Soils

Soils

1. Short-Term

   • Sands
     • Drained
       • no Ue
         • U=Uh
           • Use Effective Stress Analysis
             • s=c'+sigma'*tan(phi')
               • c'=0 (since we are looking at sands & gravels)
                 • s=is greater than it was at the beginning of construction
   • Clays
     • Undrained
       • Ue is created
         • Ue = negative
           • Causes a decrease in mean normal stress
             • Ex: due to excavation
               • Factors of Safety
                 • As Ue dissipates, s decreases
                 • Increases with time
                 • If failure will occur, it will happen right after construction
                   • Becomes a drained soil, after ue has dissipated -> thus use effective stress analysis
               • Use Total Stress Analysis
         • Ue = postive
           • Causes an increase in mean normal stress
             • Ex: loading or construction on a soil
               • Factors of Safety
                 • Low immediately after construction
                   • Shear strength is low
               • Use Total Stress Analysis
                 • s=cT+sigma*tan(phiT)
                   • Soil is fully Saturated & Loaded
                     • phiT=0 (phi'=>0)
                       • Newly applied loads are carried entirely by the pore water pressure, not by the solids ( sigma'=unchanged)
                         • Leads to "phi=0" analysis
                           • su=cT
                             • Su=Undrained Shear Strength

2. Long-Term

   • Sands
     • Drained
       • SHORT-TERM=LONG-TERM
   • Clays
     • Drained
       • no Ue
         • Use Effective Stress Analysis
           • Find c', phi', compute sigma' based on uh and post construction stresses
             • Find s=c'+sigma'*tan(phi')
               • Note: if NC, c'=0
       • Also applies when soil is loaded SLOWLY