Effects of underpinning in a house – New findings and ways of communicating risks

Abstract: Drilled foundations are often considered to be a standard method of installation of steel core piles and similar foundation components in sensitive environments such as urban areas where other common technologies such as stranding or diging cannot be applied. Much research has been made in the area of how the piles interact in the clay. BESAB, who are specialists within underpinning, have observed that it takes time for a house to settle after an underpinning. There are uncertainties when it comes to how the house adapts to an underpinning. During interviews experts have given many different possible explanations but it seems to be a lack of knowledge in the area. All agree that it takes a couple of years for a house to adapt to an underpinning. The recommendation is to wait at least 2 years before proceed with the work in the house. In the project that has been studied in this research, precision leveling points have been measured 2-3 times a week to monitor the settlements throughout the project. The measurements have shown a greater settlement on the street level than in the basement, 2 floors down from the street level during the same amount of time which lead to the question: What are the effects of underpinning in a house? The major reason for that settlement has occurred in the area is that the foundation, consisting of wooden piles, has lost its carrying capacity due to the lowered ground water level. During underpinning the house is mostly affected by dynamic forces. The structure gets temporary weakened when holes are made in walls for beams and drilling is made in already sensitive foundation. The fixated parts of the structure, where the load transferring elements are already installed between the pile and the structure, can get locally loaded when drilling is made in adjacent areas. After underpinning the house with its weight is settling on the piles. The house has to adapt to a new mode of action. This can be done either by just leaving the house and wait for the piles to take load and get compressed or by pre-load the piles with the expected final load of the house to avoid the time for the compression of the piles. The greatest risk linked to this phenomenon is that new cracks can occur due to post settlements and in worst case in an elevator shaft or in the water proofing in bathrooms or kitchen. If renovation work starts before the house have stopped settle than the risk is big that cracks will occur in the newly renovated areas resulting in that the renovation have to be re-done. 4 Effective communication is essential to assess risks involved with an underpinning and to avoid tensions between actors involved in an underpinning project if damage due to post settlement will occur. The problem investigated in this report is translated in to Kuhn’s theory on science the beginning of the crisis. Further research is required to develop the empirics in to theory. Accurate measurements must be collected and these must be processed in mathematical statistics. Models must be built and existing theories within structural- and geotechnical engineering must be studied in further depth to be able to state a new paradigm in this area.