At the request of a major retail chain, MEA, a UES Company, performed a subsidence engineering study for a proposed construction site in Alabama. This proposed construction site was underlain by a karstified limestone which has caused land subsidence. Furthermore, the site is adjacent to an abandoned limestone quarry which exacerbated the subsidence potential due to the dewatering operations which occurred during the quarry operations. The quarry pit was now abandoned and has filled with water.
From studying the subsidence activity in and around the site and other available data, MEA determined this site to have a significant risk of subsidence and associated damage during the expected lifetime of the facility. This subsidence risk was reduced as a result of the mass grading where a 8 to 16 feet cut was required from the existing grade to remove the spoils from the pre-existing quarry operations and also to remove other deleterious materials. This undercutting exposed and resulted in upward progressing subsidence activity. During this mass grading operation, 14 subsidence sinkholes were exposed. Figure 1 shows an example of one of these events. These subsidence features from the underlying karst in the quarried limestone were treated initially with the injection of low mobility grout though closely spaced injection holes. These sinkholes were then excavated out to the submerged throat of subsidence in the limestone bedrock and then plugged with high slump cement grout. This activity is shown in Figure 2. A structural slab was then placed over these grout plug areas. Once this subsidence treatment was complete, 4 to 6 feet of engineered fill was placed across the site to bring it to grade.
Because significant subsidence risk still existed after the above mitigation measures were taken, several options were considered. These included, a comprehensive compaction grouting campaign, a TDR subsidence detection network system under the building, and/or supporting the building superstructure on micropiles with and without a supported structural floor slab. Based on discussions with MEA on the future risks of subsidence and potential damage, the client determined that the superstructure supported on micropiles provide adequate protection against estimated risks.
The micropile design, and installation inspection, which included proof and verification pile testing, were performed by MEA. The installation of the micropiles into competent limestone bedrock were typically to depths of from 40 to 50 ft but extended as deep as 100 ft. Figure 3 shows the installation of one of the micropiles. For some of the piles in deeper solution features, competent bedrock could not be practically reached. To avoid significant additional costs to an unknown pile depth, the micropile design was modified by MEA in these instances to adequately support the design pile load at shallower depths.
Remarks
Given the combination of site variability of the karst conditions in addition to other project conditions and the client’s risk management needs, there are a number of approaches to handle the subsidence risk and more importantly the potential damage effects from such an event(s). Therefore the most cost effective treatment of subsidence risk from karstified limestone bedrock is very much site dependent and is related to the experience and training of the engineer on the project.