Rigid Inclusion Support of Roadways By Sonia Sorabella Swift, P.E. and Martin G. Taube, P.E., P.G.
As our cities grow and the needs of our societies change, we often look toward optimizing our infrastructure. Reducing daily traffic on heavi - ly-travelled roadways is a common focus for many state Departments of Transportation. Very often, that involves reconstructing roadways wider than the original footprint, widening the roadway into adjacent areas, and building walls to divert or improve traffic flow. However, those areas may not always provide a suitable foundation, especially when tall walls are planned. Some form of a column supported em- bankment system is commonly used to support such construction. The type of columns and their design can vary widely – from traditional pile foundations with pile caps at the individual column locations to rigid inclusions (RIs) with a load transfer platform over the entire sup - ported area to stone columns. Menard’s patented variation of a rigid inclusion is the Controlled Modulus Column (CMC) TM . In this article, we will focus on CMCs and their support of roadway projects. We will describe CMCs, explain how they are used, and review two recent projects where CMCs were used to support transportation projects. Description of Controlled Modulus Columns (CMCs) TM CMCs are small-diameter grouted columns that are installed through soft or variable soils to reduce settlement and increase bearing capac - ity. CMCs are 12.5-, 15.6-, or 17.75-inch-diameter columns installed with a displacement auger. CMCs are typically unreinforced, though structural steel can be added, as necessary to resist high compressive, tensile, or flexural forces. When supporting embankments, wall fills, or other mass structures, CMCs are not generally connected to the structure nor do they require a pile cap. CMCs are generally separated by the superstructure by a load transfer platform (LTP), which can range from 6 inches to 3 or more feet thick. The load transfer platform is generally a dense-graded aggregate that is placed and compacted in an engineered manner. The purpose of the LTP is to distribute the loads from the point of application to the CMCs while reducing stress concentrations and eliminating the need for pile caps. CMCs, as do piles, support the applied loads thereby reducing the load on soft compressible soils that would require large movements to mobilize their resistance – doing so reduces settlement. However, unlike piles, CMCs share the load with the surrounding soils. The load from the structure is shared between the CMC and the surrounding soil. See Fig. 1 for a schematic of how the CMC ground improvement system supports the applied loads. In the remainder of this article, we will focus on two roadway projects where CMC ground improvement was used to reduce settlement and improve bearing capacity. In both cases, the ground improvement was supporting mechanically stabilized earth (MSE) walls designed by The Reinforced Earth Company (RECo) and any retained or as- sociated backfill.
Fig 1. Interaction of the soil and CMC – depiction of load transfer
Case Study 1: South Capitol Street Corridor Project CMCs were used to support roadway fills and mechanically stabilized earth (MSE) walls for the South Capitol Street Corridor project in Washington, D.C. This nearly $1 billion project will include the re - placement of the Frederick Douglass Memorial Bridge, including two approach ovals east and west of the Anacostia River, the replacement of an interchange, and the construction of several traffic ramps. Ground improvement was required over a portion of the project footprint, as shown in the hatched areas of Fig. 2. Within the ground improve - ment area, the construction includes MSE walls up to 35 feet high and embankment fills up to 28 feet for which CMCs were used to mitigate settlement and enhance stability. The soil profile consisted of general fill, soft alluvial clay, dense sand and gravel, and stiff clay (Potomac Formation). The estimated settlement in the soft alluvial clay was larger than the project requirements due to the presence of the soft alluvial clay. Both 12.5-inch-diameter and 17.75-inch diameter CMCs were used at spacings ranging from 5 feet to 10 feet. More than 3,600 CMCs were installed from 7 different benches across the site to accommodate exist- ing grades and follow-on work. CMCs were installed approximately 30 to 40 feet deep into the dense sand and gravel layer for support of the embankments and MSE walls. For the more heavily loaded CMCs that supported the structural elements that will be described in the following section, the CMCs were terminated in the Potomac Formation. The proposed construction runs over multiple old, in-service utilities that could not tolerate any stress or settlement due to the construc- tion (according to the project specifications). As such, solutions that spanned the utilities were required. A concrete slab of varying thick - ness was used to span a fragile 108-inch-diameter sewer pipeline and 48-inch-square-precast-concrete box beams were used to span twin utilities that were buried just a few feet below working grades on site. CMCs with a single steel reinforcing center bar (requested by the utility owner) were used to support these structures. The center to center spacing of the CMCs perpendicular to the pipe run was 26.5 ft for the concrete slab and 62 feet for the precast--concrete box beams. The axial forces on the CMCs supporting these structures were over 250 kips. The CMCs were not structurally connected to the slab or box beams. Multiple single-element load tests were successfully per- formed to confirm the load-carrying capacity of the CMCs. The load test supporting the concrete box beams held 580 kips at a deflection less than 0.4 inch.
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