Upgrading a retaining wall design actually ment cost savings at a Scottish building site. Damon Schunmann went to see how more meant less.
A retaining wall bordering a housing development in east Scotland has been designed, redesigned, and redesigned again to provide the best long-term solution for an embankment supporting a main road.
But rather than escalate costs, a more durable solution actually saved money.
Fife's new Collinswell Park housing estate is in the village of Burntisland on the north side of the Firth of Forth. The site backs on to an embankment that rises up to carry the A921 Aberdour Road and developer Stewart Milne Holmes (SMH) wanted to cut into it to maximise development space.
SMH found sandstone up to 4m high at the embanked end of the site and thought it could get away with cutting the rock vertically and regrading the slope. But this was found to be unworkable after consultant Mason and Evans went in to identify the rock head prole and found it had an inconsistent level that disappeared in places. It was also not as competent as it would have liked and being moderately weak sandstone, would have been susceptible to weathering if left as an exposed face.
Overlying the sandstone is a shaley weathered mudstone, which contains areas of additional sandstone, that was also deemed not competent for vertical cutting.
'To minimise the excavations SMH also wanted split level houses with back gardens 2m higher than the front gardens to regulate the slope and to take 2m off the retaining wall, ' explains Maccaferri area engineer, Allan Rice. But as the vertical unsupported rock cut was not possible it came back to Maccaferri for a design and construct package price on a timber crib wall up to 4m high and 150m to 200m long.
Given the proximity of the A921 and the requirement to provide an approval in principle (AIP) for Fife Council, which owns and maintains it, Maccaferri advised that this solution might not get through on durability grounds.
'Sure enough it was rejected as it only had a design life of about 50 years, ' says Rice. Maccaferri then recommended its Anchor Landmark system, which has a Roads and Bridges BBA (British Board of Agrément) certi ate for use on or next to highways. 'We took onboard Fife Council's comments from the timber crib submission and resubmitted with the Landmark system which has a 120-year design life and this went through with no problems, ' adds Rice.
RJT Excavations won the contract for the bulk excavations with Mason and Evans responsible for supervising the bulk of this. Harte Construction was the groundworks contractor for setting out wall positions, levels and line.
Work started in July but was slightly held up when workers on the bulk excavation exposed a concrete tank, resulting in a five-day delay while it was decided that it would not be a major problem.
The Landmark wall is 168m long and covers 712m 2 with a maximum 6m height near the centre.
The site also features three Anchor Vertica walls that rise up to 3.6m.
Although Maccaferri designed and built the Vertica walls, it was less involved because these did not need an AIP as they were not bordering local authority owned property or infrastructure. The Vertica walls total 292m in length and cover an area of about 950m 2.Rice explains how Landmark has worked out cheaper for the housebuilder. 'The Landmark wall set back at a 3.8º batter would have a maximum 6m height; but a timber crib wall with a 14º batter would have been 8m high and would have been chasing the one in two slope behind. So construction costs for the 2m height difference meant Landmark offered a saving.
'The st difference you notice between the two wall systems is Landmark is a portrait block and Vertica is a conventional landscape block. We use Paragrid horizontal polymer reinforcement for a vertical wall [for the primaries that tie into the wall] and Colbond Enkagrid Pro 60 or Pro 90 for a Landmark wall for the horizontal reinforcement.'
A key distinction is the reinforcement connection with the wall. For the Vertica wall the grids are just sandwiched between the blocks and use friction to connect the grid to the block. Landmark, on the other hand, has a locking bar that grips the grid into the block.
'This is the significant difference between them that has led to approval in the UK where a retained wall is next to main roads, ' says Rice. 'You need a positive connection to satisfy the design guide for reinforced soil structures.'
CMS Construction Management Scotland was the scheme's consultant Maccaferri designed the final retaining walls.
Anchor wall systems are supplied, marketed and installed in the UK via a three-way partnership agreement. This is between American licensor Anchor Wall Systems of Minneapolis, Minnesota, manufacturer Acheson and Glover Group based in Dungannon County Tyrone, Northern Ireland and engineering support and sole supplier, Maccaferri.
The £366,000 contract for Maccaferri Construction to design, supply and build the four walls was due to nish at the end of last month. But as GE went to press there had been a hold up due to an embankment slip.
'There was an area at the Landmark wall where water liquefied the mudstone causing some [overlying] sandstone to slip. Afterwards we put up rockfall netting but the job was delayed by about four weeks so it should finish in November now, ' says Rice.
Landmark wall system
Although not yet widely used in the UK, Landmark walls are easy to build.
'I like to describe it as big Lego; there is no concrete foundation, no mortar between the blocks, it's a dry wall system, ' says Maccaferri area engineer Allan Rice. 'You bulk excavate to take out material for getting the [reinforcement] tails in if required. Next you place Type 1 foundation and compact it and lay at bottomed foundation blocks, which are half blocks with a groove in the top.
You next install a drain behind the block then place and compact backfill - here it's class 6I 6J - to the top of the foundation block level. If grid is required it's fixed into place using the locking bar. As each course of block is introduced, the drainage column and compacted backll are brought up together. Then rinse and repeat.