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In medium-depth trenches, diffferent soil conditions are very often encountered throughout the depth of the excavation , and therefore the method of timbering must be changed to suit the new soil conditions.

Hand trimming is usually required in the trench bottom to form an accurate line and level ; this process is called bottoming of trenches.

Approximately 150mm should be allowed for trimming by hand , and it is advisable to cover the trimmed surface with hardcore to protect the soil at formation level from being disturbed or drying out and shrinking .