FREESE & NICHOLS has been selected to receive the 2001 National Rehabilitation Project of the Year award from the Association of State Dam Safety Officials (ASDSO) in the US. The company was recognised for designing a rehabilitation project for the the north and south spillways of Wesley Seale dam, located near the City of Corpus Christi, Texas.

Constructed in 1956-58 at a cost of US$8.75M, Wesley Seale dam is a 24m high, 1.7km long rolled embankment dam on the Nueces river. Lake Corpus Christi, the reservoir created by the dam, is a major municipal water supply reservoir for south central Texas, and is owned and operated by the City of Corpus Christi.

The service and emergency spillways for the dam are created by 12m high, Ambursen type flat slab and buttress spillways that accommodate steel crest gates.

Serious defects were found in the dam and its foundation in 1996. Concrete bulges in the spillway hearth slabs; cracking of concrete abutment members; and downstream offsets in the alignment of the spillways of up to 10cm resulted in the City of Corpus Christie employing Freese & Nichols to investigate the apparent instability of the spillways and to design corrective measures.

Field and laboratory investigations of hydrostatic uplift, and the strength parameters of the medium plasticity clay underlying many of the buttresses of the dam, determined the cohesive strength of the foundation clay supporting the spillway buttresses was a factor of ten weaker than the 10psi that had been used in stability design calculations. In addition, the effect of a rising tailwater on the hydrostatic uplift of the spillways was not considered in the initial stability design.

The spillways were successfully rehabilitated to achieve a sliding factor of safety improvement of at least 50% during the period May 1997 to March 2001 at a cost of approximately US$22M. The rehabilitation involved:

• Construction of reference monu-ments and extensometers to allow precision monitoring of any future improvements of the spillways.

• Imposing limits on the lake level during restoration.

• Temporary stabilisation of the south spillway by using anchors.

• Final stabilisation of both spillways using 54,000m3 of concrete ballasts to increase weight of the spillways.

• Stabilisation of the south spillway chute and basin by addition of sand drains, an anchor system and a concrete overlay.

• Addition of a parapet wall to the embankments to prevent PMF overtopping of the dam.

Freese & Nichols provided practical and cost-effective designs during construction, ASDSO says. These measures, coupled with the construction management programme, resulted in a project that was completed ten months ahead of schedule and US$3M under budget.