TerraFirma Earth Technologies, Ltd.

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Scope Of Work

Project Specifications

The Project consisted of funishing and installing approximately 100lf of 60” and 4000 lf of 54” hight pressure welded steel pipe with four 54” butterfly valves in two reinforced concrete vault structures with appurtenances. The project commenced on a bluff west of the Rio Grande river, across the river and extended across the Bosque. The pipe installation included a temporary diversion of the river, and temporary dewatering of the trench excavation within the Rio Grande and the Bosque.

The twin excavations facilitating the 54” pipe beneath the river bottom ran at approximately 20’ below the reported groundwater table elevation. The twin trench excavations within the Bosque, and the single trench excavation beneath the riverside drain ran approximately 12’ below the reported groundwater table. Per the project plans and specifications, and in order for the work to be completed in a safe and efficient manner, the temporary dewatering needed to depress the piezometric water surface to a point at least 2’ below the lowest point of the excavation.

System Design

A deepwell dewatering system consisting of 62 deepwells and 12 sand drains was designed based on the project requirements. Wells spacing was approximately 100’ in the Bosque area, and reduced to 50’ in the river bottom. The sand drains were installed between wells in the river bottom to reduce lateral seepage along seams in the riverbed.

To reduce costs and improve installation efficiency, wells were installed in a linear manner along the excavation limits. Once installed, wells were developed and capped. Pumping subassemblies were installed only in the location along the active excavation area, and were leap-frogged as the work progressed.