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ACEG210 Settlement.Pdf Compression of soil ACEG210 SOIL MECHANICS I An increase in stress occurs in soil because of, for example, a geological deposit of Consolidation and settlement new soil or the construction of a new building (Figure 2). The increase in stress causes the soil to deform. The soil is compressed and reduces in volume. Objective This lecture will explain how soils deform because of changes in effective stress. An Geological deposit new of new soil understanding of soil compressibility allows us to predict the settlement of building foundations. soil element soil element Introduction If you apply load to any part of a structure (beam, column, etc.) it will deform or deformation of deflect. This is because you put stress into the material. The amount of deformation soil element or deflection is determined by the stiffness E of the material. The stiffness of steel is about 200x106kN/m2. Figure 2. Stress increase and compression of soil The same happens with soil. A new structure will load the soil under the foundations. When the volume of a soil changes, it is only the volume of the voids that changes. This causes a change of stress in the soil (which you learned to calculate in the last The soil particles do not change their volume. If there is water in the voids, the water lecture) and therefore deformation of the soil determined by the stiffness E of the soil. needs to flow out of the voids to allow them to reduce in volume. The deformation of the soil will cause settlement of the structure (the structure will move down). It is very important that the foundations are designed so that the When there is an increase in total stress in soil, such as under a foundation (Figure settlement is acceptable (less than 20mm for example) otherwise the structure may 3) this is initially supported by the pore pressure. This is because water is get damaged (Figure 1). incompressible. This creates an area of high pore pressure under the foundation which is surrounded by lower pore pressure. Since water flows from high pressure to A rising water table increases pore pressure and reduces effective stress (as you low pressure, water flows out of the voids under the foundation. This allows the soil learned in the previous lecture). This can cause heave of foundations which is the to reduce in volume and the foundation to settle. opposite of settlement. The first stage of predicting settlement or heave is to estimate the stiffness E of the building 3 new soil. Compared with the stiffness of steel, this can be about 50x10 kPa for sand and building settles as low as 5x103kPa for clay – about 40 000 times less stiff than steel. in-situ soil stresses high pore water settlement heave crack – pressure flows out crack – wider at bottom pore pressure of voids wider at top hydrostatic Weight of building supported by pore Weight of building pressure. supported by effective ∆σv = ∆u stress. ∆σ = ∆σ′ ∆u = 0 ∆σ′v = 0 v v settlement heave Figure 3. Compression of soil with voids filled with water Sands and gravels have high permeability, so the flow of water and compression of Figure 1. Damage to buildings due to settlement and heave the soil will happen quickly (in minutes). Silts have lower permeability, so compression and settlement of silts takes longer (perhaps days or weeks). Remember that clays have a permeability at least a billion times lower than gravels, so compression and settlement on clays can take years. The time-related process of soil deformation due to the slow flow of water from high to low pore pressure in silts and clays is called consolidation. The word compression is used to describe the more general change in volume of soils due to changes in effective stress without reference to the time over which the volume change happens. 1 2 One-dimensional compression Stiffness E′0 of silts and clays (the oedometer test) Since settlement and heave are in the vertical direction, to predict these we are only The measurement of E′0 of silts and clays in-situ is more difficult. Since complete interested in compression of the soil in the vertical direction. Any soil deformation in compression (or consolidation) of silts and clays can take months or even years, to horizontal directions will have a very small effect on settlement so we can ignore carry out a good test for stiffness E′0 would also take months or years. Very few these. Therefore, in soil mechanics the term one-dimensional compression is used projects have the time or money for tests that take years to complete! when analysing deformation of soil to predict settlement and heave. These tests are therefore carried out on small samples of silt or clay in the laboratory. Calculation of settlement Small samples consolidate more quickly because the water in the voids does not Where there are layers of different soil under a building, as shown in Figure 4, we need to flow very far. These tests are carried out in an oedometer. calculate the settlement of each soil layer separately. Imagine we are analysing the settlement in the soft clay layer. Oedometer test A sample of clay or silt, usually 75mm diameter and 20mm high is contained in a 1. We calculate the average in-situ effective vertical stress in the layer σ′v before steel ring (Figure 5). The steel ring is very stiff and stops the clay expanding in the the structure is built. horizontal direction. We only want settlement or heave in the vertical direction (one- 2. We calculate the average change in effective vertical stress in this layer ∆σ′v dimensional consolidation). due to the weight of the structure. You learned how to calculate these in the last lecture (using the Fadum chart). Permeable flat stones are placed on each side of the sample which allow stresses to 3. Use the one-dimensional stiffness E0′ of the soil layer to find the settlement of be applied to the sample while at the same time allowing water to enter of leave the the soil layer ∆h = h0 ∆σ′v/ E0′ (where h0 = original height of layer). sample. The sample, ring and permeable stones are placed in water to ensure the soil is saturated. building surcharge vertical load ∆h Sample Sample diameter Made ground steel height 75mm ring Soft clay. Stiffness E′0 20mm h0 in-situ σ′v + ∆σ′v soil sample in-situ σ′v Dense sand permeable stones free water ∆σ v′ ∆h = h0 = settlement of soft clay layer Soil Sample Oedometer test E0′ Figure 4. Calculation of settlement in one layer of soil Figure 5. Oedometer test To calculate the total settlement of the building we add the settlement of each soil Different stresses are applied to the sample and the settlement or heave of the layer. So we do the calculation shown above for each soil layer, calculating the sample is measured. The first stresses you apply are to get the same in-situ stresses average in-situ stress and average change in stress in each layer and using the in the sample as in the real layer of soil. After this, you then apply the changes of stiffness value E′0 of each soil to calculate the settlement of the soil layer. stress caused by your building, etc. We now need to learn how to calculate the stiffness value E′0 of a soil. The soil does not finish its settlement or heave immediately. Because the soil has low permeability, it takes time for the water to leave the voids (settlement) or enter the Stiffness E′0 of sands and gravels voids (heave). If you measure the settlement of the sample with time after you apply a stress, you get a shape like that shown in Figure 6. Settlement is usually drawn Since the compression of sands and gravels is quick, the stiffness of these soils can downwards on graphs. easily be measured in-situ. This can be done with tests carried out in boreholes (such Time, t as standard penetration test, pressuremeter, plate bearing test) or on the soil surface (such as a large plate bearing test). Stiffness E′0 of sands and gravels can also be estimated from their density and particle sizes. Settlement, ∆h equilibrium Figure 6. Settlement with time for one application of load 3 4 For the design of buildings on silt or clay you need to determine two things: 10mm. In a 4m thick layer of clay the water needs to flow a maximum distance of 1) how much will the building settle (or heave)? either 2m (‘two-way drainage’) or 4m (‘one-way drainage’). It will therefore take 2) how long will it take? longer for the 4m thick layer to complete settlement than for the 20mm thick oedometer sample. The maximum distance water needs to flow to leave a soil layer Both of these can be calculated from the oedometer results. is called the maximum drainage path length. How much will the building settle (or heave)? The time taken will also depend on the permeability of the soil. The permeability of As explained earlier in this lecture, you need the stiffness E′0 of the silt or clay. the soil will change as the voids get smaller during consolidation. So the time taken will change as the stress level changes. If you have the same soil in the oedometer ∆σ ′h test as exists under your new building AND the stress is the same, then the From the oedometer test E′ = v 0 0 ∆h permeability should be the same in both.
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