Filling a trench in an efficient way requires knowledge of the volume of material needed. In a simple trench, this is straightforward, but pipes and cables occupy space in the trench and reduce the volume. To avoid buying an excess of backfill it is necessary to subtract the volume of the pipes from the overall trench volume. With the aid of a calculator, this is process requires basic math skills.
Measure the length of the trench at the top and at the base. Compare the first and second measurements. If they differ, find the average by adding them together and dividing by two. Record this value for later use.
Measure the width and depth of the trench at regular intervals along its length, recording the data as you progress along the trench. Add together the individual width measurements and divide by the number of measurements taken. This produces the mean average width of the trench. Repeat the process with the depth measurements to ascertain the average depth.
Calculate the volume of the trench using the formula; volume = length x width x depth. For example, the volume of a 10 m long trench, 80 cm wide and 1m deep is 10 x .8 x 1 = 8 cubic metres.
Measure the circumference of the pipe. Divide the circumference by pi, 3.1415, to find the diameter. Halve the diameter to establish the radius, and express it in centimetres. For example, the radius of a pipe with a circumference of 90 cm is 28.7 cm.
Calculate the volume of the pipe, a cylinder, using the formula: volume = pi x (radius x radius) x length of the cylinder. For example, the volume of a 10 metre long pipe with a radius of 28.7 cm is 2.46 cubic metres.
Subtract the volume of the pipe from the volume of the trench to ascertain the volume of backfill required. In the previous example, the backfill volume is 8 - 2.46, or 5.54 cubic metres.
If a rectangular protective cover is placed over the pipe, calculate the volume of the cover, not the pipe.
Backfill such as topsoil will settle and compact over time. Allow a little extra to level off depressions that form as the material compacts.