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Lecture_5_-_Compaction - CHARACTERISTICS OF SOILS Physical...

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Unformatted text preview: CHARACTERISTICS OF SOILS Physical characteristics Engineering characteristics PHYSICAL CHARACTERISTICS OF SOILS Grain size distribution Atterberg limits Compaction COMPACTION SOIL COMPACTION In the field In the lab COMPACTION Compaction is soil densification via instantaneous reduction in air void contents. The reduction in the air void contents is achieved by mechanical means at a constant moisture content. Compaction is carried out by rolling or tamping, and causes compression of the soil by expelling air from the voids. COMPACTION Soil compaction is instantaneous reduction of its air void contents Air Water Solid COMPACTION OBJECTIVES Improve the shear strength of the soil and therefore its bearing capacity Increase soil stiffness and therefore reduce its future settlement Decrease the void ratio as to reduce the soil permeability DENSITY-MOISTURE CURVE It is not possible to remove water from the voids by compaction, but the addition of water facilitates compaction by reducing surface tension. There is an optimum moisture content (OMC) above which the addition of water causes an increase in voids. Dry density Maximum dry density OMC Water content COMPACTION EFFORT Dry density versus water content 150 145 140 135 130 125 120 115 110 105 100 4 6 100% saturation curve Dry density (pcf) Line of optimum 8 10 12 14 16 18 Water content (%) COMPACTION EFFORT The effect of varying the compaction effort is to vary the maximum dry density and the optimum moisture content. Increasing compaction effort causes increases in the maximum dry density and decreases increases in the optimum moisture content. In the lab, compaction effort (CE) can be calculated as the number of drops (n) multiplied by the hammer weight (W), the height of fall (h) and the number of layers (NL) as follows: CE = (N)(W)(H)(NL) USES OF LAB COMPACTION DATA The lab compaction data are typically used to: Establish the dry density-moisture content relationship (curve) Determine the optimum moisture content and the maximum dry density of the soil at the given compaction effort Establish field specifications regarding the maximum dry density (d max) and the optimum moisture content (OMC) FIELD COMPACTION Field compaction is achieved by using one or more of the following equipment: Smooth drum rollers Sheepsfoot rollers Single or double vibratory rollers Pneumatic rollers Dynamic compaction FIELD COMPACTION Smooth wheel Roller Vibratory Roller Field Compaction Equipment Pneumatic Roller Sheepsfoot Roller FIELD COMPACTION EFFORT The field compaction effort, at any given point, is a function of the roller mass and the number of passes/coverage over the point. Higher number of passes yields higher compaction effort. Higher compaction efforts may not necessarily achieve higher dry densities. The actual number of passes must correspond to the required compaction effort as to achieve the specified maximum dry density when the actual water content of the soil is around the optimum moisture content. FIELD COMPACTION - Specifications Two specification methods may be used; end product specifications and method specifications. The end product specifications are based on the lab maximum dry density and optimum moisture content data. Such specifications may state that the field dry density must be 90 percent or better of the lab maximum dry density and the field moisture content must be equal to the lab optimum moisture content + 1 percent. The method specifications addresses the type of roller to be used, the number of passes and the range of acceptable water content. FIELD COMPACTION After compaction is completed, a portable nuclear soil density meter is typically used to determine the soil density and moisture content. The engineer on site uses this information to determine the percent relative compaction (RC) and to determine whether or not the contractor has met specifications. RC (%) = field dry density/max lab dry density OVER COMPACTION In certain scenarios, it is possible to over compact the soil using higher compaction effort as detailed below. Point A represents the maximum dry density and the corresponding OMC If the soil being compacted is dry of optimum (point B) and if the compaction effort increased using higher number of passes or heavier roller, the soil will attain the dry density shown by point C, which is wet of optimum relative to point D. Although the dry density at C is higher than that at A, the soil will be much softer. DYNAMIC COMPACTION Dynamic compaction is a ground improvement process for compacting and strengthening deep deposits of loose or soft soils to support buildings, roadways, and other heavy construction. The method involves the systematic dropping of heavy weights, 100 to 400kN, from a height of 5 to 30m, in a pattern designed to remedy poor soil conditions at the proposed building site. DEEP COMPACTION - sequence of drops The sequence of drops should be designed as to overlap the pressure bulb above the top of the layer being compacted as shown below Deep layer Intermediate layer Shallow layer ...
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