Abstract:
Static loading tests and piled foundation design for a three-storey building
Scott Jacobs1), P.E. and Bengt H. Fellenius,2) Dr.Tech., P.Eng.
1) Senior Engineer 2)Consulting Engineer
Keller–North America Sidney, BC
Berwyn, PA Canada
ABSTRACT
The piled foundation design of a three-storey school building with no below-grade floor is discussed. To obtain an even final grade the school building required up to 3 m (18 ft) new fill. The fill typically consisted of reddish brown silt, sand, or clay with varying amounts of gravel and included pieces of roots, wood, and concrete. The natural soil consisted of 10 to 15 m (30-ft to 50-ft) thick compressible clay and silt overlying sand at about 15 to 18 m (50 to 60 ft) depth. The depth below final fill level to bedrock was at least 30 m (180 ft). The groundwater table was at about 7 to 9 m (25 to 30 ft) depth. Settlement analysis indicated that long-term settlement of the compressible soil after construction would amount to about 100 mm (4 in) at the ground surface and reduce more or less linearly with depth.
The piles were all 457 mm (18-inch) diameter continuous flight auger piles (CFA-piles) drilled to 22.5 m (74 ft) embedment in one continuous operation. The assigned maximum working pile load was 785 kN (160 kips). The piles were single piles and narrow pile groups comprising 2 to 8 piles. The total building area was 47,000 m2 (506,600 ft2). The total number of foundation piles installed was 1,577.
Four head-down static loading tests were performed to maximum load that ranged from 3,200 through 4,250 kN (720 through 950 kips). Measured maximum pile head movements were 18 through 106 mm (0.7 through 4.2 in). The test piles were instrumented with vibrating wire (VW) gages at several levels, allowing for determining the axial force distribution.
The difference in interpretation of the downdrag aspects of the subject project created considerable confusion for the design team. The presentation highlights the most challenging and misunderstood aspects, illustrating common misconceptions, such as adding the drag force to the sustained load when analyzing the ultimate limit states condition. The impacts associated with each misconception are discussed, including interpretation of how the code does and does not address downdrag and drag force, along with the potential consequences on the design.