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Western Dam Engineering Technical Note May 2016 26 no piezometers exist to provide an accurate understanding of the foundation pressures, field investigations to install piezometers may be warranted. A series of piezometers screened to measure pore pressures at different elevations and locations is required to estimate seepage gradients. Seepage gradients, along with the foundation material type, will control the potential to initiate internal erosion. As previously discussed, several methods are available to estimate critical seepage gradients and all cautions previously stated for these methods apply [5], [6], [7], [8]. If piezometers are to be installed, care and proper design are required to strategically select their locations and installation methods. This will help to avoid initiating a dam safety incident due to drilling, yet provide instruments that can capture the desired information. See the section Don’t Rush to Dig In for more information. An example of piezometers located to measure seepage pressures within the pervious aquifer and overlying confining layer is shown on Figure 15. Piezometers can be installed relatively inexpensively within small dams using truck or trailer mounted drill rigs. Cone Penetration Test (CPT) rigs can also be a cost-effective option for installing fully grouted piezometers while limiting disturbance. Figure 15. Piezometers Located to Measure Potential Uplift A rough rule of thumb is to assume a safety factor of 2 when the piezometric surface in the pervious water- bearing strata is at ground surface, based on the unit weight of soil typically being nearly twice the unit weight of water. Higher piezometric levels in the water-bearing strata would result in a reduced factor of safety. Seepage weirs can also provide a means of monitoring for increase seepage flows or sediment accumulation. See the previous Western Dam Engineering Technical Note article for more information on dam safety instrumentation: Does Your Dam Measure Up? Numeric Seepage Analysis A qualitative analysis using field observations and instrumentation data can sometimes provide enough information to understand the problem and guide the engineer on what to do, and just as importantly, what not to do. However, if the existing knowledge and instrumentation data are not conclusive, then a two- dimensional model of the dam and foundation can be analyzed using appropriate stratigraphy and material properties. Seepage analyses can be performed either by computer software (which is typical today) or by manual flow net generation. The results of the seepage model can be used to develop a better understanding of the seepage regime beneath and through the dam and estimate foundation pressures and hydraulic gradients. Calculated gradients can then be evaluated for the potential to initiate erosion. Typical software codes, like Seep/W by Geostudio [3], are relatively easy to learn. However, the model results are only as reliable as the model inputs. If the variation in stratigraphy and material properties is not well defined, then spending time and money to develop a model is a pointless exercise. Before investing in developing a seepage model, first do a reality check by looking at the site configuration and piezometric data (if available) and deciding whether a two-dimensional model would adequately represent the seepage regime. It is common for dams and foundations to have seepage flow (gradient) that is not perpendicular to the dam axis. If this is the case, it can be difficult to develop and calibrate a meaningful two-dimensional model. Three-dimensional models have a higher cost, are more susceptible to inaccuracies, especially with limited data, and require special expertise. All of this needs to be considered when determining the most appropriate, and justified, means of evaluation.