Evaluation of Monksville Dam Roller Compacted Concrete

In the early 1980's, the North Jersey District Water Supply Commission and United Water Resources (formerly the Hackensack Water Company) proposed to construct a dam to contain 7 billion gallons for a water supply reservoir. This dam was to be constructed immediately upstream of the 29.6 billion gallon Wanaque Reservoir. The Monksville Dam would be built in the Borough of Ringwood, Passaic County, New Jersey. This dam would be 150' high and 2200' case studies

Earthfill, rockfill, concrete gravity and roller compacted concrete alternatives were examined for the construction of the dam. The existing overburden at the dam site contained fines of low plasticity which were not suitable for earthfill dams, but were suitable for use as aggregate for Roller Compacted Concrete (RCC). During the study it was found that a separate quarry would need to be located for aggregates for either a rockfill or concrete dam. A RCC dam could be constructed using earthmoving equipment and no retaining wall forms for the downstream slope. It is estimated that the owners saved nearly $7 million and reduced construction time by 9 months with the selection of RCC for the embankment material for this dam. RCC placement began in March 1986 and the dam was completed in June 1987. Monksville Dam is one of the largest RCC dams east of the Mississippi River and the RCC mix design contains one of the lowest cement contents used for RCC dam construction in the world. Nearly 300,000 cubic yards of RCC material were placed during construction.

During several recent routine inspections of the dam, recommendations were made to obtain cores of the RCC mass in the dam core. The testing program was undertaken in November 1998. Pennsylvania Drilling Company, of Pittsburgh, PA performed the drilling of 6" vertical cores at 3 locations along the crest. Five additional horizontal cores were attempted along the downstream face.

The cores were transported to Schnabel Engineering Associates Materials Laboratory in West Chester, PA for evaluation on compressive strength (ASTM C 39), splitting tensile strength (ASTM C 496), and joint shear strength. Specially fabricated jigs were constructed for the joint shear test in order to localize the applied load to act on the joint.

Samples for testing included "hot" joints, cold joints and bedded joints (joints with bedding concrete). Evaluating the effectiveness of the various joint treatments is one of the goals of the testing program. Samples for compressive strength and split cylinder tests were 12" intact samples without joints. The results of the testing will be compared to available data from tests taken during construction to assess material strength gain over the 12 years since construction. The results of the testing will also be incorporated into the stability analysis for Monksville Dam. In addition, since the dam is experiencing freeze thaw deterioration on the downstream face, information obtained from the laboratory testing program will also be used in evaluating the severity and various alternatives for protection if deemed necessary. 9 pp., 2 references.