C+S May 2021 Vol. 7 Issue 5 (web)

We used fi - nite element P L A X I S models, both axisymmetric and plane strain to de- sign the CMCs throughout the project. Be - cause of the varying condi - tions, we ran many different analyses at different loca- tions to evalu - ate settlement

Fig. 3 –CMC-supported areas – Northbound and Southbound

tween the wall, the CMC elements, and the surrounding soil or structures. CMC design assumes the MSE wall, typically designed by a specialty MSE wall designer or geotechnical consultant, is stable against internal modes of failure. However, we still check global stability of the embank - ments using SLIDE, which performs a limit equilibrium analysis, and performed hand calculations to check both sliding and bearing capacity. During CMC installation, static load tests were performed at both projects described above, as they are for most CMC projects. The load tests are performed in general accordance with the Quick Test procedure in ASTM D-1143. The CMCs are loaded to a minimum of 150 percent of the design load or more if required for the project. The CMCs are often instrumented with strain gauges to observe the load at various depths within the element. Depending on the size of the project, the number of different sizes of CMCs used, and the variation in the bearing layer, more than one static load test may be performed. Conclusion As ground improvement becomes more widely accepted within the industry, it is being used by more and more of the state Departments of Transportation for projects that need settlement control. CMCs, specifically, can be installed for large depths and can be installed in a variety of sizes and spacings to optimize the design to the soil condi - tions and site geometry. CMCs can often replace deep foundations, especially on projects where settlement is the main concern. CMCs can be reinforced to resist flexural forces due to lateral movements, which are often present at the edges of roadway embankments and MSE walls. Because of the general lack of steel reinforcement within the elements and the elimination of pile caps on top of the CMCs, con- struction tends to be quicker and less expensive than traditional deep foundations. Although there are times when deep foundations are the most appropriate solution, RIs should be taken into consideration

Fig. 2 – Overview of CMC-supported area at S. Capitol Bridge project including the structural components

of the system, load in the CMCs, and interaction between adjacent areas. Special attention was focused on the transitions from the struc- tural elements to the surrounding embankments to avoid hard points or abrupt changes in settlement. Case Study 2: I-35 at Deep Fork Creek CMCs were used to support new MSE walls and embankment fills constructed as part of the widening of the existing Interstate 35 as it crosses over Deep Fork Creek in Oklahoma City, Oklahoma. The sup - port was a combination of embankment support and the support of an MSE wall with a backslope. A total of just over 1,000 CMCs were installed for the two phases of this project, supporting separate walls on the northbound and southbound sides of I-35. The fill heights range from 10 to 23 feet and the soil conditions consisted of sandy lean clay, over 40 feet of soft clay, silty sand, and sandstone. Given the height of the proposed construction and the thickness of the clay, estimated settlement was larger than the acceptable 1 inch post-construction set- tlement. In addition to settlement concerns, the proposed fill presented a concern for overall global stability of the widened embankment. As was discussed for the previous case study, we used PLAXIS models to estimate the settlement of the ground improvement system. We performed separate models at the locations of the highest general embankment fill and at the location of the highest fill within the MSE area. Based on our modelling, the CMCs were 15.6 inches in diameter and were spaced between 6 and 7 feet on center. The unreinforced CMCs were installed through the existing soils and were terminated on the sandstone at depths between 60 and 75 feet. The CMCs effectively spanned the soft soils and transferred the applied load to the sandstone. The maximum load in the CMCs from our models was 125 kips, which results in a fairly low stress in the CMC element. Because of the soft soils present, the design was primarily controlled by the settlement criteria, not the load in the CMCs. The key to the design was modify- ing the spacing and diameter of the elements to minimize the amount of load entering the soft soil and causing settlement. Common Threads in CMC Designs For typical roadway projects, the CMC design checks the interaction be-

for support of transportation projects over soft soils.

Fig. 4 – Typical Supported Cross section at I-35 at Deep Fork Creek


may 2021


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