Phreatic conditions

Calcite precipitation under phreatic conditions can continue uninterrupted by an air-water interface (Morse Mackenzie, 1990). Thus, isopachous or drusy, poikilotopic and blocky spar cements are most often associated with precipitation in the phreatic zone (Jacka, 1970 Folk, 1974 Retallack, 1990 Bums Matter, 1995). Sparry cements can also form in the vadose zone as calcans or crystic nodules, but they are associated with soil zonation, highly dense micritic cements and nodules (Weider Yaalon, 1982). Because these cements are not associated with such features they are unlikely to represent calcans or crystic nodules. 

Fig. 3.6 Tectonic tilting of rock strata provides the necessary condition for the formation of a partially confined aquifer. A phreatic section, located at the outcrop of the rock bed, serves as recharge intake area feeding the confined section in cases where the inclined rock bed has a drainage outlet at its lower end enabling through-flow (which is a rare case).

The Bunter sandstone <5D and <5180 data provide further support for the lack of communication between the phreatic and confined aquifer sections. The data plot in Fig. 11.25 along a line, but the data reveal an order of B->A->C. In other words, one is dealing with three distinct water groups, revealing different environmental conditions that prevailed at the time of recharge. 

Equations [3-8a] to [3-8c] are different applications of the Thiem equation, which estimates drawdown in an aquifer or well under steady-state conditions. As previously mentioned, it is assumed that the changes in saturated aquifer thickness are small compared with the total saturated depth. This is necessarily true in a confined aquifer, but not always in an unconfined (phreatic) aquifer. If drawdown becomes a significant fraction of the saturated aquifer thickness, more complicated expressions for drawdown are obtained see Bear (1979). For an unconfined aquifer in which drawdown is a significant fraction of the saturated thickness, Eq. [3-8a] must be expressed in terms of head instead of drawdown ... 

Wellpoints and wells are temporary systems, in that they must operate continuously to maintain the lowered phreatic line. A competently designed and properly functioning system can provide totally dry working condition over large areas, as seen in Figure 4.8. 

The complexity of phenomena controlling the chemical composition of waters. In the soil layers, this composition would depend principally on biological and biochemical processes, in relation to the activity of organisms but at the level of phreatic waters the chemical composition would rather be controlled by physicochemical conditions (solution-mineral equilibria). 

Carbonate cements are often among the dominant components of diagenesis and hence are of decisive importance in determining the reservoir quality of sandstone sequences. Despite this, the timing, the geochemical conditions of precipitation and dissolution, as well as the source and fate of these cements are not fully understood. In continental and near-shore sediments, cements commonly precipitate as calcretes and dolocretes in the vadose and phreatic zones, and attain a variety of mineral-ogical, textural and distribution patterns as well as elemental and isotopic compositions. These cements form lenses and layers of densely cemented... 

In this paper the terms dolocrete and calcrete are used to indicate sediments extensively cemented by displacive dolomite and calcite under conditions ranging from the soil horizon to shallow phreatic. Conversely, we use the expression carbonate cement in sandstones where is no evidence of displacive, near-surface precipitation. Kaolin is used as a... 

Consequently, we may calculate the phreatic water s seepage time of aquifer-protective mining condition s each class according to the seepage speed of Table 4 by taking 45 MPa as the comprehensive strength of overlying strata, and see Table 5 and Table 6. 

When the downstream dam site is considered, the retained reservoir level will be 2475 a.s.l. The specified phreatic surface in this model was selected to be the same level of water damming. The colluvium and heavily weathered bedrock were considered to be permeable while the bedrock was assumed to be impermeable. The shear strength parameters of the saturated deposit are selected as 0.7 times of the dry condition referring to the research of Liu (2009). The hydrostatic pressure on the slope was also considered. Figure 9 shows the pore pressure of the colluvium and the phreatic line can be obtained from the result. Figure 10 illustrates the displacement contour and vectors after 1000 iteration steps. Figure 11 shows the cross sectional map of the displacement in Y direction after partial saturation. It can be concluded that, unlike the seismic condition, the... 

As a general rule, a seepage face on a slope should be avoided by taking appropriate measures in order to keep the phreatic line as far as possible from the slope face. This can be realised by ditches, (toe) drains, impermeable layers or other means such as demonstrated in Figure 8.26. It is noted that a ditch such as shown in Figure 8.26a may affect the stability of the structure in a negative manner. Its efficiency will depend on the local soil conditions and the water level in the ditch. 

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