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This occurs when a impermeable layer (clay) lies over a permeable layer (sand). At level 1-1, when there is no excavation, full overburden pressure exists. When excavation occurs, at level BC some overburden pressure is released. At the same level 1-1, upward water pressure exists due to presence of sand layer. When no excavation occurs the total overburden pressure is greater than the upthrust, but the layer of soil below the excavation pit may not have sufficient depth to resist the uplift force. Hence if the uplift force the excavation level becomes more, the clay layer bursts open. |
