Weak Grout Areas Mapped with Tomography

Weak areas in dry pack grout pads were identified and mapped by NDT using detailed sonic/ultrasonic velocity measurements and tomographic data processing.

Grout pads under support members of a coffer dam, formed by using mechanically placed grout, were suspected of having voided and/or weak zones. Sonic/ultrasonic compessional wave data were acquired in a systematic pattern to evaluate the grout. Tomographic processing of the data generated a detailed velocity/strength contour of the grout pads. Areas with a compressional wave velocity less than 10,000 ft/sec, shown in blue above, are associated with concrete strengths less than 2,000 psi, and are indicative of areas with voided, honey combed, or cracked grout. Areas with a compressional wave velocity greater than 18,000 ft/sec, shown in red above, are indicative of refracted signals through steel overlying the grout pad. Areas above 10,000 ft/sec and below 18,000 ft/sec, shown in yellow and green, are indicative of 4,000 to 5,000 psi strength grout. Post grout testing, documented improved strength conditions of the grout pads.