Monitoring the performance of a soil nailed wall

This paper presents data from measurement of an instrumented soil nailed wall (SNW) constructed in granular soil in Elmwood Park, New Jersey. The first to be constructed in New Jersey, it was designed as a permanent structure to retain a 15 foot (4.57 m) high excavation supporting a 20 foot (6 m) high embankment roadway. The wall was required for the construction of a 22 foot (6.7 m) wide ramp, to support the excavation under an existing abutment within the granular backfill. The main concern was the lateral movement of the abutment due to removal of the soil in front of it, which was estimated to he 1.75 inches (44 mm). Accordingly, the SNW and the abutment were instrumented to monitor movement during and after construction. The SNW was constructed using the top down method and was completed with a series of 5 x 80-foot (1.5 x 25 m) excavated lifts. When each lift was excavated, drainage elements were placed at 7-foot (2.1 m) spacing and held in place by a wire mesh covering the excavated area. A 3- to 4-inch (75 to 100 mm) shotcrete layer was applied over the wire mesh and left to cure for 24 hours. Holes were drilled at an angle of 15 degrees from the horizontal. The soil nails (tendons) were inserted in the holes and grouted to the surface. A plate and nut were then placed on each 1-inch (25 mm) tendon and the nut was tightened. Upon completion of the nail installations, they were extended to be embedded in the final 12-inch (300 mm) concrete facing. An instrumentation program consisting of optical survey points, tiltmeters, and strain gages was implemented and four types of measurements were made, Six optical survey points provided the horizontal and vertical movement of the abutment. Eight digital tiltmeters provided data regarding the tilting of the abutment and the soil nail wall, and 36 strain gages provided stress/strain data along the tendons. Data collected during construction and for one year after construction indicated that there was practically no movement of the abutment or the wall Measured loads carried by the soil nails varied from 5 to 8 kips (22 to 35.5 kN) for the upper row of nails and 1 to 3.3 kips (4.5 to 14.7 kN) for the lower raw of nails.