Calculated versus measured static capacity for two pile types

Abstract

The Missouri Department of Transportation decided to replace two bridges in northeast New Madrid Country, Missouri. The bridges were in service for approximately 50 years and both were founded on driven piles. The bridge on Route WW (A-2141) was founded on driven steel shells which were then filled with concrete (referred to as cast-in-place, CIP). The bridge on Route U (N-0771) was founded on driven, precast concrete piles. The replacement bridges will be founded on new piles thus presenting the opportunity to perform a load test on one of the 50-year old piles at each site and further our knowledge of potential for re-use of existing foundations. A cone penetration test (CPT) and a boring with standard penetration (SPT) were made at each site. The objective of this thesis was to predict the axial static capacity of the load test piles based on the CPT using various methods and compare the predicted with the measured capacities. The methods included: Nottingham and Schmertmann (1975) or Schmertmann (1978), DeRuiter and Beringen (1979-commonly called the European method), Tumay and Fakhroo (1981), Bustamante and Gianeselli (1982-commonly called the LCPC or French method), and Eslami and Fellenius (1997) method. The soil stratigraphy and CPT data were compiled for both sites. The subsurface profile at Route WW consists of interbedded layers of sand, silt and clay with a thick layer of dense sand (SPN-values of 30 to 60) beginning at about a depth of 55 feet on the west end of the bridge and increases to over 70 feet on the east end below ground surface. The subsurface profile at Route U consists of a surface layer of soft to stiff clay (about the top 5 to 8 feet thick), underlain by sand with SPT N-values of about 30 at a depth of 25 feet below the ground surface. The groundwater table is near the surface at both sites. The designed exhumed pile length is 50.9 feet for the Route WW site (14-inch diameter CIP piles). The capacity measured during the load test was 124 tons while the CPT predicted capacities ranged from 58 tons to 115 tons. The predicted capacities ranged from 0.47 to 0.93 (conservative) times the measured capacity. The original design capacity was 30 tons. It is customary to use a factor of safety (ultimate capacity/design capacity) of two to three in design of deep foundations so the original design has a factor of safety of about two to four. The designed pile length is 21.3 feet for the Route U site (16-inch octagonal precast concrete piles). The measured capacity was 134 tons while the predicted capacities ranged from 90 tons to 140 tons. The predicted capacities ranged from 0.67 (conservative) to 1.05 (unconservative) times the measured capacity. The original design capacity was 21 tons. The factor of safety for the original design is about six. The analysis of the predicted versus measured capacities resulted in a range of 4 to 113 percent difference. The Eslami and Fellenius method performed best in predicting the measured capacity of the piles for the CIP pile (Qtm/Qtp = 1.07). For the precast concrete pile, the Nottingham and Schmertmann (Qtm/Qtp = 1.07), and Eslami and Fellenius (Qtm/Qtp =0.96) methods estimated best in predicting the measured capacity of piles