Structural Engineering for the Remediation of Masonry Buildings on Collapsible Soils

Two masonry buildings founded on shallow footings on collapsible soil suffered significant distress from settlement. Both buildings are of similar design and both serve to house and maintain heavy vehicles as well as office areas. Both buildings suffered differential settlement on the order of eight to nine inches, with severe damage to masonry walls. Remediation schemes considered included foundation underpinning by micro-piers, push-piles, raft foundation, compaction grouting, and soil stabilization under slabs. The pros and cons of each scheme are discussed and the reasons for the decision to utilize compaction grouting as the preferred solution for the masonry buildings are presented. Although the building sites are adjacent, the depths of collapsible soil varied dramatically owing to the geology of the debris fan upon which the buildings were founded. The owners of the two buildings had differing resources available to cover the costs, thus the scope of repair was more extensive at one of the buildings. The structural approach was to design for both soil densification and spot underpinning. The soil densification was to improve spread footing performance. The specification of discrete locations for the grout columns themselves allowed rational distribution of reactions and effects in the existing foundation elements. Water ingress was the catalyst that triggered soil collapse but the water issues were unique to each site. Unusual and extensive measures to alleviate water ingress for each building are discussed. Repairs to the masonry bearing walls, some of which were severely damaged, are presented.