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A rock and a fast place

Scotland's A77 is getting an upgrade that includes a soil nailing solution next to a busy road. Damon Schünmann reports.

As the A77 snakes its way along Scotland's west coast, it frequently dips and rises through areas with exotic names such as Hagstone (see Hagstone mythology), just north of Cairnryan and almost 100km south west of Glasgow.

Here, there is a hive of activity surrounding a project to re-profile an existing natural slope to allow the widening of the road from two lanes to three along a 2km section. This will include a new climbing lane for slower vehicles.

The excavated face is being stabilised by soil nailing with 1450 linear metres of slope being treated along a stretch, with heights varying up to 15m and slope angles up to 60˚.

But before work could begin the site needed the all-clear on any ordnance that may have been present. Main contractor RJ McLeod agent Andy Gillan explains why: "Finnarts Bay [near the site] was a staging post for Second World War convoys going over to the US so there was a lot of anti-aircraft bunkers near the site. We had to check them for ordnance and munitions, but fortunately we did not find anything."

Once the geotechnical work began in earnest it consisted primarily of the excavation and soil nailing going hand in hand. This sees RJ McLeod benching down to allow the stabilisation of the slope and access for plant and equipment. Subcontractor Ritchies is drilling and installing the soil nails as the re-profiling progresses.

Ground conditions consist of glacial till overlying greywacke (a variety of sandstone), which is both weathered and competent rock.

Prior to installing the working nails, Ritchies put in and stressed 10 test nails to confirm the design and installation process (see Test nails). By the time the scheme completes, site workers will have installed a total nail length of 20km.

Six machines are doing this work including a pair of Boart Deltabase DB120s as well as a 100, a Casagrande C6, an Altlas Copco A65, and an Ingersoll Rand Airtrack rig.

Drilling is achieved using down the hole (DTH) hammers and air flush. The nails are Dywidag GEWI Bars, double corrosion protected. Both the annulus between nail and corrugated sleeve, as well as that between sleeve and hole, are simultaneously grouted into a 125mm diameter hole. These holes vary in length between 2m and 16m. Grout is neat CEM 1 having a design 28-day strength of 40N/mm2. It has a water / cement ratio of 0.45. Selected nails are tested to 210kN by Ritchies engineers to ensure the specification is met.

Each nail is fitted with a 400mm by 400mm galvanised head plate that secures the surface facing of Maccaferri Macmat R, which acts as a passive retaining system. This is initially secured at the top when the first nails are installed and then unrolled as the work progresses. Gillan says: "The Macmat R rolls are pinned at the top by permanent 1m Duckbill anchors and we can unroll when [Ritchies senior geotechnical engineer] Stuart Jackman is working two rows of nails below us."

RJ McLeod business development manager Jamie Corser says the main problem is access on a site that is stuck between a rock and a fast place. "It's adjacent to a live carriageway," he explains," with Jackman adding: "It really is like a sliding puzzle when moving everything about."

The main contractor also had to bring in reduced tail swing excavators for the first rows of nails to prevent the tails hitting the slope's surface at the top where the benches were narrowest.

Although at the start of the job the ground was relatively free of water, rainfall prior to GE's visit made for more challenging conditions.
Jackman says: "At the south end is a gulley of glacial till and after a couple of days of rain it's unbelievable. The water gets between the glacial till and the rock-head about 7m to 8m in and when we drill into this zone the water runs out until we grout it." This has required additional temporary works in places of wooden panels to protect the face as it can become destabilised.

RJ McLeod is putting excavated material to good use on the site with intact rock acting as Type 1 carriageway sub-base. Weathered rock is getting reused as general fill Class 1 to build the road embankment on another part of the A77, a 1.4km long realignment section about 6.5km further to the north at Glennapp.

Gillan says the intention is to use all excavated material between the two sites. Corser adds: "It's economics that you don't want to be taking it to landfill, but the theory on any job is not to do this [take it to landfill] for the good of the planet."

Prior to the installation of the working nails, 10 test nails were installed in various positions throughout the site to approve the assembly and installation methods, and carry out proving tests to validate the design. The test nails were drilled, using a Deltabase 120 off the access road
in the slope above the production face. They were installed to the following depths:

TN01 and TN03 – 3m into glacial till, fully bonded
TN02 and TN04 – 2m into glacial till, fully bonded
TN05 – 5m total length, 1m bond length in non-intact rock
TN06 – 4m total length, 1m bond length in non-intact rock
TN07 – 5m total length, 1m bond length in non-intact rock
TN08 – 5m total length, 1m bond length in intact rock
TN09 – 10m total length, 1m bond length in intact rock
TN10 – 3m total length, 1m bond length in intact rock.

Once the grout cube results had returned with the 40N/mm2 strength achieved, Ritchies carried out proving tests. These were done using a 300kN hollow ram jack and hand pump. The reaction frame was made up of timber sleepers and a fabricated steel frame.

Elastic extension was measured using a dial gauge positioned on a tripod frame. A circular steel plate was attached to the end of the nail for the needle of the dial gauge to bear against.

A timber rail was erected in front of the reaction frame to monitor
any displacement of the head plate (see below).

Parties involved

Client: Transport Scotland

Engineer and client's representative: Atkins
Designer/consultant: HalcrowGroup
Design checking: Tony Gee and Partners
Main contractor: RJ McLeod
Geotechnical subcontractor: Ritchies
Ritchies agent: senior geotechnical engineer Stuart Jackman, whose experience of this type of work includes the soil nailed east catchments at Gibraltar where Ritchies installed over 8000 soil nails.

Contract, value and programme
Main contract value £18.7M
Soil nailing sub-contract £1.245M
Contract conditions are ICE (Institution of Civil Engineers) design and build
Programme duration is 18 months with soil nailing planned to take 19 weeks
Work began on site on 15 October last year with the soil nailing commencing on 7 January
Ritchies was due to finish by the end of May, depending on how the excavation progressed.

Hagstone mythology
A hagstone is a stone with a hole in it that is hung in stables and homes to keep away witches or hags at night. If hung on the bedpost, it supposedly protects the sleeper from having a hag ride one's chest and causing a nightmare. Hung in a stable, it prevents witches from riding horses all night to exhaustion.

Plant and equipment
Rigs used are two Deltabase 120s and a 100 with Eurodrill 5012 top hammers, a Casagrande C6, an Atlas Copco A65 and an Ingersoll
Rand ECM (extended crawler mounted) 350

Drilling is generally cross carriage

Both top hammer and down the hole hammers (DTH) are being used, depending upon ground conditions and the need for casing

The casing systems include the Boart Longyear 152.4mm diameter system with top hammer and Symmetrix NDT Set 120 using DTH hammers.

Working nails and surface facing
The subcontractor is installing soil nails at 1.5m centres (vertical and horizontal) into the new slope cut at 60˚ from the horizontal. Criterion for the length of the soil nails was as follows:

Soil nails, where the overburden and non-intact rock was less than 5m thick, are installed into a 2m intact rock socket. Bars are 25mm
in diameter

Soil nails, where the overburden and non-intact rock was more than 5m thick, are installed into a 3m intact rock socket. Bars are 28mm in diameter.

The first row of nails is positioned about 0.5m down from the top of the original slope. From there, a row of nails is being installed every 1.73m, slope distance, down until there are two rows of 2m long nails into intact rock. They are double corrosion protected bars. The nails are assembled onsite and grouted insitu.

The original specification was to have only the top 3m of the nail galvanised. This was thought to be difficult to manage on site and so the decision was made to purchase all bars with galvanising throughout.

The soil nail is assembled with wrapround spacers fixed to the bar 0.5m from the bottom end and then every 2m to the top. Site workers then place this bar into the ducting while adding lantern spacers on to the ducting, 0.5m from the bottom end and then every 2m to the top.

The nails are then inserted into the 125mm diameter borehole. Depending on the length of the nails, additional sections are added as it is installed. Ritchies then grouts inside and out.

A passive retaining system provides the surface covering with the head plate detail then assembled to complete the system.

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