The base of an excavation has usually to be trimmed level and cleared of disturbed or loose material so that it forms a solid base for concrete foundations, pipes or earthworks, etc. Specifications often call for the last 100 mm of excavation to be ‘carried out by hand’ – a costly procedure for the contractor which he usually seeks to avoid. The resident engineer is then faced with the problem of what alternative he will allow in lieu of hand excavation. In some types of ground, such as sandy or gravelly clay, it should be possible for the contractor to machine excavate to formation level if he uses a plain edged bucket to his machine, operates it with care, and uses the back of the bucket to re-compact any small amounts of loose material. Large open areas excavated by scraper or dozer have to be graded, and re-compacted using appropriate compaction machinery.
A formation in soft clay can be severely disrupted by tracked or wheeled excavating machinery. No amount of re-compaction of disturbed, over-wet clay will prove satisfactory; it has to dry out to a suitable moisture content before it can be rolled and compacted back. If a contractor uses a D8 to excavate down to formation level in such material, the formation surface will be so churned up by the grips of the D8 tracks that it will be rendered useless as a formation. If the contractor does not use the right method on soft clays, the resident engineer must warn him that all disturbed material will have to be removed and the excavation refilled with suitable other material or concrete at the contractor’s expense. The excavation should be undertaken by using an hydraulic hoe working backwards so that it does not have to stand on the formation. As it works backwards, suitable hardcore or other blinding material can be dozed progressively forward onto the exposed formation and compacted.
Alternatively it may be possible to use a flat tracked loader shovel to skim off the last 150–225 mm of excavation, any loose material being either removed by hand labour or rolled back with a light roller before placing of the base course for a road or blinding concrete.
The presence of springs in a soft formation material exacerbates formation finishing problems. Usually the specification will require spring water to be led away by grips or drains to a pump sump which is continuously dewatered to prevent softening of the formation. If springs are encountered and have not been anticipated, or the method of dealing with them is not specified or shown on the drawings, the resident engineer should report the situation to the engineer.
Special measures are often required to deal with springs to ensure safety of the structure to be built on a formation containing them.