Quality Control of Diaphragm Walls by Crosshole Sonic Logging

Abstract

The quality of concrete and integrity of deep diaphragm walls in any given project is very important to ensure safety within the excavation and for the long term performance of the structural element. Not only is the structural strength of the wall important, but also its effectiveness in preventing water seepage, which could be compromised by soil or slurry intrusions among other defects. For these reasons, major civil works frequently require thorough testing of deep foundation elements to assure adherence to material quality specifications. For deep foundations, Crosshole Sonic Logging (CSL) is the most common integrity test measuring the travel time of ultrasonic pulses in the concrete. A low wave speed indicates inferior concrete quality, while a completely missing signal indicates an anomaly or defect. The test is frequently used for the integrity assessment of foundation piles, but is less commonly applied to diaphragm walls. This paper describes the construction and CSL testing of several tunnel access shafts ranging from 16 to 20 m in diameter, which serve as part of a newly constructed 62 km long wastewater tunnel in Mexico. The diaphragm walls were installed to a depth of up to 45 m, and the individual panels were up to 6 m wide. Because of their great depth and large lateral extent, the maximum possible distance between CSL probes, the verticality of the inspection tubes, and the effect of variable tube distances on the CSL measurements were important questions that arose during the planning stage. This paper presents CSL test results that answer these questions.