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

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