Design - Slip Sandwich Raft.

1. Introduction
This raft is mainly used in active mining areas or where  clay is creeping on inclined sand beds where the horizontal  ground strains set up during subsidence or creep movements would cause damage to the structure, if allowed to be transferred up to it via the foundation (see Fig. 13.23).

Fig. 13.23 Effects on foundations from horizontal
ground strains.

By using a slip-plane of known resistance, the maximum force which can be transferred from the ground to the building before the plane ruptures can be calculated, and the raft designed to resist this force in any direction that it is likely to occur.

The raft can be a flat slab profile thus avoiding the use  of downstand thickenings which may pick up excessive passive load from the ground strain. Alternatively a slab with medium thickenings incorporating a design which provides a slip-plane below the hardcore dumplings (i.e. the raised areas of hardcore protruding up between the beam lines) can be used (see Fig. 13.24).

Fig. 13.24 Alternative slip sandwich rafts.

The ideal ground (i.e. uniform firm layers of non-frost- susceptible low shrinkability sub-strata) to facilitate a flat slab rarely occurs on the site to be developed. Therefore,  to prevent damage from frost, clay heave or differential  settlement, thickenings are often necessary. In such situ- ations the ground strains being picked up either have to  be designed to be resisted by the raft or a slip-plane layer  provided below the level of the downstands to reduce  the forces being transferred. The upper raft (above the  slip plane layer) of a slip sandwich raft can be any of the other rafts already designed and discussed in earlier sections  of this part.

The difference between the slip sandwich raft and the other rafts relates to the slip-plane layer below the slab and the horizontal forces produced from the ground strains transferred through the slip-plane.

The additional stresses are analysed by calculating the forces transferred from the ground strain and these forces are added to the design conditions already discussed for other rafts.

2 Design decisions
The design decision to use a slip sandwich raft will depend totally on the possible existence of critical horizontal ground strains in the sub-strata during the life of the building and the need to restrict these forces to prevent them being trans- ferred in total to the superstructure. The use of jointing to reduce the overall building into small independent robust units is part of the design process. In addition the possible need to incorporate compressible aprons around the raft requires consideration in the design and it is dependent upon the directions and magnitude of the ground strains (see Fig. 13.25).

Fig. 13.25 Section through raft and compressive apron.

3 Sizing the design
The basic sizing of the raft to sit on the slip-plane would  follow the principles already discussed in other sections of this part.

The additional requirements for the slip sandwich raft however relate to the compressive and tensile forces likely to be transferred through the slip-plane from the ground strains. If a simple rectangular plan shape raft is considered as shown in Fig. 13.26 (which would be the ideal plan shape for such a raft) and a 150 mm thick sand slip-plane, the  following simple analysis can be applied. Assume the total weight of the building and foundations to equal T and the frictional resistance of the sand slip-plane layer to be equal to µ (see Fig. 13.27), the largest horizontal force which can be transferred up from the ground strain through the slip-plane will be equal to (µT)/2. The reason for the total load acting down being halved is that the maximum force that can be transferred as tension through the building must be reacted by the other half of the load. This formula assumes that no other passive forces are being transferred to the foundation, i.e. that all forces are transferred via the sand as a frictional force. In practice the downstand beams would be cast with sloping internal faces as in Fig. 13.30.

Fig. 13.26 Simple rectangular raft.

Fig. 13.27 Forces on foundation from ground strains.

Fig. 13.30 Slip sandwich raft design example.

If however downstands project below the raft then the  slip-plane layer should be positioned below such down-stands and the downstands kept to a minimum. In addition if compressive ground strains are occurring then an apron must be introduced to prevent or restrict the amount of strain transferred from passive pressure on the raft edges. If such pressures cannot be avoided then they must be added
to the force indicated above and allowed for in the design.

Any eccentricities of such forces should also be taken into account in the design of the raft since these will produce bending in the raft foundation (see Fig. 13.28) which indicates an eccentric force on a downstand raft thickening.

Fig. 13.28 Passive forces on raft downstands.

If the plan shape adopted is not rectangular, for example, the L shape as shown in Fig. 13.29, then the two halves of the building producing the force (µT)/2 from frictional resistance below the surface will produce tensile or com- pressive forces across a line which passes through the  centre of gravity of the building and which will tend to rotate towards a line parallel to the subsidence wave.

Consideration must be given therefore to the additional stresses produced by these forces including any bending moments across this face or on lines parallel to the face  (see Fig. 13.29). Division of the slab into two separate  rectangular rafts by the incorporation of a movement joint could be considered as an alternative approach. The significant movements likely to occur here would however have to be allowed for in the detailing of the joint through the structure.

Fig. 13.29 Direction of tensile failure dependent on
direction of wave face.