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Title: Foundation precompression with vertical sand drains
Authors: Johnson, Stanley J.
Keywords: Sand drains
Soil stabilization
Soil compaction
Soil mechanics
Publisher: U.S. Army Engineer Waterways Experiment Station.
Engineer Research and Development Center (U.S.)
Description: Miscellaneous Paper
Introduction: The use of preload fills and other means to precompress soils in advance of construction of permanent facilities is a relatively inexpensive but effective method for improving poor foundation soils. The benefits of precompression are to increase shear strength and to decrease postconstruction settlements to tolerable values. Temporary surcharge loading does not accelerate the rate of consolidation; instead, it achieves essentially the same degree of consolidation under a higher load, which represents, in effect, a higher degree of consolidation under the reduced load remaining when the surcharge load is removed. Temporary surcharge loading makes it possible to eliminate, in advance of construction of structures or paving of roadways or runways, subsoil settlement that would otherwise occur subsequent to completion of construction. The amount of settlement that is generally eliminated is 100 percent of primary consolidation plus such amount of secondary compression as is required to reduce postconstruction secondary compression settlements to tolerable values. Precompression through use of surcharge loads only is economical and simple; however, many compressible subsoil deposits are sufficiently thick or impermeable so that consolidation occurs slowly, and preload fills required to precompress the soil within the time available may become so high that they become uneconomical or, because of possible foundation instability, require large and costly berms. When this happens, it may be attractive to install artificial internal drainage channels in the poor subsoils to accelerate the rate of consolidation. Permeable sand or silt layers often are found within the poor soil so that additional drainage paths are not required. Unfortunately, these natural and highly effective drainage layers are often overlooked when they are thin, especially when continuously sampled borings are not made. However, even if continuous sand or silt layers are found, they may be so thin or have such a low permeability that head losses in them become excessive if the drainage path is long. Further, it may be difficult, during the design stage, to predict their effectiveness with reasonable effort. Where the effectiveness of intermediate sand or silt layers is in doubt or when such layers do not exist, positive means for accelerating drainage may be desirable. One means for doing this consists of installing vertical sand drains. Since vertical sand drains cost money, they obviously should be installed only where subsoil studies show them to be required and cost studies have established their superiority over other construction alternatives. The use of vertical sand drains is, in effect, a detail incidental to use of the precompression technique for improving properties of compressible subsoils, arid factors involved in precompression apply also where sand drains are used.
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Miscellaneous Paper

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