Application of ERT for Quality Assurance in Jet Grouting Columns; A development of an alternative quality control

Sammanfattning: With the increased urbanization major cities are facing, expansion of larger facilities and households are inevitable. Expansion means larger areas of the surface needs to be used where the risk of varying subsurface soil properties increases with larger surface areas. This can result in soil properties that might not be stable enough to support these constructions. A potential solution to solve this problem is the soil improvement method of jet grouting. Jet grouting is a versatile soil improvement method used for various geotechnical aspects in construction projects worldwide. By eroding and mix in-situ soil with fluid binders from a high-pressure jet, improved geotechnical properties of the soil body could be achieved. When achieving the desired stability of the column it is crucial that the geometry and homogeneity correlates with the preliminary design standards. Thereby, quality controls are routinely performed on the column however, existing quality controls vary in methodology efficiency as well as the level of accuracy obtained from the controls. The thesis therefore aspired to find and compare an alternative quality control to the existing quality controls based on Electrical Resistivity Tomography (ERT) practice. ERT is a geophysical measuring technique for determining the electric properties of the subsurface. This technique consists of placing electrodes in contact with a specific medium and inject current via the electrodes to create an electric potential field. The electrodes will then measure the potential differences of the potential field which are translated to resistivity properties of the medium. In this thesis a composed cable consisted of electrode cables and temperature sensors were inserted into a freshly produced test column. By injecting current the potential differences in the column and the surrounding soil were measured and the apparent resistivity properties of the underground could be determined. The measurements were performed at different times to study the curing process. The apparent resistivity measurements were inverted for two software: Res2DInv and pyGIMLi. Due to the borehole design, the resistivity measuring was performed differently than the traditional surface ERT-measurement, a geometry adapted to the data had to be applied for obtaining suitable inversion models of the column. The measured data contained several outliers and errors which increased during the curing process of the column. This was most likely due to a corrosive action taking place on the electrode surfaces. The inversion models resulted in relatively large uncertainties as well, however the pyGIMLi inversion models showed better correlation with the intended geometry of the column compared to Res2DInv. Even though uncertainties of the inversion models were found, a resistivity contrast between the treated and untreated soil was identified throughout the curing process which decreased with curing time. The conclusion of this thesis is that the ERT-method has potential to be applicable to quality assuring jet grouting columns. The quality parameter for determining the geometry and homogeneity of the column would be defined by the boundary of which the resistivity contrast between the treated and untreated soil was located in the inversion models or how the relative resistivity changes during the curing process. However, this quality control was not ideally executed, neither with the set of inversion models developed in this thesis, nor the set of equipment used when measuring as it was less adaptive to the column conditions and the installation procedure. The developed quality control of this thesis was not validated and it could therefore not be compared with the accuracy existing quality controls have. But we believe that the ERT-method could potentially be a more time-effective and accurate quality control compared to existing quality controls with refinement in routine application, measuring equipment and inversion models.