Ground ImprovementOct 13 2017 · 0 comments · Ground improvement/Foundation design, NISHKIAN MENNINGER, Technical notes ·0
A 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.
DDC design and construction was performed by Farrell Design-Build Inc. for U.C. Berkeley’s Maxwell Family Field and Garage project in Berkeley, California. The site sits directly adjacent to the Cal Memorial Stadium, the Greek Theatre, and the Haas School of Business.
Traditionally, 2 options have been used to mitigate these conditions:
- If the soft soil is shallow, the site is over-excavated and the soft material is replaced with engineered fill.
- If the soft soil is deep or thick, drilled piers or driven precast concrete or steel piles are used to support these structures.
Both of these options have impacts on the project schedule and cost. Over-excavation requires heavy earthwork equipment, 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 traffic considerations and adequate storage, agreements with neighbors, and other environmental considerations.
A new term that has become more prevalent in soils reports and foundation design is Ground Improvement. This has become a generic term for a variety of methods that can be used to mitigate these soft soil sites without over-excavation or deep piers or piles. Ground improvement allows for a shallow foundation system to be used which will save costs and time.
Ground improvement comes in several forms, these include: deep soil mixing, drill displacement piers, and deep dynamic compaction. Deep soil mixing uses augers and other heavy equipment to pump grout and mix it into the existing soil. Deep soil mixing 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 stiffen 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.
Nishkian Engineers have utilized ground improvement techniques on several recent projects to provide less invasive and more cost-effective foundation solutions. One recent project is The Encore residential development in Redwood City, CA. This 6-story building of concrete and wood frame construction does not have huge foundation loads. However, approximately one third of the building footprint had a subgrade layer of soft material that had a high potential for liquefaction settlement. Ground improvement of this select area was a cost- and time-effective solution to mitigate these conditions in lieu of other, more costly options.
Nishkian worked with Regis Builders, the general contractor, and Farrell Design Build, the ground improvement contractor, to develop the system to support this building. Farrell quickly mobilized their equipment on the prepared site and utilized Drill Displacement Columns (http://www.farrellinc.com/services/foundation-systems/auger-cast-column-drill-displacement-column) up to 30 feet in length to provide support in compression for the foundation and ground floor slab in the soft zones. Farrell also installed displacement ground anchors for tensile resistance under lateral elements. After this quick process the shallow spread footing foundation system was excavated and installed.
Another relevant project is the Maxwell Family Field and Garage which sits directly adjacent to the California Memorial Stadium on the University of California Berkeley campus. Long ago, the site was once a creek bed. During the development of the campus, the creek was turned into a set of large culverts, and filled in to provide a flat surface. This type of loose fill makes building a seismically safe structure more difficult. Similar to the challenges of building on bay mud in San Francisco, the ground could liquefy during an earthquake, resulting in amplified forces on the structure. This condition is exacerbated by the presence of the Hayward fault, which runs just a few hundred feet away from the site. Although there are many ways to improve the soil, the best option for the Maxwell Family Field and Garage project was Drill Displacement Columns (DDC). DDC design and construction was performed by Farrell Design-Build Inc. as well.
Ground improvement installation by Farrell Design-Build Inc. for the Maxwell Family Field and Garage project at the University of California Berkeley campus.