One mix that has been used successfully is 3:2:4:2 lime putty: sieved subsoil: sharp sand: 5 to 1Omm diG fine gravel, plus chopped straw or hay (no water added to the mix). Keying is achieved by means of skew fixed 10Omm long stainless steel nails, riven oak pegs or riven hazel spars. These may be driven in, or preferably inserted tightly into predrilled holes if possible. The surface of the hollow needs to be thoroughly wetted with a gentle fine spray. A hosepipe should not be used. This would cause further erosion and loosen the surface material. It may be necessary to apply the spray several times to ensure that the surface is well wetted as the insitu work is carried out. If the existing surface is not wetted adequately, it will become too dry as the new material is applied, reducing the chances of an adequate bond. The mix should be applied in up to 25mm layers, waiting for each to dry out and then wetting the surface again before applying the next layer. Successful repairs have been carried out using layers of the original materiaI reconstituted and straw bound in, essentially following the same process. (See Figure 7a)
Where, alternatively, a cob type material is to be used for minor repairs such as this, the addition to the mix of 5 to 10% non-hydraulic lime may help reduce drying shrinkage significantly. Also, it will help the material achieve a faster set.
Figures 7b and 7c show alternative opprocches. In 7b, the material is more akin to Pise de Terre, rammed hard into place in virtually a dry state. Fairly deep repairs (up to 125mm) have been undertaken using this technique in chalk based cob walling, and so for hove held up very well. An alternative approach for deeper hollows such as this involves the insertion of small precost blocks against which in-situ fill material con be contained, as shown in 7c. This has the advantage of reducing the drying shrinkage occurring in-situ. In oil these approaches, it must be borne in mind that sufficient of the original fobric needs to be surviving intact to fulfil structural needs.
Hollows and cracks that penetrate deeply or right through the thickness of the wall need to be dealt with differently than shown at Figure 7, ensuring that the repair addresses structural defects and overloadings. It is more than likely that temporary structural support will be required for the surviving earth walling. Check that the underpin is in sound condition, and repair if necessary, taking care to avoid the use of Portland cement in mortar for bedding, pointing and rendering. Where areas of earth walling have to be rebuilt, it is usually possible to do this re-using the original material, provided it has not become contaminated. The major problem to overcome is that of drying shrinkage. Where rebuilding a section, this has been done successfully using vertical hardwood tenons, or shear keys, housed half in the existing and half in the new work. The gap that appears following drying out is subsequently filled with matching material.
Another approach is to use a very much drier mix, as described for repairing hollows, and rammed in shallow layers between
shuttering in the manner of Pise de Terre. If the material differs substantially in its physical qualities from that of the original, though, almost inevitably some tensions will arise at the interface between them. Again, it is a matter of applying the appropriate expertise to weigh up the options and decide upon the solution which is likely to result in the minimum of subsequent problems.
Another alternative is to use precast blocks made from the original material or as close a match as possible. A mortar comprising sieved sub-soil, coarse sand and lime would be appropriate. Some building conservationists may feel uncomfortable with this approach, believing that the resulting new walling is fundamentally different to the original through not having the same homogenous and monolithic qualities. Also, there may be the temptation to buy pre-made blocks from a supplier based in a different area and using material obtained from a different geological region. Nevertheless, the use of blocks offers the big advantages of reduced drying shrinkage