Primary 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. 

Matrix Formation. During this phase, a continuous matrix is formed as the consolidant solidifies. Properties of that matrix, such as elasticity, ability to recover from distortion, and stability, are critical as they are imparted to the consolidated section of the artifact. In simple consolidation, the matrix consists of homogeneous regions of consolidant. In integrated consolidation, a primary matrix is formed by the primary consolidant. In addition, an ancillary matrix is usually made from other materials selected for properties relative to the artifact s dysfunction. 

Appropriate adhesive and cohesive qualities are needed to effectively reattach degraded sections within the degraded structure. The consolidant must be dimensionally stable as it sets. Expansion and shrinkage of the consolidant can induce considerable deformation of the shape of a degraded object. Anti-shrink-effect (ASE) properties are highly useful in determining the effect the primary consolidant will have on the dissociated tissue. 

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. 

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. 

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 ... 

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. 

Simple consolidation fheory assumes a unique relationship befween the void ratio and the effective sfress. Primary consolidation ends when excess hydrosfatic pressures wifhin a sedimenf layer has been dissipated. Mosf sedimenfs continue to undergo compression after fhis point This continued compression is called secondary compression which is caused by displacemenf physicochemical behavior of fhe soil. 

Gmax )jooo IS the value of Gmax af a time of 1000 min after the end of primary consolidation... 

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. 

Consolidation of silt is influenced by grain size, particularly the size of the clay fraction, porosity and natural moisture content. Primary consolidation may account for over 75% of total consolidation. In addition, settlement may continue for several months after construction is completed because the rate at which water can drain from the voids under the influence of applied stress is slow. 

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 order to understand better the proposed approach, the example of the oedometer test is again explored. Traditional practice is to structure the laboratory session so that a student group undertakes the experiment, in essence, several times to develop a consolidation curve. Each point on the curve is obtained by applying a load to a soil specimen and recording the settlement over a period of around 30 minutes. Many clays often require a much longer period of time to consolidate and specially selected or amended clay is needed to achieve primary consolidation within 30 minutes. Usually, 6 to 8 points are needed to generate a representative consolidation curve. Hence, the time needed in the laboratory can be quite extensive and the measurement process itself is extremely dull and tedious. 

The majority of the settlement in saturated fine-grained soils, in particular in very soft to medium stiff clays, is time dependent. For computational purposes, this time-dependent settlement can be divided into two components primary consolidation settlement (sj and secondary settlement (sj. 

The primary consolidation settlement (sj occurs due to the slow expulsion of the pore water and hence the dissipation of the excess pore water pressure generated by the application of the static service loads at the rates normally expected of permanent loads. As the excess pore water pressure dissipates, the net effective vertical stress on the soil increases as the primary consolidation continues to occur. 

The total amount of settlement due to primary consolidation can be estimated using Terzaghi s onedimensional consolidation theory (Terzaghi 1943 Lambe and Whitman 1969 Peck et al. 1974 Terzaghi et al. 1996) as follows ... 

Settlements of footings on cohesive soils continuing beyond primary consolidation are called secondary settlement. Secondary settlement develops at a slower and continually decreasing rate, and may be estimated as follows ... 

Lowering of a barrier island by subsidence is compounded as barrier sand is transported into the bay by overwash diu ing storms. The existing barrier elevation is reduced, making future overwash more hkely, and the overwash deposit ( washover ) begins to load the previously imconsolidated substrate. The newly loaded sediment base then begins the primary consolidation process. Over long periods of time, these barrier islands are eroded and distorted by successive storms, potentially migrating into the bay. They ultimately may become subaqueous, such as Ship Island Shoal in Louisiana. ... 

P. T. Gayes, Primary consolidation and subsidence in transgressive barrier island systems, MSc thesis, Department of Geosciences, Pennsylvania State University (1983). 

Vertical (Primary) Consolidation Terzaghi s one-dimensional consolidation theory ... 

It is often specified that a certain percentage (90% 95%) of the primary consolidation must have occurred at the time of handover of the reclamation. It is noted that a requirement stipulating 100% consolidation is not realistic since settlement is a function of the logarithm of time. In fact the-time needed for occurrence of the last 5% consolidation settlement is, on an average, more or less equal to the time period required for occurrence of approximately 95% consolidation, even when vertical drains are installed. 

There are different theories about secondary consolidation. One approach suggests that secondary consolidation only starts at the end of primary consolidation, the other states that secondary consolidation is a different process to primary consolidation and also starts when the load is applied. The designer should be aware of these different theories, since this should also be reflected in way the consolidation parameters are derived from the consolidation tests. 

The Mesri approach assumes that the secondary compression only starts when the primary consolidation is completed. To calculate the secondary settlements, the end of primary settlement is assumed being the time when the primary consolidation reaches approximately 95%. and are calculated from the primary consolidation results. 

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