PVC Liner - Sewage Treatment Facilty
Owner: Department of National Defence
Location: Shilo, Manitoba
Material: 5,500,000 ft/2 - 20 mil PVC
The Installation of a 20 Mil PVC Geomembrane Using Wedge Welding Technologies at CFB Shilo, Manitoba.
Overview: The Canadian Forces Base (CFB) at Shilo, Manitoba is the site of one of the largest armed forces training facilities in Canada. The base consists of support facilities and training grounds on large tracts of open prairie and is founded on a vast sand deposit that extends from ground surface to bedrock. Consequently, the base is used for artillery practice by both the Canadian and German armies which takes full advantage of the presence of the sandy soils.
In 1994, the department of National Defence commissioned a study to evaluate the existing lagoon system and requested that options for a new lagoon be developed to upgrade the system. It was decided at the outset that a geomembrane needed to be incorporated into the design for the sewage lagoon upgrade given the permeability of the sand deposit. The decision to use a geomembrane is consistent with both the provincial regulations and federal government guidelines.
Design of the Sewage Lagoon Upgrade
Defence Construction Canada, acting on the behalf of the Department of National Defence, solicited the services of J.R. Cousin Consultants Ltd. of Winnipeg, Manitoba, to perform the conceptual and final design of the sewage lagoon upgrade. The upgrade generally consisted of replacing three existing unlined lagoon cells with three larger cells and incorporated the use of a geomembrane.
The purpose of the upgrade was to increase capacity so that the system could accumulate, store and treat all sanitary sewage generated by the base in a 365 day period and to improve the quality of effluent. Given the long storage period and the fact that the site is underlain by a permeable, natural deposit of sand, the Engineer recommended that a geomembrane be included in the new lagoon system. The Engineer chose to use a 0.50 mm (20 mil) PVC geomembrane for this purpose due to the long record of successful performance of PVC in similar applications, cost effectiveness and the level of quality and consistency that could be achieved by prefabricating large geomembrane panels.
The CFB Shilo sewage lagoon upgrade primarily consisted of the construction of three new cells (the Primary Cell; Storage Cell #1; and Storage Cell #2) with a combined surface area of approximately 450,000 square metres (4.8 million square feet). The maximum depth of stored sewage is 1.71 metres (5.60 feet). Treatment is to be accomplished by the combination of aerobic and anaerobic processes and the degradation of potentially harmful organisms by natural UV light. The large surface area, shallow depth and long retention time were integral to the treatment process.
The containment system selected for the upgrade is comprised of: a prepared and compacted natural soil subgrade; the 20 mil PVC geomembrane; a compacted sand protective cover; a well graded gravelly riprap bedding layer; and a coarse rock riprap erosion protection layer. Erosion protection is only included on dyke slopes. A typical section showing all of these components is shown on Figure 1.
Limitation of this Article
Layfield Environmental Services was sub-contracted to supply and install the geomembrane for this project. Due to the late start, and in order to attempt to complete the geomembrane installation prior to the advent of winter, we, in turn, sub-subcontracted one of the cells (Storage Cell #1) to another installer. This article limits discussion to the work performed by Layfield's own forces.
Project Initiation and Fabrication of the PVC Geomembrane
The General Contract for the project was awarded to Vector Enterprises Ltd. (Vector), of Winnipeg, Manitoba. In turn, Vector entered into a sub-contract with Layfield for the supply and installation of the 20 mil PVC geomembrane. The sub-contract between Vector and Layfield was executed and in force on August 18, 1995.
We fabricated all geomembrane panels required for the Primary Cell and Storage Cell #2 at two of our manufacturing facilities, one in Edmonton, Alberta, Canada and the other in Bellingham, Washington, USA. The majority of geomembrane fabrication was performed at the Canadian facility. The panels ranged in size, depending on the manufacturing facility, from approximately 2,450 sq.m. (26,350 sq.ft.) to 2,900 sq.m. (31,200 sq.ft.).
Track mounted, solid wedge units were used in the fabrication facilities, versus factory solvent bonding, in order to satisfy the requirements of the project specifications. The specifications required that Bonded Seam Strength and Peel Adhesion values for the Factory Seams meet or exceed 6.44 N/mm (36.8 ppi) and 3.50 N/mm (20 ppi) respectively. In contrast, NSF 54 - 1993 requires the same Bonded Seam Strength and a minimum Peel Adhesion value of only 1.75 N/mm (10 ppi). All factory seams were tested in accordance with ASTM standards D3083 and D413 and Layfield's Shop QC program. The factory seam test frequency was in accordance with NSF 54 - 1993. All factory seams met the specified requirements.
The installation of the 20 mil PVC geomembrane at CFB Shilo was completed in the period from September 13, 1995 to June 7, 1996 in two separate phases of construction. All of the Primary Cell geomembrane and about 15% of the geomembrane for Storage Cell #2 were completed in the first phase. The first phase of installation ended on November 6, 1995 after some significant snowfalls had occurred and the ambient temperature had fallen to approximately -15 C (+5 F). The Primary Cell geomembrane was installed during the period September 13 to October 10, 1995 where daytime highs averaged +16 C (+61 F) and lows averaged 0 C (+32 F). In contrast, daytime highs averaged +2 C (+36 F) and lows averaged -8 C (+18 F) while work was proceeding in Storage Cell #2 in late October and early November, 1995.
The remainder of the geomembrane installation in Storage Cell #2 was completed in the second phase of construction during the period May 18 to June 7, 1996. Daytime ambient temperatures averaged about +20 C (+68 F) for this period.
Overall, we achieved installation production rates ranging from 7,650 to 9,800 sq.m. (82,500 to 105,500 sq.ft.) per day, including panel seaming, CQC testing and the completion of repairs. The field crew installed 21,550 sq.m. (232,000 sq.ft.) of geomembrane on one particular day and had several days where field production rates exceeded well over 13,000 sq.m. (140,000 sq.ft.) However, the deterioration of weather conditions in the latter stages of construction in 1995 resulted in a 35% decrease in productivity, which serves to illustrate how much of an effect winter had on installation efforts.
Although we had initially intended to complete the entire installation in the 1995 calender year, the unusually early and severe winter hampered all efforts to meet this schedule. Consequently, the Owner deferred the commissioning of completed cells until the summer of 1996.
Completion and Commissioning
The Primary Cell was the first of the three cells to be commissioned and put into service. This cell was commissioned in July, 1996 following acceptance of the facilities by the Department of National Defence and Defence Construction Canada. The remainder of the cells were commissioned shortly thereafter.
Background on PVC Field Welds
It is considered common practice in North America to weld thin gauge PVC geomembrane panels together in the field by using Tetrahydrofuran (THF), which is a solvent. In fact, some manufacturers, typically in the USA, still use THF in the factory fabrication process to create panels. In addition, one should note that the NSF 54 - 1993 minimum value for Peel Adhesion, for factory seams, is 1.75 N/mm (10 ppi). This minimum standard is applied to all thicknesses of fabricated PVC geomembrane panels and reflects the use of solvents. Although the National Sanitation Foundation (NSF) specifically states that their standard applies only to fabricated geomembrane products, their standard is quite often used as a standard for field welds by the engineering industry.
However, the use of solvents for field welding or seaming leaves the geomembrane contractor open to considerable risk associated with weather. THF, a solvent typically used for field seams, is a hygroscopic material and therefore has limited use and effectiveness in the presence of moisture. In addition, its use is generally limited to temperatures above +10 C (+50 F). Generally, barring precipitation, THF can be used for at least 9 months of the year in the southern half of the USA and for 6 to 9 months in most of the northern half of the US. Canada is a land of extremes, where summer time highs can be +40 C (+104 F) and winter lows can be in the order of -40 C (-40 F). This presents a serious challenge to Canadian installers who not only have to work in more severe weather conditions for a good portion of the year but also can experience a relatively large range of weather extremes on one project.
In relation to the CFB Shilo project, the minimum specified values for Bonded Seam Strength and Peel Adhesion of wedge welded Field Seams were 6.4 N/mm (36.6 ppi) and 3.0 N/mm (17 ppi) respectively. This specification for Bonded Seam Strength matches the NSF 54 standard and the specification for factory seams. The specified Peel Adhesion value of 17 ppi for field seams is 70% greater than the NSF 54 standard. The specification did, however, allow for the use of solvent bonded field seams and required that the NSF 54 standard be met (i.e.: Peel Adhesion = 10 ppi), therefore, we had the option of either wedge welding or solvent bonding field seams.
The majority of reputable geomembrane installation contractors, in both Canada and the USA, have used wedge welding technology to seam together fabricated panels in the field. Layfield, as is the case for other contractors either side of the border, has been wedge welding PVC geomembrane in the field for several years, however, the use of this technology has been typically limited to gauge thicknesses of 30 mil or greater. By the summer of 1995, there had been some reported instances where field wedge welding of 20 mil PVC was successfully completed, albeit on a relatively small scale. The crux of the matter is that wedge welding of 20 mil PVC geomembrane had never been attempted on such a large scale.
Coincidentally, we had been running extensive trials to test and evaluate a variety of wedge welders for 20 mil PVC field use over the last 3 years. The focus of this effort was to transfer 12 years of PVC factory wedge welding experience to field installations. Upon our arrival at the CFB Shilo site, we were required by the Owner and the Engineer to demonstrate that the selected wedge welders could create quality field seams that could consistently meet the project specification . After several days of field trials with the wedge welding equipment and evaluation of the finished product, the method and equipment were accepted by the Engineer and we were permitted to employ this technology on this project.
Field Wedge Welding of the 20 mil PVC Geomembrane at CFB Shilo
We employed two different model versions of a self-propelled wedge welder or "mouse" to field seam the 20 mil PVC geomembrane at CFB Shilo. These wedge welders were modified to suit 20 mil PVC geomembrane material. The wedge welders were equipped with either a solid wedge or a split wedge made from silver or stainless steel. Nip roller combinations were selected specifically for use on PVC geomembrane. The total bonded seam area created by the split wedge measures 20 mm (3/4") wide, where the individual seams and the air channel each measure 10 mm (3/8") wide, while the solid wedge produces a bonded area measuring 38 mm (1 1/8") wide. These wedge welders performed consistently, without major failures, and created continuous seams, in both the machine and cross directions.
In order to create a field seam in the cross direction, where factory seams are oriented perpendicular to the direction of the field seam, the installation technicians were required to decrease the welding speed of the wedge welder to compensate for the additional thickness of PVC at the factory seam locations. Therefore, the welding speed was decreased as the wedge welder moved into a factory seam and was reset to the normal welding speed as the welder exited the factory seam area. Conversely, seams run in the machine direction, where factory seams are parallel to the direction of the field seam, did not require this adjustment in welding speed.
In addition, adjustments were made to both the wedge temperature and to the welding speed to suit ambient and geomembrane temperatures at the time of seaming. Wedge temperature settings varied from 325 C to 410 C (620 F to 770 F) and speed settings varied from 1.80 m to 4.90 m (6 ft. to 16 ft.) per minute depending on ambient and environmental conditions (temperature, cloudy, sunny, etc.) and the temperature of the geomembrane. As a general rule of thumb, wedge temperature was set high and speed set low during colder periods and the wedge temperature set lower and the speed increased as ambient and geomembrane temperatures increased. However, the full range of wedge temperature and speed settings was exploited in order to create the best quality seam at optimal production levels.
As is the case with all geomembrane installations projects, repairs were required to correct defects. Defects were found in the field seams, both split and solid wedge, by performing non-destructive air channel testing, air lance testing and by visual observation and inspection. Typically, the presence of a small soil particles within the weld track can result in a seam defect. The placement of a rigid slip sheet beneath the seam overlap area is an example of an action taken to minimize or to help eliminate the intrusion of these particles. This measure resulted in significantly decreasing the possibility of this occurring, however, small areas of seam required repair when soil particle related defects were found. In addition, other repairs were made to correct damage that resulted from welder burnouts, to repair seam sample cutout locations and to repair small holes intentionally placed in the geomembrane to release air that had collected under the geomembrane during backfill operations. Defects were typically repaired by using PVC patches that were welded in place with the solvent, THF.
It is noteworthy to comment that PVC geomembrane was wedge welded in a variety of ambient conditions and temperatures. Geomembrane was installed and wedge welded in ambient temperatures that varied between a minimum of -11.1 C (+12 F) and a maximum of +26.4 C (+80 F). In order to further illustrate and consolidate the point, the maximum ambient temperature did not exceed +10 C (+50 F) on 23% of the days when wedge welders were employed and successfully used.
What is most noteworthy is that not one of the field seams produced during the entire geomembrane installation contract failed Destructive Testing (shear and peel). A total of 10,650 lineal metres (34,940 lineal feet or 6.6 miles) of wedge welded field seam were produced during the contract.
Destructive Testing of 20 mil PVC Wedge Welded Seams
Destructive testing of wedge welds was accomplished by either one of two scenarios. Firstly, the geomembrane contractor Layfield was required, as is the case on most if not all geomembrane installations, to produce and test Trial or Qualification Welds at a rate set by the specifications. Depending on the weather conditions, from one to three Trial Welds were produced per seaming day. Destructive tests were performed on 44 Trial Weld samples on-site using a field portable tensiometer.
In addition and as a supplementary measure, the geomembrane temperature was measured with an electronic contact thermometer at the time that the destructive test was performed. This measure was taken so that a temperature correction factor could be applied to the field test data so that shear and peel values, adjusted to +23 C (+73 F), could be estimated. This, in turn, allowed the supervisor to make a reasoned decision as to whether field seaming activities could proceed or not.
Secondly, selected wedge weld samples were also submitted to an independent laboratory, the Alberta Research Council (ARC), situated in Edmonton, Alberta, Canada. A total of 32 wedge weld seam samples, both from Field Seams and Trial Welds, were forwarded to this facility for confirmation testing during the performance of the work. All ARC Destructive Testing was performed in accordance with ASTM D413 and D3083, as modified by NSF 54-1993, Annex A.
A summary of both ARC and contractor (Layfield) Destructive Test results is provided on Tables 1 and 2 for the Primary Cell and Storage Cell #2 respectively. **(Not ready for the web yet)**
Non-Destructive Testing of 20 mil PVC Wedge Welded Seams
All field wedge welds produced with a double or split wedge were non-destructively tested by using the Geosynthetic Research Institutes' (GRI) "Pressurized Air Channel Test method GM-6, revised in 1994. With this method, the maximum allowable air pressure for 20 mil PVC is 140 kPa or 20 psi and the maximum pressure drop allowable over a 2 minute period is 35 kPa or 5 psi.
The Air Lance method, in accordance with ASTM D4437-84, was used to non-destructively test solid wedge welded field seams. In the case of this project, the Engineer required that the air lance be positioned directly at the seam edge and not 50mm (2 inches) from the seam edge as the standard allows. In addition, all seam and geomembrane panel repairs were tested with the air lance.
Regardless of the method used, all field seams and repairs were non-destructively tested in accordance with industry accepted practice.
The sewage lagoon upgrade project at CFB Shilo, Manitoba was and still remains one of the largest geomembrane lined facilities of its kind in Canada. More importantly, this project represents the largest wedge welded, 20 mil PVC geomembrane project in Canada to date and one of the largest thin gauge, wedge welded projects of its kind in North America.
The goals set by the project requirements were not only achieved but improved on. The geomembrane was installed in conditions that would normally have prevented the use of solvents for field seaming, the amount of installed geomembrane was always maintained comfortably ahead of the cover backfill, geomembrane factory fabrication reduced the amount of field seaming by over 70% when comparing PVC with HDPE and an independent laboratory confirmed that field seam shear strength and peel adhesion values averaged 126% and 170% of the specification requirements. When these same ARC test values are applied to the NSF 54 standard (for factory seams), field seam shear and peel values were 126% and 289% of that standard. Layfield's field test data record also reflects this trend.
Layfield wishes firstly to acknowledge and thank Mr. Rick Gutz of Defence Construction Canada for allowing us to submit this article. We would also like to thank and acknowledge Messrs. Don Whitmore and Bob Heath of Vector Enterprises Ltd. for their involvement and support throughout the project.
Lastly, this article could not be considered to be complete without acknowledging the skill, dedication and perseverance of our project manager, project supervisors and field technicians, who collectively formed the field installation team.