DF-LibraryO I 1111 0 11°1 09078969 uII r Department of Fisheries & Oceans Harbours & Infrastructure, Grand Falls, NF. THE DESIGN OF A GABION RETAINING WALL FOR SNOOK'S ASV", NEWFOUNDLAND 13 August, 1990 Douglas G.Skinner TA 770 S5 21932:7. Department of Fisheries & Oceans, Harbours & Infrastructure Branch Grand Falls, Newfoundland 111E DESIGN OF A GABION RETAINING WALL FOR SNOOK'S ARM, NEWFOUNDLAND ..;„,,,,wwwwww,44541 , ;VII '' • — 41,,, .e 4 ,(.,'• 4".' - \9)(:)° _9 1.13 .....,.„),:,.. r n Prepared for : Division of Co-operative Education et1 er.:1-n Memorial University of Newfoundland St. John's, Newfoundland 13 August, 1990 Douglas GiSkinner TABLE OF CONFERS PAGE LETTER OF TRANSMITTAL TABLE OF CONTENTS (i) SUMMARY 1.0 NEEDS ANALYSIS 2 1.1 General 2 1.2 Fishing Activity 2 1.3 Existing Facilities 2 1.4 Slope Failure at the Truck Turnaround 3 2.0 SITE PREPARATION 6 2.1 General 6 2.2 Initial Stadia Survey 6 2.3 Dredging 6 2.4 Final Stadia Survey 7 3.0 GABION RETAINING WALL 9 3.1 General 9 3.2 The Case For Gabions 9 4.0 DESIGN 12 4.1 General 12 4.2 Engineering Properties 12 4.3 Fundamentals of Gabion Retaining Wall Design 13 4.4 The Plan and Cross Section 14 5.0 RESULTS 16 5.1 General 16 5.2 Design Calculation Results 16 5.3 Cost Estimate 18 5.4 Tender Documents 19 CONCLUSION 21 REFERENCES 22 APPENDIX A : Maccaferri Gabion Installation Guide 23 APPENDIX B : Calculation Sheet 29 APPENDIX C : Detailed Cost Estimates 33 APPENDIX D : Invitation to Tender & Tender Documents 34 The embankment at the edge of the newly constructed truck turnaround in Snook's Arm collapsed this past Spring. In order to provide enough room for a truck from the National Sea fish plant in nearby LaScie to maneuver when it arrives to collect each day's fish landings, the bank must be rebuilt. A rock filled gabion wall is proposed to solve this problem for socio-economic reasons. The installation of gabions requires very little skill and the local unskilled labour force could be employed to work on this project when the fishing is slack. Such employment is eligible for funding through various Federal Government Agencies. Even if the locals are unavailable for work on this job, it is still a worthwhile pursuit to utilize gabions as an experiment to determine the feasibility of installing gabions on future projects, since this project is very small and low in cost. The engineering properties of a gabion manufactured by Maccaferri Gabions of Canada Ltd. are well suited to this project. The gabions are constructed of hexagonal zinc coated steel wire mesh with a PVC coating applied for further protection in the corrosive marine environment. The individual gabions are filled with rock and wired together to form a flexible and permeable structure which - is capable of resisting any type of stress. The wires act as reinforcement and a gabion structure becomes stronger and more efficient with age. The gabion retaining wall designed for Snook's Arm will effectively resist failure by overturning, sliding, and settling. The design calculations show this unambiguously. The project could be constructed for under $5,000.00 if local labour was available. Since this is not the case, tender documents have been prepared for local contractors to prepare bids for this work. It is expected that the cost will be 30 to 50% higher than estimated. It may not be possible to finance this project from the 0 &M budget of 1990. This means that the rebuild of the truck turnaround in Snook's Arm may have to wait for funding from next year's budget. More damage and higher repair costs will result if the work is not completed before next spring. 1.0 NEEDS ANALYSIS 1.1 General This section of the report deals with the present state of the harbour facilities at Snook's Arm and identifies the need to rebuild the truck turnaround area near the wharf. 1.2 Fishing Activity The community of Snook's Arm is located on the western side of Green Bay, approximately 18 kilometers from LaScie. The fact that there are 26 full and part-time fishing licenses issued for the 1990 season from a total population of 57 is indicative of the importance of the fishery to this village. For the past five years the fishermen of Snook's Arm landed an average of 150,925 killograms of all species of fish with an average yearly value of $115,450.00. 1.3 Existing Facilities The most important criteria for the allocation of funds for the upgrade of any facility are safety and efficiency for use by fishermen. By 1988 the wharf at Snook's Arm had deteriorated to the point where it was no longer considered safe for use. For this reason and the fact that the community itself would probably not survive without a proper wharf, a new treated timber wharf was constructed last year at a cost of $490,000.00. The construction of this wharf ensured the survival of the community and provided the facilities for the local fishermen to operate safely and efficiently. During the construction of the new wharf, it was suggested by the local Fisherman's Committee that the approach to the wharf be widened to provide a truck turnaround area. The landings at Snook's Arm are stored on ice in containers at the wharf for collection by a straight body truck from the National Sea Products Fish Plant in LaScie. The driver 3 had to back up to the wharf because the approach was not wide enough for turning. This meant that he had to turn around at the top of some very steep hills above the village. It was easier for the truck to negotiate these steep hills in reverse gear going down the hill empty than to have to do so going up with a load. Since the contractor was already on site to construct the wharf, it was decided to proceed with the construction of the truck turnaround at that time to avoid extra mobilization and administration costs associated with hiring another contractor at a later date. The truck turning area that was constructed was a compromise with what the Fisherman's Committee suggested and the budgetary constraints of the Department. At a cost of $36,500.00, a 20 foot long section of treated timber retaining wall with associated rock and gravel fill was constructed adjacent to the wharf. This campramise provided enough room for the trucks to maneuver, while the timber retaining wall kept the backfill from reducing the water depth at the wharf, thereby maintaining the amount of berthage available to users. Beyond the retaining wall, the backfill was allowed to slope to the water's edge. The embankment so formed followed a curvilinear shape towards a privately owned shed across from the wharf. The original suggestion called for a 40 foot section of timbei retaining wall and the embankment was to proceed straight across the shoreline. This proposal was estimated to cost $53,000.00. 1.4 Slope Failure at the Trick Turnaround Over the past winter, the embankment at the edge of the newly constructed truck turnaround area and adjacent to the timber cribwork retaining wall collapsed. Sufficient room is still available for the National Sea Products trucks to maneuver adequately. However, the deterioration of the slope caused other difficulties that need to be addressed. Much of the backfill material adjacent to the retaining wall at the wharf fell into the water in front of the retaining wall. This 4 deleterious material severely hampers the safe mooring of boats near the end of the wharf. During low tide, the water depth in this location is insufficient for vessels to maneuver without striking the rocks. The fishermen avoid this area and, as a consequence, at least two berths are unavailable for use. This material must be dredged out as soon as possible. When the slope of the bank collapsed at the shoreline, the guardrail installed at the top of the bank was left suspended between the first and last posts. This is evident from Photo # 1 below. Photo # 1: Slope Failure at Truck Turnaround - Snook's Arm Four posts are left suspended in air. This could be a safety hazard to anyone who is unfamiliar with the washout and who decides to visit at a time when the lighting is dim, say early morning or late evening. This person could quite conceivably drive a vehicle over the bank. Granted, the chances of this happening are quite miniscule, but the hazard should still be addressed. A more insidious difficulty exists in the natural processes of nature. The spring run-off will further erode the bank to the point where the available space for trucks to turn is considerably reduced. 5 Furthermore, the forces exerted by the trucks themselves can lead to greater slope failure. The foregoing arguments lead to the inescapable conclusion that the bank must be rebuit so as to maintain the advantages that the original construction was supposed to provide. Yet, the proffered solution must ensure that the problem does not re-occur for a reasonable period of time. The cost to repair the damage should be reasonable. 2.0 SITE PREPARATION 2.1 General This section of the report is concerned with the collection of information pertaining to the physical layout of the site before and after the spoil that had fallen from the bank was excavated. 2.2 Initial Stadia Survey In order to get a full appreciation of the extent of the damage to the slope, a stadia survey was conducted on June 26, 1990.