Soil suitability card — minimum foundation depth

Areas where load-bearing sand layers are shallow in the soil profile are generally more suitable for construction than areas in which these layers lie at greater depth. In the legend of this card this is indicated by a gradation from “Minst suitable” to “Most suitable”. Based on information from the 3D subsurface model, the depth of the Pleistocene sand body was determined. This depth position is incorporated into a separate grid file. The depth of the Pleistocene sand often indicates, but not always, what the minimum length of foundation piles will be. It can also be based on the sand packs of Holocene ridges cut into the Pleistocene sand. Such ridges are also referred to as the term “funded flow backs”. To locate these well-founded current ridges, the 3D subsurface model and the resulting sand-thickness grid file were used again. The thickness information from these grid files is combined in a GIS with the Soil Map of the Netherlands 1:50:000 and the Geological Surface Map of TNO. This has shown that the boundary of the well-founded Holocene current ridges can be determined by looking for soil profiles with a total sand package thickness of more than 2 meters in the 3D subsoil model. The thickness of the well-founded Holocene sand bodies is then added to the depth of the top of the Pleistocene Sand Package. The resulting grid file shows the depth of the top of the load-bearing “solid sand package”. This is the depth on which, for example, heipals can be based. In practice, depending on the intended gravity of the construction, a heipal is driven to a greater or lesser extent in the sand package. That’s why the card also shows the minimum foundation depth. The actual foundation depth varies by type of construction. For the municipality of Vijfheerenlanden, which has been part of the province of Utrecht since 1 January 2019, no data is yet available.