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Bottom heave in clay


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Stability of braced cuts

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Bottom heave in clay

  2. 2. β€’ A braced-cut may fail as a unit due to unbalanced external forces or heaving of the bottom of the excavation. β€’ If external forces acting are unequal, the stability of the entire system has to be analyzed. β€’ If soil on one side of a braced cut is removed, stability of the system will be impaired. β€’ Two cases may arise. They are 1. Heaving in clay soil 2. Heaving in cohesionless soil
  3. 3. Heaving in Clay Soil β€’ The danger of heaving is greater if the bottom of the cut is soft clay. β€’ Two types of failure are possible. They are:- Case 1: When the clay below the cut is homogeneous at least up to a depth equal 0.7B Case 2: When a hard stratum is met within a depth equal to 0.7B
  4. 4. Case 1: Formation of full plastic failure zone below the bottom of cut Anchorage load block of soil 𝑸 = 𝜸 𝑯 𝑩 βˆ’ πœπ‘― = 𝑩 𝑯 𝜸 βˆ’ 𝒄 𝑩 Vertical pressure q per unit length 𝒒 = 𝑸 𝑩 = 𝑯 𝜸 βˆ’ 𝒄 𝑩 Bearing capacity qu per unit area at level ab 𝒒 𝒖 = 𝒄𝑡𝒄 = πŸ“. πŸ•π’„
  5. 5. Factor of safety against heaving is 𝑭 𝒔 = 𝒒 𝒖 𝒒 = πŸ“.πŸ•π’„ 𝑯 𝜸 βˆ’ 𝒄 𝑩 Further, 𝑭 𝒔 = πŸ“.πŸ•π’„ 𝑯 𝜸 βˆ’ 𝒄 𝟎.πŸ•π‘© This indicates that the width of the failure slip is equal to 𝑩 𝟐 = 𝟎. πŸ•π‘©
  6. 6. Case 2: When the formation of full plastic zone is restricted by the presence of a hard layer If a hard layer is located at a depth D, width of the strip sink is also equal to D. 𝑭 𝒔 = πŸ“.πŸ•π’„ 𝑯 𝜸 βˆ’ 𝒄 𝑫 For cut in soft soil with const. value of Cu , 𝑭 𝒔 = πŸ“.πŸ•π’„π’– πœΈπ‘― = πŸ“.πŸ• 𝑡 𝒔
  7. 7. BJERRUM AND EIDE METHOD OF ANALYSIS β€’ This method is used when, braced cuts are rectangular, square or circular in plan or the depth of excavation exceeds the width of the cut.
  8. 8. β€’ For rectangular footings, the value of Nc may be computed by the expression 𝑡 𝒄 𝒓𝒆𝒄𝒕 = 𝟎. πŸ–πŸ’ + 𝟎. πŸπŸ” 𝑩 𝑳 𝑡 𝒄 β€’ Factor of safety for bottom heave is 𝑭 𝒔 = 𝒄𝑡 𝒄 πœΈπ‘― + 𝒒 β‰₯ 𝟏. πŸ“