Rock Netting

The Installation of Rock Netting

Following the cleaning of the Green Moor Quarry RIGS, all parties directly involved in this project agreed that the decision by the geotechnical engineer to apply netting to the rock face was good practical common sense. Although the conditions for the planning permission, to conserve the RIGS, were strictly adhered to by the developer, the elimination of potential Health and Safety hazards for the occupants of Stoneway Manor was considered to take priority.

The Green Moor Quarry RIGS after cleaning

After the scaffold had been dismantled and all remaining loose material had been cleared from the base of the quarry face, I arrived very early on site to take photographs of the cleaned RIGS - before the rock netting contractor commenced work.

Tools and materials

Even though I wasn’t paid for my time to take these photographs, having been involved with the Green Moor Quarry RIGS for 17 years, I was not only curious to see how this was done, but I also thought that it was needed to record this project - from start to finish.

In the time that I was on site, I was only able to observe the setting out of safety lines and drilling and netting to the south of the RIGS - the highest point on the old quarry face - but it was enough time to provide me with a good introduction to the techniques, tools and materials used to undertake this kind of work.

Looking at the materials – the 80 mm mesh rock netting, rock anchors, cables, various other nuts, bolts and epoxy resin – these were obviously of a grade that would be used when unstable geological conditions make the rock susceptible to rock falls and landslides. 

A detail of rock netting

As a geologist, with an appreciation of structural defects and structural loading - acquired in the general construction and specialist restoration industries - I agreed with the opinion of the engineer.

Conservation projects like this are so rare but, ideally, I think that there might be scope for the rock netting industry to make a “conservation grade” product which allows unwanted plants to be removed and still let the geological features be seen.

Drilling

That said, with Stoneway Manor essentially complete, except to finish off some road surfaces and gardens, I was very impressed by the way that the whole team worked in the tight spaces available; I was also quite surprised by the speed at which they progressed.

I particularly liked the way that the mobile access platform was used. The operative skillfully manoeuvred it into position - lifting rock netting to the top of the rock face, where other site operatives had previously sunk ribbed stainless steel dowels, ready to receive it.

When the cleaning work took place at the beginning of May, after a long winter that had extended well into April, the vegetation along the quarry face had largely died back but, 6 weeks later, the root balls that had been left on the RIGS had sprouted into plants. To the south of the RIGS, where vegetation covering the overburden had been left untouched, large ferns formed obstacles to the placing of the rock netting and, in some instances, they were very difficult to remove.

The removal of a deeply entrenched fern

A Site Report

Ironstone nodules found at the Green Moor Quarry RIGS

A condition for the planning permission to develop Stoneway Manor was that, once the Green Moor Quarry RIGS  was cleaned, a report on the geology would be made, along with a systematic photographic record of the exposed rock face.

In the old quarries that I had seen around Green Moor, there were exposures of relatively thick beds of sandstone, once capable of supplying very large,slabs of paving stone and I envisaged that the cleaned quarry face would reveal rocks with the same character.

Once the cleaning started, it was soon obvious that the Greenmoor Rock here was full of oblique fractures and sections of rock parallel to the quarry face, 50 – 300 mm wide, were becoming detached. 

Most of the rock that had fallen down from the quarry face was, at best, only suitable for dry stone walling, except for one large piece of rock, which one of the general workers on site moved by himself from the quarry face to the back of a small van – to everyone's amusement.

A section through the Greenmoor Rock

After dead vegetation from the top lift had been removed, only the uppermost section of relatively solid rock, with flaggy beds, could be seen, with the overlying material being made up of soil, waste rock and man made items.

Here, natural weathering processes have opened up beds, joints and fractures in the rock and these have been deeply penetrated and further disrupted by the roots of brambles, ferns and other invasive species.

Lower down, the weathered sandstone passes into more massive blocks and, except for fine partings of silt that are weathered to yellow clay, with occasional ferruginous beds and associated nodules, most of the exposed rock has a similar character. 

Details from the cleaned Green Moor Quarry RIGS

The spur of rock, the most prominent feature of the RIGS was found to be unstable and large blocks of stone were taken down during cleaning work. At the back of the spur, a wide joint has been infilled with loose breccia and finer material - also weathered to clay.

Especially where soil and weathered rock had accumulated on ledges, the rock was extremely loose and very easily disturbed when cleaned with a trowel, and it was usually pervaded by fine rootlets.

Although every attempt was made to leave the quarry face in a safe condition, the oblique fractures, which are an inherent feature of the Greenmoor Rock here, constitute a structural weakness and a decision was made by the geotechnical engineer to apply rock netting to the entire length of the old Green Moor Quarry face. 

The Green Moor Quarry RIGS - after cleaning

On the Scaffold

Scaffolding to the Green Moor Quarry RIGS

Six months after I had submitted my report, and the Scheme of Work to clean the quarry face was agreed with the local authority, I was again commissioned to supervise the work on site, to write a report and to take a good set of photographs that would record the condition of the RIGS (Regionally Important Geological Site).

Clearing vegetation from the top lift

With the greatest part of the old Rock Inn site prepared for the installation of the infrastructure and services, construction work to the estate started immediately and was largely completed, when I returned to site a year later.

At a pre-contract meeting attended by the developer, specialist contractors and the geotechnical engineer, I outlined the purpose and scope of the work and recommended the techniques to be used to clean and conserve the RIGS.

By now, most of the old waste rock and soil had been removed from beneath the old quarry face and, once the full height and lateral extent of the RIGS had been exposed, the problem of providing a safe means of access to undertake the programme of work had to be resolved.

Although access to the rock face using a mobile platform was considered, it was rejected on Health & Safety grounds and a decision was made to carry out the cleaning work from a fixed scaffold.

Having extensive experience of working on scaffolds, in the specialist building cleaning and restoration industry, I had always thought that this was going to be the best option for providing good access, although perhaps the most expensive one.

With trees and saplings to be removed, where they interfered with the geological features, and the root systems of brambles and ferns penetrated deeply into fissures within the rock, a good solid working platform was needed.

The developer and the site clearance contractor didn't have any previous experience of undertaking a project like this but, when I climbed up to the top lift of the scaffold to start work on the first morning of a planned 3 day programme of work, it seemed like second nature to me.

Views along the scaffold

Working from the top down, a team of 3 - supervised by myself - set to work using a wide variety of hand tools that are familiar to a gardener: a spade, shovel, trowel, croppers, shears, secateurs and a wood saw, amongst others. All of the soil and loose or unstable rock was carefully removed from the quarry face and ledges were formed, to arrest the fall of small blocks of stone, which might later be washed down by rainfall.

A root ball of bracken

The uppermost section of the quarry face, comprising weathered rock and soil, was not disturbed and only the dead vegetation was removed, with the plants cut back down to the ground. However, at lower levels, it soon became evident that the removal of deeply penetrating plants from  the beds and joints would cause substantial damage to the very friable rock face and - where these were encountered - they were left growing in situ. The use of biocide to treat residual roots was not considered to be appropriate, for good environmental reasons.

The top and 2nd lift after cleaning

 

The 1st lift after cleaning

Finally, all ledges and faces were cleaned down with soft brooms and hand brushes, ready for an inspection by the Geotechnical Engineer...