The final design for Phase 2 of the Pryces to Crickheath channel was produced after a long process of ground investigation, laboratory studies, design proposals and numerous consultations. This article attempts to explain the design in non-technical terms.
Before the restoration started anybody walking along the towpath between LAF and Crickheath basin would have noted that many sections of bank had subsided over the years. The problems had been around for many years; the Shropshire Union Canal Company employees naming part of this area near the present solar farm ‘the hell hole’. It was known from the beginning that this length warranted special attention, and a professional ground investigation was the first task. The extent of the problems quickly became apparent. The geologist responsible remarked that he had never encountered a site which had such a variety of soil types or where the ground conditions changed so abruptly and so frequently. The resulting geotechnics report made it clear that majority of Phase 2 has a deep bed of peat underneath it, and other areas have equally unpleasant water bearing gravels, running sands and loams.
The design had the twin aims of addressing the ground conditions to ensure that channel construction that would not subside when the restored canal was in use, and being realisable by a volunteer workforce. Because of the variety of ground conditions the design included bespoke bank constructions for the many and various ground types. The upshot of was that the channel/bank configurations varied markedly along the length of Phase 2 due to both the different bank constructions and the need to fit these into the available land. The computer generated fly-through of the channel (see below – the viewpoint is from the LAF towards Crickheath) shows this variation which required considerable attention to detail to build accurately. Note that the design included a constant width navigable channel.
The first stage of actual construction was to make the bed of the channel strong enough to take the weight of the excavators and dumpers. This was done by mixing cement into the peat and covering it with geogrid and imported stone. The banks were constructed of layers of compacted earth and imported stone held together by geogrid. The latter is a very strong fabric mesh which when placed horizontally prevents the bank spreading under load. In areas of particularly poor ground the banks were initially constructed to a height which took account of the predicted subsidence over the next 25 years. In some places this was in excess of two metres above the then existing towpath level. The banks were then loaded for six months using 1000 litre capacity water filled containers (IBCs) stacked on the top of the bank. These are visible on the computer fly-through. The loads were calculated to cause the equivalent of 25 years of subsidence in six months, that being irreversible. The Society was responsible for on-site testing of the soil which formed the banks, and for monitoring the rate of settlement of the loaded banks. After the banks had settled within prescribed limits the loads were be removed. Only at this point could the channel be shaped and lined in a manner similar to that used in Phase 1.
As can be appreciated the work of this nature is a very complex bit of engineering of a type not usually done by volunteers. However a combination of a the well thought out design, the right materials and equipment and support from CRT and the designers, the talents of our workforce saw us through. It was for this work that the Society, together with Arcadis and Canal and River Trust were recipients of a 2021 Ground Engineering Award (see here for more details).