More detailed analyses were performed to estimate downstream flood levels, as wastewater can have a significant impact on stability. The HEC-RAS flow modeling software package developed by Corps was used to model the flow in the creek and floodplain downstream of the dam. For the overflow section, the discharge water calculated with HEC-RAS was adjusted to reflect the effective runoff against the dam, which is affected by the high-speed flow through the overflow and the roller bucket. The choice of physical and mechanical properties of the dam and foundation is crucial for assessing the stability of a gravity dam. The unit weight of concrete or masonry is a key element of the analysis. Estimates of the shear and tensile strength of concrete in the dam can be estimated from laboratory tests on representative samples and/or using available guidelines.4,5,6 The following case studies include discussions on the analysis of the stability of gravity dams, the importance of parameter selection, remediation to address stability issues and the risk of erosion and abutment failure. In addition to failures through the foundation and along the dam/foundation interface, stability analysis should consider failure through the dam, typically along horizontal construction joints. This «partial section» analysis is generally performed using the same methods used to assess the stability of the entire structure. Greg Paxson, PE, Director at Schnabel Engineering, was the designer of the Sugar Hollow Dam Project, conducted a stability analysis for the Green Lane Dam and acted as an evaluator for the Stony Creek Dam Project. Dave Campbell, PE, Director of Dam Engineering at Schnabel, was the Director responsible for the Sugar Hollow and Green Lane Dam projects.

Mike Canino, PE, West Chester, Pennsylvania, Branch Manager for Schnabel, has been involved in the evaluation of numerous gravity dams. Mark Landis, PE, Director at Schnabel, is the Project Manager for the Stony Creek Dam Project. One of the most common methods of rehabilitating gravity dams that do not meet stability criteria is to support or anchor. The support consists of adding mass to the downstream part of the structure to resist slipping. This can be achieved with conventional mass concrete or roller concrete. Since the 1960s, high-tension, high-capacity anchor bolts have been used to stabilize gravity dams, with more than 300 dams anchored across North America.12 Vertically installed prestressed anchors increase normal force, increase frictional resistance, and prevent the development of strains at the dam heel. Sloped anchors provide additional slip resistance by directly compensating for applied horizontal forces, but installation can be more expensive than vertical anchors. The Rivanna Water and Sewer Authority owns the Sugar Hollow Dam near Charlottesville, Virginia. This 80-foot-high concrete gravity dam was completed in 1947 and consists of overflowing, non-overflowing gravity sections with barrier walls extending into earthen abutments.

In the mid-1990s, the Virginia Dam Safety Program determined that the dam did not have sufficient overflow capacity, and analyses showed that the dam did not meet stability criteria for extreme flood loads. Gravity dams about 100 feet tall and less often require special considerations when assessing the stability and rehabilitation of these structures. Three case studies are presented, illustrating some of the unique challenges in assessing the stability and modernization of these dams. This method of checking the stability of the dam can be divided into two parts: These forces have very little influence on stability and are therefore usually neglected in stability analysis. (viii) Determination of principal and shear stresses on the toe and heel in accordance with Article 13.5. Buoyancy forces inside the dam, at the base of the structure and in the rock mass are important in stability assessments. For structures without internal drainage system or other features, and with fairly uniform foundation conditions, it is typical to assume that buoyancy varies linearly from full source at the heel to full discharge at the top of the dam. In the case of dams equipped with a drainage system, a reduction in these pressures should only be permitted if it can be demonstrated that the drainage system is effective. These case studies show approaches to the rehabilitation of gravity dams with stability issues or abutment erosion potential. The Green Lane Dam case story illustrates the importance of a detailed review of dam construction records, advanced hydraulic analysis to estimate stormwater efficiency, and laboratory testing (particularly in terms of unit weight and bonding) when it comes to stability analysis results.

An underground investigation revealed that the concrete from the dam was of good quality. Most horizontal construction joints were not visible on visual inspection, indicating a connection to these joints. However, the rock at the concrete/bedrock interface was heavily fractured, indicating that cohesion at the interface is unreliable in a stability analysis. In 2004, an updated analysis was conducted taking these results into account, and the results showed that the Green Lane Dam met the Corps` criteria for gravity-dam stability, eliminating the need for upgrades. The stability analysis of flood conditions should consider a range of floods to determine the combined reservoir load (headwater) and runoff that gives the lowest safety factor. The largest hypothetical flood or the maximum probable flood is not always the most critical flood load scenario. Green Lane Dam is a 103-foot-high, 800-foot-long concrete gravity dam northwest of Philadelphia. The dam, owned by Aqua Pennsylvania, was built in the mid-1950s for water supply. The design flood for this highly hazardous dam is the PMF, which was reassessed in the late 1990s and dominates the non-overflowing sections of the dam by about 2 feet. A preliminary stability assessment concluded that the dam did not meet generally accepted criteria and the former owner`s advisor recommended further field studies and analysis to support a reclamation design.

Initial estimates for stabilizing the dam with prestressed rock anchors were $1 million to $3 million. Stony Creek Dam is a 35-foot-tall concrete gravity dam built in the late 1920s for water supply. The dam, owned and operated by the city of Burlington, North Carolina, has a 200-foot overflow section with no-overflow concrete sections that bind to the earthen abutments. State dam safety regulations require the safe passage of half of the MFP. Although the concrete of the dam is in good condition, the sections and abutments without overflow show storms of about 100 years, and stability analyses show that the dam does not meet the safety factor required for events larger than a storm estimated at 300 years. For the modeled half-CMF, the abutments exceed 12 feet, which would cause the tank to rupture. Smaller weight dams are typically evaluated using simplified two-dimensional analyses with conservative resistance assumptions along the dam/foundation interface and inside the bedrock. For dams that do not meet the stability criteria, stabilization is often carried out with anchors or prestressed rock supports. In addition to stability issues, many small gravity dams are not «inserted» into the bedrock of the abutments, creating a risk of erosion and abutment failure. Shear strength along the dam-foundation interface or across the foundation is probably the most important parameter to be defined. The shear strength is composed of the angle of friction and the cohesion of the material(s) or interface. Typical shear strength values are available.6,7,8 The angle of friction is often estimated by material testing and/or correlation with empirical data for similar materials.

Estimating cohesion (or adhesion along the base of the dam) is more difficult and the value chosen has a significant impact on the results of the stability analysis. FERC recognizes the difficulty of accurately defining cohesion along the base of the dam and provides additional stability requirements if cohesion is not used in the analysis.9 Geological surveys and stability assessment methods are often less rigorous and complex for small structures. The behavior of large dams requires a better understanding of foundation conditions and further analysis of the structure`s performance under different load conditions, including finite element and deformation analyses.