Closed Hydraulic Piezometer

Rapid drawdown

In the case of lagoons a sudden drawdown in the level of the slurry is unlikely, but the problem is important in the case of a normal earth dam. Bishop (1954) considered the case of the upstream face of a dam subjected to this effect, the slope having a rock fill protection as shown in Fig. 5.20. A simplified expression for u under these conditions is obtained by the following calculation:

Fig. 5.19 Determination of excess head at a point on a flow net.

Fig. 5.19 Determination of excess head at a point on a flow net.

Water level before

Water level before

If it is assumed that the major principal stress equals the weight of material, then the initial total major principal stress is given by the expression:

ci0 = 7c he + 7rhr + 7whw where 7C and jr are the saturated unit weights of the clay and the rock. The final total major principal stress, after drawdown, will be:

Clp = 7chc + 7drhr where 7^ equals the drained unit weight of the rock fill.

Change in major principal stress = <tif — ai0

Note Porosity of rock fill, n, Vv/V or, when we consider unit volume, n = Vv. Hence (7dr - >) = -7wn.

The pore pressure coefficient B can be obtained from a laboratory test but standard practice is to assume, conservatively, that B = 1.0. In this case

and the expression for u becomes u = 7w[hc + hr(l - n) - h']

5.5.2 Measurements of in situ pore water pressures

For any important structures the theoretical evaluations of pore pressures must be checked against actual values measured in the field. These measurements can be obtained by an instrument known as a piezometer of which there are four basic types: open standpipe, hydraulic, pneumatic and electric.

Open standpipe

For fully saturated soils of high permeability, such as sands and gravels, the water pressure can be obtained from the water level in an open standpipe placed in a borehole. The borehole must be sealed at its top to prevent the ingress of surface water (see Fig. 14.4).

Hydraulic piezometer

For soils of medium to low permeability open standpipes cannot be used because of the large time lag involved. In this case pneumatic, hydraulic or electric piezometers are employed. These piezometers and the relevant time lag effects have been discussed by Penman (1960). Only the hydraulic piezometer will be described here.

The hydraulic piezometer can exist as an open system (i.e. the standpipe arrangement used for highly permeable soils), or as a closed system in which the value of the pore water pressure is taken as equal to the pressure required to prevent flow through a porous filter placed at the point of measurement. It is important that free air is prevented from entering the system. The difference in value between the pore air pressure and the pore water pressure tends to force air into the piezometer and the value at which this pressure difference allows air into the system is known as the air entry value. Obviously the pore spaces in the filter must be small enough to ensure that the air entry value is so high that free air cannot gain entrance. Such a filter is known as a high air entry filter.

Two nylon tubes, of approximately 3 mm internal and 5 mm external diameter and coated with a 1 mm thick layer of polythene, lead from the filter to the gauge house where the pressure is measured by an electrical transducer, a Bourdon gauge or a mercury manometer. Although the latter is more accurate and requires no calibration it must be remembered that, for a dam some 30 m high, a manometer up to 4 m in height will be required.

The need for two tubes running from the filter is to enable the system to be filled with air-free water. In spite of a high air entry filter, free air will eventually penetrate the system which must then be flushed out by the use of pressurised de-aired water forced round the system.

The main disadvantage of the hydraulic piezometer is that the measuring device and connecting tubes should be no more than 7 m above the lowest piezometric level to be measured in order to avoid cavitation. They should also be below the frost line.

It is often convenient to build the instrument houses into the downstream slope so as to be partly (or wholly) buried.

5.5.3 Effective stress analysis by Bishop's method Bishop's conventional method

The effective stress methods of analysis now in general use were evolved by Bishop (1955). Figure 5.21 illustrates a circular failure arc, ABCD, and shows the forces on a vertical slice through the sliding segment.

Let Ln and Ln+] equal the lateral reactions acting on sections n and n + 1 respectively. The difference between Ln and Ln+i is small and the effect of these forces can be ignored with little loss in accuracy.

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