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extending up to 150mm above the pipe could be utilised, the additional imported bedding material will either partially or wholly replace the selected excavated backfill material.
Where groundwater exists at a level above the interface, between the hardcore and the new stabilised trench bottom, a procedure detailed in Waterlogged ground (below) should also be applied. The geotextile should surround both the material in the base of the trench as well as the pipe bedding material.
Alternatively, a stable pipeline base can be constructed via a geogrid reinforced granular mattress below the bedding. The unique interlock mechanism between grid and aggregate creates a supporting mattress which provides an efficient load spread into the weaker ground, and also helps to control any longitudinal differential settlement which may take place.
Laying clay pipes in waterlogged ground
Moving groundwater at a level above the trench formation in fine grained soils can reduce the support that pipe beddings provide. Granular bedding material encourages water movement as it becomes similar to a French drain and this washes fines out of the surrounding ground, causing a loss of support to the bedding and pipeline. This can occur particularly in peat, silty ground, soft to very soft alluvial clays, running sand or artificially filled ground.
The traditional method of dealing with this problem was to include a proportion of coarse sand in the bedding material to fill the gaps which might otherwise take up the fine material from around the trench. This limits the movement of fines, but the bedding material requires much more compaction than if it were single sized or graded.
A more effective method is to wrap the whole of the bedding construction, including any additional compacted material in the trench bottom (as detailed above) in geotextile fabric. This will allow the movement of water through the bedding material, but prevent the movement of fine material, and keep it in the ground around the trench. In such conditions measures are also needed to prevent similar movement of fines under manholes. The geotextile construction should be continued around the outside of the manhole excavation and under any manhole bedding material. The specification for the geotextile, particularly the pore size, should be related to the nature of the fines in the ground.
It is essential to dewater the trench prior to commencing pipe laying. Any well point dewatering must also be filtered to prevent continuous removal of fine sand and silts. Sump pumping from the end of the trench isn’t recommended even when filtered, as instability of the formation can arise. ‘Drag box’ or similar trench support systems should be used carefully in waterlogged fine grained soils because the pipe and bedding are likely to disturb the trench wall when the support is removed. Steel or timber sheet trench support systems are recommended for their incremental approach and application. They should not be driven or extracted with vibratory hammers and backfilling should proceed progressively as the support system is removed.
It is important that due consideration is given to the selection of a geotextile if it is going to perform adequately over the design life of the pipeline. The performance of any geotextile filter depends on the introduction of a number of factors, including:
• The particle size, distribution, structure and chemistry of the base soil
• The geotextile pore size and permeability. If care is taken to match the correct geotextile to the soil conditions, after a period of instability due to the establishment of equilibrium conditions, no further in situ soil should be piped out of the filter system.
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Use of geotectile material around pipe bedding in waterlogged ground
Installation of geotextile
To ensure continuity, overlap adjacent layers. Side- by-side rolls should have a minimum overlap of 300mm and ends of rolls should overlap 600mm. Over soft or uneven soils these overlaps may be increased.
About Hepworth Clay
Hepworth Clay is the overarching brand of Hepworth SuperSleve, Hepworth HouseDrain and Hepworth Terracotta product ranges with manufacturing facilities at Hazlehead near Sheffield and Forest Works near Burton Upon Trent.
In the next issue, we will cover laying vitrified pipes through and under structures such as buildings.
| December 2016 | drain TRADER 13
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