Tuesday, January 22, 2013

Design - Jacking Raft

Raft foundations suitable for jacking are specifically designed versions of crust rafts, cellular rafts, lidded cellular rafts, beam strip rafts or other foundations whose stiffness and behaviour is designed to resist the jacking forces and moments involved in the process of re-levelling.

The raft is designed to cater for the bending and shear forces likely to be produced during subsidence and relevelling activity. Jacking points are built into the foundation to allow for re-levelling and the type of raft and
number of jacking points depends to a large extent on the proposed use, the size of the structure and the predicted subsidence likely to occur.

The need for a jacking raft tends to be determined by the unpredictability of subsidence and the practicalities for the building user of re-levelling within the life of the building.

As mentioned previously, domestic sites in areas of brine mining are typical of sites where such foundations have been adopted and basically two design conditions need to be considered.

(1) To design for the normal subsidence condition for the site including bending moments and forces as previously discussed for the raft type.
(2) Additional analysis and design to incorporate the structural requirements to resist stresses and distortions during the jacking operations.

Sizing the design
It can be seen that the initial sizing of the foundation for trial design would be to adopt generous sizes for a standard raft of the type being proposed in anticipation of embracing the jacking stresses.

The stresses produced during jacking are dependent on the restrictions and methods of jacking and therefore tend to be one-off designs for set conditions.

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