8.2.1 Details
The ground immediately around the base of the wall should be well drained and footings should be protected from water infiltration. Where termites are a problem, adequate protection should be provided. Since termites are only very rarely a problem in the UK this type of protection is considered beyond the scope of this document. On the other hand rat infestation is a fairly common problem in cob construction, where broken glass or pure clay laid at the base of the wall during construction or raised footings have been used as deterrents (Pearson, 1992). Extent of rat infestations in historic rammed earth is unknown.
8.2.1.1 Drainage
Accumulation of standing water adjacent to the footing should be avoided by installing necessary surface and underground drainage.
Surface drains usually consist of paving units sloping away from the footing to a specified collection point. The Australian earth building handbook (Standards Australia, 2002) suggests that the slope should not be less than 1 in 20 and the collection point no further than one to two metres. Houben & Guillaud (1994) on the other side claims that impermeable surfacing over the soil around footings should be avoided in order to allow moisture in the soil to evaporate. Hard paving such as rubble with permeable filled joints can provide such a surface. In this case, a suitable soil gradient of 2% or more should be used.
Underground drainage should be built to a convenient ditch close to the foundations at a short distance, 1.5 metres approximately (Houben & Guillaud, 1994). Channels of burnt clay or other suitable materials can be laid at the bottom of the trench, which then collect the water and remove it by means of a regular gradient. Alternatively trenches with at least 200mm deep fine gravel, often known as “french drains” can be used to remove the excess water (Keable, 1996).
8.2.1.2 Damp-proofing
In modern rammed earth building damp proof courses are provided in order to prevent moisture penetrating from the subsoil into the building. In traditional earth building damp proofing was not provided, any moisture entering the wall was able to dissipate through the wall mass as part of breathing wall system of construction. A convenient position for DPC is immediately on the top of the foundation (CSIRO, 1996).
It is important that any damp-proof courses are consistent with the local building codes and at the same time are relatively flexible and therefore are unlikely to fracture due to shrinkage in the wall or minor foundation movement (Middleton, 1952). A widely used form of damp-proofing in rammed earth is bituminous paint (Hodder; Houben & Guillaud, 1994; Middleton, 1952; Middleton, 1953; NZS 4299:1999, 1999). However, this is not acceptable for masonry walls in the UK. Damp-proofing materials used more widely in masonry, and some rammed earth walling, include polyethylene (Norton, 1997), bitumen/sheet metal composites, bitumen DPC, mastic asphalts, lead or copper sheeting
Foundations
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(Middleton 1952), bitumen polymer, dense bricks and slate. Other materials used in rammed earth internationally include water repellent cement (Houben & Guillaud, 1994) and chemical treatments.
8.2.2 Construction
Concrete footings can be either poured separately or in conjunction with the stem wall. Excavation for foundations should remove all topsoil and if the bottom of the trench is not firm, a compacted layer of fill with minimum bearing capacity between 50 and 100kN/m2 (Standards Australia, 2002) should be placed. Any required formwork is set in
place and the reinforcement installed. Any services going through the foundation are ducted and the trench is cleared of vegetation and any other organic material or debris. Concrete is then poured in the trenches, compacted using a concrete vibrator and moistened with water and covered with plastic sheeting. Forms are then left in place for as long as necessary and concrete curing with water is continued for at least seven days before rammed earth works commence (Easton, 1996).
Stabilised earth footings are constructed in a similar manner. Trenches are excavated and at the bottom a 20 to 50mm thick blinding layer of clean quarry sand or weak concrete is placed (Standards Australia, 2002). If chemical damp-proofing is not used, the damping proof membrane is installed to line the footing. The cement stabilised mix is then poured in the trench in layers approximately 100mm thick (Norton, 1997) and compacted in order to obtain at least 98% of the Proctor dry density. During compaction, care should be taken not to punch the damp-proof course and any tearing should be adequately repaired and sealed. After compaction the material should be covered with plastic sheeting or damp hessian sacks and left to cure undisturbed for at least seven days (Standards Australia, 2002).
8.3 Conclusions
Foundation design for rammed earth buildings is very similar to that for similar low rise buildings. Slab and strip footings are the most common types of footings encountered in earthen construction. Concrete strip footing is the most widespread modern mode of foundation used for rammed earth buildings, though cement stabilised earth footings are also used in Australia. The size of footings depends on the type of the supported structure and the soil bearing capacity underneath the foundation. It is important that foundation is of sufficient depth to avoid frost underneath the foundation and that the ground immediately around the base of the wall is well drained and footings well protected from water infiltration. The installation of surface and underwater drains and damp-proof courses is therefore generally considered essential.