ANALYSIS OF PERFORMANCE OF PILE GROUPS ADJACENT TO DEEP EXCAVATION

Analyses and performance of groups of step-tapered piles located adjacent to a 50-ft-deep tieback excavation are described. The excavation is made through primarily granular soils within an existing framed structure. The main columns are supported by groups of 70-ft-long, unreinforced or lightly reinforced concrete piles. The temporary tieback sheet-pile wall is located as close as 2 ft to the pile caps. Field observations, including lateral deformations of the sheet pile and lateral and vertical deformations of the main columns, reveal movements of several of the pile caps as large as 3 in. toward the excavation. This movement is twice as large as normally expected for excavations made under similar conditions. An analysis of the existing foundation conditions and a finite element computation of the bending moments induced in the piles during construction of the cut indicate that the resulting moments are not large enough to cause cracking in the piles. Sheet-pile extraction as close as 2 ft to the main-column pile caps cause the pile caps to translate toward the excavation by an amount no larger than one-half the displaced width of the sheet pile; it does not significantly alter the magnitude or distribution of computed, excavation-induced bending moments in the piles.