Application of ground penetrating radar in grouting evaluation for shield tunnel construction

Uniformity and quality of the grout behind the lining segments have a great influence on the long-term settlement in shield tunnel construction in soft soil areas. In order to evaluate the effectiveness of the grouting treatment before the tunnel operation, a nondestructive testing method using ground penetrating radar (GPR) was proposed to detect the grout thickness behind the lining segments of metro lines in Shanghai, China. GPR has shown to be a viable approach due to the facts that: (1) the detecting objects (concrete segments, grout and soil) were in the depth of one meter or less; (2) dielectric parameters of all the materials can be obtained from the laboratory; (3) the contrasts in the dielectric properties among these three materials were large enough; (4) only the boundary between the grout and the soil needed to be found since the concrete segments had a known even thickness of 0.35 m. Three GPR frequencies 250 MHz, 500 MHz and 1 GHz were used in the field tests in Shanghai Metro line 9. The results showed that the 250 MHz GPR had a low resolution while the 1 GHz GPR had a shallow detecting depth. Frequency at 500 MHz showed the most promising results. These tests results demonstrated that nondestructive geophysics techniques such as GPR detection can be used to mitigate the risks of long-term ground settlement, a critical issue of shield tunnel construction in soft soil areas such as Shanghai.