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Ensuring the safety of a “high hazard” dam

A horseshoe-shaped outlet tunnel was completed despite challenging conditions, including weak and friable rock and squeezing ground conditions.


2011 Public Works Project of the Year – Structures, American Public Works Association

2010 Project of the Year, American Public Works Association – Silicon Valley Chapter

2010 Ruby Level Partnered Project of the Year, International Partnering Institute

2010 Marvin M. Black Excellence in Partnering Award, Associated General Contractors of America

2010 Excellence in Partnering Award, projects under $50 million, Associated General Contractors of California

2009 Outstanding Project in the large project category, American Society of Civil Engineers – San Francisco Section


The Lenihan Dam and its Lexington Reservoir are located in Los Gatos, California, the prosperous Silicon Valley town where John Steinbeck once wrote his classic novel The Grapes of Wrath. Only 1.3 miles (2.1 kilometers) from the San Andreas Fault, Lenihan is considered a “High Hazard” dam because of its proximity to dense populations.

A thousand-foot-long earthen dam completed in 1953, the Lenihan dam has an outlet pipe whose original capacity was 410 cubic feet per second (cfs). But beginning in the late 1980s, sections of the outlet pipe began buckling, and the maximum outflow was limited to 70 cfs.

In 1998, the Santa Clara Valley Water District was directed to find the causes of the failure and implement a long-term solution. During the 1989 Loma Prieta earthquake, brick buildings in Los Gatos were destroyed or damaged, underlining the importance of being able to lower the reservoir quickly in order to prevent the dam from failing in the event of another earthquake.


A horseshoe-shaped tunnel 2,000 feet (0.6 kilometers) long, 14 feet (4.3 meters) wide, and 13 feet (4 meters) tall was designed to house a new outlet pipe 54 inches (1.4 meters) in diameter. The plan called for a 37-foot (11.3-meter) vertical tunnel to be sunk from the back of the dam to intersect with the tunnel below.

As Construction Manager, Mott MacDonald provided project management, quality assurance, cost/schedule monitoring, environmental monitoring, and other services for the project. The tunnel was completed despite challenging conditions, including weak and friable rock and squeezing ground conditions.

Mott MacDonald established a project site office with networking capability for processing submittals and RFIs. We monitored the budget, schedule, safety, and quality of the project, organized project partnership workshops and a Disputes Review Board, involved the Division of Safety of Dams to satisfy permit requirements, hosted tours by visiting dignitaries, and prepared monthly summary reports.


The new outlet structure returned the dam’s outflow capacity to the original 410 cfs. The project was completed nine days early and $1.4 million under budget, and after more than 155,000 hours of work, there were no recordable or lost-time incidents that resulted in a lost work day.

The local habitat was monitored during the project and a population of Dusky-Footed Woodrat (a type of packrat) was relocated. A “fish-friendly” inclined intake enables water from different levels of the reservoir to be discharged downstream, allowing the temperature and oxygen level of the discharge water to be regulated.

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