I am building a 6' double skin recon stone garden wall. I amm digging down to a depth of 650mm (width 500mm). I have to negotiate a manhole which has a depth of 800mm. One of the pipes will run under the wall foundations, i.e. 200mm below bottom of trench. I am concerned that the weight of concrete & wall will compress pipe. Should I dig down to 800mm at point of pipe and construct a duct with lintel over?
Yes. Dig down to the underside of the pipe for 400mm either side of it. You will then have a thickening of the foundation in these areas. See our foundations project on the website which ( using a stepped foundation as an example) demonstrates this thickening. Lay the blocks up to the pipe either side and place a lintel over with at least 400mm bearing either side of the pipe. This will sread the load of the blockwork above the pipe all the way back to the main foundation.