I have been passed your email by my colleague, James Agg. I am the lead for HS2 in Hertfordshire and North London Area and am best placed to respond to specific queries in this area of the proposed route.

I work closely with James and the other members of our HS2 team. We have a common objective to maintain the current state of the River Misbourne, both in terms of water quality and flow.

As I am sure James has explained, we are working closely with HS2 Ltd to secure the protection of the chalk streams throughout the Chilterns. We have had significant input into the design of the ground investigation that is currently being carried out by HS2 Ltd along the line of route in this area. This includes the installation of monitoring points at key points between the line of route and sensitive environmental receptors, such as the River Misbourne. Baseline monitoring is currently being undertaken at these monitoring points. Once the baseline is established this information will be combined with the long, historical dataset that we already hold on the Misbourne.

Once HS2 Ltd have the findings of their ground investigation and baseline monitoring information, we will be in a position to agree what monitoring will be required during the construction of the scheme. We will use this information to agree a comprehensive response procedure for triggering investigations and actions in relation to potential impacts. Triggers could include significant deviations in monitoring from the baseline, for example, an unexpected decrease in the flow of a watercourse or a drop in groundwater levels.  Actions could range from undertaking additional monitoring to halting tunnel construction while the issue is investigated further.

The solution for discharging water abstracted as part of the vent shaft construction has not yet been agreed. We are aware of the sensitivity of this area and will be highlighting our concerns to HS2 ltd and their contractors. Calculations using the data obtained in the preliminary ground investigation will provide an estimate of how much water will be encountered during the shaft construction. This will then feed into an appropriate design of discharge system including any treatment requirements including if the ground is already saturated. I note your suggestions on water treatment from both excavation dewatering and other construction effluent, we will explore the options that are available in our discussions with HS2 Ltd.

You are correct that the tunnel is likely to pass through areas of rubbly, weathered chalk along its length. The borehole logs have revealed a weathered upper surface to the chalk that varies in thickness significantly over relatively short distances. I believe that David McCann has explained to you that the tunnel boring machine can inject grout ahead of it to stabilise the ground, facilitating the tunnelling process. The rate of grout injection is very carefully monitored during tunnelling. If grout is being lost to the surrounding rock at a rate that is not expected, the mixture is changed so that it “sets” close to the line of the tunnel without filling the fractures of the chalk unnecessarily. This process ensures that stabilisation only occurs around the tunnel, which leaves the wider aquifer as undisturbed as possible and preserves groundwater flow pathways away from the tunnel. If the tunnel does cut flow pathways, the grout prevents water from flowing into the tunnel and so groundwater finds a way around the tunnel.

We continue to work closely with HS2 Ltd to establish the ground conditions across the proposed route, including the depth below ground that they expect to encounter solid chalk. This information will be available from HS2 Ltd when they publish the ground investigation report.

With regard to your query regarding the cutting above Leather Lane, our historical monitoring suggests the groundwater level is beneath that of the cutting therefore the likelihood of the cutting intercepting groundwater in this area is very low. HS2 Ltd must seek consent from us where there are potential impacts on; the flow or purity of a main river, surface or ground water or where works affect the conservation, distribution or use of water resources. We will be working closely with HS2 Ltd and their contactors to identify and mitigate the risks of all their tunnelling work including those posed by the tunnel rising to the portal near South Heath.

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Yours sincerely,

Matt Parr

Matt Parr 
Account Manager, Hertfordshire and North London Area
Environment Agency | Alchemy, Bessemer Road, Welwyn Garden City, AL7 1HE

Since the completion of the preliminary ground investigation, HS2 Ltd commissioned their consultants to carry out a detailed review of the findings along the length of the Misbourne valley, paying particular attention to the river crossing points and the vent shafts along the tunnel. This review has shown that the chalk along the Misbourne is very heavily faulted and fractured; the cross-sections produced have identified numerous features that have not been previously picked up by the BGS or by historic investigations. This information is very important not only for the construction of the HS2 route but also for the wider understanding of the chalk aquifer and its interaction with the Misbourne. The Environment Agency are already looking to incorporate the data gathered by HS2 Ltd in our existing groundwater models to assist with future decisions affecting the aquifer. The groundwater monitoring that has been carried out to date has confirmed the presence of a layered system, with groundwater perched at various levels throughout the chalk. This critical information has highlighted to HS2 Ltd the need for very careful design throughout the Misbourne valley. I should also hasten to add that the monitoring programme is ongoing and will continue from now, through the entire construction period and into the operation of the route.

The report has highlighted the need for additional detailed investigation around the vent shafts and river crossings; this will be essential ahead of the detailed design of the vent shafts and final choice of tunnel boring machines used. The Main Works Civils Contracts have recently been awarded and so the design process will start soon; my colleagues and I will be working hard to influence the final designs to be as environmentally sensitive as possible.

Following the completion of the preliminary ground investigation, HS2 Ltd commissioned their consultants to carry out a detailed review of the findings along the length of the Misbourne Valley, paying particular attention to the river crossing points and the vent shafts along the tunnel.

This review has shown that the chalk along the Misbourne is very heavily faulted and fractured. The cross-sections produced have identified numerous features that have not been previously picked up by the British Geological Survey or by historic investigations. This information is very important not only for the construction of the HS2 route but also for the wider understanding of the chalk aquifer and its interaction with the River Misbourne.

The groundwater monitoring that has been carried out to date has confirmed the presence of a layered system, with groundwater perched at various levels throughout the chalk. This critical information has reinforced the need for very careful design throughout the Misbourne valley and the requirement for further ground investigation studies of the Chilterns Aquifer.

We have discussed the findings of the draft ground investigation report with HS2 Ltd, however the report has not been issued to us. As the information is not held by us we are unable to supply it to you as you have requested. Our understanding is that HS2 have not yet formally accepted the report through their internal acceptance procedures and therefore are not in a position to make the report available. Once the report is available you will be able to request the report from HS2 Ltd as it is their data and information.

246. MR GLADWIN: Can I ask a couple of questions? Mr Smart, it seemed to me when you were talking about Chalfont St Giles, it was tunnelling on the precautionary basis: well we’re going to monitor this; we’re going to monitor that. What you didn’t explain was, if your monitoring then identified a problem what you were going to do about it?
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Now, if you’re taking somewhere like Chalfont St Giles, what happens – you put a tunnelling machine under there; you’ve got maybe five or six metres of competent chalk; you’ve got rubbly chalk above it, all that’s – the tunnelling machine is going to be shaking the ground isn’t it? It’s going to have vibrations and what is the impact if you’re going through 16 metres of rubble? Are we going to get a drop of 2cm, or a drop of 4cm, or a drop of 2 metres? That doesn’t seem to have been taken into account in designing where this route is going.

247. MR SMART: Well, yes it has, because based on the geological information and the bore hold records in the national archive, we have designed the tunnel for the best alignment, both for the railway and indeed the material that we are tunnelling through. Now, as has already been said, we are about to embark on some very extensive geotechnical investigation across the whole route, which informs the decision that we’ve made and, in the event of some really difficult ground, it may be that we will just lower the alignment slightly, by a metre or something if we had to. But based on the information available, we believe that the sort of risks that you’re talking about are not actually going to manifest.