A timber heading is where a tunnel is manually excavated, and supported using timber sets, enabling the installation of sewer pipes where access is limited, and standard methods of sewer connection can’t be employed. This method of trenchless sewer connection dates back centuries, but it is very much still relevant today – it’s regularly used in London and in other large city centres, where space constraints, busy roads, and dense development makes laying sewers in trenches impossible.
Our client wanted to connect onto the public sewer within the highway in central London, but with the carriageway being a designated Red Route, a standard approach would have meant incurring a very high cost in lane closures and TFL rental scheme daily charges. With this in mind, we decided to propose a timber heading, which actually worked out cheaper than open-cut trenching when the extra costs of road closures were borne in mind. Using a timber heading, our work could be carried out from the site boundary, by tunnelling out to the main public sewer within the highway, keeping all traffic flowing as normal.
After taking level measurements from the existing upstream and downstream manholes within the highway, to give us our excavation depth for the shaft, the timber heading began as a shaft at the site boundary, excavated down to invert level. At the correct depth, the actual process of excavating towards the sewer does change depending on ground conditions, but does not change the way it is installed. Once we reached the existing sewer with the timber heading, we installed pre-formed clay saddle to complete the connection to the public sewer.
Sewer connection made, the back-fill process involved laying the pipe, and dry packing with concrete by hand, until the heading is filled back to the shaft. At this point we used the shaft to construct a new pre-cast concrete boundary manhole, leaving a stub pipe for our client to connect to.