The worked examples in this chapter consider the design of a T-shaped gravity wall retaining dry fill under undrained conditions (Example 11.1); the same wall under drained conditions (Example 11.2); the same wall again, retaining wet fill under drained conditions (Example 11.3); and a mass concrete wall retaining granular fill (Example 11.4).
Specific parts of the calculations are marked O, ©, ©, etc., where the numbers refer to the notes that accompany each example.
11.11.1 T-shaped gravity wall retaining dry fill (undrained analysis)
Example 11.1 considers the design of a T-shaped gravity wall retaining dry fill, as shown in Figure 11.10.
Even though the wall is founded on clay, the backfill has been assumed to be granular (which would be typical for this type of wall). A fully effective drain at the heel of the wall has been included so that water pressures may (for simplicity) be ignored.
© The effect of the weight of the back fill is favourable for sliding and toppling but unfavourable for bearing, thus for design different characteristic values for the weight density should be used depending on whether the weight of the back fill is favourable or unfavourable. However, the variation in weight density is likely to be small and in order to demonstrate more important features of the calculations a single weight density for the backfill has been used.
© Retaining walls are the one geotechnical structure where it is normal to apply an adjustment to the dimensions to allow for the potential for unplanned excavation on the passive side of the wall.
© The characteristic surcharge acts across the whole backfill and on top of the wall, where it is considered to be unfavourable (for bearing). Where the surcharge is considered to be favourable, it is excluded from the calculation.
© The partial factor sets are those given in Annex A. They are unmodified by the UK National Annex to BS EN 1997-1.
© For the assumption of a virtual back with Rankine earth pressures to be true a check needs to be carried out to ensure that it can develop fully. It should be noted that using standard design rules for proportioning T-shaped concrete walls will generally result in the conditions for a virtual back with Rankine earth pressures not being satisfied (as is the case here for Combination 2).
T-shaped gravity wall retaining dry fill (undrained analysis) Verification of undrained strength (limit state GEO)
Consider a T-shaped gravity retaining wall, ts = 250mm thick, which retains
H = 3.0m of sand fill and sits upon a medium strength clay. A drain will be installed at the wall heel to keep the fill dry. The base of the wall is B = 2.7m wide and tb = 300mm thick, and its toe extends x = 0.5m in front of the stem.The underside of the base is d = 0.5m below formation level. The weight kN
density of reinforced concrete is Yck = 25-(as per EN 1991-1-1 Table m
A.1). The fill has characteristic drained strength ^ = 36° and c'k = 0kPa.
The fill's weight density is Yk = 18-." The clay below the wall has m characteristic undrained strength cukfdn = 45kPa and weight density kN
Yk fdn = 22-. A variable surcharge qQk = 10kPa may act at the top of the m wall during persistent and transient situations.
Design Approach 1
Unplanned excavation AH = min(10% H, 0.5m) = 0.3 m © Design retained height Hd = H + AH = 3.3 m
Characteristic vertical actions and moments (about toe) due to self-weight:
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