Design Procedures for Retaining Walls.

a. Criteria forselecting earth pressures.

     (1) The equivalent  fluid method should be used  for  estimating  active earth pressures on retaining structures up to 20  feet high, with  the  addition  to  earth pressures resulting from backfill compaction (fig 14-8).
     (2) For walls higher than  20  feet,  charts, equations, or graphical solutions should  be  used  for computing  lateral  earth  pressures,  with the addition of earth pressures resulting from backfill compaction.
     (3) Use at-rest pressures for rigid retaining structures resting on rock  or batter piles.   Design cantilever walls  founded on rock or restrained  from lateral movement  for at-rest pressures near the base of the wall, active pressures along the upper portions of the wall,  and compaction-induced earth pressures  from  the top  to the depth at which they no longer increase lateral earth pressures (fig 14-8).  Generally,  a linear variation in earth pressure coefficients with depth  may  be assumed between the sections of wall.
     (4) Consider passive pressures  in  the design if applied loads  force  the  structure  to  move against the soil.  Passive pressures in  front of retaining walls are partially effective in resisting horizontal sliding.

Estimates of increased pressure induced by compaction.
Figure 14-8.  Estimates of increased pressure induced by compaction.


b. Overturning.  Calculate the factor of safety, FS, against overturning, defined as  the  ratio of  resisting moments to the overturning  moments.  Calculate the resultant force using load diagrams shown in figure 14-1, as well  as  other loadings that may be applicable.  Use only half of the ultimate passive resistance  in calculating the safety factor.  The resultant of all forces acting on the retaining wall should fall within the middle third to provide a safety  factor with respect to overturning equal to or greater than 1.5.

Load diagrams for retaining walls.
Figure 14-1.  Load diagrams for retaining walls.

c. Sliding.
     (1) The factor of safety against sliding, calculated as the ratio of forces resisting movement to the horizontal component of earth plus water pressure on the back wall, should be not less than 2.0.  If soil in front of the toe is disturbed or loses its strength because of possible excavation, ponding, or freezing and thawing, passive resistance at the toe, Pp,  should be neglected and the minimum factor of safety lowered to 1.5; but if the potential maximum passive resistance is small, the safety factor should remain at 2.0 or higher.
     (2) For high walls,  determine the shearing resistance between the base of wall and soil from laboratory direct shear tests in which the adhesion between the concrete and the undisturbed soil is measured.  In the absence of tests,  the coefficient of friction between concrete and soil may be taken as 0.55 for coarsegrained soils without silt, 0.45 for coarserained soils with silt, and 0.35 for silt.  The soil in a layer beneath the base may be weaker, and the shearing resistance between the base of wall and soil should never be assumed to exceed the soil strength.  Consider maximum uplift pressures that may develop beneath the base.
     (3) If the factor of safety against sliding is insufficient, increase resistance by either increasing the
width of the base or lowering the base elevation.  If the wall is founded on clay, the resistance against sliding should be based on su for short-term analysis and φ’ for long-term analysis.

d. Bearing capacity.   Calculate from the bearing capacity analysis preously.  Consider local
building codes or experience where applicable.

e. Settlement and tilting.   When a high retaining wall is to be founded on compressible soils, estimate total and differential settlements using procedures outlined previously.  Reduce excessive total settlement by enlarging the base width of the wall or by using lightweight backfill material.  Reduce tilting induced by differential settlement by proportioning the size of the base such that the resultant force falls close to the center of the base.  Limit differential settlement to the amount of tilting that should not exceed 0.05H.  If settlements are excessive,  stabilize compressible soils by surcharge loading or a support wall on piles.

f. Deep-seated failure.   Check the overall stability of the retaining wall against a deep-seated foundation failure using methods of analysis outlined previously.  Forces considered include weight of retaining
wall, weight of soil, unbalanced water pressure, equipment, and future construction.  The minimum safety factor is 1.5.

g. Use of piles.  When stability against bearing capacity failure cannot be satisfied or settlement is excessive, consider a pile foundation.  Use batter piles if the horizontal thrust of the lateral earth pressure is high.

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