Gmac Utilities Ltd overcame difficult ground conditions to install 520m of 500mm rising main to a depth of 48m whilst drilling under a SSSI wetland and river replacing the existing infrastructure, as part of EA scheme to return 68 hectares back to wetland. The project involved pipe ramming 60m of 1000mm steel casing through dense river terrace gravels to depth of 18m to the sandstone bed rock, before undertaking the horizontal directional drilling. Not only was the pipe ramming and hdd completed on time and within budget with no environmental impact to surrounding area, the hdd crew performance, site set up and work ethic received praise from some very knowledgeable local residents.
The picturesque seaside town of Budleigh Salterton in Devon was the location for the Lower Otter project, which involved reclaiming past intertidal marshland area, replacement of storm water outfall and a new pedestrian foot bridge. The Lower Otter SSSI area designation was made for both ecological and environmental reasons. The marshland designated area is important for water birds, which rely on the collective ecosystems of plants and invertebrates within that habitat. The geological designated area is the cliffs of Otterton Point ( hdd exit site) which are of palaeontological importance. One significant issue however was that overtime, the top of the existing storm water outfall had become exposed in the bottom of the river estuary, so relining was not an option. Discussion with Gmac Utilities Ltd, water authority, consultants and the Tier 1 construction company led to the need to replace the existing rising main using HDD. Whilst the exit side ground conditions were hard sandstone, from entry side to a depth of 18m were dense river terrace gravels. Gmac utilised the Goliath Grundoram from Tracto UK Ltd and in a 2-week programme before Xmas 2022, installed 60m of 1000mm diameter ( 12mm w/t) in 12m sections ( one of the largest pipe rams to date in UK) The lead face of the front casing was prepared with a strengthening band a cutting teeth. The rammer was connected to 2 no 850 cfm compressors and installing time per 10m took between 1 and 2 hours. The pup section with rammer attached was gas cut from the casing and attached to a new 12m length. Then lifted into position with 2 no 21 tonne excavators and welded to the length already in the ground. Welding time per joint was approx. 6 hours. In early Jan 2023, Gmac utilised their American Auger 440 ( 220t ) hdd rig and along with supporting equipment established the works compound.
The initial 1000m casing was cleaned of all gravels using a 700mm hole opener pushed down the pipe and flushed using a high viscosity drilling slurry that carried the pebbles and cobbles out of the casing. After flushing out the 1000mm casing, a 500mm casing with central stabilising wings was installed down to the rock head through the 1000mm casing. The 500mm casing was delivered in 14m lengths and a PN16 ring welded to each end so casings could be bolted together. The pilot bore was then undertaken using an 8 1/2″ mud motor with 12 1/2″ tricone bit and the highly accurate gyro steering system. Whilst the deepest point of the drill was 48m below ground level, the critical point was on the up hill climb directly below the base of the cliff exit side approx. 480m out from HDD rig, where ground cover to the hdd bore was less than 10m. a further 50m drilling took the drill head to the exit pit. Throughout the entire length of the drill, slurry and cuttings flowed back to the rig side for recycling treatment, right up to the point of break out at the exit side. Rates of penetration on the pilot drill varied from 3 metres in the hardest deepest section to 10 metres an hour, with the pilot bore completed in 7 days. To eliminate the need for tractors and bowsers transporting slurry from exit side to entry ( a 5km drive along narrow country lanes and town centre), Gmac proposed forward reaming the bore, using a 660mm hole opener to enlarge the bore for the 500mm pipe. Again, during the forward reaming, drilling slurry returns flowed back to rig side and were recycled. All the solid cuttings that came off the recycling system were recycled by a local company as sand. The 540m pipe string was welded in one piece across adjacent fields and pressure tested above ground. Following completion of pre-ream, the hole opener was turned around and pipe attached behind it and then installed over one shift.
Due to the pipe buoyancy over the length, Gmac ballasted the 500mm pipe by pumping water down from tractor and bowser as it was installed , thereby greatly reducing the overall pulling forces. The pie was then connected to a new shaft that was sunk by another contractor on to the existing sea outfall. Benefits to the local community included removing the need for a massive open cut along the beach and river, hdd provided a fast solution to the problem and varying hdd technique to forward reaming removed need for transportation of slurry so no 30t tractors and bowsers along country lanes and town centre. Gmac site staff took time to talk to local residents as the Budleigh Salterton car park was a hive for walkers and bird spotters going around the tidal flats, with consideration given to residents during the noisy pipe ramming phase. Further uses of Mudd dry agent to hydrate the drilling slurry eliminated the need for waste tankers, the drying agent creating an inert material. The pipe ramming phases took 3 week from mobilisation to removal of all plant from site before Xmas 2022, whilst the hdd works took 9 days to pre-ream and 1 day to install pipe with maximum pull force of 24t during installation.