Underground, Overground. It is ironic that having set up Save St.Giles to protect Stone Meadows above ground we found ourselves submitting a petition to the House Of Lords HS2 Select Committee to draw attention to concerns we have for the exact same area of green space — underground.
The Dream In May 2011 the HS2 Ltd Road show came to Blizzards Yard Car Park, Chalfont St Giles. Representatives presented maps that reassured the community that tunnelling would bye pass the centre of St Giles to the North east at a depth of 100 m. Villagers left the HS2 Road show reassured that someone, somewhere had the sense to avoid the heart of a peaceful, historic community if the project ever went ahead.
The Reality Months later home owners in the centre of the village received letters stating that HS2 Ltd could not be certain the community would not experience problems relating to vibration until after the line was built in 2026. It was discovered that the direction of the tunnel had been diverted and the seventy foot wide, double bore tunnel was now directed to run directly under the centre of the village, close to the grade one listed church.
Late to the Party Save St.Giles was only formed at the end of February 2016 so we accept that we are late to the party but we believe HS2 Ltd have over looked the physical and environmental cost to our community.
HS2 Ltd describe the alignment of the tunnel to pass under the outskirts of the village yet they could not have tunnelled more directly under the centre of the village.
Concern Concerned about the lack of knowledge HS2 had about the historical and geological ground under our village, Save St. Giles decided to enlist the help of three independent experts.
Ian Cloke BSC, MSC, PHD works for a UK company and has experience drilling through chalk and in sensitive areas including The North Sea, Uganda and Kenya where he has dealt with sensitive areas and earthquakes.
Bob Older Retired chartered civil engineer, Fellow of the Institute of Civil Engineers, experience in tunnel boring including Cross Rail under Tottenham Court Rd and London Underground at Victoria. Chairman of the River Misbourne Action Group with 300 local members he guided the River Misbourne back to health in recent years.
Haydon Bailey PHD in Chalk Palaeontology, years of experience in the oil industry, contributed to a book detailing the chalk beds in Herefordshire. Most relevant to this argument he has been studying and logging sections of the Misbourne Valley for 10 years and perhaps understands the geological make up of the Misbourne Valley better than anyone.
Save St. Giles Meet HS2 Ltd Save St Giles met with HS2 and the above experts in the summer of 2016 and asked what data they were using to assess whether the ground they were proposing to drill their twin bore tunnel through was geologically suitable. They admitted that they had used desk based data which we believe to be close to one hundred years old and they had undertaken no physical surveys of the ground around the proposed tunnel in the four years since the line had been re directed under our village. Their reason for this was that they had not been able to secure permission to access the necessary land. We checked with Chiltern District Council, who own Stone Meadow which is directly above the tunnel in our village, they confirmed that HS2 had never requested permission from them as land owners to conduct any tests or gather any data.
Ground Collapse During the meeting with HS2 our experts were able to express their concerns which were based on modern data and years of study and local knowledge. Geologist Haydon Bailey does not live in the village but he has studied the geological make up of the valley for ten years. Haydon's view is that if HS2 bore a twin tunnel under Stone Meadow 19 meters below the surface, with only 3 meters of solid chalk above the roof of the tunnel, it is not a matter of IF there will be ground collapse, it is a matter of WHEN. A tunnel will be drilled through fractured rock. Inevitably the fractures over time will dissolve and eventually some form of collapse will happen. No one knows where or when but this sensitive area is not in the middle of a remote field, it runs under a public space, Stone Meadow. Our community welcomes 7,000 visitors to the annual show, 5,000 spectators for the Fireworks display, funfairs with Ferris Wheels pitch in the meadow. The tunnel also passes under the children's playground, the bridge feeding the village, the library and high street. If it is considered possible or even certain that there will be some form of ground collapse somewhere, at some point, we believe taking the tunnel deeper and reducing or removing that risk is essential. See article HERE See Haydon’s presentation to the House of Lords Select committee : 2.40pm 15th November 2016 HERE
The River Thames and Chalfont St Giles Ian Cloke expressed his own concerns to HS2 when we met. He explained that the upper layer of ground under Chalfont St Giles is made up of what is known as Beaconsfield Gravels. This is river gravels that go back more than half a million years when interestingly, the River Thames or proto Thames flowed, pre glaciation, through the centre of Chalfont St Giles. This is what makes the ground under the village unsuitable for drilling a twin bore tunnel at a depth which is suspected to be only three meters below rubble, fractured chalk and gravel. Ian agrees with HS2 Ltd that tunnelling in the modern world is extremely reliable and accurate, especially if you render as you go. If you are drilling through rock or clay or indeed solid Seaford chalk you can bore through with little or no adverse effects but what is completely unpredictable with regard to ground displacement, is the process of drilling through a fragmented medium like the rubble, flint and broken chalk such as the ground directly below our community. The tunnel itself will be fine but there is a serious and very real threat that at this shallow depth, there will be ground disturbance and possible collapse, not necessarily immediately but in the future and at the very least, disturbance caused will add additional fractures that will most likely cause our precious river to dry up. If this happens then all communities up and down the Misbourne Valley will lose the river. Ian believes any geologist or engineer will agree that the last place you should tunnel is under a valley because you have so little solid ground above, but by drilling under the Misbourne Valley at a minimum depth of 19 m, when that ground above the tunnel is rubble is in Ian’s experience, asking for trouble.
Old and New available Data In contrast to the 100 year old, desk based data HS2 used to assess the area before directing it under St Giles, we have at our disposal a bore hole drilled to 19 m depth directly over the area of the proposed tunnel. We have access to modern geological maps produced in the mid nineties, we have access to decades of data explaining the extremely unpredictable nature of the Chalk Stream that runs through this area Of Outstanding Natural Beauty.
The Very Worst Place to Tunnel? All information available to us suggests that HS2 have drawn a line through Chalfont St Giles on their map with little investigation and do not realise that they have perhaps chosen the very worst point in the whole of the Misbourne Valley under which to tunnel. When presented with our extensive research and decades of data HS2 simply say that they have faith in their experts and in their judgement. if anything goes wrong with the drilling of the tunnel, if there is ground collapse, or if the river stops flowing through the valley then they will simply ‘make good’ but this approach is considered wholly inadequate by those who understand the geology of ground under our village and the nature of the river.
HS2 Ltd Off to a very poor Start HS2 assured Save St. Giles in person that drilling and all related work would take place under strict supervision with complete respect to the community, undertaking the very best and most stringent working practice. The fact that HS2 did not research gather physical data from the ground beneath our village before directing the twin bore tunnel through it at such a shallow depth does not, to us, suggest that they are respecting our community. Our first physical exposure to their ‘stringent working practice’ was when a team of engineers arrived in late October 2016 to take data from Stone Meadow (Only two weeks before HS2 Ltd were due to face our petition in front of the House Of Lords). They arrived with heavy drilling machines, vans and engineers and started drilling bore holes in Stone Meadow. It soon became clear that HS2 Ltd had not asked permission from the land owner to drill in the Meadow and as a result they were shut down and asked to leave immediately and not return until they had been granted permission. Their departure was then delayed as their bore hole drilling machine had broken.
The Risk of the River Misbourne being Lost Forever Bob Older, chairman of the River Misbourne Action group, expressed concern to HS2 Ltd that there is significant risk to the river being damaged and drying up completely if mitigation is not undertaken. Seventy-five percent of the worlds chalk streams are in Southern England, they are so rare they should be considered the UK’s ‘Amazon Rain Forest’. There are more Panda’s in the world than there are chalk streams yet the proposal of HS2 to tunnel without taking any mitigation may well see the Misbourne disappear forever. HS2 Ltd reassured Bob and Save St. Giles that they would respect the sensitive nature of the chalk stream whilst tunnelling and that they would monitor the flow of the river for a year during construction. This is considered to be completely inappropriate by Bob. Because of the nature of the chalk stream, every year the recorded flow is different. Some years it dries up, some years it floods so monitoring for one year is pointless. There are a host of variables including abstraction, rainfall, the condition of the bed, they all effect flow and depth. By the time you’ve monitored and identified a problem, if water goes down a new fracture caused by drilling for example, it’s gone. HS2 Ltd will not know where the fractures are, they will be working blind, so by that stage, its all over, its too late. The river has been lost. The nature of fragmentation in chalk is such that issues of losing the river above the twin bore turn tunnel could appear in 3-5-10 years time. We do not believe that HS2 Ltd will be motivated to return to the site years after they have tunnelled and accept that a future problem with flow or drying up was caused by them years before.
Mitigation for the River Misbourne HS2 can’t establish a base line on flow but they can mitigate. They have accepted there is a risk to river and they can mitigate by fitting 400 meters of Bentonite matting to protect the area of river over the tunnel. This is a process approved by the environmental agency as it preserves the Hypopheric Zone (Natural wildlife and habitat in the river) . This process is considered essential no matter where the tunnel crosses the river but HS2 Ltd do not accept that it is necessary.
After reviewing HS2 Ltd position and with increasing concern that not enough was being done to protect our village we decided to apply to petition the House Of Lords HS2 Select Committee.
SSG petition to The House Of Lords Select Committee: HERE
On 7th November 2016 Save St. Giles appeared before the House Of Lords HS2 Select committee and presented their case.
Video of petition HERE Transcript of Petition HERE
What are Save St. Giles asking HS2 Ltd to do? We are asking HS2 Ltd to respect the geological make up of the ground under our village and accept that they need to bore the tunnel significantly deeper. As you go deeper you get away from rubble and flint and hit more solid chalk. We understand the restraints of the line and of the process but we believe HS2 Ltd can easily set their tunnel deeper under the whole of this area which would not only make sense for the ground make up, the river, the community but also for HS2 Ltd. Currently the deepest point of the tunnel section is not under the area which is potentially the weakest. We are not asking for a tunnel depth of 100 m that the village was originally promised, we accept that tunnel depth is determined by the landscape but during our petition to the House Of Lords, Ian Cloke presented HS2 Ltd with our own version of tunnel depth that takes the tunnel under our village at a depth of 40 m instead of 19 m this will create extra competence beneath the rubble zone. HS2 Ltd will avoid drilling through potentially difficult rubble and flint, they achieve a better canopy over the tunnel and bore through solid chalk with less chance of damaging the river and risking ground collapse.
HS2 Ltd response HS2 Ltd accepted that the tunnel could possibly go deeper but they argued that tunnels had successfully been through chalk with Cross Rail, the Channel tunnel and HS1 but these are very different environments as they relate to solid clay or solid chalk or rock that behaves completely differently to the rubble and flint that makes up the ground below Chalfont St Giles. The major projects HS2 Ltd did not mention, perhaps because they resulted in ground collapse was the tunnel at
And the collapse on the Channel Tunnel Rail Link HERE
HS2 also referred to a letter from the environment agency which claimed that they could see no reason why preventative measures should be taken but this letter only referenced their view on the lining of the river, not the make up of ground under the village.
What happens next? The proposal to tunnel deeper was made by Save St.Giles on 7th November in the House Of Lords and we await a response.