Primary and Secondary Consolidation
Settlement of saturated soils under increased loading consists of two successive phases, commonly referred to as the primary and secondary consolidation phases. The primary consolidation phase is dominated by pore pressure dissipation and effective stress increase; whereas, the secondary consolidation phase is dominated by creep (viscous deformation) at almost constant effective stress. The consolidation settlement consists of two parts. In conventional soil mechanics, Terzaghi’s theory of one dimensional consolidation is widely used despite some limitations.
In order to know the compressibility behavior of soil, we can make a plot of voids ratio versus log time using one dimensional consolidation tests as shown in figure 1.
Types of consolidation tests:
Conventional incremental loading test (Oedometer test)
Constant rate of strain test (CRS)
Calculation of settlement can be made as follows:
Total settlement (S)=C_c/((1+e_0)) log〖(P_v0^'+∆P)/(P_v0^' )〗+C_α/((1+e_0)) log〖t/t_p 〗
This equation assumes that the secondary consolidation settlement occurs after the dissipation of excess pore water pressure.
One of the most practical questions concerning consolidation is to know how to define the relevant consolidation curves for in situ conditions. The existence of creep during primary consolidation is evident, but there exist opposing opinions on the role of creep in the primary consolidation phase. Many researchers pointed out that, there are two extreme possibilities. Hypothesis A assumes that creep occurs only after the end of primary consolidation and consequently that the stress strain curve followed in situ is the same as the one obtained in the laboratory at the end of primary ...
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... the land reclamation. Finally, the consolidation process is not the only one controlling the rate of settlements in clay. Even when the excess pore water pressure has completely dissipated, the settlement continues, which is called creep or secondary compression. Towards the end of primary consolidation, a contribution of the secondary compression becomes more pronounced and may produce significant increases in settlements long after the primary consolidation is over.
It is important to remember that the consolidation process is not the only one controlling the rate of the settlements in clay. Even when the excess pore water pressure has completely dissipated, different physical phenomena cause continued settlement. While this secondary compression is present from the beginning of the consolidation process, towards the end of primary consolidation its contribution
• For the spacing, a consideration should be done for the depth of compressible layer, permeability of soil and location of ground water table. As well as that when compacting for deeper layers a wider spacing should be used compared to upper layers.
This is related to the blending of the fluids and rocks of the reservoir. Skeletal properties of interest to reservoir engineers include porosity, pore size distribution, compressibility, and absolute permeability of the rod. Interaction or dynamic properties of reservoir rocks are affected by the nature and by its interaction with present fluids, as...
Desert pavements are common landforms in arid regions. They consist of flat or sloping surfaces where stones are closely packed angular or rounded, and generally exhibit low relief (Mabbutt, 1977). Pavements tend to form on both alluvial fan toposequences and on weathering volcanic flow fields in arid regions. Soils are often found under desert pavements and they play an important role in the evolution of pavements (McFadden et. al., 1987). In the past there have been several theories as to the formation pavements and soil development beneath them. Deflation, or the erosion of finer grained particles from a surface, stone concentration by wash erosion and upward displacement of stone due to shrink and swell clay characteristics were at one time believed to be the main factors in the formation of desert pavements (Mabbutt, 1977). However, more recent research has shown that desert pavements are born and maintained at the surface, and that the soil below them is mainly eolian in origin. Slow accretion of eolian dust below the pavement is a process that eventually develops cumulate horizons. Eolian dust in environments where pavements often develop is rich in carbonate salts and clays due to the fact it often originates from nearby playa lake evaporate basins (McFadden et. al., 1987). Soils that form below the pavements over time develop calcic horizons and clay rich structure due to the influx of these eolian fines through the pavement surface. In turn the development of mature or plugged calcic horizons effects the form of the pavement surface because it alters the water drainage infiltration rate and causes pavements to decline.
A reservoir is considered as a compaction drive is when the pore volume contraction takes prominently to overall expansion while the reservoir is saturated. This drive is supplemented by solution gas drive and may or may not by water/gas cap drive. This reservoir acts like their non-compaction counterparts except that they exhibit enhanced recoveries. For example, the oil recovery will be greater for a solution gas drive by which the compaction drive will act like a normal solution gas drive reservoir. This is because of the direct consequence of the extra rock expansion that compaction drive reservoirs actually have. Due to the extra compaction, some production occurs. For instance, the permeability may decline, fracture may happen and subsidence but all there problems are manageable and the result of compaction is very favorable.
Soil liquefaction describes a phenomenon whereby a saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid. The phenomenon is most often observed in saturated, loose (low density), sandy soils. This is because the loose sand has a tendency to compress when a load is applied; dense sands by contrast tend to expand in volume. If the soil is saturated by water, then water fills the gaps between soil grains. In response to the soil compressing, this water increases in pressure and attempts to flow out from the soil to zones of low pressure (usually upward towards the ground surface). However, if the loading is rapidly applied and large enough, or is repeated many times (e.g. earthquake shaking, storm wave loading) such that it does not flow out in time before the next cycle of load is applied, the water pressures may build to an extent where they exceed the contact stresses between the grains of soil that keep them in contact with each other. These contacts between grains are the means by which the weight from buildings and overlying soil layers are transferred from the ground surface to layers of soil or rock at greater depths. This loss of soil structure causes it to lose all of its strength. According to the
Recompression method was developed by Bjerrum and Berre (1973) at the Norwagian Geotechnical Institute (NGI). In this method, soil specimens are reconsolidated to in-situ effective overburden pressure before sheared under undrained condition. Bjerrum mentioned that the principle behind this technique has been the swelling of sample that occurred before testing is so small and elastic in nature that the mechanical disturbance caused by such swelling can be eliminated by reconsolidating sample exactly as that it is in the in-situ stress condition before testing. Berre and Bjerrum (1973) highlighted that the volumetric strain during recompression should be less than 1.5 to 4%. This method should be used in the case of highly structured, brittle and sensitive clay with high quality sample. It is evident that the low plastic clay samples are found to be somewhat more disturbed than that of high plastic clay samples and reduced water content due to sample disturbance would cause gain in strength. On the contrary, increasing disturbance cause a reduction in strength and an increase in strain for the reconsolidated specimen at the verge of failure. Furthermore, there is some criticism on recompression method that the decrease in void ratio due to densification measure overestimating of strength even for bad quality sample.
Dense to very dense natural and compacted foundation soils had settled a maximum of 4inches or four-tenths of a percent of the wall height. By the comparison of the
Each type of soil has its characteristics in terms of water holding capacity. The first type is sand. Sand has the largest particles, with huge spaces between them, this is why sand doesn’t have the ability to hold water. Clay has the smallest particles compared to the other type, so it has good water storage qualities. It’s sticky to the touch when wet, but smooth when dry. () Clay has many fine pores, which gives it a higher capacity to hold water, than other types of soil. Eventually, it holds a higher amount of water than sand does.
Vertisols are mainly soils that have a high content of expending clay and that have at some time of the year deep wide cracks. They shrink when drying and swell when they become wetter. Vertisols are mineral soils that have a exist in a well-balanced supply of moisture or warmer soil temperatu...
Most changes in processes of soil formation and soil erosion are indirectly affected by the presence of livestock and more directly associated with the geomorphic changes these anima...
While the material is changing from liquid to solid Contraction takes place at varying rates, which causes irregularly shaped shrinkage cavities depressions.
White, W. A. 1949. Atterberg Plastic Limits of Clay Minerals. The American Mineralogist, Vol. 34, Nos.7 & 8; Public Roads, 22, 508-512.
Surface Creep occurs when landing sand particles remove the larger and heavier particles, pushing them forward.
Hydrometer test is needed as more than 10 % of soil sample passes the 63 µ m sieve (BS 1377-2:1990). It covers the quantitative determination of the particle size distribution in a soil from coarse sand size to clay size. Particles settle under gravity during testing (Head, 1984). The results of hydrometer analysis can be referred to Appendix C1. The calibrations which used in the hydrometer analysis and water viscosity are shown in Appendix C2 and Appendix C3.
This step is followed by Mixing to break big lumps and to homogenize the softened clay mixture. The mixing could be done mechanically, by the use of some ani...