Need help with question 5. A retaining wall, of the geometry showed in Figure 1,

Question

Need help with question 5.

A retaining wall, of the geometry showed in Figure 1, is back filled with some draining material The structure is 2.5 m high and 0.2 m thick (footing and face). A drain is placed at the bottom of the wall with the centre of the drain located at 0.5 m from the underside of the footing and 0.5 m from the face of the wall. For the sake of this exercise, consider that the drain is sufficiently small that all points of the drains are at the same elevation (that of the centre). The drain is connected to atmospheric pressure. The drain has to be sized (not by you), which requires an estimate of the flow rate under the worst condition i.e. backfill fully saturated, water level at the top of the wall elevation 2.5 m). The backfill material has a saturated unit weight of 19kN/m3 A constant head test was conducted on the backfill material to determine its permeability. Results of the test are given in Table 1. The cross section of the specimen is 31.65 cm, its length is 10 cm and the head difference is 50 cm. The backfill material can be assumed to be isotropic. Table 1: mass of outflow water recorded as a function of time Time [sec] Mass of water [gl 30 60 90 120 251 513 765 1008 1286 1. Identify the boundary conditions that you deem relevant to establish a flow net. (10 MARKS). Draw a flow net for the stea of water remains at the top of the wall (there enough water supply that no draw down is produced) (10 MARKS). Determine the hydraulic conductivity of the backfill material from the permeability test data (5 MARKS). 2. dy state seepage from the surface to the drain. Assume the level 3. 4. What is the flow rate through each flow tube in your flow net and what is the potential drop between each pair of equipotential lines in your flow net? (10 MARKS) Determine the seepage rate to the drain and express it in meaningful units (5 MARKS). Identify (label/number) each of your equipotential lines and prepare a table showing the value of total potential for each. (10 MARKS). Produce the distribution of water acting on the retaining wall (20 MARKS) Determine the effective stress at points h, g and fusing the flow net you have drawn. What is the effective stress in h, g and fif the drain gets fully blocked (water does not flow in the drain anymore) (15 MARKS). 5. 6. 7. 8.

Explanation / Answer

As you need help in 5th only, so I am going to explain it to you. In this part it is asking about seepage rate in the drain, so for that first we need to find out the seepage head at the level of drain and its nothing but the total head at the level of drain. Now assuming unit length of drain into the plain of paper multiply the permeability of soil with the seepage head, and you will get the rate of flow per unit length that is discharge per unit length.

For computing permeability, given constant head test so

Permeability(k)= (m*L)/(p*T*h*A)

where,

m=mass of water collected in gm

p=density of water i.e. 1gm/cc

L= length of the specimen in cm

T=time taken by test

h=head lost in test cm

A- cross section area of specimen cm2

Put all these values and you will find permeability in cm/sec

Now there are many test result so for each result calculate permeability and then take average value.

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