Heavy or settlement-sensitive facilities that are located in areas containing soft, loose, or weak soils are often supported on deep foundations. Such deep foundations are typically made from driven pilings or concrete piers installed after drilling. The deep foundations are designed to transfer structural loads through the soft soils to more competent soil strata. Deep foundations are often relatively expensive when compared to other construction methods.
Another way to support such structures is to excavate out the soft, loose, or weak soils and then fill the excavation with more competent material. The entire area under the building foundation is normally excavated and replaced to the depth of the soft, loose, or weak soil. This method is advantageous because it is performed with conventional earthwork methods, but has the disadvantages of being costly when performed in urban areas and may require that costly dewatering or shoring be performed to stabilize the excavation.
Yet another way to support such structures is to treat the soil with “deep dynamic compaction” consisting of dropping a heavy weight on the ground surface. The weight is dropped from a sufficient height to cause a large compression wave to develop in the soil. The compression wave compacts the soil, provided the soil is of a sufficient gradation to be treatable. A variety of weight shapes are available to achieve compaction by this method, such as those described in U.S. Pat. No. 6,505,998. While deep dynamic compaction may be economical for certain sites, it has the disadvantage that it induces large waves as a result of the weight hitting the ground. These waves may be damaging to structures. The technique is deficient because it is only applicable to a small band of soil gradations (particle sizes) and is not suitable for materials with appreciable fine-sized particles. What is needed in the field is a system that can rapidly improve cohesionless, cohesive, and semi-cohesive soils without inducing damaging vibrations.
More recently, ground reinforcement with aggregate columns has been used to support structures located in areas containing layers of soft soils. The columns are designed to reinforce and strengthen the soft layers and reduce settlements. Such piers are constructed using a variety of methods including drilling and tamping methods such as described in U.S. Pat. Nos. 5,249,892 and 6,354,766 (“Short Aggregate Piers”), driven mandrel methods such as described in U.S. Pat. No. 6,425,713 (“Lateral Displacement Pier”), and tamping head driven mandrel methods such as described in U.S. Pat. No. 7,226,246 (“Impact®” system).
The “Short Aggregate Pier” technique referenced above, such as described in U.S. Pat. No. 5,249,892, which includes drilling or excavating a cavity, is an effective foundation solution, especially when installed in cohesive soils where the sidewall stability of the hole is easily maintained. The Short Aggregate Pier method may, theoretically, also be applied to multiple holes at once. However, this technique has the disadvantages of requiring casing in granular soils with collapsing holes and of necessitating the filling of the holes prior to tamping. When theoretically applied to multiple holes at once, the system is limited to very shallow treatment depths such as those needed for improvement below pavements. Needed in the field is a system that overcomes these deficiencies by allowing soil improvement to a wide range of soil conditions without the necessity of filling the holes between tamping passes and of being able to treat to deeper depths required for the support of shallow spread footings.
The “Lateral Displacement Pier” and “Impact®” system methods were developed for aggregate column installations in granular soils where the sidewall stability of the cavity is not easily maintained. The Lateral Displacement Pier is built as described in U.S. Pat. No. 6,425,713 by driving a pipe into the ground, drilling out the soil inside the pipe, filling the pipe with aggregate, and using the pipe to compact the aggregate “in thin lifts.” A beveled edge is typically used at the bottom of the pipe for compaction. The Impact® system is an extension of the Lateral Displacement Pier. In this case, a smaller diameter (8 to 20 inches) tamper head is driven into the ground as disclosed in U.S. Pat. No. 7,226,246. The tamper head is attached to a pipe, which is filled with crushed stone once the tamper head is driven to the design depth. The tamper head is then lifted, thereby allowing stone to remain in the cavity, and then the tamper head is driven back down in order to densify each lift of aggregate. An advantage of the Impact® system, over the Lateral Displacement Pier, is the speed of construction.
The “Rampact®” system is yet another displacement method in which a single conical shaped mandrel is driven into the ground and then filled with crushed stone as described in U.S. Pat. No. 7,326,004. The mandrel is hollow and fitted with a sacrificial plate or a valve mechanism at the bottom. The mandrel is later lifted to allow the rock to flow out of the bottom of the mandrel. The mandrel is then redriven back down into the cavity to compact the stone. The pier is constructed incrementally upwards in thin lifts from the bottom.
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