AN expert in surveying and demining has warned that the recent landslip on the A40 is a “timely warning”.

Monmouth resident Andrew Smith is a specialist in Humanitarian Mine Action and has worked as a Chief Technical Advisor for UNDP country programmes in a number of countries such as Tajikistan, Libya and Lebanon.

He has told the Beacon that several studies made since the A40 was completed in 1968 have identified a risk to the eastbound carriageway from landslides starting above that carriageway.

“This risk arises because the sedimentary sandstone layers in the area are inclined at 30 degrees with layers of slippery “mudstone in between”. Two severe landslides occurred during the road’s construction in 1964 and 1965, one at ‘Whipping Green’’ and one at Chapel Farm,” he added.

The Chapel Farm slip destroyed both of the new carriageways and the farm buildings close to the river Wye. In fact, the earliest landslips in that area date back to Roman times, so its instability was always well known and it was never a suitable place to build a dual carriageway.

He explained that the cutting of carriageways increased the slope in some areas and that combined with the weight of the new road (not its vehicles) were identified as the causes of the landslips during construction.

“When there have already been landslips in an area, the risk of similar events in parts that have not fallen before are obvious,” he warned.

He said that a Prime monitoring system was installed at Leyes Bend between the sites of the former landslides in July 2017 for a “trial monitoring period”.

It measured groundwater and land movement over a very limited area until February 2019. The equipment remains above the carriageway and may still be providing data for an extended study but there is no indication of this in the assessment report published in Transportation Geotechnics in 2023.

The ‘trial’ monitoring equipment is closer to Monmouth than the current landslip which appears to be a continuation of previous slips, Its positioning between areas that had already slipped was logical but it is apparent in retrospect that the trial area was too limited and the duration of the study was too short.

It is known that a high level of rainfall increases water ingress between fractures already present in the sandstone layers, so lubricating the layers of mudstone.

The current closure of the carriageway has allowed evidence of inadequately maintained drainage to be safely observed. It is also possible to see evidence of increased water accumulation in dips behind the carriageway where immature trees have fallen exposing rotten root systems.

He warns that It appears to be “inevitable” that further landslips on the Eastbound carriageway will occur. “These do not have to be large to be disastrous on a busy dual carriageway,” he added.

Walkers will remember that a slip blocked the riverbank footpath in 2021 almost in line with the slip that has now occurred higher up. That slip appears to have been ignored. If a section of the entire road were to slide away as it did during construction the consequences would be devastating.

The official estimate (no one is actually counting) is that more than 2,000 vehicles use the road every hour during peak times – and often at high speed.

He says that locals need reassurance that the area above the carriageways will be monitored over the entire area of risk and its drainage will be both maintained and improved. “The at-risk area between and beneath both carriageways leading down to the river should also be surveyed, reinforced where necessary, and constantly monitored for movement.”

He suggests that as the road was designed for 50mph traffic in the 1960s, the Highway Authority could re-route it to bypass Monmouth, so avoiding landslips and making it safe enough for modern traffic volumes.

“The existing road could then become weight and speed restricted single carriageways using the lower cutting. This would have the added benefit of restricting traffic volumes through Monmouth that would allow the town to access its river frontage as it did before the A40 was built.”

If all through-traffic went elsewhere, Monnow Street could become one-way because the old town cycle could be safely reinstated on a part of the A40. Tourism and trade would probably increase and the residents would be relieved of the pollution associated with having a de-facto motorway cutting through the town.