Evaluation of Hydraulic Fracturing Phenomena in Earth Dam

Abstract

Knowledgebase for hydraulic fracturing (HF) in dam is crustal information in expert system for dam internal erosion identification. The saddle dam (Dike 4) of the Sirikit Dam was used in the current case study where is located in the regional geology of metamorphic rock. The intrusion of igneous rock in the area creates foliation by tectonic forces. Consequently, rock foundations have become overturned folds. The case study on dike 4 of geology, field monitoring and Finite Element Method reveals that the foliation of foundation rocks leading to HF phenomena. Hydraulic fracturing shows the initiation when the reservoir water level reaching +148 m. MSL. and progress as RWL. is increased. This behavior was detected by the piezometers installed in the interface of residual soil and phyllite. The pore pressure ratio (P_R) between piezometric pressure and reservoir pressure can indicate the degree of HF. Four levels of HF are very likely, likely, unlikely and very unlikely when P_R are 1.0-0.8, 0.8-0.6, 0.6-0.3 and 0.3-0.0 respectively. The geometry of foliation, depth of HF from dam crest, elevation head of reservoir water level and modulus of foundation soil and rock can formulate the “Hydraulic Fracturing Index, HFI”. Four similar levels of HF are indicated by HFI of 4.0-2.5, 2.5-1.0, 1.0-0.25 and 0.25-0.0 respectively. HFI is also confirmed on another dam in the same project. Verification and validation can be done using dam data from Teton dam. The validations are agreed to failure modes caused evidently HF. Compilation of these knowledges into decision flowchart can yield the levels of possible risks of hydraulic fracturing in the dam. Three categories of identification are from 1) geology data, 2) pore pressure ratio (P_R) and 3) hydraulic fracturing index (HFI). If the results from 3 categories are agreed then the confidence of hydraulic fracturing level is very high.