It is vital when comparing the costs of each method to consider all the potential benefits and limitations. Among the many elements to consider, it is particularly important to plan to achieve the following:

• Minimise trench excavation volume;
• Maximise re-use of trench spoil for either trench backfill or third-party use;
• Minimise the quantity of excavated material going to waste;
• Reduce or eliminate truck haulage and handling involved in the export of waste and import of intimate backfill (bedding and padding);
• Minimise negative environmental effects of truck haulage and waste disposal; and
• Reduce or eliminate risks and costs arising from use of explosives.

Some important factors in the selection of an appropriate trench excavation method are given in Table 1, and a realistic estimate of trenching and backfilling costs can only be achieved when all of these have been taken into account.

Some methods are more appropriate in particular terrains and rock types, and the choice of a particular method can have a significant effect on overall project costs and environmental impact. For instance, blasting is inherently dangerous and the resulting trenches are generally too large, but it can be used in a wide range of rocks. Chain trenchers can produce neat, regular trenches and readily re-useable spoil but become uneconomic in very strong, abrasive rock.

Sweeney, Leng et al. (2005) compared progress rates of chain trenchers with various rock properties. It emerged that fracture spacing in the rock mass had only limited influence on productivity and that rock strength (measured as unconfined compressive strength) and rock hardness or abrasiveness (expressed in terms of Mohs’ scratch hardness) were the most important properties. Increasing machine power did not achieve a corresponding increase in advance rate (metres per day), although of course production rate (cubic metres per day) was greater for larger machines cutting wider, deeper trenches.

Mohs’ scratch hardness characterises the resistance of minerals through the ability of a harder material to scratch a softer material. The test has a relative scale of 1 (talc) to 10 (diamond), with calcite at 3 and quartz and hardened steel at 7. Application to rocks becomes difficult in coarser-grained varieties composed of several minerals. One technique is to count each mineral in a petrological thin section and then calculate an average Mohs’ value from the weighted percentages. This method also allows quartz content to be measured. It should be noted that there is some degree of correlation between Mohs’ scratch hardness and indentation hardness measured in the Vickers and Knoop tests.

Table 2 summarises trench excavation and backfilling methods in various rocks. It should be noted that when estimating costs of trench excavation in permafrost, similar criteria must be adopted as for strong, hard rock, and the behaviour of steel in very low temperatures also has to be taken into account.

Figures 1, 2 and 3 illustrate a large chain trencher, the Trencor 1860HD. This weighs 227 tonnes and uses an 895 kW engine for the digging chain, and a separate 224 kW engine to power the crawler tracks. Operational variables include machine forward rate and operator skill. Pockets welded onto the cutting chain hold 300–350 teeth arranged in a chevron pattern. Wider tooth spacings often give optimum overall performance, but closer spacings may be preferred in strong, hard rock.

In Figure 2 the cutting chain is obscured by rock dust and the maintenance sledge towed behind the trencher. The windrow on the left shows an abrupt change between finer and coarser spoil corresponding with the change from rubbly limestone to strong limestone visible on the trench wall. Individual trencher performance on this project ranged from 15 m per day in localised very strong calcareous sandstone, with 600 teeth replaced in a single day, to 1,500 m per day in mudstone, with only 15 teeth replaced in a day (Sweeney, Pettifer et al. 2005). Downtime for replacing teeth and pockets was significant in resistant rocks, and the tungsten tip of teeth was often worn away or plucked out. This emphasises the need to maintain stocks of teeth and other replacement parts on a project. The cutting chain is clearly seen in Figure 3.

Figure 4 illustrates a small chain trencher, the Tesmec TRS1085, which uses a 240 kW engine for all purposes. The trench is being cut at the side of a highway in andesite, tuff and agglomerate overlain by up to 0.2 m of road base. Because of space restrictions and contract specifications for road reinstatement the spoil is being discharged directly to a dump truck for removal from site.

An important advantage of chain trenchers is that they generate well graded spoil which can be processed for reuse as intimate backfill. The size and shape of spoil particles probably reflect both the nature of the rock material and the change in trencher tooth action (Verhoef 1997 and others) from ripping in highly fractured rocks to chipping and scraping in more massive rocks. Spoil produced from very strong, siliceous rocks is particularly likely to contain a high proportion of larger, sharper particles which may damage the pipeline coating. The cost of spoil processing must therefore be balanced against the cost of using thicker pipe coatings, for example.

Figure 5 shows a Laurini Somico self-propelled screener and padder placing padding. The spoil is from a trench excavated by backhoe in sand and requires minimal processing. Figure 6 shows a Trench-Tech crusher, screener and padder processing spoil from a pipeline trench excavated by a bucket wheel trencher. This machine and comparable units manufactured by Laurini use a hammer mill impactor to reduce particle size, while others use roll crushers. Recent developments in vertical shaft impact crushing technology specifically designed to improve the shape of aggregate particles may eventually be incorporated in self-propelled padding plant.

Selection of trench excavation method must therefore take into account a range of rock and machine properties. It is suggested that the advantages of using chain trenchers in suitable rock outweigh the limitations and may have cost benefits and fewer adverse environmental effects compared with alternative methods.

References

• Sweeney M., Leng S.A., Pettifer G.S. & Haustermans L. (2005). Performance of chain trenchers in rock: a database and a preliminary predictive model. In: Sweeney M. (ed.) Proc. Conf. on Terrain and Geohazard Challenges Facing Onshore Oil and Gas Pipelines, Thomas Telford, London.
• Sweeney M., Pettifer G.S., Shilston D.T., Bel-Ford P. & Stockbridge M. (2005). In Salah Gas Project, Algeria – Part 3: Prediction and performance of large chain trenchers on a desert pipeline project. In: Sweeney M. (ed.) Proc. Conf. on Terrain and Geohazard Challenges Facing Onshore Oil and Gas Pipelines, Thomas Telford, London.
• Verhoef P.N.W. (1997). Wear of Rock Cutting Tools: Implications for the Site Investigation of Rock Dredging Projects. Balkema, Rotterdam.