APPLICATION OF LATERAL EARTH PRESSURES TO RETAINING WALLS
Description:
Field and laboratory tests have not confi rmed the Coulomb and Rankine theories. In particular, field and laboratory test results showed that both theories overestimate the passive lateral earth pressures. Values of Kp obtained by Caquot and Kerisel (1948) lower the passive lateral earth pressures, but they are still higher than experimental results. Other theories, for example, plasticity theory (Rosenfarb and Chen, 1972), have been proposed, but these theories are beyond the scope of this book and they too do not significantly change the Coulomb and Rankine passive lateral earth pressures for practical ranges of friction angle and wall geometry.
Rankine’s theory was developed based on a semi-infi nite “loose granular” soil mass for which the soil movement is uniform. Retaining walls do not support a semi-infi nite mass but a soil mass of fixed depth. Strains, in general, are not uniform in the soil mass unless the wall rotates about its base to induce
a state of plastic equilibrium.
The strains required to achieve the passive state are much larger than those for the active state (Figure ). For sands, a decrease in lateral earth pressure of 40% of the at-rest lateral earth pressure can be suffi cient to reach an active state, but an increase of several hundred percent in lateral earth pressure over the at-rest lateral earth pressure is required to bring the soil to a passive state. Because of the large strains that are required to achieve the passive state, it is customary to apply a factor of safety of about 2 to the passive lateral earth pressure.
We have assumed a generic friction angle for the soil mass. Backfi lls are usually coarse-grained soils compacted to greater than 95% Proctor dry unit weight. If samples of the backfi ll were to be tested in shear tests in the laboratory at the desired degree of compaction, the samples might show peak shear stresses resulting in f9p. If you use f9p to estimate the passive lateral earth pressure using either the Coulomb or Rankine method, you are likely to overestimate it because the shear strains required to develop the passive lateral earth pressure are much greater than those required to mobilize f9p in the lab. For granular materials, f9p is mobilized at shear strains <2% and less. The use of f9p in the Rankine or Coulomb equations is one reason for the disagreement between the predicted passive lateral earth pressures and experimental results.
Alternatively, you can multiply the active earth pressures by a factor 1.1.202 to account for compaction stresses.
You should consider active lateral stresses due to surcharges. Surcharges are taken as uniformly distributed pressures, qs, at the surface. Typical surcharges are:
Buildings on shallow foundations including mat foundation 10 kPa
Highways (live load) 20 kPa
Rural main roads (live loads) 15 kPa
Light traffi c roads, footpaths (live loads) 5 kPa
The active lateral pressure is Ka qs and is assumed to be uniformly distributed over the depth of the wall for backfi lled wall. For natural retained soil, the lateral surcharge pressures would be uniform over each layer, and their magnitude would depend on Ka for that layer. In summary, you should use:
1. because:
(a) Uncertainties of loads and soil properties exist.
(b) Tolerable rotations of walls (,0.005 Ho) would not normally mobilize peak shear strength or critical state shear strength.
c) Prior to wall completion, shear bands (thin zones of soil that reached critical state) in the compacted backfi ll may develop.
(d) Quality of construction methods and construction loading, environmental conditions (floods, heavy rainfall, etc.), human and animal interventions (excavation at toe, dumping of materials on top of wall, burrowing of holes, etc.) cannot be estimated (at least, accurately) in advance.
(e) You should design the wall so that the backfi ll soil behaves in a ductile manner. Using f'cs or lower values would allow the soil to respond in a ductile manner.
2. Total stress analysis in conjunction with an effective stress analysis for fi ne-grained soils.
3. Conservative values for wall frictionand wall adhesion (sw 5 0.5su, but < 50 kPa for active state and sw # 25 kPa for passive case).
4. A high-quality drainage system to drain water from the backfi ll and away from the wall.
5. For a retaining wall with a sloping back, you can use an artifi cial wall face of height, H, from the heel to the soil surface (Figure ) to calculate the active force. The active horizontal and vertical forces acting on the artifi cial wall face arerespectively. In calculating the active lateral earth pressure coeffi cient use h 5 0 in Equation .
What’s next . . . In the next three sections, we will analyze retaining walls to determine their stability. We will consider an ESA (effective stress analysis) and a TSA (total stress analysis). We begin by considering the possible failure modes