Foundations are essentially built in the same way as roads up to the base course. Layer thickness can differ depending on the nature of the load the structure is expected to carry and the quality of the underlying materials.

Poor sub-grade conditions can require stabilization or even pile driving to create a stable foundation.

Foundation work is well defined and limited to the site of the actual building or bridge. Strict demands are given in the case of bridge abutments. Both the load of the bridge structure and the traffic must be supported.

Layers cross-section
dynapac_lager

Compaction of the sub-grade material is done if possible, this is then followed by a graded material with proper load distributing properties. A non-woven geo-textile is often used to separate the sub-grade from the layer above.

Methods for material placement and compaction vary depending on the size of the foundation and the amount of material. A small scale foundation can have the material placed by hand or by an excavator, with additional manual raking to attain the correct level. Larger sites require dozers and large single drum vibratory compactors in order to achieve the expected productivity.

A method specification giving the required roller size and the minimum number of passes is often used for foundation work. Additional quality requirements can be a minimum stiffness, or a specific degree of compaction that must be reached.