Secondary consolidant

The rate at which the pore fluid is expelled is related to the length of the sample and the pore size. A typical graph of the rate of settlement, and, therefore, pore fluid extraction after the addition of a load (Fig. 10.12) shows both primary and secondary consolidation. 

Most of the excess pore pressure dissipates during primary consolidation. Secondary consolidation involves the movement of particles as they adjust to the increase in effective pressure and the dissipation of excess pore pressure from very small pores. The pore water extracted during squeezing is mainly because of primary consolidation. 

Selection of the primary consolidant is based on the materials it will interact with and the method of introduction. It may be selected for high reversibility, or to isolate a less reversible secondary consolidant selected for strength enhancement, or it may be selected for its ability to integrate extremely disintegrated fabric. 

Selection of a secondary consolidant or interfacing material is based on the properties that are to be imparted. The secondary consolidant may be the final consolidant, or it may provide an interface to a tertiary component. In the case of an external reinforcement matrix, load is transferred to external components, and adulteration of the intrinsic fabric is thereby reduced. 

Compaction of Holocene sediments includes dewatering of sediments (primary consolidation), rearrangement of mineral structure of the sediment and subsequent loss of volume (secondary consolidation), and the decomposition of organic matter in the sediment. This is the most important process affecting subsidence in Louisiana coastal areas however, it is a very difficult process to evaluate. 

Shalliker, R. A., Broyles, B. S., and Guiochon, G, Evaluation of the secondary consolidation of columns for liquid chromatography by ultrasonic irradiation. Journal of Chromatography, A 2000, 878, 153-163. 

Void ratio-effective stress Coef. of consolidation (cj Coef. of secondary consolidation (CJ Permeability (k) Coef. of compressibility m ... 

Volume change resulting in settlement of a sediment under load is the result of three interacting processes immediate or elastic compression (Sj), primary consolidation settlement (sj, and secondary consolidation settlement (sj. The total settlement (St) from these three factors is a combination as given in the following equation ... 

Discussion of immediate or elastic settlement is covered in most texts on soil mechanics such as Holtz and Kovacs (1981). In the following, only primary and secondary consolidation will be considered. 

Primary consolidation is a hydraulic phenomenon that is controlled by the rate at which water can escape from the sediments interconnected pore spaces. Secondary consolidation, by contrast, is controlled by the rate at which the soil structure can deform. This is sometimes referred to as volumefric creep. 

During secondary consolidation, the relationship between the void ratio and the log of time is usually linear line sloping downward over the range of times of interest. There is no... 

Using the results of secondary consolidation an estimate of the effect of age on NC clay can be determined. This is shown in Figure 8.19 assuming linearity between void ratio (e) and the log of vertical pressure (a ). A review of this figure shows that at a constant vertical pressure, a clay material undergoing secondary consolidation will exhibit a decreasing equilibrium void ratio with time. The void ratio of the clay will therefore depend not only on the type of material loading but also on its undisturbed age. 

Bjerrum (1972) presented Figure 8.19 in which he proposed that the secondary consolidation over a protracted period of time results in an apparent overconsolidation. In the example that he showed, the apparent overconsolidation would therefore be the difference between and as shown in the following equation ... 

Inorganic cementation phenomena Oh—Biological cementation phenomena Op—Laboratory testing artifact O —Secondary consolidation effect... 

The coiq)lexity of such equations follows the attempts to describe the primary consolidation where the total voidage e dependmt on local con res e pressure and a secondary consolidation, the so-called creep effect. 

If the organic content of a soil exceeds 20% by weight, consolidation becomes increasingly dominated by the behaviour of the organic material (Berry and Poskitt, 1972). For example, on loading, peat undergoes a decrease in permeability of several orders of magnitude. Moreover, residual pore water pressure affects primary consolidation, and considerable secondary consolidation further complicates settlement prediction. 

Settlement can present a problem in clayey soils, so that the amount that is likely to take place when they are loaded needs to be determined. Settlement invariably continues after the construction period, often for several years. Immediate or elastic settlement is that which occurs under constant-volume (undrained) conditions when clay deforms to accommodate the imposed shear stresses. Primary consolidation in clay takes place due to the void space being gradually reduced as the pore water and/or air are expelled therefrom on loading. The rate at which this occurs depends on the rate at which the excess pore water pressure, induced by a structural load, is dissipated, thereby allowing the structure to be supported entirely by the soil skeleton. Consequently, the permeability of the clay is all important. After sufficient time has elapsed, excess pore water pressures approach zero, but a deposit of clay may continue to decrease in volume. This is referred to as secondary consolidation and involves compression of the soil fabric. 

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. 

Mesri (1977) summarized the test results of the 22 kinds of secondary consolidation of clay for the same kind of undisturbed soil, the coefficient of secondary consolidation and compression index ratio CJC is a constant. Mesri (1987) after a large number of engineering practices, it is recommended that the ratio ranges from 0.02 to 0.07 between... 

Zhang Junhui, Miao Linchang, Huang Xiaoming. Study on secondary consolidation deformation of soft clay. Journal of Hydraulic Engineering, 2005, (1) 1-5 (In Chinese). 

Sui, WH. Zhang, G.L. et al. 1994. High pressure consolidation and secondary consolidation characteristics of deep soil layers in a coal mining area. Coal Geology Exploration 22(2) 37-40. 

The secondary consolidation settlement (s ) occurs after the completion of the primary settlement (i.e., after the complete dissipation of the generated pore water pressure) due mainly to the reorientation of the soil particles under essentially constant effective vertical stress. 

We have applied the MD/HA method to bentonite, and have uncovered the mechanisms involved in the extremely low permeability, delayed diffusion and secondary consolidation (Ichikawa et al. 2002, 2004) water molecules close to the clay mineral surface are strongly constrained due to the charged state of the mineral, and the clay minerals themselves form a nano-order of stacked structure. 

Results for the long-term consolidation are shown in Fig. 10.2, which indicates that a secondary consolidation process takes place and then, after about 100 days, a tertiary consolidation. 

This result is shown in Fig. 10.6. We understand that the permeability obtained by the MD/HD procedure is different from the experimental data at these values of low void ratio. However, as we will discuss later, if we use both sets of data for the ID consolidation analysis, the results in both cases are similar, and we cannot reproduce the secondary consolidation response. Conducting permeability experiments on these low permeable materials is extremely difficult. Thus we cannot conclude that the MD/HD results are incompatible with the experimental results. 

In soil mechanics it is common practice to adopt a viscous model to describe the process of secondary consolidation (see Mitchell 1993) however, by using an MD simulation it can be shown that a smectite clay stack does not possess a high-enough viscosity to account for the secondary consolidation process (Ichikawa et al. 2004). Therefore, it is necessary to consider an alternative mechanism to represent the secondary consolidation. 

From the results shown in Fig. 10.9, it can be seen that for the case of a constant permeability k = ko for all time-steps and in all regions of the specimen (dashed line) the consolidation is terminated at an early stage. If the permeabilities given by the MD/HA simulation (line with o) and the results of Pusch 1994 (line with x) are used, the consolidation terminates later. However, none of these cases correctly represents the secondary consolidation. 

We assume that at the early stage of consolidation, water is drained from the space external to the stacks in the specimen of bentonite at the intermediate stage, the water is drained from both the external space and the interlayer space, and at the final stage, the water is drained from the interlayer space. Bentonite has a double space structure and shows secondary consolidation behavior because of this structure. From this point of view it is clear that even for permeability changes over time, the secondary consolidation behavior is not represented in these analyses because of the homogeneous permeability in the specimen. 

The result obtained using this model with non-homogeneous permeability in the specimen is shown in Fig. 10.10, which represents the secondary consolidation of bentonite more accurately. 

Fung YC (1969) A first course in continuum mechanics. Prentice-Hall, Englewood Qiffs Gibson RE, Lo KY (1961) A theory of consolidation for soils exhibiting secondary consolidation. 

Schiffman RL, Ladd CC, Chen ATE (1964) The secondary consolidation of clay. Rensselaer Polytechnic Institute, Troy... 

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