Ground Improvement By Martin G. Taube, P.E., P.G. and Sonia Sorabella Swift, P.E.
Historically, when building on sites with poor soils, builders had two primary options to choose from – piling and removal and replacement. While some ground improvement techniques such as dynamic com- paction have origins tracing back to the middle ages, it wasn’t until the past half century that ground improvement began to gain wide ac- ceptance and recognition as a viable option for addressing sites with problematic soils. The last few decades have seen a proliferation of the types of ground improvement techniques, installation methods, equip- ment capabilities, design methodologies, QA/QC requirements and procedures, and number of contractors offering ground improvement. At many sites, excavating the problematic soils and importing and compacting fill is a viable solution. However, removal and replacement has its limitations, particularly when the problematic layers extend be- yond a few feet. Where groundwater is high and deeper excavation is required, it may be necessary to dewater or install groundwater barriers such as cutoff walls or sheet piling. It may be necessary to protect adjacent structures by underpinning or with support of excavation sys- tems. Deeper removal and replacement becomes very costly and cause schedule issues, especially when faced with preparing subgrades and placing fill in wet or frozen conditions. And of course, the prospect of unearthing contaminated soils or buried hazardous materials will strike fear in any developer or property owner. Most civil and structural engineers are very familiar and comfortable with multiple types of piles and other deep foundation elements since these systems have traditionally been the default foundation systems for sites with problematic soils. These systems are tried and true, but are they always necessary? In many cases, ground improvement can result in savings by both eliminating deep foundations and allowing structures to be supported on shallow foundations systems, as if you were building at a site with competent soils. To select the appropriate type of ground improvement for a site, it is important to understand what challenges the ground may pose for the support of the structure. Could excessive settlement occur due to soft or loose soils? Is the factor of safety against bearing capacity failure too low due to the presence of weak layers? Are slopes or embank- ments unstable? Would excessive differential settlement occur due to differential loading conditions or variability of soil conditions across the site. Is undocumented fill or buried debris present at the site? Is liquefaction a risk at the site? With all the potential issues that sites may face, the importance of a thorough geotechnical investigation can- not be overstated. To allow for the selection of the appropriate ground improvement system, and its optimization, it is critical that sites are characterized with sufficient coverage (and depth) of borings or sound- ings, and with ample laboratory testing to determine the characteristics and strength properties of the on-site soils. Another vitally important aspect of ground improvement selection and design optimization is having a good understanding of the layout and ac-
tual loads for the proposed structure. In some cases, the highest building column loads are adapted as the design case for the entire structure – this results in an inefficient ground improvement design. Optimization of ground improvement comes from a deeper understanding – of the site, of the ground, and of the structure. There are many different ground improvement systems adaptable to a wide array of site conditions, soils, and structure types. In general terms, there are three typical modes of soil improvement – densifica- tion, reinforcement, and drainage enhancement. Some systems provide Rapid Impact Compaction (left) and Dynamic Compaction (right) are used to densify fill material to accommodate building construction at a site in New Jersey.
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csengineermag.com
march 2021
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