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Figure 8. Major Joint Planes 1 10

Figure 9. Locations of the major rock blocks shown in the 3-D model view

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Joint Plane 1 has an orientation of 77/289 (dip/dip direction). This plane corresponds to Joint Set A and was identified primarily using the LiDAR data and from the downhole optical televiewer survey data for DH09-03. Joint Plane 2 is a Set C joint. This joint plane was identified based on the findings of the drilling program. The optical televiewer survey identified shallow open joints intersecting boreholes DH09-01 (at el. 433) and DH09-02 (el. 431) with orientations corresponding to Set C. Joint Planes 3, 4, and 5 are essentially parallel to Plane 2 and were identified during geological mapping and can also be identified from the breaks in slope shown in the LiDAR survey data. These planes are approximately parallel and are oriented at 33/303. Planes 3 and 4 are spaced approximately 15 ft apart and Planes 4 and 5 are spaced approximately 20 ft apart.

Joint Planes 2 and 3 are over 90 ft apart with no evidence of any persistent subparallel joints occurring between these planes. Typically, in both the left and right abutments, it has been noted that the zones of higher permeability are typically associated with open joints inclined at 30 to 40 degrees (belonging to Joint Set C or a random joint). In addition, as described in Section 2, several open and persistent Set C joints with apertures of 0.25 to 0.5 inches were noted during the geological mapping of the plunge pool and at outcrops at the base of the left and right abutments. The drilling results indicate that the left abutment rock mass has a very low permeability between Planes 2 and 3. Although several Set C joints are visible in this portion of the left abutment, and were intercepted in Boreholes DH09-01 through DH09-04, it should be noted that there is no evidence that any of these are sufficiently persistent to be considered a major joint Joint Plane 6 is a Set B joint, oriented at 86/196 and is located upstream of the arch dam. Plane 7, oriented at 90/227 is also a Set B Joint and the contact between the arch dam and the rock. Joint Plane 8, oriented at 83/335 partially follows the closely spaced joint zone located immediately downstream of the dam. Joint Plane 10 is also a Set B joint and has an orientation of 90/198. Figure 9 displays the locations of the various rock blocks and the block geometry. In general, Joint Sets 1 and 3 dominate and define most of the blocks, with the Set 3 joint being the sliding plane in most cases. It should be noted that the orientation of the sliding plane may not necessarily correspond to the exact direction of sliding as this is a function of the intersection of the various joint planes as well as the direction of the forces exerted by the dam. The critical blocks are Blocks 2, 4, 5, 6, and 7. The stability analyses for the blocks should therefore be focused on these blocks.

NEXT STEPS

Stability analysis will be performed for rock blocks subject to dam loading. Figure 8 shows both the joint planes and the rock blocks that have been identified for stability investigations. Our stability analysis and stabilization design process will start with Block 7 and work upstream. All of the rock blocks shown on Figure 8 will be analyzed with the exception of Rock Blocks 1, 3, and 8 since they are not in direct contact with the

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existing or raised dam. Joint Plane 2, which forms the sliding plane for Blocks 1 and 3 does not appear to daylight within these blocks. However, portions of Blocks 1 and 3 may be inundated by the dam raise and this will require consideration in the stability analyses. Block 8 is located downstream of the existing and raised dam, thrust block, and grout curtain and any proposed drainage system. In addition, the sliding plane (Plane 9) has a low angle (17 deg dip), which is considerably lower than the estimated friction angle. As a result, blocks 1, 3, and 8 are unlikely to be a concern. The extent of dam loading has been conservatively estimated for each block. For example, the dam load attributed to Block 7 (7A+7B+7C) extends from el. 205 to el. 437 even though the dam actually appears to contact Block 7 from el. 250 to el. 437. This is because the potential dam load could be transferred through the upstream blocks to Joint Plane 8 and into Block 7. Time history analysis results for the existing and raised concrete arch dams are used in this rock block stability analysis. The analysis will include the entire mass of each block and consider any three dimensional issues that may not be reflected in the two dimensional analysis. To date, only preliminary analysis of Block 7 has been completed. Results indicate that drainage will be a key part of the stabilization design. The following load cases will be analyzed: Static with normal maximum pool (el 425) FOS > 1.5 Static with PMF flood FOS > 1.3 Seismic with normal maximum pool (el. 425) FOS > 1.1 The safety factors will be reviewed based on parametric analyses and review of key variables influencing stability. Rocplane (Rocscience 2008) and Swedge (Rocscience 2008) will be used to carry out limit equilibrium analyses for Rock Block 5 since this block will slide on two planes. Toppling failure will be considered in the analyses as well. Limit equilibrium analyses will be undertaken to evaluate the potential for toppling failure. These forces included the water forces acting on the sides and bases of the blocks, seismic loading, and the loading from the dam and thrust block. Sensitivity analyses will be undertaken by varying selected parameters including joint orientations, joint shear strength parameters (effective friction angle with no cohesion), water pressure and seismic forces. The values for these parameters will be varied over the anticipated range variance.

CONCLUSIONS

Utilizing the LiDAR topographic survey data, the results of geological and geotechnical drilling and downhole testing, a total of 15 major joint planes were identified. As can be seen in the model, Sets 1 and 3 play a major role in defining the rock blocks as well as the

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topographic expression at the left abutment. These joints form a total of 8 rock blocks on the left abutment within the bounds of the geological model. The blocks that would be subject to direct loading from the dam or thrust blocks include Blocks 2, 4, 5, 6, and 7. These blocks considered to be the critical cases. Pseudostatic stability analyses should be undertaken on these blocks using simple limit equilibrium analyses to assess the stability of the critical blocks. Pending the results of the psuedostatic analyses, dynamic analyses may be considered. The results of the stability analyses will used in the design of stabilization measures. The development of the 3-D model provided a more effective representation of the geological conditions at the left abutment than could be done using the 2-dimensional plans and sections. The development of the model facilitated the identification of the major planes and rock blocks and provided a means of compiling geological data from multiple sources into a 3-D platform that allows the project team to visualize the kinematics of the left abutment stability.

REFERENCES

Barton, N. and V. Choubey. 1977. The Shear Strength of Rock Joints in Theory and Practice. Rock Mechanics, Vol. 10, No.1. Barton, N.R.1973. Review of a New Shear Strength Criterion for Rock Joints. Engng Geol. 7, pp 287 332. Barton, N.R. 1976. The Shear Strength of Rock and Rock Joints. Int. J. Mech. Min, Sci. & Geomech. Abstr.13 (10), pp 1 24. Barton, N.R. and S.C. Bandis. 1982. Effects of Block Size on the Shear Behavior of Jointed Rock. 23rd U.S. symp. on rock mechanics, Berkeley, pp 739 760. Barton, N.R. and S.C. Bandis. 1990. Review of Predictive Capabilities of JRC-JCS Model in Engineering Practice. Proc. Int. Symp on Rock Joints, Loen, Norway, (eds N.Barton and O. Stephansson), pp 603-610. Rotterdam: Balkema. Bowes, D.E., 1994. Periodic Safety Inspection Report. Blue Lake Hydroelectric Project. FERC Project No. 2230 Ak. Prepared for City and Borough of Sitka. De Rubertis, K. 2004 Review of Safety. Blue Lake Dam FERC No. 2230. Prepared for City and Borough of Sitka, Alaska. De Rubertis, K. and McArthur, M. Report on Investigations. April 25 27, 2009. Prepared for City and Borough of Sitka, Alaska.

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Duke Engineering and Services. 1999. 1999 Review of Safety. FERC NO. 2230 AK. Prepared for City and Borough of Sitka. Duke Engineering and Services. 1997. Spillway Plunge Pool Inspection Report. FERC NO. 2230 AK. Prepared for City and Borough of Sitka. Haeussler, P.J., Gehrels, G. and Karl, S.M. 2004. Constraints on the Age and Provenance of the Chugach Accretionary Complex from Detrital Zircons in the Sitka Graywacke near Sitka, Alaska. Hoek, E. and Marinos, P. 2000. GSI- A Geologically Friendly Tool for Rock Mass Strength Estimation. Proc. GeoEng. 2000. Conference, Melbourne. Loney, R.A., Pomeroy, J.S., Brew, D.A., and Muffler, L.J.P. 1964. Reconnaissance Geological Map of Baranof and Kruzof Islands, Alaska. Map I-411. Department of the Interior. United States Geological Survey. R. W. Beck and Associates. 1989. Blue Lake Hydroelectric Project FERC Project 2230. Periodic Safety Inspection Report. Prepared for the City and Borough of Sitka. R. W. Beck and Associates. 1984. Blue Lake Hydroelectric Project FERC Project 2230. Periodic Safety Inspection Report. Prepared for the City and Borough of Sitka. R. W. Beck and Associates. 1983. Blue Lake Hydroelectric Project FERC Project 2230. Supplemental Safety Inspection Report. Spillway Plunge Pool. Prepared for the City and Borough of Sitka. R. W. Beck and Associates. 1974. Blue Lake Hydroelectric Project FERC Project 2230. Periodic Safety Inspection Report. Prepared for the City and Borough of Sitka. Rocscience 2008. Rocplane Version 2.035. University of Toronto. Rocscience 2008. Swedge Version 5.003. University of Toronto. United States Department of the Interior Bureau of Reclamation. 1954. Preliminary Report on the Blue Lake Project. Sitka, Alaska. Alaska District Office, Juneau, Alaska.