Lyneham Banks Subsidence
On the 1st June 1981 a large slip occurred on the A420 Swindon to Chippenham Road at Lyneham Banks. This road was the main western route between the two towns before the M4 was completed and then subsequently renumbered to the B4069 after the prioritisation of traffic re-routed traffic along the A3102 Swindon to Calne. The slip caused the northern part of the road to drop by about 250mm over a length of approximately 25 metres.

Initially, using traffic signals, one lane of the A420 was kept open causing extensive delays to traffic. It was considered however that either a formation of a regressive slip scar or a vehicle dropping into the slipped area could endanger life and therefore it was unsafe to keep the road open to traffic.

On the 3rd June 1981 Wiltshire County Surveyor staff requested advise in dealing with this hazardous slip. Initial inspection and a number of proposals were discussed in order to allow the road to be re-opened as quickly as possible, at the same time ensuring that the solution was more that a short-term expedient.

The report produced discussed the relevant geology of the slip area, the past history of events, the site investigation, testing and analysis, and records the advice given. As a result of a close collaboration between the advisors and Wiltshire County Council surveyors the road was re-opened on the 17th July.

History of Instability
It is apparent that this portion of the B4069 has suffered sporadic slips over a long period. Although on sidelong ground, the south side of the road has little cut, but on the north side it is often 1 -2 metres above field level. Failure and subsidence occur generally on the low, northern side of the road - the side which the heavier vehicles travel bring countless thousand tonnes of out of area industrial waste to Compton Bassett landfill site.

Following a long history of small failures, in 1967 the County in conjunction with the Department of Transport discussed what action should be taken to ensure long term stability of the escarpment road. The following was discussed that a new, lower route avoiding the main unstable sidelong ground and a steeper route more directly down slope. To maintain acceptable grades this would necessitate a deep cutting in the hillside.

Installation of Major Damage
In order to assess the depth of the slips below a surface veneer, slip indicators were installed. The records of the County Surveyors staff show that at the position of the slip indicators the shear surface was shallow, generally 1-2metres below ground level. It was decided however not to undertake any major works as the M4 was soon to open and hence the traffic on this main road was likely to decrease.

A summary of memories of events to supplement the factual records showed that during the Second World War when Lyneham Airbase was developed, the use of heavy vehicles on this road led to increased failures and at least locally part of the down slope side on the highway was replaced by large concrete longitudinal slabs.

More recent times, as the airbase is being heavily utilised to convey enormous volumes of air freight to worldwide operations, this route is heavily used by military vehicles and indeed civilian contractors supplying essential supplies for the troops overseas. The increased use, following the opening of the M4 motorway and shortcut route from junction 17 via Sutton Benger to Lyneham the wear and tear of the road has increased, with the additional slip damage.

Due to an uneven and cracked road surface part of the road was excavated to 1.35 metres in the summer of 1974. This was probably near the concrete discussed above. The first 1.2 metres was backfilled with lean mix concrete of a 16 to 1 mix before being emulsioned and covered with 150mm of dense bitumen macadam.

Four years later during the summer of 1978, again owing to uneven and cracked surface, a part of the road across Lyneham Banks with approximately 50mm of macadam was replaced.

In March 1981, a large crack with a 50mm deep subsidence occurred. When excavated it was found that a solid block of concrete up to 1 metre thick occurred under the northern edge of the carriageway, probably placed during the Second World war. Below the concrete was a layer of reinforcing mesh overlying the sub-soil. Between the concrete underlying the northern carriageway and the southern side were several layers of concrete and bitumen macadam. This was removed and replaced with 400 --500 mm of clean stone, 400 mm of granular sub-base type 1 followed by 200 mm of dense macadam. In order to expedite drainage for this layer a section was cut through the concrete beam at the northern side. At the same time a ditch on the south side was deepened from 450 mm to 1200 mm and backfilled with 50 mm size clean stone.

On the 6th April 1981 a 9.6 metre long crack appeared at the eastern end of the Lyneham Banks slip area SU 001799. The road surface was removed and replaced to a depth of 100 mm. Less than a month later two cracks up to 9.6 metres appeared at the same location and another at the west end. The road surface at both positions were removed to 450 mm and replaced.

Geology of the Area
The slips occurred in the Oxford Clay of Upper Jurassic age. From the 1:50,000 geological map it is apparent that to the south, up slope of the Lyneham Banks there is a change in relief due to the incoming of the overlying Coral Rag and Osmington Oolite. In general the regional strata dip is very gentle being under 5 degrees eastwards. A full descriptive geological background of the Lyneham area can be found here

Lyneham is situated on a small plateau approximately 500 feet above sea level and to the south is the Marlborough Downs escarpment. The underlying water table is considerably close to the surface and the excess underground water was a major factor when the Air Ministry decided to acquire the Lyneham Court estate for future use of the Lyneham Airbase. There are main ground springs in the hillside and numerous mapped sites of water wells in the hamlet. The majority of the springs and water courseways converge and flow northwards to Lilybrook stream then topping up the Tockenham Reservoir. Up to the early 1900's this water supply was used to propel the paddles in the water mill at Lyneham.

Nowadays we often see the underground water, bleed out of the springs along the Lyneham Banks carriageway roadside and then meandering from side to side down the road to the former Dauntsey Wharf Canal. The under surface water is causing serious layer erosion to the softer foundations and suggestions have been made to improve the drainage along the area. To reduce further slip, a series of under road trenches with better converging irrigation would help reduce this regular under ground erosion and hence the subsidence.

The B4069 Lyneham Banks road is notoriously infamous for the hazardous surface and frequent road subsidence. The former main western route was in such a serious and dangerous condition in the early spring of 2004, many villagers campaigned to the local council and county highways department to get the road repaired. The road was closed for two weeks and consequently repaired during May 2004 at a staggering cost of over £250,000. The road is liable to continual subsidence, hence the numerous warning signs scattered along its route.

This roadway was the former A420 Swindon to Chippenham main trunk route prior to the M4 motorway being opened. The Wiltshire HGV Freight Network Plan actually pinpoints the B4069 road as unsuitable for heavy vehicles and consequently advise HGV to use the approved freight route from the M4 Junction 17 via the A350 Chippenham and A4 Calne and A3102 primary roads.

We have regularly witnessed numerous accidents along the route, hazardous subsidence causing further dangers to road users, costly repairs at tax payers expense.

History shows that the underlying water activity will always be there, we can help the situation from deteriorating, by drainage improvement, use a form of lateral support such as a piled structure, use a grib or gabion wall after ensuring the area was stable by installing sufficient drainage and most importantly prolonging the costly maintenance intervals, by reducing or prohibiting the flow of over weight vehicles.

The B4069 has been the subject of lengthy discussion on the village forum and the census of opinion is for an enforced weight restriction to be imposed on the B4069 Lyneham Banks as soon as possible.

During the spring of 2004, we raised concern about the amount of HGV waste being transported through our village, after being routed up the B4069 Lyneham Banks road. Opinion polls were conducted and the results indicated the majority of residents, equally share a concern over the frequency of out-of-area industrial waste vehicles in our community.

Viridor Waste Management from Somerset, which we understand part own the landfill site in Compton Bassett, have been depositing tens of thousand tonnes of industrial waste in the Wiltshire landfill site. We went into dialog with the Somerset company, requesting the need to use Compton Bassett Landfill site and why they have to use the unsuitable roads with over weight vehicles.

The Viridor Waste website highlights company policy "Viridor has a powerful commitment to customers and the community, and an excellent track record in protecting and enhancing the environment" The responses and lack of initiative, Viridor displayed show the opposite approach to their company statement, regarding protecting the environment.

We did eventually receive a reply from Viridor External Affairs manager "Viridor has investigated the use and possibility of alternative routes to Calne Landfill and of alternative disposal points for the materials being transported. The findings of the investigation are that the alternatives do not currently offer viable options due to the time, inefficiency and availability restraints of other routes and sites. Viridor therefore intends to continue to use the B4069 to access Calne for the foreseeable future and unfortunately the motorway junctions direct that traffic through Lyneham."

Further questioning Viridor on the requisition to use this unsuitable route, Viridor advised that they would now use the approved freight network via Chippenham and Calne. However, people have still reported seeing the large vehicles travelling along the B4069 before the morning rush-hour.

The Wiltshire County Council "Freight Routes in Wiltshire" guide clearly shows that the B4069 is not warranted as a local lorry route for HGV traffic, especially over 7.5 Tonnes.

Further Information
A letter sent to North Wiltshire Mr Gray from Mr G.C. Batten, Environmental Services Department, Wiltshire County Council, dated 1st July 2004, following concern about Lyneham Banks subsidence and the reply outlines HGVs do typically cause more damage to the fabric of roads and response to the weight limit request - Read more.