Stabilization of Fill Slopes Solved Using Deep-High Capacity Tiebacks

The Stonepoint Sweetwater Reconstruction Project consists of a 172 unit residential development of single family homes on shallow concrete slab foundations built in 1987 near San Diego, California. The project has experienced various levels of distress and damage from geotechnical related causes attributed to unstable fill slopes and associated lateral fill slope extension, volcanic rock fills with voids and inadequately compacted clayey fill soils, and soil creep on fill slopes as high as 18 meters and as steep as 1 1/4 to 1. Differential building settlements of up to 20 cm and horizontal movements of building and slope improvements of several centimeters forced immediate action and the expenditure of several million dollars to solve the problem over a 5 year design/construct period from 1995 to 2000. Considerable investigation and evaluation of numerous options resulted in the selection of a unique but economic solution of tied back individual anchor blocks as the primary slope restraint system to maintain the homes from further movement. Over 100 wire tendon tiebacks were installed and pressure grouted into the volcanic bedrock using down hole percussion drilling techniques. The tiebacks were drilled to depths as deep as 61 meters and tested to capacities of 1423 kN. Concrete and steel anchor blocks measuring as much as 2.4 meters square and 0.6 meters thick were then constructed for each tieback on the rock and clay fill slopes below the homes. When tested and locked off these tied back anchor blocks formed the primary line of defense to mitigate the shallow building foundation and slope improvement failures. This paper will discuss the design of the tieback restraint system, the difficult construction constraints encountered on the steep slopes, and the current successful results of slope and building monitoring.