L08-1_d_settlement - Soil Mechanics CIVL2410 Lecture 11 1...

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Soil Mechanics CIVL2410 Lecture 11 1 8. ONE DIMENSIONAL SETTLEMENT BEHAVIOUR Contents 1. One Dimensional Loading Conditions 1 2. The Oedometer 2 3. Relation of volume strain and vertical strain 3 4. Relation between volume strain and voids ratio 4 5. Behaviour of soil under one dimensional loading 4 a) Preconsolidation Stress (pressure) 5 b) Normally consolidated soils 5 c) Overconsolidated soils 6 d) Estimation of the preconsolidation stress 7 6. Idealised soil behaviour 8 7. Compression and Recompression Indexes 9 1. One Dimensional Loading Conditions Soils are often subjected to uniform loading over large areas, such as shown in Figure 1, from an embankment. Under such conditions soil which is remote from the edges of the loaded area undergoes vertical strain, but no horizontal strain. That is strains, and hence surface settlement, only occur in one-dimension. Figure 1: Embankment loading on a layered soil x z Rock Soil layer 1 Embankment Soil layer 2 The accuracy of this assumption depends on the relative dimensions of the loaded area and thickness of the soil layer. If the area is relatively large and the thickness of the soil layer relatively small then the assumption of 1-D conditions will be reasonable.
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Soil Mechanics CIVL2410 Lecture 11 2 It is possible to make approximate estimates of surface settlement using the 1-D approach even when the loaded area is not relatively large. The procedures for doing this are discussed in section 9 on the calculation of settlement. 2. The Oedometer The behaviour of soil during one-dimensional loading can be tested using a device called an oedometer, which is shown schematically in Figure 2. The one-dimensional condition in which the vertical strain, ε zz ≠ 0 , and the lateral strains, ε xx = ε yy = 0 is also referred to as confined compression. Figure 2: Schematic diagram of an oedometer Cell Loading cap Load Displacement measuring device Soil sample water Porous disks The following points may be noted: The soil is loaded under conditions of no lateral strain (expansion), as the soil fits tightly into a relatively rigid ring. Uncontrolled drainage is provided at the top and bottom of the specimen by porous discs (two way drainage). In more sophisticated oedometer apparatus control of drainage is possible. A vertical load is applied to the specimen and a record of the settlement versus time is made. The load is left on until all settlement ceases (usually 24 hours although this depends on the soil type, impermeable clays may take longer). The load is then increased (usually by a factor of 2, so the vertical stresses might be e.g. 20, 40, 80, 160 kPa). When the maximum load is reached, the soil is unloaded in several increments. If desired reloading can be carried out. At each step time-settlement records are made. The relationships between voids ratio and effective stress, and settlement and time are found
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L08-1_d_settlement - Soil Mechanics CIVL2410 Lecture 11 1...

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