September/October, 2016

Ground Improvement – Coming Full Circle

By Vernon R. Schaefer, PhD, PE, M.ASCE

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Ground improvement has been a part of geotechnical engineering and geoconstruction activities since the evolution of modern soil mechanics in the early 1900s. But as a 1978 report by the ASCE Committee on Placement and Improvement of Soils states, soil improvement methods have been used much longer than today’s engineers might imagine:

 

Soil, nature’s most abundant construction material, has been used by man for his engineering works since prior to the beginnings of recorded history. Virtually all construction is done on, in, or with soil, but not always are the natural soil conditions adequate to accomplish the work at hand. The basic concepts of soil improvement — densification, cementation, reinforcement, drainage, drying, and heating — were developed hundreds or thousands of years ago and remain unchanged today.

The ancients used soil for burial mounds, religious structures, dwellings, roadways, and military emplacements. In the previous century, our forebears, quite understandably, built mostly in areas with good ground conditions, and have used many such sites. As today’s populations have continued to expand, there has been a corresponding need for infrastructure, so modern-day engineers find ourselves increasingly building on more challenging sites that may have soft or loose, saturated, and/or problematic soils that can be expansive, collapsible, contaminated, or organic.

When these conditions are encountered, our remedial options include:

  • Bypass the poor ground by relocating the project to a more suitable site or by using a deep foundation
  • Remove and replace the unsuitable soils
  • Design the planned structure to accommodate poor/marginal ground either in-place or by removing, treating, and/or replacing the existing soils
  • Modify (improve) the existing soils,either in-place or by removing, treating,and/or replacing the existing soils

Through the years, a variety of terms have been used to describe how we might modify or “fix” the ground: soil stabilization, soil improvement, ground improvement, and ground treatment are perhaps the most common. While the main purpose is ground improvement, ground treatment is the actual process that we go through. Ground modification is the result of that process.

Today’s modern ground improvement and geoconstruction technologies make it possible to improve marginal sites and unsuitable in-situ soils to meet demanding project requirements. In fact, ”fixing” poor ground conditions to make them better suited for a project’s needs is often the economically preferred solution.

While earthwork construction has occurred for centuries, the development and evolution of machines has greatly increased construction efficiency and allowed significant improvements in the quality and quantity of work undertaken. Drainage methods to improve poor ground conditions, and compaction and densification of soils to improve engineering properties, were among the earliest forms of ”engineered” ground improvement. In recent decades, with our continuing use and expansion of improvement techniques and drastic improvement in equipment and technology, ground improvement has come of age and reached a high level of acceptance within the geotechnical community. Indeed, the functions of ground improvement encompass nearly all that we do as geotechnical engineers.

Today, there are many drivers for using ground modifica- tion. They include: (1) the increasing need to use marginal sites for new construction purposes; (2) the need to expand current infrastructure; (3) designing to mitigate risk of failure or potential poor performance; (4) designing for longevity and asset management; and (5) accelerating construction to minimize delays to users and the public.

From the geotechnical design engineer’s perspective, ground modification means changing relevant engineering properties (e.g., increasing soil shear strength, reducing soil compressibility, and reducing soil permeability) — or transferring load to more competent support layers. From the contractor’s perspective, ground modification may mean reducing construction time and/or construction costs. Both points of view are valid reasons to consider the use of ground modification techniques, and they are often mutually inclusive.

Many of the current ground modification techniques originated in Europe and the Far East, and were subsequently brought to the U.S. Contractors often led the development of the techniques as they wrestled with poor ground conditions and made improvements in equipment and methods to make their efforts more efficient and cost-effective. The contractor-led developments often meant that the techniques were experience-based and sometimes proprietary. Thus commercial and technological innovations by contractors’ technologies have almost always preceded research studies of fundamental performance and the development of engineer- ing guidelines.

Exciting advances in column-supported embankments, soil-mixing techniques, grouting, reinforced soil technologies, slope stabilization methods, and many other applications are occurring and pushing the profession forward. A key issue facing these advancements is the conservative and skeptical nature of owners and engineers, who ask, ”Who has used it?” There’s often reluctance to be the first or an early adopter of new technologies. More case histories of lessons learned and successful implementation of these technologies will help to increase their use.

With new technologies also come questions about performance monitoring and acceptance criteria. Movement towards performance rather than method specifications opens the door to competing technologies to meet specific settlement or similar performance requirements. We only need to look at how relatively new technologies such as mechanically stabilized earth (MSE) walls and soil nail walls have revolutionized earth retention design and construction to understand the great opportunities available and progress that can occur. Improvements in heavy equipment, tooling, and computer-based data acquisition and control systems have greatly expanded use of soil-mixing techniques, grouting methods, and shallow and deep densification methods. The new field of biogeotechnics is burgeoning and seemingly ready to make a large impact on our profession.

As population growth accelerates, the need to construct on poor sites will become imperative. This demand, and all of the developments in ground modification techniques and technologies, makes it an exciting time to be a geotechnical engineer involved in this field. Many of our recent developments are mimicking nature through reinforcement or cementation, thus bringing us full circle from our earliest uses of soils. Learning from nature and applying its lessons will

allow us to advance and develop better solutions to our vexing soil challenges.

VERNON R. SCHAEFER, PhD, PE, M.ASCE, is a professor of civil engineering at Iowa State University. He was chair of the Geo- Institute’s Soil Improvement Committee from 2002 to 2008 and led development of the GeoTechTools geo-construction information and technology selection guidance system from 2007 to 2012. He can be reached at vern@iastate.edu.