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| This type of limestone rock is very prevalent in Frederick, Maryland, where Morgan-Keller Inc. was contracted to build the Riverside Corporate Park development.
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Aboveground builders have it relatively easy. Depending on a building’s design, function, number of stories and location, they know it will require a certain amount of concrete, steel, glass and other materials. But when a project requires ground stabilization, especially in a geologically unpredictable environment, there is less certainty and potentially greater risk for differential settlement. Call it the curse of karst.
Karst topography describes irregular ground conditions characterized by sinkholes, caves, streamless valleys and underground streams--all developed by the flow of surface and underground water in soluble rock. Karst is especially common in limestone formations. In extreme cases, the distinctive caves and large sinkholes of karst are potentially devastating when roads and structures are built on top of them.
Morgan-Keller Inc. knows this. When the general contractor was contracted for the construction of Riverside Corporate Park in Frederick, Maryland, it anticipated problems because the site sat within a 35-square-mile section of a known karst formation. Before beginning work on the fifth building, RIV-5, Morgan-Keller contracted GeoStructures of Purcellville, Virginia, for design/build services and Specialized Engineering of Frederick, Maryland, for geotechnical engineering services. Ultimately, their combined efforts and teamwork led to a cost-effective solution for stabilizing the building’s foundation.
RIV-5 ReconSince Morgan-Keller had worked on karst sites before, it was very careful to research the RIV-5 site terrain. Borings drilled at the site revealed that the karst rock formation was inactive with no imminent sinkholes, but the site still required special attention. The limestone bedrock was “pinnacled,” meaning that the rock had been eroded in a way that the load of a foundation could potentially be supported only by ridges, or pinnacles, of rock in between sections of soil. This created a potential for excessive differential settlement that could produce cracks in walls and floors.
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| The unsuitable soils of the RIV-5 site, as shown in this cross section, could have caused differential settlement.
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In addition to the test borings, the engineers used an air-track percussion hammer to advance holes into the ground and further define the depth to bedrock, the layer of consolidated rock that can carry high loads. This drilling technique, according to Ken Lightbody, engineer and project manager for GeoStructures, is particularly helpful on karst sites. “It is a quick and relatively easy way to see whether there are voids or soft soil layers within the rock,” Lightbody explains.
Searching for a SolutionIn karst environments, geotechnical engineers often recommend a solution of micropiles, the small-diameter drilled and grouted foundation piles that provide vertical and lateral support. Often designed with steel casings and tension rods, micropiles are drilled through the top layers of soil and then “keyed” into suitable bedrock so that structural loads are transferred from a footing to the rock. The original plan for the RIV-5 site called for micropiles, but the steel casings and the installation time they require pushed the cost too high. Alternative solutions include drilled shafts or total excavation and replacement with fill soil, but these also were deemed too expensive.
Against this backdrop, the geotechnical engineers proposed the use of Rammed Aggregate Pier (RAP) elements, which are essentially rock piers stiff enough to minimize differential settlement. The piers directly support the concrete footings and provide the necessary capacity for a foundation. But they differ from micropiles in that the loads within the pier are transferred to the surrounding soil instead of directly to the bedrock below. Built by GeoStructures, the RAP elements raise allowable bearing pressure by two to three times that of the unreinforced soil and create more uniform foundation support. And because they can be installed quicker and don’t require the expensive steel casings, RAPs cost up to 75% less than micropiles.
“Both micropiles and RAP elements have a long history of supporting foundations,” Lightbody says. “But in this situation, we utilized RAP elements because of their lower cost and [because] soil reinforcement was sufficient for minimizing differential settlement.”
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| A specialized beveled tamper mounted on the Cat 315 excavator compacted the stone in 1-foot lifts to stiffen the soil.
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Installing the SystemDuring the installation process, GeoStructures excavated soil from 30-inch-diameter holes using an LM
30 LoDril by Bay Shore Systems Inc. mounted on a Caterpillar 315 excavator. This gave the engineers a good view of the soil stratigraphy across the site so they could confirm soil conditions and verify the RAP depths.
T300 Bobcat skid steers dumped the aggregate into the open RAP cavity using a specially designed graduated bucket. Then a specialized beveled tamper mounted on the Cat 315 excavator compacted the stone in 1-foot lifts. As each lift was compacted, the aggregate was pushed downward and outward against the cavity wall, which created lateral pressure on the soil and confined the RAP. A total of 258 RAPs were initially installed under perimeter walls and columns at an average depth of 15 feet. Another 18 RAPs were installed after a redesign of the building resulted in the need for more spread footings.
Because of the karst, GeoStructures also took extra steps to guard against erosion. With a technique patented by Geopier Foundation Company, the developer of the RAP system, GeoStructures used cement-treated aggregate to construct the RAPs in order to lower the permeability of the pier and avoid potential destabilization.
Keeping it Under ControlControlling surface water is of primary concern on karst sites because of the possible complications caused by further erosion. At RIV-5, crews installed a stormwater management system consisting of a temporary sediment trap where silt settles to the bottom. After the site is developed and the silt is scooped out of the sediment trap, the area will be converted to a stormwater pond where roofs, parking lots and any other hard surface will drain.
The choice of RAP elements underneath the perimeter walls and columns was cost efficient and allowed Morgan-Keller to establish a faster construction schedule. But, as demonstrated at RIV-5, it takes a certain amount of value engineering to determine which technology is best suited for site-specific conditions. “Construction is a mature industry,” Lightbody says, “but it still must innovate and use approaches that are unique enough to handle the marginal sites or those with tricky characteristics like RIV-5.”
