Jacking Raft.

The jacking raft is used in areas where the expected subsidence would tilt or distort the structure to an unacceptable degree and where re-levelling of the raft produces an  economic and viable foundation for the design conditions.

The jacking raft is used in locations of excessive or unpredictable subsidence, for example, in areas subjected to brine or other mineral extraction. A typical jacking raft for a domestic property is shown in Fig. 9.30.

Fig. 9.30 Jacking Raft.

Buoyancy (or ‘Foating’) Raft.

A buoyancy raft is similar to a cellular raft and is a deep raft with large voids. The main weight of removed earth is replaced with practically weightless voids of the raft (see Fig. 9.29). Basement accommodation can be provided in this form of construction. Basement slabs together with retaining walls form the raft.

It is used for heavily loaded structures in areas of low ground-bearing capacity.

Fig. 9.29 Buoyancy raft.

Beam Strip Raft.

The beam strip raft consists of (ground-bearing) downstand beams in two or more directions which support the heavy uniform or point loads from the structure. The beams are tied together by a ground-bearing slab supported on the hardcored dumplings, i.e. the raised areas of hardcore protruding up between the beam lines (see Fig. 9.28).

This raft is mainly used in areas of either mining activity or soft alluvial deposits where a stiffened beam is required  on the main load lines. The tying of the ground floor slab into the beams prevents lateral distortions of the beam and evens out any local differential settlements. This type of raft is more economic than the cellular form and is used where conditions are not as severe.

Fig. 9.28 Beam strip raft.

Lidded Cellular Raft.

The lidded cellular raft is very similar in profile to the cellular raft and is used in similar locations, i.e. severe
mining conditions, areas of poor ground where the raft will be subjected to large bending moments, etc. The main  difference however is the use of a lighter form of upper  slab designed to be separate to the main foundation (see Fig. 9.27).

The detail at the seating of the upper floor depends upon the need for re-levelling and the possible number of times adjustments to line and level may be necessary.

Fig. 9.27 Lidded cellular raft.

Cellular Raft.

A cellular raft consists of an arrangement of two-way interlocking foundation beams with a ground bearing slab at  the underside and a suspended slab at the top surface. The upper and lower slabs are usually incorporated within the beams to form I sections. The intersecting beams effectively break the large slab into two-way spanning continuous small panels (see Fig. 9.26).

The top slab is cast using precast soffits or other forms of permanent formwork such as lightweight infill blocks.

These rafts are used on sites subject to severe mining activity or in areas of poor ground where large bending
moments are to be resisted. They are also used in locations where a valuable increase in bearing capacity can be achieved by the removal of the overburden and where deep foundation beams are required.

Fig. 9.26 Cellular raft.

Slip-Plane Raft.

The slip-plane raft consists of a concrete raft constructed on a slip-plane layer, such as sand of known friction or shear resistance, which is located between the raft and the substrata. The slip-plane is constructed in sufficient thickness to ensure that a straight failure plane could occur under excessive longitudinal ground strain (see Fig. 9.25). The depth of penetration of the raft into the ground is kept to a minimum to avoid picking up loading from ground strains.

However, the depth below finished ground level must take account of potential frost heave.

Fig. 9.25 Slip plane raft.

Blanket Raft.

The blanket raft consists of a concrete crust raft constructed on a stone blanket which in turn is built up in layers off the reduced sub-strata level (see Fig. 9.24). The crust raft and blanket interact to support and span the loading over any localized soft spots or depressions. The main difference between this and the crust raft is the introduction of the stone blanket. This blanket effectively disperses any heavy point and edge loads or imbalance of load. Composite action between the crust raft and the stone blanket is the basis of the action and design of this foundation system.

Fig. 9.24 Blanket raft.

Nominal Crust Raft.

Nominal crust raft
A nominal crust raft is basically a ground-bearing reinforced concrete floor slab with nominal thickenings around the edges. Internal thickenings are sometimes incorporated in the raft (see Fig. 9.23).

The slab acts as a surface crust to the sub-strata thus evening out any small local differential settlement movements which could result from variations in imposed loading on the top of the slab and/or local variations in settlement characteristics of the sub-soil. The design is generally carried out either by sizing the raft from previous experience or by calculation based upon nominal assumptions.

Fig. 9.23 Nominal crust raft.

Crust raft

The crust raft is a stiffer version of the nominal crust raft.

The ground slab and thickening which form the crust are combined into a total raft design. Heavier loads on soil of low bearing capacity determine the size and depth of the thickenings. The thickness of the slab is dictated by the overall raft design which generally exceeds the nominal slab requirements.

Surface Spread Foundations.

Surface spread foundations consist mainly of rafts and are generally used where the normal ground bearing
sub-strata is relatively poor and the depth to suitable loadbearing soils is excessive or the load-carrying capacity of the soil deteriorates with depth. Surface spread foundations are therefore employed to distribute the superstructure/substructure loads over a large area of the ground thus reducing the contact bearing pressure. Since most structures also require a ground floor slab it is usually  economic to incorporate it with the foundation into one structure/element. This can be done by making the upper surface of the raft foundation coincide with the top surfaceof the floor slab. A simple example is shown in Fig. 9.22.

Surface spread raft foundations are often adopted in areas of active mining as the best means of resisting excessive distortion, tensile and compressive forces, etc., resulting from the ground subsidence. These and other types of surface spread foundations are discussed in the following sections.

It should be noted that rafts do not necessarily distribute the loads as a uniform contact pressure to the sub-strata, on the contrary, most rafts are relatively flexible foundations and will have higher contact pressure under loaded points and edge thickenings than below the main slab areas.

Fig. 9.22 Typical raft foundation.

⇒ Nominal crust raft
⇒ Blanket raft
⇒ Slip-plane raft
⇒ Cellular raft
⇒ Lidded cellular raft
⇒ Beam strip raft
⇒ Buoyancy (or ‘?oating’) raft
⇒ Jacking raft