Vibro stone columns (VSC’s), also referred to as stone columns or aggregate piers, are a form of ground improvement that utilize an array of crushed stone columns to increase the bearing capacity of in-situ soils. The techniques used to install the VSC’s displace weaker soils and leave a column of compacted stone in its place. VSC’s typically terminate in a stronger layer of soil or on refusal and allow applied loads to be transferred to the bearing layer through the new matrix of improved soil and VSC’s.
As with other forms of ground improvement, VSC’s are not designed to eliminate settlement. Instead, they are designed to limit settlement to acceptable levels and to control differential settlement. One of the primary advantages of this type of ground improvement is the ability to provide a defined improved soil bearing capacity at working grade. This means that foundations can be constructed using the same approaches that would be used for conventional shallow foundations. A specific advantage of VSC’s over other types of ground improvement is that no spoils are generated. This is an important consideration at locations where contaminated soils may be present.
H.B. Fleming uses a VIPAC system to install both VSC’s and rigid inclusions. The VIPAC system is a patented installation system developed by Bauer/RTG. The VIPAC system consists of a mandrel and feed hopper that is driven into the ground using a variable moment high frequency vibratory driver/extractor. The vibratory driver/extractor is attached to and powered by an RTG RG-19T, a state of the art universal piling rig with an on-board computer system that monitors and records VSC installation parameters.
The VSC installation process consists of driving the VIPAC mandrel to a target depth and filling the mandrel with crushed stone. The mandrel is then extracted and re-driven in a repetitive cycle that compacts the stone and forces it against the cavity created by the initial installation of the mandrel. Each time the mandrel is partially extracted, the mandrel tip opens and allows stone to flow out. As the mandrel is re-driven, the mandrel tip closes and compacts the stone. This process continues from the bottom of each VSC up to the proposed foundation subgrade. The end product is a matrix of compacted columns of stone surrounded by improved soil. The improved soil/VSC matrix has a defined bearing capacity that is verified with testing in the field.
H.B. Fleming recognizes that not all projects are good candidates for deep foundations. In cases where ground improvement is a technically viable and economical alternative to deep foundations, we offer design-build capabilities to our customers. Below are some photos from recently completed projects.