CONTROLLING GRADING EXCAVATION
This type of setting out generally occurs in drainage schemes where the trench, bedding material and pipes have to be laid to a specified design gradient. Manholes (MH) will need to be set out at every change of direction or at least every 100 m on straight runs. The MH (or inspection chambers) are generally set out first and the drainage courses set out to connect into them.
The centre peg of the MH is established in the usual way and referenced to four pegs, Alternatively, profile boards may be set around the MH and its dimensions marked on them. If the boards - are set out at a known height above formation level the depth of excavation can be controlled,
(a) Elevation, and (b) plan
Use of sight rails:
Sight rails (SRs) are basically horizontal rails set a specific distance apart and to a specific level such that a line of sight between them is at the required gradient. Thus they are used to control trench excavation and pipe gradient without the need for constant professional supervision. SRs being used in conjunction with a boning rod (or traveller) to control trench excavation to a design gradient of 0.5% (rising). Pegs A and B are offset a known distance from the centre-line of the trench and levelled from a nearby TBM. Assume that peg A has a level of 40 m and the formation level of the trench at this point is to be 38 m. It is decided that a reasonable height for the SR above ground would be 1.5 m, i.e. at a level of 41.5;
thus the boning rod must be made (41.5 − 38) = 3.5 m long, as its cross-head must be on level with the SR when its toe is at formation level. Consider now peg B, with a level of 40.8 m at a horizontal distance of 50 m from A. The proposed gradient is 0.5%, which is 0.25 m in 50 m, and thus the formation level at B is 38.25 m. If the boning rod is 3.5 m, the SR level at B is (38.25 3.5) = 41.75 m and is set (41.75 − 40.8) = 0.95 m above peg B. The remaining SRs are established in this way and a line of sight or string stretched between them will establish the trench gradient 3.5 m above the required level. Thus, holding the boning rod vertically in the trench will indicate, relative to the sight rails,
whether the trench is too high or too low. Where machine excavation is used, the SRs are as in Figure 12.18, and offset to the side of the trench opposite to where the excavated soil is deposited. Before setting out the SRs it is important to liaise with the plant foreman to discover the type of plant to be used, i.e. will the plant straddle the trench as in or will it work from the side of the trench and where the spoil will be placed to ensure the SRs will be useful.