Pile mattress for building on low bearing ground

Constructing roads, railroads or embankments on low load-bearing soils is a common challenge. The use of a geosynthetic as internal reinforcement of the soil mass prevents settlement and limits differential settlement. What about overall settlement? If the subsoil does not have sufficient bearing capacity, it will be necessary to provide additional support. For infra works, the pile mattress is the solution to allow, with a limited number of piles, uplift.

Pile cover
To increase the bearing surface of the pole, a lid is placed on top. Typical dimensions of the pile lid are 0.75 x 0.75 x 0.45m. Because the area of the lid is larger than that of the pile head, the distance to be bridged between them reduces. 

Granulate mattress
A strong and little deformable geotextile or geogrid is tightly rolled out over the pile covers, typical size of the standard rolls is 5m x 100m. This requires a sophisticated laying plan, as the proper width of the rolls and their positioning must be chosen so that sufficient anchoring will occur through friction after backfilling. Since the geo plastic has a limited width and thus no continuous strength in the transverse direction, a second layer of geo plastic will be rolled out perpendicular to the first layer. These two layers will come into contact with each other, alternatively a layer of granulate will be applied between them. Marco Hazenkamp of TenCate, with around 15 years of experience in this field, points out the importance of this laying plan because complicated geometries often arise, for example when connecting with bridges. Ideally, in the design phase, the pattern of the piles should be determined so that it matches the dimensions of the reinforcement layers. It is important to be well prepared during execution with the correct roll lengths, widths and a drawn-in laying plan.

The principle of pile mattress works by arching. Like vaults in a cathedral, the gripping forces are transferred to the supports. (Image: Suzanne van Eekelen, Deltares)

Arching
A pile mattress works by arch action, the mattress bends the load toward piles. These transfer the forces to the subgrade. After backfilling, compaction, construction of earth course with road foundations and asphalt, little or no residual settlement occurs after commissioning. 

Two layers of geotextile (or geogrid) are placed at right angles, this to ensure high strength in the two main directions.

Advantages of pole mattress
Steven De Maesschalck of Texion points out that due to a good spreading of the load to the subsoil and as a result of the strength of the geosynthetic, it also counteracts lateral displacements. The piles may therefore be driven quite far apart, resulting in savings for the builder.

Designing pole mattress
For free advice and calculation using a specially developed calculation program, for projects in the Netherlands please contact m.hazenkamp@tencate.com