Water & Wastewater Treatment

28 | JULY 2014 | WWT | www.wwtonline.co.uk In the know Technically speaking: Thrust blocks F or many of us designing thrust blocks for the UK water indus- try, the first port of call is CIRIA R128, 'Guide to the Design of Thrust Blocks for Buried Pressure Pipelines', published in 1994. The guide deals with thrust restraint in cohesive un-drained soils, and granular drained soils. Though it has served well for the last 20 years, it presents a number of chal- lenges to the second comer looking to connect to existing networks. Thrust forces that were previously not present in straight pipe lengths make an unwelcome appearance when tees and valves are introduced to exist- ing pipelines. For instance, designers tasked with upgrading borehole disin- fection methods from chlorine contact tanks to UV plants are o‹en faced with the situation of needing both methods online during commissioning. The solution to thrust on the new pipework is straightforward; end restrained joints Dealing with thrust restraint in network extensions Thrust block design for additions to existing pipework requires careful consideration. We examine the problem and potential solutions. such as anchored gaskets, anchored couplings/flange adaptors, or welded joints. However, the existing pipework is unlikely to be end restrained and it is on these sections that the newly intro- duced thrust forces have the capability to dislocate joints. The solution then is usually to design a thrust block to resist the unrestrained portion of the thrust force acting on the existing pipeline. This requires the following parameters to be determined:- • the unrestrained thrust force (a function of the cross sectional area of the pipe and the inter- nal water pressure) • shear strength of the soil (for un-drained cohesive soils) • soil density and friction angle (for granular drained soils). The soil and the block provide a resistance, divided by a thrust reduc- tion factor, which needs to be greater than the thrust force. This is where the design o‹en runs into the first hurdle. The first hurdle CIRIA R128 is designed, justifiably, to limit displacement to very small values (when the ultimate resistance is neared, soil displacements can be very large resulting, essentially, in the block fail- ing to resist the thrust force). The thrust reduction factors recommended by CIRIA R128 are designed to limit these displacements, and are never less than 2, rising to 5 for what are considered to be poor soils. As these reduction factors essentially multiply the net thrust force, even relatively small forces can result in the need for large thrust blocks in poor ground. This problem is exacerbated at treat- ment works; sites are congested with numerous buried services jockeying for space. This, and the shallow pipeline cover depths o‹en encountered not only limits the available space for thrust blocks but also necessitates their founding in made ground or disturbed soils. More and more, designers find themselves in situations where assign- ing a shear strength or a friction angle to the soil is difficult to do with any degree of certainty. In such cases the passive soil resistance, mobilised by the vertical face of the thrust block and which usually comprises the bulk of the thrust resistance, has to be abandoned, with the entire thrust resistance burden being taken in shear on the block base (see figure 1). This has the net effect of vastly increasing the plan of the block, the precise opposite of what is required in a congested site environment. Introducing Eccentricity The reliability of soil strata and the depth at which they are founded intro- duces geotechnical structural design issues that are beyond the scope of CIRIA R128. For instance CIRIA's thrust guide assumes that the pipeline requir- ing restraint is centred in the middle of the block, or that the line of thrust passes through the centre of the block at whichever is the relevant dimension. In order to found blocks in soil strata that can be assigned reliable design parameters o‹en requires the height of the block beneath the pipeline to be significantly greater than the height of the block above it. This introduces an eccentricity between the block and the line of force, resulting in an overturning moment. Given the weight of concrete and the relationship between the block size and the thrust force, it is unusual for the restoring moment to be less than AdrIAn dAvIEs-JordAn WaTer and WasTeWaTer pipeLines engineer MWH gLobaL Thrust blocks transmit the thrust forces to the soil to. prevent movement of the pipe and sepa- ration of the joints www.wwtonline.co.uk | WWT | july 2014 | 29 What is thrust force and how it is exerted? Thrust forces are a result of static internal water pressure and occur at bends, tees, and valves. The forces have the effect of trying to straighten the pipeline such that unrestrained thrust forces have the capability of dislocating pipe joints. What is a thrust block and where and when is it used to mitigate the above? Thrust blocks are essentially lumps of concrete placed at the location of bends and tees which are designed to transmit the thrust forces to the soil behind and beneath the block. In this way they prevent movement of the pipe and separation of the joints. the overturning moment. The problem, however, is one of bearing pressure. The moment, though not sufficient to overturn the block, may be large enough to cause the vertical reaction at the base of the block to occur outside the middle third. The effect of this is to cause the shear resistance on the base to tend to zero at a distance less than the full length of the block (see figure 2). The block base dimensions must then be increased to cover the shortfall. Once again, our thrust block gets bigger. This particular issue is compounded when designing for the long term drained condition in cohesionless soils by the presence of groundwater. Where groundwater is known (or may be reasonably assumed) to exist at the founding level of the block, it is taken to extend to ground level. This requires the deduction of the water density from that of the soil covering the thrust block. Given an average soil density of approxi- mately 19.6 kN/m3 (BS EN 1295-1:1997 Structural Design of Buried Pipelines under Various Conditions of Loading) this roughly halves the value of the soil density, and as the density is a multiply- ing factor in the determination of the thrust resistance, this too is halved. So, where thrust restraint is required in cohesive soils, i.e. the soil is consid- ered un-drained, can we assume this problem does not occur? For temporary load conditions, such as net thrust forc- es due to line stopping or the intermit- tent closure of valves, it is reasonable to assume that a cohesive soil will remain un-drained. However, where the load is permanent, e.g. on bends and tees (bearing in mind that valve/tee arrange- ments which are designed to perma- nently divert flow from decommissioned process plant to newly installed plant effectively become 90 degree bends), it is arguable that over time the soil will, at least locally, achieve a drained condi- tion. At this point the un-drained shear strength may no longer be sufficient to provide adequate thrust restraint. In this instance it is only prudent to design for drained conditions, which introduces the constraints already outlined. Thrust force Shear resistance Competent soil Incompetent soil - no passive resistance Valve Pipeline Concrete thrust block Shear toe Flow direction Figure 1- Thrust block resisting thrust force as a result of valve operation. Only the base of the block is in contact with compe- tent soil, therefore only base shear resistance can be utilised to resist thrust forces. Figure 2- Thrust block resisting thrust force as a result of valve operation. Pipeline is not centred in the block as block depth has been extended to reach competent soil. As a result moment forces are introduced which determine whether the vertical pressure is located within the middle third of the block. How can we mitigate second comer thrust issues? There are alternative thrust restraint methodologies that can be used. US AWWA pipeline structural design standards allow for the dissipation of thrust forces through soil/pipe friction. The method determines the number of joints either side of an anchored bend that require anchoring to dissipate the thrust force. This is fine for new installations but would involve digging up existing pipelines to provide the necessary joint anchorage. Not only is this an added cost, but is unadvisable on cast iron pipework. This is because it's difficult to assess whether the an- chored joints are gripping on iron or on graphitised corrosion products. It is also possible to design piled thrust blocks to take advantage of su- perior soils at depth. However, this has cost implications and methodological constraints. Water utilities are also nerv- ous of using such methods on borehole sites as they can cause turbidity. Early consideration of thrust restraint is needed A better way to limit thrust block size is to consider the need for thrust restraint early on in the design. Thrust values are a function of the bend angle, so specify- ing bends ≤ 45° results in much smaller thrust forces. Proper design of ground investigation studies can ensure that potential sites for pipeline installation and/or connection are assessed on how far below ground level competent soils are located. Hydraulic assessment to accurately gauge the internal pressure of the pipeline at the thrust location, rather than conservative assumptions, can also significantly reduce thrust forces. MWH's pipelines team is currently working on a method of designing thrust blocks based on geotechnical first principles. Working from first principles allows the designer greater flexibility to model site conditions, and make use of any advantageous conditions. This methodology has been incorporated in a design tool that can determine the global stability of a thrust block, based on these principles. The tool outputs thrust block dimensions that account for overturning forces, slid- ing resistance, and allowable bearing pressures. Development of the method- ology is ongoing to show that Eurocode serviceability partial factors will limit block displacement in a less conserva- tive manner than CIRIA R128's thrust reduction factors. The result, hopefully, will be smaller thrust blocks. 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