CSO 52 - Temporary Support of Excavation and Deep Foundations for Flood Protection-Paper

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The CSO-52/Blackwater CSO Tunnel project in Lynchburg, Virginia, is intended to reduce combined sewer overflows by providing up to 4.7 million gallons of storage. The paper focuses on construction at the CSO 134 site, beside Blackwater Creek, where flood exposure, contaminated soils, variable rock, restricted access, and undocumented buried obstructions complicated the work.<br /><br />The approximately 98-foot-deep pump shaft required temporary excavation support that could withstand a 100-year flood. The selected system used a circular sheet-pile enclosure, four levels of steel ring bracing, and pre-excavation grouting to limit groundwater seepage through fractured rock. Rock excavation was supported with rock bolts and fiber-reinforced shotcrete. The electrical building, designed for the 500-year flood, was founded on 4-foot-diameter caissons with permanent casing and 12-foot rock sockets. Soldier piles and timber lagging supported excavations for other structures.<br /><br />Construction revealed buried concrete slabs, walls, columns, and footings, as well as substantial variation in rock elevation. These conditions delayed pile installation and required drilling through some obstructions. Actual sheet-pile toe elevations differed from design assumptions, prompting adjustments to bracing elevations. Constrained work areas and the need to pour caissons soon after socket excavation also demanded careful scheduling.<br /><br />Survey monitoring showed that sheet-pile movements generally matched predictions. Greater movement in one area was linked to an unexpectedly large stockpile surcharge, which was redistributed after the issue was identified. No flood event had occurred during the reporting period.<br /><br />The authors conclude that coordination, planning, monitoring, and timely responses to unforeseen conditions enabled a cost-effective support system that met project requirements.

Keywords

CSO-52

Blackwater CSO Tunnel

Lynchburg Virginia

combined sewer overflow

sheet-pile enclosure

pump shaft

flood-resistant excavation

rock excavation

groundwater seepage

construction monitoring