Consolidation Under a Constant Load—Primary Consolidation
Description :
Let us now open the valve and allow the initial excess porewater to drain. The total volume of soil at time t1 decreases by the amount of excess porewater that drains from it, as indicated by the change in volume of water in the burette (Figure ). At the top and bottom of the soil sample, the excess porewater pressure is zero because these are the drainage boundaries. The decrease of initial excess porewater pressure at the middle of the soil (position C) is the slowest because a water particle must travel from the middle of the soil to either the top or bottom boundary to exit the system.
You may have noticed that the settlement of the soil (Dz) with time t (Figure ) is not linear. Most of the settlement occurs shortly after the valve is opened. The rate of settlement, Dz/t, is also much faster soon after the valve is opened compared with later times. Before the valve is opened, an initial hydraulic head, Duo/gw, is created by the applied vertical stress. When the valve is opened, the initial excess porewater is forced out of the soil by this initial hydraulic head. With time, the initial hydraulic head decreases and, consequently, smaller amounts of excess porewater are forced out. An analogy can be drawn with a pipe containing pressurized water that is ruptured. A large volume of water gushes out as soon as the pipe is ruptured, but soon after, the fl ow becomes substantially reduced. We will call the initial settlement response soon after the valve is opened the early time response, or primary consolidation. Primary consolidation is the change in volume of the soil caused by the expulsion of water from the voids and the transfer of load from the excess porewater pressure to the soil particles.
Secondary Compression:
Theoretically, primary consolidation ends when Duo 5 0. The later time settlement response is called secondary compression, or creep. Secondary compression is the change in volume of a fi ne-grained soil caused by the adjustment of the soil fabric (internal structure) after primary consolidation has been completed. The term “consolidation” is reserved for the process in which settlement of a soil occurs from changes in effective stresses resulting from decreases in excess porewater pressure. The rate of settlement from secondary compression is very slow compared with that from primary consolidation. We have separated primary consolidation and secondary compression. In reality, the distinction is not clear because secondary compression occurs as part of the primary consolidation phase, especially in soft clays. The mechanics of consolidation is still not fully understood, and to make estimates of settlement it is convenient to separate primary consolidation and secondary compression.
Drainage Path:
The distance of the longest vertical path taken by a particle to exit the soil is called the length of the drainage path. Because we allowed the soil to drain on the top and bottom faces (double drainage), the length of the drainage path, Hdr, is
'
where Hav is the average height and Ho and Hf are the initial and fi nal heights, respectively, under the current loading. If drainage is permitted from only one face of the soil, then Hdr 5 Hav. Shorter drainage paths will cause the soil to complete its settlement in a shorter time than a longer drainage path. You will see later that, for single drainage, our soil sample will take four times longer to reach a particular settlement than for double drainage.
Rate of Consolidation:
The rate of consolidation for a homogeneous soil depends on the soil’s hydraulic conductivity (permeability), the thickness, and the length of the drainage path. A soil with a hydraulic conductivity lower than that of our current soil will take longer to drain the initial excess porewater, and settlement will proceed at a slower rate.
Effective Stress Changes:
Since the applied vertical stress (total stress) remains constant, then according to the principle of effective stress (Ds9z 5 Dsz 2 Du), any reduction of the initial excess porewater pressure must be balanced by a corresponding increase in vertical effective stress. Increases in vertical effective stresses lead to soil settlement caused by changes to the soil fabric. As time increases, the initial excess porewater continues to dissipate and the soil continues to settle (Figure ). After some time, usually within 24 hours for many small soil samples tested in the laboratory, the initial excess porewater pressure in the middle of the soil reduces to approximately zero, and the rate of decrease of the volume of the soil becomes very small. Since the initial excess porewater pressure becomes zero, then, from the principle of effective stress, all of the applied vertical stress is transferred to the soil; that is, the vertical effective stress is equal to the vertical total stress (Ds9z 5 Dsz).