TECHNICAL MEMORANDUM 002 EMORANNO. 001

Transcription

1 TECHNICAL MEMORANDUM 002 EMORANNO. 001 To: Jack Synder, P.E. EES Consulting From: Mort McMillen, P.E. Paul Larson, SE Date: October 13, 2010 Project: Cc: Taylor Bowen Subject: Technical Memorandum (TM) No. 002 Diving Lake Bottom Observation Report 1.0 INTRODUCTION 1.1 Purpose This memorandum summarizes the observations from the diving of the lake bottom at Sullivan Lake Dam on October 6, 2010 by Harbor Offshore, Incorporated (HOI). The purpose was to observe the conditions of the lake bottom and to take soil samples along the proposed pipeline. 2.0 OBSERVATIONS 2.1 Method HOI Arrived at Sullivan Lake at 8:00 AM with four divers to launch a work platform from which to dive. Once the platform had been launched the diving gear was loaded and work began to install a guide cable. The cable was run from a buoy, which had been set by the survey crew at a depth of 120 feet, to the center of the bridge. The cable was to approximate the proposed pipeline location. The dive proceeded from the buoy toward the bridge at about 10 feet along each side of the cable. Five soil samples were taken, one at the buoy, one at the bridge and three at intermediate points in between. An additional dive was performed at about 200 feet to the west of the buoy to verify that the lake bottom conditions off the centerline of the projected pipeline route were similar. 2.2 Observations The dive can be broken into 5 segments that were performed by 3 divers of the 4 man crew. The segments are the four lengths between each sample and the additional dive 200 feet toward the west of the buoy. See Figure 1 for reference to the dive observations. A video was taken of the dive. A. Segment 1 The diver proceeded to the bottom of the lake at a 120 depth. As he made it to the bottom he was approximately waist deep in very soft mud. A soil sample was McMillen, LLC Page 1 EES Consulting October 14, 2010 Tech Memo No. 002

2 taken and hoisted to the surface. The diver then began to follow the guide cable toward the bridge in a zigzag pattern. No substantial debris was seen at this segment, but the diver had to always proceed forward due to the mud cloud that was created by moving. At a depth of about 75 a second sample was taken. At this location the diver was about knee high in soft mud. B. Segment 2 After the second sample the first diver was removed and the second diver was placed into the water. The second diver moved through the mud until a depth of about 55 where the lake bottom turned to large cobles and gravel. At this location a third sample was taken. C. Segment 3 After the third sample was taken the lake bottom was cobles and gravel. Debris started to become more apparent but was still small in nature. At a depth of about 45 logs began to appear that were about 12 in diameter and 40 to 60 long. The logs were still small enough that they could be moved by divers. At about a 30 depth some steel parts about 8 from the guide cable were deposited on the lake bottom in a pile. At a depth of about 20 the logs subside and the lake bottom changes to a more angular 4 inch minus material where a fourth sample was taken. D. Segment 4 Moving toward the bridge from the fourth sample the lake bottom began to change to smaller angular gravels. At a depth of about 15 feet a pronounced channel started to become visible to the diver and the gravels are 1 to 2 minus material. The diver moved along the channel to under the bridge where a fifth sample was taken. E. Segment 5 It was decided to make another dive approximately 200 feet west of the buoy perpendicular to the cable line to verify the lake bottom conditions in an arc form the bridge. As third diver performing this dive reached the bottom at a depth of 96 he was up beyond his knees in soft mud. The diver walked about a 20 radius circle and the conditions did not change. 3.0 ACTION ITEMS Based on the deep deposits of surface soft mud the proposed foundation of the intake screen will need to be modified. The proposed foundation assumed a gravel lake bottom, but the soft mud that was encountered may require a pile or pier foundation system. The depth of the mud deposits is unknown and the depth the bearing material will be encountered is unknown. To be able to properly design the correct foundation system the depth of the bearing material is required. McMillen proposes engaging a geotechnical engineer to drill a soil boring at the proposed intake location to determine the bearing depth for the intake foundation. McMillen would like to discuss this and our options with EES Consulting and the PUD at your earliest convenience. McMillen is currently holding on the intake foundation until further information can be obtained. The remainder of the design is progressing as scheduled. McMillen, LLC Page 2 EES Consulting October 14, 2010 Tech Memo No. 002

3 Figure 1: Dive Survey McMillen, LLC Page 3 EES Consulting October 14, 2010 Tech Memo No. 002

4 DRAFT TECHNICAL MEMORANDUM 003 EMORANNO. 001 To: Jack Snyder, P.E. EES Consulting From: Mort McMillen, P.E. Paul Larson, SE Date: Project: Cc: Files Subject: Technical Memorandum (TM) No. 003 Construction Methods, Sequence and Permitting 1.0 INTRODUCTION 1.1 Purpose This memorandum summarizes alternative construction methods, sequences, and the potential permitting requirements associated with the in-water pipeline construction. The primary focus of this memo is the in-water work through the channel upstream of the dam. This section of the pipeline was originally proposed to be buried in an excavated trench behind a cofferdam during construction. 2.0 PIPELINE ALIGNMENT 2.1 Proposed Alignment As illustrated on Figure 1, McMillen, LLC (McMillen) is proposing to place the new intake 1,000 feet from the dam into the lake at an intake elevation that is 120 feet below the low water surface elevation. From the downstream face of the dam, the pipeline begins with a pneumatic knife gate which enters a square steel conduit through the dam that then transitions to a 48- inch round steel pipe as it exits the upstream side of the dam. A flanged connection connects the steel pipe to a 48-inch diameter high-density polyethylene (HDPE) pipe. A transition section is provided from the concrete inlet apron to a pipe invert elevation approximately 8 feet below the top of the concrete apron to ensure a minimum of 2 feet of cover over the pipe through the channel (see Figure 2). The pipe is embedded through the channel and remains approximately level until it daylights 335 feet from the dam. From this point, the pipeline will be laid on the lake bottom out to the intake screen structure. Concrete anchors will be installed on the pipeline to weight down and secure the pipeline to the lake floor. 2.2 Pipeline Profile Approach Channel to Dam The existing Sullivan Lake Dam was constructed in the natural outlet channel from the lake. The dam is located approximately 400 feet from the natural drop off to the lake depth. Two alternatives were considered for the design of the pipeline within the existing approach channel. The first alternative was to bury the pipeline in the floor of the approach channel. McMillen, LLC Page 1 EES Consulting

5 The concrete elevation of the low level inlet apron is feet, and the minimum channel elevation of the approach channel is about feet. To maintain a minimum pipe cover over the pipeline of 2 feet, the invert elevation is set at feet. Assuming the pipe invert remains level through the channel curving downward to meet the slope of the lake bottom, an excavated pipe trench is required from the dam to a point approximately almost 400 feet from the dam. The second alternative consists of placing the pipeline along the approach channel at a higher pipeline invert to minimize excavation. Option 1 would be to raise the pipeline such that the top of the pipe is flush with the top of the concrete intake apron. The pipeline would be imbedded in a trench through the channel and anchored with concrete anchors along its entire length. Option 2 would be to raise the pipeline such that it remains level through the dam and the approach channel. In this case, the pipe would be anchored to the channel bottom with concrete anchors along its entire length (See Figure 3). Alternative 2 was identified to address several constructability issues associated with the cofferdam, dewatering and excavation. A brief discussion of these potential issues is presented in the following paragraphs. 3.0 COFFERDAM 3.1 Lake and Pipeline Elevations The lake elevation behind the dam varies from a High Water Surface Elevation (HWSEL) of feet to a Low Water Surface Elevation (LWSEL) of feet. It is currently assumed that the lake elevation could be drawn down to the LWSEL during the construction window to aid in construction and to minimize the size of the cofferdam. A higher WSEL may be accommodated during construction but will result in a taller and longer cofferdam. The discussion in this memo is based on the LWSEL during construction. The proposed baseline configuration results in a cofferdam that is approximately 400 feet from the dam with a height of about 12 to 16 feet and a length of about 300 feet, as illustrated on Figure 4. The Alternate 1 pipeline location results in a cofferdam that is approximately 360 feet from the dam with a height of about 6 to 10 feet and a length of about 200 feet (see Figure 5). The Alternate 2 pipeline location results in elimination of the cofferdam since minimal excavation would be performed below the water surface. Alternate 2 may result in substantial cost savings due to the elimination of the cofferdam and the reduced requirements for dewatering. 3.2 Subterranean Conditions Currently, the subterranean conditions below where the divers could obtain samples are unknown. The channel bottom is composed of gravels, but the location of bedrock is unknown. The bottom of the excavation is below the bottom of the old dam and within two feet of the bottom of the cutoff wall, which was added in the 1994 rehabilitation of the dam. Encountering rock above the bottom of the trench elevation will increase cost and the construction schedule. If McMillen, LLC Page 2 EES Consulting

6 the pipeline is buried, an additional soil boring should be taken in the channel concurrent with the proposed intake borings. 3.3 Log Zone The cofferdam location falls in the zone of the lake with substantial log debris. The debris at the lake bottom will need to be removed before the cofferdam is constructed. The extent of the log removal will depend on the type of cofferdam that is selected by the contractor. As can be seen in Figures 4 and 5, the baseline pipeline elevation will require the most log removal and Alternate 2 the least. 3.4 Dewatering and In-Stream Flows Before construction begins behind the cofferdam, the work area must be dewatered by dewatering pumps. The water is pumped into portable settling tanks before it is released into Outlet Creek. Once work begins in the work zone, pumps and the settling tanks will be operated around the clock to remove the seepage that occurs through the cofferdam. Since the cofferdam will block the only outlet from Sullivan Lake, stream inflows must be either pumped around the cofferdam and piped below the dam, stored behind the cofferdam by adding sufficient height to the cofferdam to create storage capacity, or a combination of the two. Minimum in-stream flows in Outlet creek will need to be maintained. This could be provided by the water from the settling tanks (if sufficient) or auxiliary pumps pumping from the lake. The Option 1 pipeline construction approach will reduce the length and head required for the pumps and the amount of seepage that is required to be pumped. The reduced seepage will reduce the settling tank requirement, but lake inflows and in-stream flows may not allow the pump capacity to be reduced. The Option 2 pipeline construction approach could potentially be permitted with a temporary turbidity exemption allowing work to proceed without settling tanks. Since there is no cofferdam to block the outlet stream, inflow can be released through the dam low level outlets. Some pumping may be required to provide for minimum stream flows in outlet creek. 4.0 PERMITTING 4.1 Fish Construction Window The normal construction window for fish extends for six weeks in July and August with possible extensions for either the start or the finish. Stream flows into the lake are a maximum in May and June and taper in July. Construction of the cofferdam is an early activity such that the cofferdam and dewatering system will need to be designed for the higher flows. The cofferdam also extends the construction schedule requiring longer extensions for the permitted in water work. McMillen, LLC Page 3 EES Consulting

7 5.0 CONCLUSION Burying the pipeline through the channel increases the cost, schedule, and construction risk of the project; however, it does improve the channel hydraulics. The velocities through the channel are low even during the peak spill condition. They are estimated to be approximately 1 feet per second (fps) average and 2 fps peak. The area of the pipe is less than 0.5% of the channel area at the HWSEL. The flow characteristics of the channel are adequate for any of the alternatives. Table 1 below gives the benefits and disadvantages of the different alternatives. Based on the preliminary construction estimate the cost of the cofferdam and associated dewatering is approximately $95,000 and the trench excavation and fill is approximately $70,000. Any of the alternatives are feasible. The selected alternative is should be based on the PUD s preference. Table 1. Alternative Matrix Benefit/Disadvantage Proposed Baseline Alternate Option 1 Alternate Option 2 Hydraulics No interference with hydraulics. No interference with hydraulics. Pipe is exposed in the channel at low water levels. Pipe is minimal interference Vandalism Cofferdam Construction Schedule Dewatering Construction Safety Subterranean / Excavation Risk Construction Cost Not susceptible to vandalism. Largest cofferdam of alternatives. Additional construction time to build and dewater cofferdam. Dewatering will require settling tanks. Increased risk working behind a cofferdam. Unknown sub-surface condition of channel. Additional soil boring will provide required information. Additional cost to build cofferdam and excavate trench. Top of pipe is susceptible to vandalism at low water levels. Reduced Cofferdam Size. Reduced construction time to build and dewater cofferdam. Dewatering will require settling tanks. Increased risk working behind a cofferdam. Unknown sub-surface condition of channel. Additional soil boring will provide required information. Additional cost to build cofferdam and excavate trench. at high water levels. Pipe is susceptible to vandalism at low water levels. No Cofferdam. Shortest construction schedule. No Cofferdam. Minimal dewatering required Reduced risk with no cofferdam. Minimal excavation reduces risk. Reduced construction cost. McMillen, LLC Page 4 EES Consulting

8 Figure 1. Pipe Alignment McMillen, LLC Page 5 EES Consulting

9 Figure 2. Pipe Profile McMillen, LLC EES Consulting

10 Figure 3. Trench Profile McMillen, LLC EES Consulting

11 Figure 4. Baseline Configuration McMillen, LLC EES Consulting

12 Figure 5. Alternate 1 Configuration McMillen, LLC EES Consulting