Deep Soil Mixing Solves Difficult Soil Conditions

Replacing failing sewer lines is challenging under even normal circumstances. The conditions that make the historic district of Alexandria, Virginia area beautiful, including 100 year old trees and proximity to a vast coastal area, and old hsitoric communities, make traditional dewatering and open cut pipe reconstruction nearly impossible. After initially relying on traditional well point dewatering techniques to keep the trench dry for pipe replacement, the hydraulic connection to the adjacent river proved to be overwhelming and unsuccessful. Microtunnelling techniques were used to replace the aging sewer in this established neighborhood, which minimized impacts to historic growth trees and the community. The techniques minimized surface land disturbance, traffic impacts, noise pollution, and by-pass pumping. The microtunnel launching and receiving pits were constructed using deep soil mixing methods and jet grouting methods to keep the tidal waters from seeping into the pits. Excavation support was completed using steel H-piles and walers and carefully controlled jet grouting, which avoided significant utility conflicts. Combining these methods, although costly, eliminated impacts to nearby structures due to minimal ground settlement, achieved tight vertical and horizontal alignment tolerances and target invert elevations at manhole connections, eliminating weeks of the sheeting and shoring vibration and ramming noise that would have greatly disturbed this quiet old world neighborhood.