Ground improvement mitigates soft soil sites, saving time and money

Deep dynamic compaction design and construction was performed by Farrell Design-Build
for U
niversity of California, Berkeley’s Maxwell Family Field and Garage project. The site sits
adjacent to the Cal Memorial Stadium, the Greek Theatre, and the Haas School of Business.


The challenge of constructing larger and larger projects in dense urban environments is placing those buildings on sites with sub-optimal soil conditions. These sites may include soft compressible layers of native or fill materials, soils that may be subject to settlement during an earthquake due to liquefaction, sites that may be subject to lateral spreading during an earthquake, or conditions that require a high capacity foundation system. 

Traditionally, two options have been used to mitigate these conditions: 

  1. If the soft soil is shallow, the site is over-excavated, and the soft material is replaced with engineered fill. 
  1. If the soft soil is deep or thick, drilled piers, driven precast concrete, or steel piles are used to support these structures. 

Both options, however, impact a project’s schedule and cost. Over-excavation requires heavy earthwork equipment and a large site for material storage and creates significant environmental conditions that must be addressed. Installing drilled piers or driven piles can be expensive, time consuming, and loud. Driven piles require the owner and design team to address traffic flow, adequate storage, agreements with neighbors, and other environmental considerations. 

A new option for mitigation is ground improvement, a term that has become more prevalent in soils reports and foundation design. Ground improvement includes a variety of methods that can be used to mitigate these soft soil sites without over-excavation, deep piers, or piles, and allows for a shallow foundation system to be used, saving clients time and money.  

Ground improvement comes in several forms, including deep soil mixing and deep dynamic compaction.  

Deep soil mixing uses augers and other heavy equipment to pump grout and mix it into the existing soil. The mix can be spread over a site to support a mat foundation or can be closely spaced to support concentrated loads. These drilled elements can vary in diameter and depth and produce small amounts of spoils. Another type of deep soil mixing uses vertical blades to cut a trench in existing soil while mixing in a cement slurry. This is called cutter soil mixing with machinery that has blades that can cut through in situ soil up to 130 feet in depth. These improved trenches can be used as stiffened vertical support elements, retaining walls, and to restrain liquefiable soil.  

Deep dynamic compaction uses rams or deep soil vibrators to consolidate and stiffen existing soil or existing soil with added aggregate. Adding grout to the existing soil increases the shear strength, lateral stiffness, and bearing capacity and allows for use of shallow foundation systems on top of the improved subsurface.  

Since each of these methods involves a specific type of specialized heavy machinery, the exact type of ground improvement will depend on the contractor selected. The result is that ground improvement is typically provided on a design-build basis. 

 For more information about ground improvement, contact Kevin Menninger. 

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