Emergency Recovery at the highest Dam in the USA using DYWIDAG THREADBAR® Anchors

Context

Oroville Dam, completed in 1968, is located east of the town of Oroville, California. At a height of nearly 235m (770ft), it is the highest dam in the USA. The adjacent storage lake has a capacity of over 4.3km³ (3.5 million ac-ft) of water.

In 2017, Northern California experienced the wettest winter in over 100 years, which lead to an extremely high water inflow into Oroville Lake from the Feather River. To reduce the pressure acting on the dam, the spillway was opened in February.

As a result of the extremely high water volume rushing down the 931m (3,055ft) long spillway, the concrete lining was heavily damaged and a large crater formed in the rock below. Water flowing over the emergency spillway resulted in erosion at the bottom of the weir. The potential failure of the weir and the possible collapse of the spillway gate structure made it necessary to evacuate more than 180,000 residents in the neighboring communities.

Solution

Emergency recovery of the spillways was carried out in two phases. In the first project phase, a large part of the spillway chute was re-built in only 165 days. Furthermore, the adjacent emergency spillway had to be re-built and strengthened. Work had to be accomplished before the start of the winter season.

The first project phase included placement of approx. 267,000m³ (350,000 yd³) of roller-compacted concrete in the area where the crater had formed along with the construction of a 265m (870ft) long upper chute section and of a 107m (350ft) long lower chute section.

The new concrete slab for the chute was placed in approx. 9 x 11m (30ft x 37ft), 0.76m (2.5ft) thick concrete blocks on top of a 1.5m (5ft) thick layer of leveling concrete. Each block was reinforced by two layers of rebar, one of which was epoxy coated, and anchored into stable rock using 14 DYWIDAG THREADBAR® Anchors.

Work for the first phase was successfully completed in November 2017. In the spring of 2018, work for the second project phase started.

Within the scope of the second phase, new drainage systems were placed and more than 213m (700ft) of the upper chute was replaced with a steel-reinforced structural concrete chute. In addition, a 0.76m (2.5ft) thick steel-reinforced structural concrete slab will be placed over the roller-compacted concrete that was constructed in the middle chute during phase 1. The slab in the chute was anchored down with 36mm Ø DYWIDAG THREADBAR® Anchors (#11 GR75) similar to what was done in the first phase. The temporary concrete walls in the middle chute were replaced by higher structural concrete walls.

In both phases, a total of 6,936 epoxy coated, 4.6m to 7.6m (15ft to 25ft) long 36mm Ø DYWIDAG THREADBAR® Anchors (#11 GR75) including accessories were used for anchoring the concrete slab. DYWIDAG-Systems International USA also supplied the necessary stressing equipment – up to 6 jacks were used simultaneously to carry out the testing of the anchors in order to meet the tight schedule.

At the emergency spillway, a concrete splash pad was placed between the emergency spillway monolith and the secant pile wall. 160 epoxy coated 36mm Ø DYWIDAG THREADBAR® (#11 GR75) dowels were utilized to connect the concrete to the spillway monolith as well as the secant pile wall.