PEOPLE, PROJECTS AND EQUIPMENT Controlled Modulus Columns: Innovation in Ground Improvement The common factor for CMC instal- ISSUE : INNOVATION SPECIAL foundations (e.g., piles and shafts). Within this wide range, the field of ground improvement has grown substantially. Development of CMC Ground improvement columns for use in vertical support of facilities include vibro stone columns and variants on this technique. Vibro columns are ideal when soft granular soils can be easily penetrated and improved with such equipment. When very soft organic or peat layers are present, a stone column is not very effective under vertical load. Historically companies offered to place concrete columns using vibro equipment, although this technique was never widely applied. Since a vibro probe is not a drill rig, penetration is not readily achievable in many cases without external assistance, e.g., pre-drilling. For such cases, where compacted stone Installation of CMCs Geotechnical engineering has a strong culture of innovation. Change is driven by the need to solve difficult problems or make a process or technique simpler, easier to build or less time consuming. Micropiles, for example, solved the problem of working in structures with low overhead or very tough ground. Drilled shafts introduced an easier solution to piers, which were dug down by hand and filled. Over time, pile driving equipment has become more efficient and quieter. Jet grout- ing was developed to make soil cement in place, strategically targeting certain shapes. More recent innovations in ground improvement have been driven by the need for a more elegant solution for supporting structures on soft ground that uses less external material and is more economical. There is a wide gap between facilities constructed on good soils (shallow bearing) and facilities that require deep elements were not feasible, Controlled Modulus Column (CMC) technology was developed. In the early 1990s, Menard in France developed the innovative concept of CMC when challenged with the problem that no economical, ground improvement technique was available to support structures over very soft soils except piles or other deep foundations. CMC was the solution to make the process of bridging very soft layers of soil simple and elegant. Piling has been used for centuries – driven through soft layers and sub-layers from above to support structures and facilities. The cost of building such facilities is inherently greater since the piling represents discrete points of support and the structure has to span over the piles. Using CMC allows development of vertical capacity similar to that gained from piling, with the elegance of structural support on a shallow foundation. This innovative technique requires under- standing the load transfer mechanism between the CMC elements and the upper soil layer (Load Transfer Platform – LTP), the participation of the surrounding soil and how these factors interact with the structure on shallow foundations. lations is that the facility can be configured for conventional footings and slab-on- grade construction. Once the LTP is installed above the CMC-reinforced ground, the facility behaves as if it is on inherently capable ground. This feature differs fundamentally from any building where the structure has an integral connec- tion to the deep foundation elements. The most dramatic demonstration using this technique is the design and construction of the foundation for the Rion-Antirion Bridge in Greece (see Deep Foundations, Fall 2007), led by Vinci of France, the parent company of Menard. Menard engineers consulted with Vinci to conceptualize and solve the unique problem of providing vertical and rocking stiffness under the cable-stayed support towers, without drawing excessive structural loads. The driven steel pipe piles, covered with 3 ft (1 m) of gravel, acted to provide stiffness as needed without drawing large base shears. (i.e., the CMC concept acts as a seismic fuse.) Sustainable and Cost-Effective CMC are a sustainable and cost-effective ground improvement technology that trans- mit load from the foundation into the bear- ing soils relying on soil-element interaction. CRUST SOFT FIRM Typical CMC installation procedure AUTHOR: Seth L. Pearlman, P.E., D. GE President and CEO, Menard USA DEEP FOUNDATIONS • MAY/JUNE 2012 • 73
