skip to Main Content
Project Spotlight: Rehabilitation Of Lake Surprise Dam

Project Spotlight: Rehabilitation of Lake Surprise Dam

The Lake Surprise Dam is located in Mountainside, Union County, New Jersey in the Watchung Reservation. The dam is approximately 200 feet long with a maximum height of 13 feet, which impounds the waters of the Blue Brook to create a recreational lake. Prior to rehabilitation the existing dam configuration consisted of an earthen embankment steel sheet piling embedded along the upstream face, and a stone masonry wall on the downstream face. Since its original construction, the dam structure deteriorated and was in serious disrepair requiring remedial measures to avoid further deterioration and the possibility of a dam failure. Several large diameter trees were growing throughout the dam embankment and the stone masonry wall on the downstream side of the earthen embankment had collapsed in several locations.

This project involved the Rehabilitation of Surprise Lake Dam. The work required the contractor to maintain and divert the existing stream flow and perform lake lowering, including acquisition of a lake lowering, fish collection permit, and fish restocking permits. To protect their work, the contractor installed temporary dewatering measures to divert flow away from the work during construction. Trees and brush were initially cleared from the existing dam embankment. The contractor installed cofferdam steel sheeting and H-piles downstream of the proposed concrete wall and spillway. The contractor constructed a cast-in-place concrete footing, as well as a mass concrete retaining wall and spillway structure. The footing was anchored with 44 post-tensioned rock anchors, each with a design load of 360 Kips. The retaining wall was faced with a stone and mortar. The contractor installed a precast concrete valve chamber with a new low level outlet pipe and gate valve to allow drawdown of the impoundment created by the new dam. A stamped cast-in-place concrete slab was constructed at the crest of the dam to provide uniform dam crest and spillway crest elevations. Gabion mattresses and gabion baskets were added downstream of the concrete walls to prevent erosion of the soil during extreme rainfall events.

The existing dam structure was composed of an earthen embankment and a dry stacked unreinforced stone masonry wall, which were subjected to erosion and hydraulic loads causing deterioration of the structure.  Present NJDEP Dam Safety Standards require the structure resist greater design loadings for ice load and seismic loads.  Reinforced concrete was selected and designed to carry the higher design loads, in order to enable the dam to be rehabilitated to meet these greater design standards. A reinforced concrete wall on a footing anchored into the bedrock was constructed downstream of the earthen dam. The new concrete was faced with stone masonry to provide a natural appearance, consistent with the previous dam structure.

  • A total of approximately 1,800 cubic yards, of 5,000 psi mass concrete was used for most concrete. The mix consisted of Type I Portland Cement, High Range Water Reducer, Air Entrainment, and a Hydration Stabilizer.

  • Grout mix used for rock anchors consisted of Type II Portland Cement, as well as an expansive hydraulic cement (K) which was chemically compensated for shrinkage, a high bond and compressive strength value, and was crack resistant for permanent anchor installations. The grout had a maximum w/c ratio of 0.45, and was required to attain a compressive strength of 3,000 psi prior to testing. Approximately 1,000 cubic feet of grout was used for the rock anchors.

  • Portland Cement Mortar, one part Portland cement, two parts fine aggregate, was used to reconstruct or repoint the stone masonry.

Back To Top