plate load test is no longer
commonly run in practice. Instead, subgrade reaction values are estimated from theCalifornia Bearing Ratio or from the soil classification.
If a high-quality, well-compacted granular subbase is used under the floor slab, the kvaluewill increase. On large projects it may be feasible to construct a test section andperform plate load tests on top of the subbase. Typical values of subgrade modulusrange between 100-400 pci for coarse grained soils and 25-150 pci for fine grainedoils.
Subbases
A subbase—the layer of granular material placed on top of prepared subgrade—is notmandatory for floors on ground. A granular subbase, however, can provide benefitsduring the construction process and afterwards (to the completed floor). Duringconstruction, the subbase functions as a stable work platform for heavy equipment.When grading and compaction operations do not produce a uniform subgrade, a
granular subbase will provide a cushion for more uniform slab support by equalizingminor subgrade defects. The cushioning effect and increased uniformity can be veryimportant for cohesive soils that are susceptible to reduced bearing support withincreases in moisture content. A subbase also serves as a capillary break, reducing
moisture migration towards the bottom of the completed slab. A granular subbase canalso serve as a collection layer for radon gas.
Since uniform rather than strong support is the most important function of the subgradeand subbase for a concrete floor, it follows that floor strength is achieved mosteconomically by building strength in to the concrete slab itself-with optimum use of lowcostmaterials under the slab.This article is excerpted from the PCA publication Concrete Floors on Ground(EB075.03). For full treatment of the subject the reader is advised to consult the originaltext of the publication.