21 November 2013 Meet with HS2 Notes

Document Number: 
Meeting Date:                                       Wednesday 21 November 2012
Meeting Location:                                Chalfont St Peter Community Centre
Meeting Type:                                       Technical
Zone/Area References:                         Country South
GIS/Land Registry Ref. (if known)
HS2 Contact Person or Group:            Terry Stafford, 020 7944 0660
Stakeholders:                                        Misbourne River Action Group / Chiltern Society / Chilterns                                                                                             Conservation Board


Topic Keywords:

John Gladwin (JG),       Trustee, Chiltern Society
Bob Older (BO),            Chairman, Misbourne River Action Group
Hayden Bailey (HB),      Geological Adviser, Chiltern Society
Allen Beechey (AB),       Chalk Streams Officer, Chilterns Conservation Board
Roger Lerry (RL),          Chairman, Rivers and Wetlands, Chiltern Society
John Norris (JN),           Rivers and Wetlands, Chiltern Society
Rob Sage (RS),             Senior Water Resource Planner, Affinity Water
Mike Overall (MO),        Chairman, Chiltern Society
Martin Wells (MW),        Area Stakeholder Manager, HS2 Ltd
Mark Bailey (MB),          Senior Environmental Manager – Route Wide, HS2 Ltd
Charles Jones (CJ),       Senior Hydrogeologist, Mott Macdonald (working on Environmental Statement)
Barnaby Harding (BH),   Hydrogeologist, Mott Macdonald
Ian Gee (IG),                  Tunnelling Engineer, Atkins (Responsible for tunnelling and underground structures,                                                Country South)
Terry Stafford (TS),        Area Stakeholder Adviser, HS2 Ltd

This meeting had been requested by the Misbourne River Action Group through the Chalfonts and Amersham Community Forum. The meeting began with a round of introductions. JG explained that HB and BO would be late. It was agreed that HB’s paper, circulated previously, would be discussed after HB arrived.

MO gave a brief description of the Chiltern Society’s history, membership and work. He explained that the society had first raised issues relating to the potential impact of HS2 on the River Misbourne in 2010, and that HB had produced a technical paper on the subject. MO said he was delighted HS2 Ltd had agreed to meet the society and others to discuss the paper and the local concerns.

BO joined the meeting during MO’s introduction.

AB explained that chalk streams such as the River Misbourne were a rare global habitat (with 85% in the UK), and a key feature of the area. The Misbourne had long suffered because of over-abstraction and other human activity. AB said that HS2 was due to cross the river in two underground locations and that this could impact on the river’s flow.

The river was currently failing to meet its Water Framework Directive status of ‘Good’ – its status was currently ‘Moderate Ecological Potential’.  AB also noted that 76% of rivers in the Thames region were failing, and that a full assessment of the Misbourne had taken place in 2009, with the next one due in 2015. The board did not want this work prejudiced by HS2. AB was particularly concerned about the potential for adverse effect on flow in the river and the aquifer beneath as well as its impact on the landscape.

BO explained that while the Misbourne River Action Group had no direct affiliation to the Chiltern Society, the organisations worked closely together towards full restoration of the river through both local cleaning and developing wider projects such as localised lining and support of Affinity’s proposals for augmentation of flow.

BO said that the River Misbourne was unique in having three zones – upper, middle and lower – each with its own characteristics. The middle zone, which HS2 could affect, had an existing and ongoing problem of low flows, particularly around the Amersham dump and the leat at the Chalfonts. This zone was ‘perched’, meaning that it was above the water table. It was noted that HS2 tunnelling could also affect the upper section of the river at Shardeloes Lake.

BO stated that such flow as his group managed to sustain was very precious. The river was much loved by local people and a vital part of the character of the Misbourne Valley. BO explained that his concern primarily focused on the risk of “unintended consequences” of the development, but noted that HS2 is likely to be a best-practice promoter. He and his supporters wished to ensure that everything possible was done to avoid harm.

JG pointed out that 22% of London’s water currently came from the River Colne, which was fed by the Misbourne aquifer and expressed concern at the possibility of pollution of the aquifer.

MO pointed out that the Chiltern Society had been pursuing a number of projects to improve the flow in the river. Also that the society had been taking borehole readings and measuring flows in the river for a number of years. RL added that the flow measurements had identified two particular areas of loss. These were the areas around the Amersham Dump and the leat.

AB noted that the river had long been the subject of investigation into its low flow. Following a report by Halcrow, abstraction had been substantially reduced as the first stage of an Alleviation of Low Flow scheme. However the river still suffered from low flows and new studies were on going to determine the scope of a second stage.

CJ asked: “What have you been able to do about the river?”

JG explained about Amersham Dump and the leat: Until 1950 there had been a sewage works at Amersham. This was dismantled and sewage pumped to Maple Cross. Before the sewage works were dismantled, river flow was augmented by treated sewage effluent. The river was re-profiled and the new course created with gabions filled with the rubble from the dismantled sewage works. Also the gradient of the river was changed creating a level zone with a sharp decline at the end. None of this was ideal for preventing water loss from the river.

The leat fell into disrepair when the mill no longer required water power. With periodic drying out and rabbits there was significant loss of water when the river started again. MRA had carried out extensive works on the leat, filling in holes removing excess debris etc. This had significantly reduced the loss of water, as shown by our river flow measurements.

RL explained about the Chiltern Society’s relining project for the Amersham dump. This involved re-profiling the river and lining with bentonite in a geo-textile sheet.

CJ indicated that he understood the issues.

AB pointed out that HS2 would run underneath the major sewer that connects balancing tanks at the location of the old sewage works to works lower down the valley.

AB said that continuity of flow was essential as it prevented the riverbed from drying, which reduces flow further and he drew attention to the Environment Agency’s Restoring Sustainable Abstraction programme for the river.

Referring back to problems at the leat, BO said that he suspected that the cleaning discharge pipe from the pumping station might have formed a swallow hole, which he believed had affected the chalk below, near where HS2 would cross the river.

BO said that swallow holes were a well-documented phenomenon in the area, and that approximately two swallow holes appear every year.

CJ said that HS2 Ltd’s Environmental Statement (ES) would have to take account of the current state of the river for which HS2 Ltd’s approach was to take all the historical data they could get. The statement would also have to take account of planned schemes for improvements to the river as part of the future baseline, as well as the Environment Agency’s expectations to meet Water Framework Directive targets.

MO said the ES could evaluate different base line assumptions and recognise alternative solutions.

CJ said that HS2 Ltd expected that managing the hydrology would ensure that ecology in the area was not adversely affected.

RS said that Affinty Water were approaching the end of their surveys on the River Misbourne as required by AMP 5, and that they were discussing options with the Environment Agency for Projects to be included within AMP 6. This included setting a minimum flow rate for the river and looked at various options, including lining, borehole augmentation and reducing abstraction. He noted that Affinity was currently negotiating with the Environment Agency and he was hopeful that the issue would be resolved soon.

RS said that the flow had improved in the upper part of the river as a result of reduced abstraction limits established in the last century, but in the perched area it still struggled because of its height above the water table. RS said that Affinity believed that without further measures, (e.g. lining and/or augmentation) this part of the river would be dry even without abstraction.

Affinity are concerned about any intervention (especially tunnelling) on abstraction. Affinity is holding bi-lateral talks with HS2 Ltd.

HB joined the meeting during RS’s comments.

HB’s paper on ‘Concerns arising from the Geology and Hydrology of the ground underlying the original and re-aligned High Speed (HS2) routes through the Chilterns’ had been distributed before the meeting. 

HB said that HS2 crosses the valley near Affinity’s pumping station. He acknowledged that the concerns he was raising were ‘not insurmountable’ but that concerns remained because the route was aligned to pass through medium to high-quality aquifers.

HB summarised his concerns:

  • Initial reaction that line choices should be in higher ground. That fault lines in aquifers tend to follow river valleys. Thus in higher ground less risk of disturbing the flow of water through the aquifer.
  • The crossing at Chalfont St Giles was on the line of the proto-Thames, with the associated degradation of the chalk. 
  • Boreholes show 16m of chalk rubble.
  • The impact on the Chalfont St Giles pumping station, where the tunnel is within 170m
  • Concerns that the tunnel is going through medium to high quality aquifer.
  • Water supply to the river would decrease in the short term and probably medium term.

 HB concluded that, while “not insurmountable”, he was concerned that:

  • The river might be damaged permanently by tunnelling because the chalk was unstable. He suggested that the scheme could completely remove the river.
  • There will be a serious knock on effect on hydrogeology.
  • It would be better to tunnel through the interfluve between the Chess and the Misbourne.
  • It looked like a flexicurve had been used to choose a route.

JG suggested that the chosen route would therefore be unnecessarily expensive to tunnel.

IG disagreed that tunnelling would affect the river in the way HB reported.

IG explained that for a high speed railway the route position locally was constrained by a range of considerations particularly the need for relatively straight alignment. Furthermore as required by the project specification the railway alignment design criteria required certain vertical gradients affecting journey time performance,  and associated traction power supply, and that HS2 Ltd had to find an appropriate economic balance in all these and other considerations. Therefore geology alone could not define a local alignment. A lowering of the alignment for example would mean additional cost, including that of the depth of intermediate shafts.

He stated that the current route alignment corridor had been selected following public consultation and was the option HS2 Ltd was developing toward Hybrid Bill submission, and further, that they, as consultants, were required to deliver within the chosen route alignment.

CJ said that there had recently been some small horizontal and vertical changes to the route, specifically: alignment of the River Colne viaduct and position of Mantles Wood tunnel portal and lowering at each river crossing by approx two tunnel diameters to approximately 20m cover over the crown. He said the vertical changes had been made to improve drainage and increase cover under the riverbed.

IG confirmed that no change to levels at Mantles Wood were required as a result of these vertical realignments and undertook, subject to HS2 Ltd approval, to confirm the exact cover figures, subject to ongoing design development.

MW said that these changes had been shared with the community forum.

HB asked if any borehole tests had been done. IG confirmed that HS2 Ltd had not undertaken additional intrusive ground investigation in the area and that they were using information available from existing boreholes, geological memoirs and other records obtained by desk study. 

HB asked whether the cover was deep enough?

IG stated that various options had been reviewed, but there is a need to balance engineering costs along the route. He noted that deeper tunnelling increases costs, including structures and where to place intervention shafts and one could not always find the best geology.

MO asked whether these alternatives would be in the public domain? CJ stated that some alternatives would be made public during the select committee process of the hybrid bill.

JG stated that in his opinion the geology should have been looked at before the route was chosen/finalised, and in his opinion if the consultants thought there was a better route they were bound professionally to report that.

JG mentioned that there was anecdotal evidence of a large cavern under Shardeloes Farm, which was indicated by boring undertaken for a groundwater source about two years ago by a local farmer. He also pointed out that swallow holes were not uncommon in the Chalfont St Peter and Chalfont St Giles area. This was confirmed by BO who referred to two significant incidents, which closed roads on Chiltern Heights in recent years.

IG said that they were employed to develop the chosen post consultation route. CJ said that the decisions taken and consideration of alternatives would be in the public domain during the select committee stage of the hybrid bill process.

IG said that HS2 Ltd’s current proposal was to use Tunnel Boring Machine (TBM) methods similar to those that were currently being used for Crossrail.  This would produce a tunnel lined with pre-cast concrete segments grouted in to position.  The tunnel would be specified to achieve a very high level of water tightness, resulting in very little water ingress in the operational phase of the railway.

RL commented that with fissures in the chalk, there was risk of introducing substantial amounts of grout into the aquifer.

IG explained that the grout is used to fill an annular overcut made by the TBM (typically 10-15cm) to secure the linings in the ground, and that the grout itself was designed so as not to migrate in an uncontrolled fashion.

RL asked how water would be prevented from entering the tunnel at tunnel mouths.

IG explained that the tunnel portal would also have drains and pumps to keep the tunnel dry from collected rainwater. Collected water would go back into the ground or into the river via appropriate interception.

AB said he was concerned about the impact this would have on water quality.  IG said that the statutory authority’s approval processes, (in this case the Environment Agency) would not allow water releases to have an unacceptable impact on water quality.

CJ said that there were already examples of tunnelling through chalk, including the Docklands Light Railway tunnels under the Thames for the Woolwich Arsenal Extension, and that the technology HS2 Ltd would use was well tried and tested.  It was noted that this method had also been used for tunnel construction through drinking water aquifers.

MO asked what a slurry-based TBM was. IG explained that this is a type of ‘closed face’ tunnelling machine where the ground was continuously supported by a pressurised fluid (slurry) in the front of the machine, which was used to convey mined material from the cutterhead via pipes in continuous circulation back to a slurry treatment plant at the supporting worksite. In chalk tunnelling, the slurry was a mixture of water and the excavated material.  The TBM slightly over-excavates the bore and fills the annular void with slurry. As the machine moves forward, concrete segments forming ‘rings’ are placed and grout is injected into the annulus between the concrete sections and the outside of the tunnel excavation.

IG continued that boring of the Chilterns tunnel would be from south to north.  The major construction site for the tunnel will be within the M25.  Slurry arising from the boring process will be transported here for processing.  Water will be extracted from the slurry (e.g. in lagoons) and re-used, generating chalk arisings.  IG stated that this recycling of water would significantly reduce but not eliminate the requirement for water.

IG said the way in which the chalk spoil will be disposed of following processing is not yet known and subject to development of HS2’s proposals. MW opined that there might be planned infrastructure projects (not known to HS2 Ltd at the present time) that may be in construction at the same time as HS2 that the spoil could be used for.

IG confirmed that there would be a smaller work site at the Mantles Wood tunnel portal to deal with the TBM’s as they emerge, and for constructing the infrastructure at the tunnel mouth. The exact location and size of this site is currently being worked on.

CJ stated that more detail of the operations at the work sites would be set out in the ES.

Responding to a question about the possible use of ground freezing, IG explained that the slurry TBM method is selected to work in wet rock and does not require ground freezing or large scale de-watering of the surrounding ground to be carried out during operation. However, ground freezing and/or de-watering MAY be required for the construction of some aspects of the tunnel design such as emergency, cross-tunnel access shafts.  These techniques may also be required for operational reasons, such as if there is a problem with the cutting face of the TBM.

CJ confirmed that water arising from these operations would be discharged to a local watercourse or groundwater once appropriately treated. CJ agreed that the treatment process might well involve the use of settlement ponds and the details of these would be confirmed at the detailed design stage.

IG confirmed that there would be small ground movements (or ‘settlement’) at surface level above the tunnels as part of the construction process.

IG confirmed the tunnel excavation outside diameter as 10.5m.

CJ said that the exact tunnelling method for work in very wet chalk rock could remain an open question until a contractor was appointed, but that the technology will be capable of dealing with the ground. CJ also said that there were many Environment Agency boreholes that allow HS2 Ltd to be confident about where the water table was. He added that there was a presumption against major de-watering, although there would be a requirement for some localised de-watering to construct cross passages or other such tunnel junctions.

RL and BO noted that borehole readings were made at 13 sites along the valley every month and substantial historical records of flows and aquifer levels are available.  IG confirmed that recommendations would be submitted to HS2 shortly for a programme of ground investigation which HS2 may choose to implement in 2013.

Without being specific, CJ stated that “historical” data sets were being used as baseline for work towards the ES.

HB said that a tunnel at Stonehenge had had to be postponed because of the high water content of the chalk. IG said this was not the case and that care should be taken in comparing HS2 with other schemes. He remarked that one of the UK’s largest and most recently constructed road tunnels; the A3 Hindhead Tunnel, was constructed largely above the groundwater table, permitting alternative construction methods, not currently being proposed by HS2 Ltd.

CJ said that the technology HS2 Ltd would use was designed to go through wet chalk and more highly saturated substances.

HB said that the chalk was mostly free of flint so tunnelling should be relatively easy. However, he believed there would be considerable faulting in the area that was not shown on maps. IG agreed the rock mass would be faulted but did not consider that the varied geological conditions along the route would present any difficulty for TBM tunnel construction. 

BO asked what, given the agreed faulted nature of the chalk, would prevent vertical ground movement below the riverbed causing an increase in bed leakage.  He asked for further reassurance on this point. IG said that predicted ground movements and any associated changes to ground permeability would be very low. CJ said that the ES would report on this and, if it was proved to be necessary at the crossings, HS2 Ltd could provide some protection against extra vertical flows from the river.

CJ suggested that the settlement “trough” could be in the order of 60m wide crossing the river. It was agreed with BO that that the possibility of providing lining to the river bed along these sections could be considered in the Environmental Statement to provide some protection against extra vertical flows from the river if this was determined to be an issue.

IG stated that Atkins are assessing the predicted ground settlements from tunnel construction, and how these might potentially affect existing assets, including services such as water infrastructure and the foul water main that runs along the valley. IG confirmed that there would be settlement, but it was too early in the work to quantify movement.

MW expressed confidence that an impact will be avoided.

MO asked what Design Standard criteria had been incorporated into the tunneling design philosophy, as this could range from “minimum legal compliance” to "global best practice”.  He felt there was a reasonable expectation that the latter would be appropriate for tunneling through an AONB.  BO echoed this query, commenting that he had raised it similarly regarding other matters at the Community Forums. No clear response was given to this question but MW referred to the high investment cost of the scheme.

Moving on from discussion of settlement to impact on groundwater flow, HB said he suspected that rivers follow fault lines and that there would be faulting in the valley. He added that tunnelling would impact on hydrogeology. HS2 Ltd said that these were typical challenges that have to be met during tunnelling and that the specification and methods would prevent this.

JG noted that the route had been altered to avoid the inner protection zone of the Amersham pumping station but still impacted the outer protection zone.

RL asked if the pumping station would need to be moved. CJ and RS said that HS2 Ltd and Affinity were discussing how Affinity would continue to meet their customer needs and these discussions could not be examined at this meeting.  RS said that, although the impact of HS2 on water supply infrastructure would be “quite significant” availability of drinking water would be unaffected.

AB asked if HS2 Ltd had done any modelling of the impact of grouting on groundwater flow.

CJ said that 9.7 metres of vertical section – plus rings and grout – would be taken up by the tunnel. He suggested that this was a relatively small number in the context of the cross section of the aquifer and likely impact would be perhaps a small number of centimetres rise in groundwater level.

HB pointed out there is a need to understand the impact of fractures, faulting and jointing in the aquifer. Elsewhere the Chilterns show a lot of faulting. This could significantly impact the water flow through the aquifer. The assumption of a homogenous isotropic material was invalid when actual flows were determined by faulting and fractures and that as the route may intercept preferential paths of groundwater flow, there could be significant “mounding” of groundwater upstream of the tunnel.

CJ accepted this point and said that calculations of groundwater flow were a part of the assessment process, which would be included in the ES.

Moving on to longitudinal effects, IG stated that the construction method for the tunnel would not lead to preferential groundwater flows along the outside of the tunnels.  He explained that annular grouting would be used to confine the concrete segments into the ground excavated by the TBM, and said that contractors would not want to use more grout than was necessary.

JG asked about the risk of contaminating groundwater while tunnelling. JG said European Union water directives prevent the introduction of even non-hazardous pollutants to the groundwater. CJ said that HS2 Ltd knew what process they would use, but not the materials. The process, including all reagents, would be set out in the Local Environmental Management Plan, which would form part of the Code of Construction Practice (CoCP). MB confirmed that the draft CoCP was due to be publicly available before the end of the year and would be consulted on, in parallel with the draft ES, next year.

JG asked if they were aware of the Buncefield to Heathrow fuel lines.

CJ said that they had consulted all utility maps related to the routes as none of these could be fractured. IG said that all utilities crossing the route would be investigated. 

JG said that pollution to the aquifer would be considerably more severe and longer lasting.

AB again asked about how spoil would be disposed of. G said that a large site was needed to establish a spoil processing facility.  Generally the desire is to re-use spoil. The site would be at the M25 tunnel portal site.  When pressed he confirmed it was not known where the spoil would be disposed of and that this will be dealt with in the ES.

AB asked how the impacts on the AONB would be made public. MB said that HS2 Ltd shared information through the community forums; however it was the main purpose of the ES to report on likely significant effects. 

MO asked who the safety regulator for the project was. HS2 Ltd said that they believed it was the Office of Rail Regulation.

JG commented that the impact of HS2 on the Hillingdon Outdoor Activities Centre (HOAC), the SSSI and sites for overwintering birds would be very serious. MW said that HS2 Ltd were aware of this, were working with local stakeholders, and were looking at realigning the viaduct to reduce adverse environmental impacts.

JG and BO thanked HS2 for arranging this meeting and the consultants for their engagement which, it was agreed had been illuminating and would enable MRA and CS to better inform their responses to many questions being raised by their members and correspondents.