Foundation Reuse by increasing the Bearing capacity of Foundation Soil

Polyurethane based ground injections are most frequently used to stabilize shallow foundations that have under- gone settlement due to soil shrinkage, softening of the ground by water (e.g., heavy rainfall or defective sewer pipes), or planning errors regarding the bearing capacity. The expansive reaction of the injected polyurethane, together with the counter-pressure of the foundation, results in a ground improvement of the soil layers underneath the foundation. When more material is injected, a releveling of settled shallow foundations or concrete slabs is possible. Due to more frequent reuse of existing buildings in urban areas, we have seen an increase in requests regarding the reuse of the existing foundations under increased loads. Sometimes the load increase can be accommodated by the existing foun- dations without any modifications, but it is often the case that the new load exceeds the bearing capacity of the old foundation. In this case, there are several options to strengthen the foundation. Most commonly, sectional underpinning and retrofitting micro- piles along the foundation are used to increase the bearing capacity. An alternative option is the strengthening of the ground under the existing foundations to increase the bearing capacity and to minimize settlement due to the new load. In the last few years Uretek, a provider of polyurethane based ground injections, has frequently applied these injections to successfully perform the necessary ground improvement in the context of building reuse or the addition of new stories to buildings. This article will present a case study of a residential building in Hamburg, Germany, where an additional story was planned and the bearing capacity of the existing shallow foundations was not sufficient for the new load. In this case expanding geopolymer injections were used to gain the necessary bearing capacity and the effect was monitored by an in-situ load test. A second case study will briefly illustrate an experimental project where ground injections were used to increase the skin friction of a micropile foundation at a parking garage in Munich before the addition of three new stories.