Foundation Design for Large Water Storage Tanks with Marginal Subsurface Soil Conditions and Challenging Differential Settlement Criterion-Paper

Paper

Pdf Summary

East Bay Municipal Utility District evaluated foundation options for replacing Oakland’s Central Reservoir, an aging, oversized reservoir on weak Holocene alluvium over weathered bedrock. The proposed tanks would sit on a raised pad requiring up to 30 feet of fill, and the project sought to limit differential settlement to approximately 0.3 inches over 50 feet.<br /><br />The study compared seven foundation alternatives, including lightweight cellular concrete (LCC), roller-compacted concrete, rammed aggregate piers (RAP), cement deep soil mixing (CDSM), and vibro stone columns. Settlement estimates combined conventional elastic and consolidation calculations with simplified finite-element soil–structure interaction analysis. Because subsurface deformability is uncertain, the authors assessed four settlement scenarios, varying whether the alluvium and underlying Upper San Antonio Formation could consolidate.<br /><br />Results indicate that improving the existing soil is necessary: unreinforced soil and fill, as well as LCC or roller-compacted concrete alone, did not adequately control settlement. LCC fill over RAP-reinforced alluvium was the only option reported to meet the settlement objective across all scenarios, with estimated maximum total settlement of 0.53–0.81 inches and differential settlement of 0.14–0.31 inches. CDSM and stone columns also performed effectively in many scenarios, but their outcomes were more sensitive to assumptions.<br /><br />The study is preliminary and intended to compare alternatives, not provide design-level predictions. Its findings highlight the need for additional investigation and testing, particularly to determine the consolidation behavior of the bedrock formation. Settlement at the tips of rigid inclusions may transfer additional load to the alluvium, making accurate assessment of tip settlement essential to selecting and designing a suitable foundation.

Keywords

Central Reservoir

Oakland

foundation alternatives

settlement analysis

rammed aggregate piers

lightweight cellular concrete

cement deep soil mixing

vibro stone columns

soil-structure interaction

differential settlement