Instrumentation of a 50-ft (15.2-m) Deep Braced Excavation

An extensive instrumentation program was carried out over a 6-month period on a 50-ft (15.2-m) deep braced cut in dense sand. The temporary bracing system consisted of interlocking steel sheet piles (Larssen type IIIN), wales, and struts from wide-flange steel sections. The soil consisted of fine, uniformly graded sand with varying relative densities from medium to very dense. The loads of the struts on each side of the center bay for three strut levels were obtained by measuring the strains in the struts with a mechanical dial gage over an 8-in. (20.3-cm) gage length. The reliability and precision of these measurements are discussed. The active and passive earth pressures were measured with 13 vibrating-wire, earth pressure transducers mounted flush on two adjacent piles at the center of the wall. The pressure cells were attached to the piles prior to driving and were subjected to severe conditions due to the extreme driving resistance of the piles. The calibration methods of these transducers prior to driving and on retrieval after completion of the construction are discussed in detail. The ruggedness and precision of the cells were evaluated. The relatively large deformations of the cut were measured from lines of sight in the horizontal and vertical planes. In its final construction stage the wall had moved out more than 3 in. (7.6 cm) at the top and over 7 in. (17.8 cm) at the bottom of the sheet piles. The large inward movement of the bottom of the wall was mainly due to a hydraulic failure which significantly reduced the passive resistance of the soil inside the excavation. Since the earth pressure cells measured total pressures only, it was necessary to measure the stresses due to water pressure to calculate the effective earth pressures. The water levels were obtained from borehole and open-standpipe observations. Each water pressure indicator consisted of plastic tubing with an open-end, screened fitting tapped through the sheet piles. The standpipes were installed during the excavation, and proved to be simple, inexpensive, and accurate. The instrumentation program was considered successful despite many problems which were encountered mainly during the excavation period.