REACHING SETTLEMENTS: USING DESIGN MEDIATION TO RESOLVE SUBSURFACE CONFLICT BETWEEN FLEXIBLE AND RIGID FOUNDATIONS WITHIN LEVEE SYSTEM TRANSITION ZONES

摘要

The Joint Venture design team recognized during the USACE Independent TechnicalReview (ITR) process that the transition from the proposed the T-wall section of thelevee system to a proposed 16 feet high earthen levee required a detailed understandingof settlement response within this transition zone. The design review was preformed ontwo segments of the New Orleans West Bank and Vicinity (WBV) Hurricane StormDamage Risk Reduction System (HSDRRS) where the estimated settlements made itclear that careful design consideration was imperative given the anticipated settlements ofthe pile supported T-wall monoliths were less than one inch, and settlements of theabutting earthen levee embankment were estimated in excess of several feet.Ground improvement through preconsolidation of thick soft to very soft compressible siltand clay soils was selected for the transition zone where T-wall construction supportedby deep pile foundation ends and earthen embankment placed on existing subgradebegins. Preconsolidation was achieved by the use of vertical and lateral drains (i.e.,geosynthetic composite drains), a sand blanket drain at the surface and a designed preloadsoil mass. The preconsolidation of the transition zone addressed two significantdesign concerns: 1) Overstressing of steeply battered piles as a result of free field soilmass movement across the piles during consolidation; and 2) Large settlement beneaththe T-wall bases, leading to separation and development of voids as the subgrade settlesaway from the pile supported base.A geotechnical instrumentation plan was developed to monitor settlement and porewaterpressure response within the foundation soils in response to preloading and evaluatewhen sufficient consolidation had been reached. This paper presents the data collectedfrom the instrumentation which include vibrating wire settlement gauges and piezometersas well as standpipe settlement platforms and inclinometers.Using the observed settlements, this paper presents an evaluation of battered pile bendingstresses that were mitigated through pre-consolidation. Bending moments are estimatedusing the method developed by researchers at Virginia Tech and the USACE who werecommissioned by the USACE to develop a practical, straightforward approach forengineering design of T-wall pile foundations using the computer program LPILEbyEnsoft, Inc.