Remediation of Differential Settlement in Residential Structures on Collapsible Soil in Western Colorado

A group of detached single family houses founded on shallow footings on hydro-compressible soils suffered differential settlements up to 46 cm (18 inches). Some adjacent houses suffered no discernable damage. Most of the houses had box-like foundation wall structures with no interior or isolated footings. The foundation walls were well reinforced and surprisingly undamaged, but the wood superstructures of some of the houses were damaged beyond economical repair. Some of the houses had previously undergone remedial repairs with mixed success. The nature of most of the damage was economic (serviceability concerns), although some safety issues did exist. The time rate of settlement and geotechnical predictions for future movement combined with the nature of the damage to define the objectives for design of the remediation. That design depended on a complex interplay of geotechnical, structural, drainage, and construction engineering. Several schemes were considered for the remediation including underpinning and soil stabilization. Underpinning schemes studied included push-piles, helical piers, drilled micro-piers, and large diameter drilled piers with new grade beams. All underpinning schemes were designed to resist future downdrag from the continuing consolidation of the underlying soil. The soil loses much of its strength and stiffness when wetted, thereby reducing the lateral support to the small diameter piers. This meant that buckling of underpinning solutions had to be considered. Conventional rules of thumb for driven piling were not directly applied to the small diameter piles and piers. Instead second order analyses were performed using upper and lower bounding assumptions for stiffness of the soil. Soil densification by compaction grouting was also studied, and was the best engineering choice at some sites.