Design Example: Slip Sandwich Raft.

The nominal crust raft for a pair of semi-detached properties in Design Example 1 is now assumed to be located in a mining area. It will therefore be reworked as a slip sandwich raft, to accommodate the associated ground strains.

The slip sandwich raft is designed on the assumption that the two halves of the raft – on either side of the centreline – are moving away from each other (tension), or towards each other (compression). The maximum horizontal force across the centreline of the raft, arising from the horizontal strains in the underlying ground, is equal to the maximum frictional force which can be transmitted across the slip-plane into one half of the raft.

Vertical loadings
Loads are as Design Example 1 (see Fig. 13.15):

Fig. 6.15 Raft detail – low-rise/lightly loaded buildings.

Horizontal force across raft centreline
The raft is 10.0 m × 12.0 m. With reference to Fig. 13.30, the total ultimate vertical load on one half of the raft is




located at the level of the underside of the raft thickenings to act as a slip-plane (see Fig. 13.30). The raft will be assumed to behave as illustrated in Fig. 6.14 and Fig. 13.27.

Fig. 6.14 Effect of ground strain on raft.

The Coal Authority guidelines(4) recommend the use of a coefficient of friction of µ= 0.66 for a sand slip-plane. The length of the centreline is B = 10.0 m. The horizontal force per metre length across the centreline of raft is therefore given by

Reinforcement design for raft tension
Provide high yield reinforcement to resist this force in  tension such that

Provide two layers of A193 mesh throughout, as shown in Fig. 13.30.

Design for raft compression
The same tensile force calculated above can also act in com- pression. By inspection the raft concrete can accommodate this magnitude of compressive stress.