Design-Build for SFPUC Biosolids Digester Facilities Project

San Francisco Public Utilities Commission (SFPUC) is a department of the city that provides drinking water and wastewater services to San Francisco along with, green hydroelectric and solar power to San Francisco municipal departments.

SFPUC decided early in the process to use an alternate delivery method to utilize the expertise of the entire construction industry by engaging Trade Core partners at an early stage in the design-build process. Malcolm Drilling was chosen to be the Foundation Trade Core partner, after an intense pre-qualification process.

We worked on the pre-construction and construction during the design and construction of all deep foundations and Support of Excavation (SOE) for various structures of the Biosolids Digester Facilities Project for the SFPUC Sewer System Improvement Program. Malcolm working as a trade core partner, developing design, and meeting the SFPUC budget and construction schedule from an early stage, proved to be a remarkable success to the project.

The foundation and site dewatering package included various deep foundations and SOE techniques including Cased Drilled ShaftsContinuous Flight Auger (CFA) PilesDiaphragm Walls (D-walls)Tie Downs (Micropiles), Tie Backs, Bracing, Mass Excavation, Earthwork and Asphalt, Shotcrete, Demolition and Dewatering. Coordinating the various scope of works that Malcolm self performs was another challenge for us. Working closely together with the projects CM/GC (a JV of Webcor and MWH constructors) and our main subcontractor Bertco Inc., as well as two concrete suppliers (Central and Cemex) in a real partnership was the key to the successful delivery of this massive project.

Drilled Shafts & Continuous Flight Auger (CFA) Piles

Malcolm’s expertise for various deep foundation methods helped the designer to optimize the use of Continuous Flight Auger Piles (CFA piles) in lieu of the traditional Cased Drilled Shafts. On this project Malcolm installed over (140) 48-inch diameter Continuous Flight Auger Piles (CFA piles) up to 80-feet deep and (200) 36-inch diameter CFA piles up to 70-feet deep bringing considerable schedule and cost savings to the Project. Where the future structure was below grade and CFA piles were not a viable option, Malcolm installed over (210) 48-inch diameter drilled shafts up to 100-feet deep, utilizing temporary steel casings to stabilize the weak bay mud soils during drilling. From our highly specialized fleet of equipment, we utilized several of our Bauer drilling rigs, Leffer oscillators, Putzmeister boom pumps, and Liebherr cranes to complete this work.

Diaphragm Walls (D-walls)

As a result of the proximity of the active railroad lines, the 200,000 square feet  support of excavation (SOE) wall for building 610 had to also act as a cutoff wall reaching a depth of 160-feet to key in the low permeable old bay clay layer. Malcolm chose a 36-inch-wide structural diaphragm wall, which was laterally supported by tiebacks and corner bracing. Reinforcement was terminated at about 75-feet, tie back sleeves and bracing embed plates were installed within the diaphragm wall reinforcement. For the excavation of each diaphragm wall panel, we utilized several Bauer BC40/BC48 cutters mounted on Bauer MC96 cranes, mechanical grabs, and hydraulic grabs. One Liebherr LR1300 and one Bauer MC96 service crane were utilized to support the rebar cage installation and tremie concrete operation. The D-wall achieved the required water cutoff and kept the maximum deflection and settlement within the tight project requirements.

Shoring Tiebacks

The support of excavation (SOE) wall for building 610 included over six hundred tiebacks with length of up to 80-feet. All tiebacks were installed at 5-feet on center and had design loads of up to 245 kips. They were installed at 30-degree angles with sufficient unbonded length to create a bond zone in the competent upper layers of sediments that exist beneath the young bay mud. The tieback strands varied from five to ten strands across the entire excavation. The tiebacks were installed through pipe block outs that were pre-installed within the D-Wall rebar cages.


Dewatering was required for two deep building excavations as part of this project. However, final designs of the Support of Excavation (SOE) walls and the required dewatering scheme was developed during the pre-construction phase. We performed an early dewatering pump test program to help guide the design of the Support of Excavation (SOE) system and its potential effects on the two nearby active railroad tracks. Due to contaminated soil conditions in some parts of the excavation, we had to obtain a National Pollutant Discharge Elimination System (NPDES) permit to allow the treated, pumped water to be discharged back to the San Francisco Bay. In total, we designed and installed fifty dewatering wells targeting three different aquifer zones (shallow, intermediate, and deep) so mass excavation could safely occur in the dry without the risk of bottom heave. We also installed twelve observation wells to monitor groundwater levels outside the excavation. Malcolm partnered with Clear Creek Systems to provide the water treatment solution as well as WSP, to help provide water sampling, testing, and compliance with the NPDES permit.