Q&A: Langan has provided geotechnical services on a number of projects in the Mission Bay neighborhood of San Francisco; what makes The Exchange project different?

The Exchange, Mission Bay, San Francisco

The Exchange, Mission Bay, San Francisco

Mission Bay is a reclaimed area of San Francisco that was previously a shallow bay. The site was reclaimed in the late 1880s and early 1900s using a variety of soil materials and debris, including debris from the 1906 San Francisco earthquake. The fill was not properly placed and is typically potentially liquefiable. Underneath this fill is weak, compressible marine clay known as Bay Mud.

The Exchange project is different because we used an alternative foundation type that had never been used in Mission Bay. Strong foundations are particularly important in this neighborhood because of its poor soil conditions.

With these poor soil conditions, most buildings in Mission Bay are being supported on deep foundations that gain capacity in either a dense sand layer, which is present in part of the area, or in bedrock.

Driven piles have typically been the most economical foundation type to support the buildings in Mission Bay. However, driven piles could not be used for The Exchange project because of concerns of noise and vibrations during installation that would impact the nearby UCSF Medical Center.

Langan worked closely with the owner, structural engineer, and general contractor to evaluate alternative pile types. The team asked us to participate in foundation subcontractor interviews so that we could provide input on the different proposals and ask pertinent questions.

Malcolm Drilling proposed a foundation type that had never been used in Mission Bay: rock socketed drilled shafts. With shallow groundwater, poor fill, and weak clay, drilled shafts were not typically feasible in this neighborhood; however, The Exchange site is near the shoreline of the original bay, and bedrock becomes shallower and Bay Mud becomes thinner toward the bay margin.

To evaluate the constructability of the drilled shafts and determine their capacity, Langan drilled the test location, including 30 feet of rock coring. With information we gathered from the test location, the team asked Malcolm to design a test pile, including Osterberg load cells for load testing.

The drilled shaft was successfully installed and the load test provided data for design of the foundation — which resulted in cost savings to the owner. The key to moving forward with this foundation type was good communication, strong relationships, and productive meetings among the design and construction teams.

Answer provided by Lori Simpson, PE, GE, Principal
Lori Simpson has over 26 years of experience in providing geotechnical investigation, design, consultation, and construction observation services for a variety of geotechnical engineering projects, especially sites with reclaimed land. Her projects include commercial and residential buildings, high-rises, infrastructure improvement for new developments, sports facilities, and investigations for seismic upgrades.

 

Q&A: Why is groundwater a major issue in the Middle East, despite many projects being located in desert environments?

Langan provided dewatering services for the Four Seasons tower on Al Maryah Island in the UAE.

Langan provided dewatering services for the Four Seasons tower on Al Maryah Island in the UAE.

Dewatering the Desert 
Despite being in a climactic environment where summer temperatures can reach in excess of 120 degrees, groundwater is generally very close to the surface in many developed cities throughout the Middle East (Dubai, Abu Dhabi, Bahrain, Doha, and Jeddah, in particular).  The nature of the relatively high permeability, near surface geology in these areas means that up to five kilometers from the coast, there is a often direct hydraulic connection between groundwater and the adjacent sea. Typically in these areas, groundwater is within two meters of the surface.  This means that almost every basement excavation and a large percentage of excavations for utility lines require some form of temporary dewatering to allow for construction in a dry environment.  For shallow excavations in sand (up to around four meters below the water table), we typically utilize drilled or jetted wellpoint systems.  For deeper excavations, a combination of staged wellpoint systems, or deep wells, are utilized, to draw the groundwater down anywhere from five to 25 meters.

Specific issues arise when we attempt to dewater areas where solution features exist, which is particularly problematic in certain parts of Abu Dhabi where layers of karstic gypsum exist.  These solution features are irregularly distributed and flow rates can be extremely high, requiring large scale pumping to allow sufficient groundwater drawdown.  In many cases, long term pumping can actually exacerbate the groundwater flows. Increased flows gradually increase groundwater velocities which causes further erosion of the gypsum karst, until the flow volumes effectively become unmanageable without resorting to ground treatment (i.e. grouting) to reduce flows to manageable levels.  Cut-off walls are often ineffective in reducing flow volumes as flow is effectively vertical (rather than horizontal) through the base of the excavation.

The most notorious area in the Middle East with respect to groundwater issues is Jeddah, particularly along the ‘Corniche’ north of the city.  A combination of proximity to the Red Sea and the presence of highly permeable porous coralline limestone at relatively shallow depth causes major issues when trying to control groundwater flows into a typical basement excavation.  For example, it is common in Jeddah for a 50 x 50 meter footprint site with three basement levels to have groundwater flows as high as 5,000m3 per hour (1.1 million gallons per hour) without some form of grout-based ground treatment (again cut-off walls are largely ineffective as most flow is vertical rather than lateral).  Even if pumping capacity is available, there are major issues associated with both water treatment and discharge of such large volumes of water.  These issues among many others, certainly make life as a geotechnical engineer interesting in the Middle East.

About Paul Gildea
Paul  is a geotechnical specialist with 27 years of experience as both a specialist contractor and consultant in the UK, Ireland, Hong Kong, China, Australia and throughout the Middle East. He currently manages international projects for Langan, primarily in the Middle East, Asia, and Africa with specific expertise in the design and construction aspects of foundations for high-rise structures and deep basements.