Back-fill grouting quality evaluation of the shield tunnel using ground penetrating radar with bi-frequency back projection method

The quality of back-fill grouting plays an essential role in the settlement control of shield tunneling, especially in the urban core area. Consequently, the tunneling professionals need practical ways to detect the grouting distribution behind the tunnel segment. Unfortunately, up to now, the traditional detection methods are not yet available for professional use. This paper presents a contribution to fill this gap, which is based on ground penetrating radar(GPR). The bi-frequency back projection(BBP) algorithm is used to obtain a high-quality visualization of the grouting distribution. By employing an advanced segment-grouting-ground model using GPRmax 3.0, the correlation between two domain frequencies radar data is used to obtain the profile of the segment, the grouting, and the soil. Furthermore, the comparison of results of the BBP method with those obtained by the BP and CBP methods shows the high performance of the proposed method. Indeed, on the one hand, the diffraction and multiple reflections are reduced dramatically, and on the other hand, the anti-noise ability is significantly enhanced according to the Integrated Sidelobe Ratio(ISLR). The use of the proposed method in the construction of line 12 of the Shanghai metro allowed engineers to determine the location of rebars, the thickness of the grouting layer, and the grouting defects.