5 The graph on the next page shows water level fluctuations

5. The graph on the next page shows water level fluctuations observed in a piezometer near a railroad station. A train pulls into the station and stops at a time of about 7 minutes. The train departs from the station at a time of about 18 minutes. Explain the water-level fluctuations in terms of effective stress, total stress, and fluid pressure

Solution

Answer:

As is seen in the graph, the pore water pressure suddenly increases with the arrival of the train at 7m. This happens due to the generation of the excess pore water pressure. This excess pore water pressure equals to the excess stress caused due to the weight of the train. Because water can not dissipate or permeate instantly all the stress caused by train weight is converted into the excess pore pressure, but rapidly decreases because water starts to flow out under this huge pressure, this is the reason for a steep initial decline in the pressure after 7m.

Then the curve becomes gentle but moving downwards, because the water flow is there, but stress has reduced, so it is slow. Then at 18 m, there is a steep decline in pore water pressure, this happens due to the generation of the negative pore water pressure. Suddenly the excess overburden stress of train is removed, and this causes negative pore water pressure, and therefore pore water pressure is steeply decreased. But it is steeply regained also, because suddenly the water will start flowing into the empy pores, which earlier was flowing out under the excess overburden. Once enough water is inside the pores again, the pore water pressure nears to its normal initial level.