(1) Excavation for any section of a trench for drainage works shall not commence until the nature, location and size of existing utilities which may be affected by the excavation have been ascertained and the setting out details have been approved by the Engineer.(2) The effective trench width of trenches for drainage works shall not exceed the relevant effective trench widths stated in Table 5.9 for the different diameters of pipe. The effective trench width shall be measured as stated in the Contract.