Coefficient of Consolidation

Soil samples are generally taken vertically consolidation samples are taken from a horizontal slice. The coefficient of consolidation from such a test is based on vertical flow, which is applicable for much field work. (For projects using, for example, vertical sand drains, much of the flow is in the... 

Drains are placed in a square or triangular pattern. Empirical relationships have been developed to design the size and spacing of drains. Values for the diameter and the distances between drains are selected. Then, using the appropriate value of the coefficient of consolidation and the time factor for the desired percent of consolidation, the time can be computed. If this is not satisfactory, the process is repeated with different selected values. The design process is illustrated in the example which follows ... 

Disturbance and remolding occurs to some extent both with sand drains and wicks. It is probable that the coefficient of consolidation in the remolded zones is somewhere between the vertical and horizontal values. This will result in actual consolidation times greater than those gotten from computations using Ch. 

The Standard recommends plotting of the deformation of the specimen (as thickness of the specimen, as strain or as void ratio) as ordinate on a linear scale and the corresponding applied pressure in kN/m2 as abscissa on a logarithmic scale. From this, the coefficient of volume compressibility or the coefficient of consolidation may be evaluated as specified in the Standard. 

The deformations of the seabed under load have not been measured with sufficient accuracy for determination of either compressibility or coefficient of consolidation because of the limited number of tests that have been carried out. However, the results of tests in a dense sand and using bearing capacity theory have been used to back-calculate a friction angle of about 42 as shown in Figure 4.32 (Andresen et al., 1979). 

Permeability can also be determined from the normal consolidation test. The permeability of a saturated sample undergoing consolidation under a load is used to model the dissipation of pore pressure with time (Terzaghi, 1925,1943). The coefficient of permeability (k) is a function of the coefficients of consolidation (Q) and volume compressibility (mj which arise from one-dimensional consolidation theory. 

A relationship of the form E = aqc, where is the cone tip resistance, has also been suggested. For very dense sand, a is of the order of 3-5. The time history of consolidation settlements is computed using the coefficient of consolidation, Cy, as determined from oedometer or triaxial tests. For normally consolidated sand it is of the order of 1.5-2.0. 

Pre-liquefaction coefficient of consolidation (c ) values for each specimen were calculated based on measured hydraulic conductivity (k) and volume compressibility (m ) data for virgin loading shown in Figure 4a. Post-liquefaction c, values were back-calculated using time histories of pore pressure dissipation and volume change measurements obtained during post-liquefaction dissipation tests for each incremental change in effective stresses. Back-calculations of c were... 

Effect of Silt Content. Silt content has a significant influence on c. At the same confining stress, the c values are smaller for silty soils by more than one to two orders of magnitude compared to sand (Figure 8). Figure 9 illustrates this more clearly. It shows the change in normalized coefficient of consolidation (c )/(c )q for Ottawa sand silt mixes, where, (c ) is the coefficient of consolidation value for clean Ottawa sand, and c =coefificient of consolidation of Ottawa sand-silt... 

Post-liquefaction coefficient of consolidation of a soil is similar to that of a freshly deposited soil at the same void ratio. It is highly influenced by silt content. The influence of silt content on c is significant through its effect on hydraulic conductivity than through its effect on volume compressibility. Both parameters influence c. It is also highly stress-dependent. 

The unconfined compressive strength of chalk ranges from moderately weak (much of the Upper Chalk) to moderately strong (much of the Lower Chalk of Yorkshire and the Middle Chalk of Norfolk). However, the unconfined compressive strength of chalk undergoes a marked reduction when it is saturated (Bell et al., 1999). For instance, the Upper Chalk from Kent may suffer a loss on saturation amounting to approximately 70%. Chalk compresses elastically up to a critical pressure, the apparent preconsolidation pressure. Marked breakdown and substantial consolidation occurs at higher pressures. The coefficients of consolidation, Cy, and volume compressibility, triy, are around 1135 m a- and 0.019 m MN- respectively. 

Sills, G.C. and Hird, C.C. 2005. Coefficient of consolidation from piezocone measurements. Geotechnique, 55, 597-602. 

The thickness of soft soil in Lianyungang is 6-15 m, there have a generally soft soil with high plasticity, high water content, high compressibility (fli.j = 0.9-4.0 MPa" ), high void ratio, and low-intensity ( = 2.33-5.07 kPa) and so on, but also has a uniform deposition of soil, poor permeability coefficient of consolidation (C = 2.2 X 10" -4.7 X 10" cmVs), sensitivity high (iSt = 2-6, 12), characteristics of the physical... 

In general, with respect to the conventional consolidation experiments, the coefficient of secondary consolidation of soft soil needed to have a certain amount of experience to the appropriate coefficient accurately analyzed. Commonly used approach is to estimate the size of the secondary consolidation coefficient with easy conventional experimental determination of the coefficient of consolidation. 

In the absence of dilation, the time required for consolidation to be complete is a function of the permeability k) and compressibility of the ground (m and of the path-length (/ ), which is the distance water has to travel as the transient pressures dissipate. The compressibility and permeability terms can be combined with the weight density of water ()/ ) to give the coefficient of consolidation Cy = klm, yw The time required for essentially complete consolidation is given by h lc. ... 

In the past an attempt was also made to construct a consolidation curve from increments applied by different teams. The experience was not successful, but it is believed that inadequate communication of the results was then the root of the problem. Nowadays all interaction of the course is made by means of a Moodle interface and students populate a database with the results of their experiments, so there is renewed hope of including consolidation - undoubtedly a most relevant topic -into the laboratory-based learning process. For time being (2011) each team has just been asked to apply one load increment (see Table 3 for scheduled sequence of load increments), and calculate the coefficient of consolidation for that increment. The good results of 2011 suggest that an attempt at construction of the full compression curve may be justified for 2012. Figure 6 summarizes student opinion about several aspects of Experiment C. 

A coefficient of volume compressibility or coefficient of consolidation can be calculated from the thickness, volume or void ratio versus logarithmic applied stress relationship. 

In natural sediments the horizontal permeability can be up to three times larger than the vertical permeability. The same ratio may apply for the horizontal and vertical coefficient of consolidation. However, when using PVDs caution must be taken in using a larger horizontal coefficient of consolidation The closer PVDs are installed to each other, the more the effect of smear will become important and the positive effect of a larger horizontal coefficient of consolidation might be diminished. Imanshi 2000 even reported that in case of close PVD spacings, the horizontal permeability became equal or smaller than the vertical permeability. 

It is recommended to use the formulas taking into account smear effects and to use the correct ratio between the horizontal and vertical coefficient of consolidation. This means that the horizontal permeability needs to be defined by means of laboratory or field testing. 

The scenario shown in Fig. 13.19(a) can be approximated as in Fig. 13.19(b) with a coefficient of consolidation c = c/fp for the soil. A vacuum pressure is applied at the GCD level. This approximation may overestimate the average vacuum pressure... 

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