Strutted Excavations.

a. Empirical design earth pressure diagrams developed from observations are shown in figure 14-11.

In soft to medium clays, a value of m = 1.0 should be applied if a stiff stratum is present at or near the base of the excavation.  If the soft material extends to a sufficient depth below the bottom of the excavation and significant plastic yielding occurs, a value of 0.4 should be used form.  The degree of plastic yielding beneath an excavation is governed by the stability number N expressed as


where  γ, H, and su, are defined in figure 14-11.  If N exceeds about 4, m < 1.0.

Pressure distribution-complete excavation.
Figure 14-11.  Pressure distribution-complete excavation.
b. For stiff-fissured clays, diagram (c) of figure 14-11 applies for any value of N.  If soft clays, diagram (b) applies except when the computed maximum pressure falls below the value of the maximum pressure in diagram (c).  In these cases, generally for N < 5 or 6, diagram (c) is used as a lower limit.  There are no design rules for stiff intact clays and for soils characterized by both c and  φ such as sandy clays, clayey sands, or cohesive silts.

c. The upper tier of bracing should always be installed near the top of the cut, although computations may indicate that it could be installed at a greater depth.

Its location should not exceed 2su below the top of the wall.

d. Unbalanced water pressures should be added to the earth pressures where the water can move
freely through the soil during the life of the excavation.

Buoyant unit weight is used for the soil below water.

Where undrained behavior of a soil is considered to apply, the use of total unit weights in calculating earth
pressures automatically accounts for the loads produced by groundwater (fig 14-11).  Pressures due to the
surcharge load are computed as indicated in previous sections and added to the earth and water pressures.

e. Each strut is assumed to support an area extending halfway to the adjacent strut (fig 14-11).  The strut load is obtained by summing the pressure over the corresponding tributary area.  Temperature effects, such as temperature increase or freezing of the retained material,  may significantly increase strut loads.

f. Support is carried to the sheeting between the struts by horizontal structural members (wales).  The wale members should be designed to support a uniformly distributed load equal to the maximum pressure determined from figure 14-11 times the spacing between the wales.  The wales may be assumed to be simply supported (pinned) at the struts.

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