SHOTCRETE DESIGN FOR IRRIGATION CANAL LINING

Transcription

1 SHOTCRETE DESIGN FOR IRRIGATION CANAL LINING Syed Hasan 1, Gunvant Vaghela 2, James Yip 3, Ben Chung 4 ABSTRACT: The major artery of Lake Wyangan irrigation system is the 16km long Lake View Branch Canal (LVBC) which was installed mainly in The 75mm thick reinforced concrete lining is currently in a poor condition with significant water loss through seepage and needs replacement as part of Lake Wyangan Modernisation project. The main factors to be considered in the selection of a lining type include durability, seepage loss, construction cost and time, and maintenance cost. For irrigation canals, the use of concrete (cast in place or precast) with or without reinforcement is the most common practice of canal lining. A shotcrete lining (unreinforced or fibre reinforced) is uncommon for irrigation canals and there has been limited (if any) documentation of its recent usage in Australia. A trial comprising approximately 27m long prototype (with fibre reinforced and unreinforced) shotcrete lining of different joint configurations constructed on ground having similar properties to the LVBC was undertaken. On the basis of the trial result together with cost and performance, unreinforced shotcrete lining with transverse and longitudinal control joints was selected as the preferred canal lining option for un-obstructed flow of water without excessive seepage. This paper provides details of the LVBC renewal, including a literature review of irrigation canal lining, the development of design and the validation process. KEYWORDS: Shotcrete, Prototype model, Fibre-reinforced, Unreinforced. 1 INTRODUCTION Lake Wyangan Modernisation project (a key component of Murrumbidgee Irrigation s Modernisation Program) involves the replacement of the ageing concrete lined irrigation channel system with a modern and efficient water delivery system to service around 200 land holdings in Lake Wyangan catchment. The Australian Government is providing funding of $50 million for this project as part of the Private Irrigation Infrastructure Operators Program in NSW. An Alliance, MIA Renewal Alliance, comprising Murrumbidgee Irrigation Limited, GHD, UGL Infrastructure and John Holland Group was set up to plan, design and construct the modernisation project. The 16km long Lake View Branch Canal (LVBC) was built mainly in The existing 75mm thick reinforced concrete canal [1] is at the end of its design life and is being refurbished as follows: Canal bank lining is being demolished and replaced with 85mm thick unreinforced shotcrete lining. Majority of the canal is being widened to increase its capacity. Canal bed lining will be retained where lining condition is good, otherwise the bed will be upgraded by overlaying 75mm thick unreinforced concrete. 2 CURRENT PRACTICES OF LINING CONSTRUCTION A limited literature review of canal linings has indicated the following: Most concrete linings installed in older channels in the United States were reinforced. Recently, to reduce construction cost, reinforcement has been omitted wherever possible. (Stevenson, 1999) has reported that it is generally better to use unreinforced concrete [2]. Reinforcement cannot be justified, except under unusual conditions such as high back pressure, high flow velocities and where movement of the subgrade is a possibility. Unreinforced concrete linings are more susceptible to damage by 1 Syed Hasan, GHD Pty Ltd, Sydney. syed.hasan@ghd.com 2 Gunvant Vaghela, GHD Pty Ltd, Sydney. gunvant.veghela@ghd.com 3 James Yip, GHD Pty Ltd, Sydney. james.yip@ghd.com 4 Ben Chung, GHD Pty Ltd, Sydney. ben.chung@ghd.com

2 hydrostatic pressure and subgrade movement, however this is more than offset by the difference in cost when compared to a reinforced lining. Unreinforced concrete fractures more readily than reinforced concrete, thus relieving the pressure and reducing the area of damage. Steel reinforcement, controls crack widths however it complicates construction and will lead to deterioration associated with corrosion. A recent option for reinforcement in concrete involves the addition of fibre reinforcement, synthetic or steel fibres (approximately 20-40mm long) [2]. A successful concrete lined channel must be durable and remain substantially watertight to give many years of low-maintenance service life. The success of a concrete liner is highly dependent on its installation and materials used. [2]. Concrete, cast in place or pre-cast, with or without reinforcement is a common type of lining for irrigation canals. The disadvantages of this type of lining are its relative high cost, the long period of time required for its installation and its lack of capability to adjust itself to differential settlement of the underlying soil [3]. A nominal weldmesh reinforcing layer in canal lining has proved to be of assistance in keeping maintenance and repair costs to a minimum by distributing stresses caused by temperature effects and other minor operational loading. For this reason it is usual practice in the Department of Water Affairs, South Africa [4] to place this reinforcement. However, there are certain cases where an unreinforced concrete lining is both preferable and acceptable. Construction joints, placed transversely across the canal at about 3m intervals, are convenient for hand-lining in which alternate slabs are cast sequentially thereby limiting tensile stress development. Corps practice [5], prior to the 1960 s, was to utilise concrete pavement with expansion and contraction joints for paved trapezoidal channels. The experience with these channels shows that substantial joint maintenance is required. When concrete paving is used for trapezoidal channels in soils, it should be continuously reinforced concrete pavement (CRCP) [5] [It must be noted that these channels are designed for vehicular traffic, hence the use of the term pavement ]. Unreinforced shotcrete is considered suitable for applications where there are no applied loads, or when, tension is not applied to the shotcrete material [6]. Channels, reservoirs and spillways can be constructed by excavating the required shape and shotcreting directly freeform onto the exposed rock or earth. Shotcrete has the ability to be placed, compacted and finished (possibly in one pass) in cases requiring high access, freeform or very thick linings. Some examples include the Olympic Whitewater Stadium Channel in Sydney and Shannon Creek Dam Spillway in Grafton NSW [6]. Unreinforced shotcrete canal lining was used in North Unit Canal, USA with excellent condition after 10 years of service [7]. 3 DESIGN OF CANAL LINING 3.1 DESIGN CRITERIA The criteria used in the design of canal lining are as follows: Convey the required flow (as defined by the hydraulic analysis, this requires an increase in the available waterway area); Be constructible and suitable for construction within the window available at the winter shutdown; Substantially water tight but not required to be a water retaining structure; Prevent erosion and dispersion of the soil behind the canal lining; Retain the existing canal bed where the condition of the existing bed allows this; Level of durability sufficient to achieve design life of 80 years (with maintenance as required); and Not to be designed for equipment or traffic loading 3.2 DESIGN DEVELOPMENT OF CANAL LINING MATERIAL The main factors considered in the selection of canal lining material include; durability for 80 years design life, seepage loss, maintenance cost, construction cost

3 and time (for construction during winter shutdown periods). The lining materials considered during detailed design stage are covered below Reinforced concrete lining cast-in-situ or precast The existing lining design of approximately 75mm thick reinforced concrete was reviewed and discounted on the basis of inadequate cover to the reinforcement to comply with current design standards. A conventionally reinforced concrete lining would need to be in the order of 140mm thick to provide longevity and robustness. However, it will result in a substantially higher cost than unreinforced lining. For this reason reinforced concrete cast-in-situ or precast lining was not considered to offer value for money Fibre-reinforced shotcrete Fibre reinforced shotcrete has higher capital cost and greater difficulty in construction in comparison with unreinforced shotcrete. Two types of fibre reinforcement were considered in this project: steel and synthetic (polypropylene) fibres. Steel fibres can assist with the long term control of drying shrinkage cracking by providing resistance to widening of cracks. To achieve this, sufficiently high dosage of steel fibres will be required, with the following implications: Cons: significant capital cost, and difficulty in achieving homogeneity and finishing of the shotcrete; and Pros: provide an added degree of tensile strength and larger contraction joint centres could be achieved. Synthetic fibres provide no material benefit for long term drying shrinkage, but can assist in the reduction of plastic cracking (cracking while the shotcrete is in a fresh state) and slumping during construction Unreinforced shocrete/concrete lining Unreinforced shotcrete/concrete lining requires contraction joints at close intervals to reduce the risk of crack development from drying shrinkage, temperature and moisture variation. A panel length of 3m was selected to match the typical existing panel joints of 6m spacing Selection of canal lining material On the basis of cost, durability and performance of prototype trial test panel s unreinforced shotcrete/concrete lining with transverse control joints at 3m spacing and expansion joints at 30m spacing was selected. 3.3 DETAILED DESIGN OF UNREINFORCED CANAL LINING During the detailed design of the unreinforced canal lining, comprising a shotcrete bank and a concrete bed, the following design considerations were taken into account : Hydrostatic uplift pressures The results of the geotechnical investigation have indicated that the canal is predominantly underlain by low permeability clay material. On this basis, groundwater inflow beneath the canal lining is unlikely to develop damaging uplift pressure when the canal is empty, and as such, no treatment has been specified. At localised areas of the canal where high ground water and sandy soil are encountered, weep holes and flap valves are proposed to allow drainage of the subgrade to minimise the effect of hydrostatic uplift Shrinkage and temperature effect To prevent cracking of unreinforced lining due to shrinkage and temperature effects, transverse contraction joints are designed at 3m spacing. The short panel length will minimise any cracking and enable easy replacement of panel in the event of further cracking or breaking. Transverse expansion joints at 30m spacing are also provided. Rearguard waterstops under contraction and expansion joints are required to mitigate seepage losses Reactive soil movement Due to the presence of medium to high plasticity clay along the canal alignment, an average 50mm of foundation movement is expected normal to the canal lining [8]. Such movement is expected to occur predominantly in the upper soil strata. A finite element model using STRAND-7 software was used to analyse the effect of soil heave on unreinforced panels. Heave effects in the transverse direction is expected to occur at the top of the bank with the potential to lead to the canal lining panels being unsupported. The structural analysis carried out has indicated that the resulting lifting is likely to lead to longitudinal stresses and cracking in the unreinforced lining. Therefore a longitudinal control joint at the mid height of the panel was introduced to allow the panel to articulate with the movement in the upper soil profile. A longitudinal joint at the junction of the canal base and bank was also provided to allow further articulation in the event of soil movement.

4 Furthermore, to limit direct infiltration and evaporation behind the lining, a bitumen coated geofabric is incorporated behind the crest of the bank extending 500mm back from the shotcrete lining Durability According to AS3735 the exposure classification of canal water is B2. The 80 year design life calls for a 85mm thick unreinforced S40 shotcrete lining, which includes15mm sacrificial thickness as an allowance for surface erosion Joint detailing Joint details were developed in consultation with MIA Renewal Alliance taking into consideration the design requirements. The canal bank subgrade would be covered with geofabric (Bidim A29) before placing shotcrete in order to reduce the risk of fine material loss through the lining cracks. The types of joints used in the canal lining are covered below: Transverse contraction joints Transverse joints are provided in the shotcrete lining for shrinkage and temperature movement in the longitudinal direction. Bending due to heave effect along the canal is negligible for joint spacing of 3.0m. Cold jointed 3m wide shotcrete panels are constructed with a hit and miss fashion and application of bond breaker at the cold joints thereby removing the need for a saw cut or crack initiator. A rear-guard waterstop is provided to mitigate against seepage loss. Sika the supplier of the nominated waterstop confirmed that the waterstop will accommodate the calculated joint movement. Joint sealant was not selected due to its lower design life, requiring maintenance and periodic replacement. Transverse expansion joints With the provision of expansion joints at 30m spacing thermal expansion is estimated as 6mm and short term (7 days after curing) shrinkage movement as 2mm. Therefore net movement of expansion joint is estimated to be approximately 4mm. The proposed 10mm wide expansion joint is capable of accommodating this movement. Similar to the transverse contraction joints, a rear-guard waterstop is provided without any joint sealant. Longitudinal joints Longitudinal joints are required to cater for bending of the lining resulting from reactive soil movement, covered in Section A longitudinal control joint is provided at the mid-level of the bank shotcrete lining to allow articulation of the panels in the event of foundation movement. Rear-guard waterstop is provided under longitudinal control joints. A 3mm wide wet formed tooled joint (D/3 depth) is provided to initiate cracking at the joint, where D is the thickness of shotcrete lining. Longitudinal joints in base slab The effect of foundation movement can extend to the junction between the base and bank of the canal causing rotation of the lining. A longitudinal joint is therefore provided at the base of the canal, 200mm away from the junction to mitigate the risk of an uncontrolled crack. A 3mm wide wet formed tooled joint and a rear-guard waterstop are provided. Details at top of bank The shotcrete at the top of the bank is provided with a 100mm return to avoid hanging up the shotcrete during placement. This will mitigate the formation of horizontal cracks at the top of bank as the fresh shotcrete naturally seeks to move down-slope. 4 TRIAL AND POST DESIGN VALIDATION In order to validate the performance of the lining design, a trial was carried out in April Approximately 27m long prototype canal, comprising nine panels was constructed in ground having similar properties to the LVBC. The panels were either fibre reinforced or unreinforced shotcrete. The performance of the lining was reviewed and the following observations were made: Addition of fibre in the shotcrete did not result in an improved performance when compared with the unreinforced panels. More time was required to clean the pump and hoses for fibre reinforced shotcrete. Shotcreting alternative panels controls thickness as edge forms act as thickness guides. Between the shotcrete panels, crack widths up to 1.5mm were observed. The cracks did not follow the cold joint due to bonding between adjacent panels as no bond breaker was applied during trail test. 5 CONCLUSION The trial test result validated the canal lining design and assisted in fine tuning the design to achieve better service performance. A 85mm thick unreinforced shotcrete lining with transverse contraction joints at 3m c/c and expansion

5 joints at 30m c/c was selected on the basis of cost, trial test and performance. The construction of the canal lining commenced in May 2013 with all the works scheduled to be completed by August Two winter shutdown periods will be required to complete the upgrading work. Photograph 1 to 4 shows the construction work of canal lining. Photograph 4: Concrete pouring on canal bed Photograph 1: Preparation for shotcrete application Photograph 5: Completed lining showing joints ACKNOWLEDGEMENT Photograph 2: Application of shotcrete to canal bank The author would like to acknowledge the contributions made by the entire Alliance Team. The innovations that are outlined in this paper are a direct result of successful Alliancing relationships. The author would also like to acknowledge Murrumbidgee Irrigation Limited, and the Department of Environment, Commonwealth of Australia for their approval to present this paper. REFERENCES Photograph 3: Roller finish to bank shotcrete panel [1] Canal Condition Assessment Lake View Branch; Report by GHD, 20 July, [2] e.html. [3] andardsandguidelines/canal%20lining.htm. [4] Guidelines for the design of canals and related structures; Department of Water Affairs, Forestry and Environmental Conservation, Pretoria, Republic of South Africa.

6 [5] Structural Design of Concrete Lined Flood Control Channels; US Army Corps of Engineers Department of the Army, EM , 30 April, [6] Recommended Practice Shotcreting in Australia; Prepared by Australian Shotcrete Society for the Concrete Institute of Australia, 2 nd Edition, September, [7] Canal lining demonstration project year 10 durability report; US Department of the Interior, Bureau of Reclamation, November, [8] LVBC Lining Uplift and Reactive Ground Movement; GHD Geotechnical Memorandum, 21/20216/187610, 05 February, 2013.