Subsurface & Foundation Stabilization Design

There are several different methods for remediating confirmed sinkhole activity.

Compaction grouting is a common method for remediating sinkhole activity. The purpose of compaction grouting is to seal the limestone surface and to densify/consolidate the weak or loose overlying soils. SUBSIDENCE_subsurfacefoundation stabilization design_01The grout consists of sand, cement, and fly ash. Injection holes are drilled around the perimeter of the structure on approximately 9 to 10 foot centers. The grout points are generally advanced to the depth of the competent limestone. The injection holes are either straight down or at various angles to reach further under the structure and cover the area of identified sinkhole activity. Grout is pumped through the injection holes at depth, sealing the limestone surface. Structural movement and grout line pressures are monitored during the process. Once a predetermined pressure is reached, the casing within the injection holes is raised 5 to 10 feet. Grout is again pumped through the injection hole creating a bulb of grout, compacting the adjacent soils. This process is continued in each injection hole up to 10 to 15 feet below land surface. Compaction grouting is ideal for sinkhole activity at depth, as well as geographical areas with more than 25 feet of soils between land surface and the limestone unit.

OLYMPUS DIGITAL CAMERAUnderpinning is another form of stabilization. Underpins are structural steel piles designed to transfer the load of the structure onto the competent limestone, bypassing the soils between land surface and the limestone unit. They are typically installed around the perimeter of the structure on approximately 6 foot centers. On occasion, interior underpins may also be used to support interior load bearing walls or floor systems. Underpins are tubular, steel piles advanced to the competent limestone by means of a hydraulic push. Once the underpins have been advanced to the competent limestone they are permanently fixed to the existing foundation of the structure with a steel bracket. Some instances also call for steal spreader beams connected to more than one underpin to better spread the load of the structure. Underpinning is ideal for sinkhole activity in geographical areas where the limestone unit is shallower than 25 feet below land surface. Additionally, underpins are effective in foundation repair in situations with detrimental shallow soils such as highly plastic shrink/swell clays and decomposing organic soils.

SUBSIDENCE_subsurfacefoundation stabilization design_03Injection piers are a form of structural underpinning that incorporates the use of compaction grouting. The injection piers are hydraulically advanced to bear on the competent limestone. The piers are then fixed to the structures foundation with steel brackets. Finally, compaction grout is pumped through the piers which have been perforated at depth. The grout essentially locks in the piers while compacting the surrounding soils. Injection piers are ideal for remediating sinkhole activity at depth in areas with shallow limestone.

Chemical grouting is a remediation technique that can be used in conjunction with any other form of foundation and subsurface stabilization. The purpose of chemical grouting is to densify shallow soils. The loose shallow soils can be the result of erosion, downward migration due to sinkhole activity, or inadequate compaction prior to construction. Chemical grouting is an effective way to densify these loose soils, creating a stronger foundation for the structure. Chemical grouting is done in conjunction with and after any other form of remediation. The process of chemical grouting includes injecting polyurethane foam liquid at a low pressure through 5/8” holes spaced 3 to 5 feet apart. The polyurethane foam expands slowly; filling any cavities or voids, as well as compacting the loose, shallow, load-bearing soils. The chemical grout quickly hardens, to create an adequate, strong, base material. Chemical grouting is normally a shallow application averaging around 10 feet in depth.

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