Proceedings - DFI Specialty Seminar, "Augered Cast-In-Place Piles Seminar- 2000", St. Louis, MO, (DFI)
Static Load Testing-How important is it?
Jack Hayes, Bob Simpson
The design and construction of deep foundations involve many uncertainties. For the designer there are uncertainties regarding soil and ground water variability, regarding sampling and lab testing, regarding analytical techniques and regarding construction technique. The latter being the uncertainty over which the engineer has least control and which may have the greatest impact on capacity. Foundations with a high design factor of uncertainty can be very costly. Foundations with a high design factor of uncertainty can be very costly. Foundations designed with a high factor of uncertainty may hav ea low factor of safety, which can be even more costly. The only way to reduce these uncertainties to manageable levels and, at the same time, produce reasonably economical deep foundations is to carry out static load tests on actual production piles. The methods for analyzing loads in a load test should mimic, as closely as possible, the loads that will be applied by the structure. If the pile foundation resists primarily static loads then test loads should be static. If the foundation resists impact loads then a dynamic impulse-type test would be more appropriate. Why do engineers carry out load test? Some do them to: 1-confirm design ultilmate capacities 2-assess movement characteristics under load, 3-confirm design soil parameters, 4-assess construction technique, 5-detect deficiencies, 6- control quality. In our experience, the majority of engineers are concerned primarily with the first three in the foregoing list (sometimes only with the first one). In other words most are interested only in showing that their own design work is OK. (A "CYA" mentality?) In real life, however, the last three items can account for more grief during construction that the first three. It seems clear to us that static load testing of production foundations is the only way to rmove uncertainties regarding all six items. Dynamic and indirect test methods provide a reasonably economical way to reduce uncertainties about the last three items. Therefore a good test program probably should include static testing to provide absolute data and dynamic, impulse or indirect testing to provide comparative or relative data.
|article #216; publication #52 (ACIP-2000)|