Driven piles in central Texas expansive soils

Abstract

Expansive soils cause more damage to structures annually than a combination of other major natural disasters. Because of the cost to our society, all means and methods need to be fully explored to mitigate the problems associated with expansive soils. This study will present a foundation design approach that is under utilized in this application, driven piles. The main objective of the study is to present pile test results and analysis from four driven pile project sites in three types of expansive soils found in central Texas: Del Rio formation, Taylor/Navarro formation, and expansive alluvium. Observations of the pile driving operations will be reported to highlight pile design considerations like predrilling and open versus close-ended pipe piles and the type of equipment involved. High strain dynamic pile tests were conducted on each of the four studies with rigorous signal matching analysis from the CAse Pile Wave Analysis Program (CAPWAP). Ultimate pile capacities ranged from 73 to 311 kips with an average of 61% of the total capacity coming from the pile shaft and were two to six times the structural capacity needed. Static design methods under-predicted, dynamic formulas over-predicted, and wave equation analysis conducted with GRLWEAP closely modeled test results. Average unit skin frictions ranged from 0.55 to 4.7 ksf. Restrike pile tests of 1 to 17 days after initial driving reported 30 to 100% shaft capacity gain. All open-ended pipe piles driven produced soil plugs ranging from 4 to 14 feet thick, and it was observed that harder driving conditions produced thinner soil plug thicknesses. Small diameter, thick-walled, open-ended pipe piles reached penetration of twice the depth of designated zone of seasonal moisture change without problem. The observed production rate of the driven piles was on average 8 minutes which implied daily production of 15 to 40 piles. Predrills or augered holes should be specified for underground obstructions found in soil investigation. Future studies on pile-supported foundations should measure localized movement correlated with seasonal moisture changes in expansive soil, or active zone, to confirm long-term performance. Also uplift forces need to be observed from tests on fully-instrumented and loaded driven piles to determine required pile embedment length below the active zone to withstand movement.