Many good level development sites on poor ground are avoided due to lack of reasonable programmes and systems. When increasing levels over peat, soft clay or existing fill Geoman Ltd promotes the traditional method of surcharging.
This method is very inexpensive compared to modern piling and ground improvement solutions. There is no heavy plant or specialised equipment required. The natural soils are loaded slowly and so gain in strength with consolidation whereas modern methods all involve soil displacement and mixing that weaken the soil strength permanently.
The existing vegetation is normally trimmed with the topsoil left in place. A high strength geotextile is then laid over the formation to separate and strengthen. A drainage layer is placed over the geotextile to allow upward migration of water from the consolidating soil below.
Levels are increased to about 600mm above finished level and then left for two to three months. This is normally sufficient to complete primary consolidation though every site is different. The increased level during the surcharge stage means that the ground is then unloaded to provide the finished level.
Any excess fill remaining from the surcharge process can then be used in other parts of the site.
Flexible services are usually placed in the finished road after surcharging when future differential settlement has been minimised.
Geoman Ltd provides advice at every stage of the process as follows:
Trial pits and soil tests to assess compressibility
Base geotextile design strength
Advice on compaction method in relation to plant equipment and soil fill type available
Analysis of levels during surcharge
Prediction of long-term behaviour