Reinforced Polyethylene - Irrigation Canal Liner

 

Lateral 11B Springhill Rehabilitation

Geosynthetic liner used in a Canal to prevent seepage onto adjacent lands

INTRODUCTION

The irrigation system within the Eastern Irrigation District was originated by the Canadian Pacific Railroad Company in 1907. The CPR determined that in conjunction with the construction of the trans-continental railway, they would initiate the development of Canada's most ambitious agricultural project. The CPR began construction of a network of canals, flumes, syphons, drains, spillways and reservoirs. Through irrigation, crop production increased and the irrigated farmland benefited from less erosion than a dry land farm. With the eventual transfer of ownership to the Eastern Irrigation District (EID), the district grew to include presently 1,500,000 acres of land, of which the EID delivers water to over 275,000 acres of cultivated crop land. The water provided by EID is also utilized for domestic livestock, poultry production, and is critical to the 40,000 acres of wetland habitat that is located within the district.

EID CANAL SYSTEM

Eastern Irrigation District conducted a study on one of the canal sections, Lateral 11B Springhill, because of concerns that surrounding farm areas of the canal had become more alkaline. Eastern Irrigation determined that the canal was affecting the adjacent lands through seepage and there was a considerable amount of erosion occurring within the canal cross section. Further investigation revealed that the 11B Springhill canal was lacking adequate banks and freeboard, thus impacting the capacity of the canal. In order to minimize water loss and prevent surrounding land deterioration due to seepage, a major rehabilitation for 11B Springhill was planned involving reconstruction of the banks, installing a geomembrane and gravel armouring the inside slopes. The sub laterals from Lateral 11B Springhill would be rehabilitated by installing closed buried PVC pipelines.

OVERVIEW OF THE PROBLEM

Seepage is the primary concern for the irrigation districts because of the alkali contamination that occurs, which in turn, damages the soil and prevents effective crop growth and development. The physical process of seepage from the canals results in the rising of the ground water below the liner, called water mounding. This occurs as the soils beneath the canal become saturated from the seepage and the water table rises to meet the saturation point. The salts and minerals in the soils migrate to the surface surrounding the canals causing a change in the chemical composition of the soils making them not conducive to crop growth. The soil types occurring in the 11B Springhill area primarily consisted of a silty clay till with the presence of occasional sulphates and oxides.

The ground water table ranged from 1m to 6m below the surface and was subject to seasonal variations, usually being the highest in the spring.

In 1993, an infrared photographic study was completed for the 11B Springhill area. The photos indicated how many acres each farmer owned and what areas were suffering from high salinity, thus affected by seepage. EID assesses what amount is to be paid by each farmer based upon what they are irrigating. If the land has been damaged by seepage, EID cannot assess the area and will only account for it in the assessment once the land has returned to the fertile state. Therefore, in order for the EID and the landowner to maximize the benefits from the irrigation system, the canals must be in top operating condition.

Through their investigations, EID identified the following as being the primary problems with the 11B Springhill canal:

  • The lands adjacent to the canal were being adversely affected by seepage,
  • The canals were lacking in capacity due primarily to deterioration of the banks and poor original canal design.

PROPOSED REHABILITATION

In order to rehabilitate the affected land, the EID determined that a geomembrane must be installed to minimize seepage to prevent the worsening of the seepage. The rehabilitation for 11B Springhill was to include:

  • Rebuilding the banks, where required, to attain proper freeboard and bank width to accommodate access for operations and maintenance.
  • Install a geomembrane for the entire canal
  • Reconstruct the canal cross section, including the side slopes
  • Gravel armour the inside slopes

DESIGN CONSIDERATIONS

The criteria for design is determined by the size of the canal and flow requirements to meet specific demand. Based upon this information EID determines whether PVC pipe or an open canal design will be utilized. Typical flow rates for the varying sections of canal are as follows:

  • main canal 1000 - 2500 cubic feet per second (cfs)
  • laterals 200 - 800 cfs
  • sublaterals 10 - 100 cfs

The sublaterals, which are considered to have a low flow velocities, are primarily accommodated by the installation of a closed buried PVC pipeline. The pipelines are gravity fed and are a side lateral from the laterals and feed back into them.

The main canal and laterals involve much higher flow rates and the design was based upon an open canal system incorporating a geomembrane. Two separate methods of constructing the open canal were utilized based upon the quality and location of borrow material available.

The first method of installation utilized a combination of borrow material and gravel on top of the liner because the source of borrow was not readily available to the site and hauling costs deemed uneconomical. Construction involved sub-cutting the existing bed to an elevation of one metre below design grade. The geomembrane was installed and covered and compacted with one metre of the borrow fill material. The side slopes were then armoured with 150 mm of gravel to an elevation of 300mm above the F.S.L.

The second design was chosen if the borrow source was not located near the area under construction. This method involved excavating the canal section 300mm below design grade, installing the geomembrane and then backfilling the entire liner with 300 mm of gravel armour.

THE GEOSYNTHETIC SOLUTION

Because the water within the canals is not shut off until late fall, typically mid-October, construction of the canal and liner installation cannot commence until after the canals are sufficiently drained. This region at this time of year can experience severe winter temperatures and weather conditions. Therefore, the liner material must be able to survive installation and backfilling stresses in a cold temperature environment. EID specified Reinforced Polyethylene as the geomembrane to meet these requirements because of it's cold temperature properties and economics. Layfield Plastics was awarded the contract to supply the geomembrane specified by the EID.

RPE 15, supplied by Layfield Plastics, is a light weight woven polyethylene material consisting of high density woven tapes coated with a low density polyethylene. This material has been used for canal linings for over thirty years due to its low cost and ease of installation in cold temperatures. For EID's canals, Layfield Plastics Edmonton manufacturing plant prefabricated the RPE 15 into custom panels based upon the width of the canal and suitable handling weights. Layfield Plastics is capable of prefabricating the RPE 15 into panels up to 8800 m2, which is roughly 1800 kg. Welding the panels was done utilizing a method of hot air fusion, which provides a fully fused, water tight weld. Panels are accordion folded in one direction, rolled in the other direction and finally wrapped in a UV stable packaging with clear panel identification. Each panel is clearly identified, tightly packaged and easily deployed. Layfield fabricated 42 panels at 18m x 110.1m and 20 panels at 13m x 151.5m. The 18m and 13m widths were the exact width of the canal, minimizing the wastage of material incurred during installation.

Quality control is an essential part of Layfield's fabrication process. Layfield Plastics has established a comprehensive Quality Control program ensuring all fabricated products meet or exceed both industry and project specifications. The RPE 15 seams are tested in Layfield's quality control laboratory for both full delamination and shear strengths (one test is performed on every fifth seam or every 280 lineal metres). Test results and material manufacturers mill certificates are provided with every shipment. This QC/QA program leaves a complete audit trail of quality assurance documentation.

Installation of the RPE 15 on site was performed by rolling out the panel in one direction and unfolding it in the other. EID follows the method of only excavating and grading as much subgrade as can be installed with the liner on that day. They have chosen to do this to prevent frost build up in the subgrade, which would result in frost lumps that could potentially damage the liner during installation. Individual fabricated panels are overlapped using a 1m interlocking overlap, backfilled and compacted. The earthmoving equipment then backfills the liner by pushing the fill onto the liner and "rolling" it ahead of the equipment.

THE RESULT

Irrigation rehabilitation is a time sensitive process for the districts and Layfield Plastics was able to meet all material and delivery requirements. With the ability to fabricate, test, package and ship up to 17,000 m2 of RPE 15 per day, EID's rehabilitation project was completed on schedule. The use of the RPE 15 panels has proven to be the most cost effective route for EID in the past and they are determined to continue with this type of rehabilitation. This technique of lining the canals with RPE 15 has controlled the problem of seepage for Eastern Irrigating District and established the surrounding farmland to begin the long term process of reclamation.

Acknowledgements:

Erwin Braun, R.E.T. Eastern Irrigation District

Harry Schoorlemmer, P. Eng. Eastern Irrigation District

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