Causal Factors Associated with Hydraulic Fracturing During Foundation Grouting at TVA's Boone Dam-Paper

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Between 2015 and 2021, the Tennessee Valley Authority built a composite seepage barrier at Boone Dam, Tennessee, to reduce the risk of internal erosion. The work included low-mobility grout (LMG), followed by high-mobility grout (HMG), and a secant-pile cutoff wall. During HMG grouting in late 2016 and early 2017, dam instrumentation recorded embankment movement. Grouting was suspended in February 2017 while investigators assessed the cause.<br /><br />The forensic investigation concluded that HMG injections caused hydraulic fractures that propagated from bedrock into the foundation soils and embankment. Evidence included movement that tracked grout-injection activity, abrupt pressure drops accompanied by increased grout flow, grout recovered in drilling samples, grout emerging at the surface, and thin, longitudinal grout seams exposed in exploratory trenches. Pressure analyses indicated that injection pressures exceeded estimated soil-fracturing pressures in the areas where evidence of fracturing was strongest. The investigation found that these pressures could initiate fractures without other contributing activities; sustained pressure and grout volumes allowed fractures to propagate and produce the observed deformation.<br /><br />The grouting program nevertheless helped control slurry losses during construction of the cutoff wall and treated deep bedrock. For a subsequent drilling and grouting program, specifications were revised to reduce fracture risk and improve control. Changes included using a faster-setting grout mix, increasing separation between open holes, measuring pressure at the injection point with instrumented packers, imposing stricter pressure and volume limits, defining mix progressions, limiting water-pressure testing, and improving real-time monitoring of pressure and flow.<br /><br />The case highlights the need to tailor grouting methods and limits to site geology, manage both pressure and injection volume, and use instrumentation to monitor dam response throughout construction.

Keywords

Boone Dam

Tennessee Valley Authority

seepage barrier

high-mobility grout

hydraulic fractures

embankment movement

cutoff wall

forensic investigation

grouting pressure

dam instrumentation