19-Dec-2023

Events in the Aquifer

Documents disclosed by HS2 in response to FOI requests by Hillingdon Green Party have revealed several events affecting the aquifer, and the River Misbourne, which we were previously unaware of. The Chiltern Society submitted FOI 23-5341 on 26-Nov-23, requesting details of the events which were disclosed, and requesting a list of all such events associated with the Chiltern Tunnels ( occurring since the TBMs passed the Chalfont St. Peter vent shaft; this is to reduce the amount of data requested, as FoI requests can be declined if too much work is involved.)

Request A (loss of water in the Misbourne) relates to para. 1.18 of the Colne Valley water monitoring report, May 2022

There was additional concern over a sudden drop in water level within the Misbourne river on the 3rd May, following which a possible subsidence location was identified within the river bed. This was infilled by the 9th May, with ongoing monitoring not indicating any additional concern regarding water levels related to ALIGN’s works.

Request B (Subsidence at an unmapped landfill) was reported in "Chemical Composition of Contaminated Water...", p11 (Conclusions)

It was noted in November 2021 that a subsidence event revealed an unmapped historical landfill nearby (approx. 100 m away from the tunnel alignment). It is possible that additional unmapped landfills/illegal dumping areas are located along the path of the tunnel route.

Response !

A response was received from HS2 on 15-Dec - within the 30 day limit !

Request A - Water loss in the Misbourne

This is described in great detail in an incident report (1MC05-ALJ-EV-REP-CS02_CL04-000228). Fluctuations in the river level were observed on 3rd May in the region of the Pheasant Rise bridge in Chalfont St. Giles, 2 days after the northern tunnel TBM (Florence) had passed under the river, and on the day that the southern tunnel TBM (Cecilia) did so. The water level fell by 13mm in just over an hour. A site visit on 4th May identified a 'depression' under the road bridge, above the northern tunnel. The northern TBM was 76m beyond the bridge at this time (para 2.1.5).

The depression was filled on 6th May with clay and chalk gravel (1.1.3). 9 further drops in level (of lesser magnitude) were recorded over the next 3 months (Figure 6). However, no corresponding effects are observed at monitoring locations up or downstream of the bridge. The report concludes

5.1.2 The recorded sudden drop in river level is likely to be either anomalous readings collected by Align or local effects within the river that are not linked to TBM operation. The data do not indicate that a sustained change in river flow characteristics has occurred to the River Misbourne as a result of the TBM crossings.

Comments

• The 'anomalous readings' explanation is improbable, as two separate systems recorded similar traces on 5 occasions (Figure 5).
• The riverbed under the bridge & immediately above the tunnel was not inspected before the arrival of the TBM (1.1.2), so the depression may have been a pre-existing feature.
• The fluctuations coincided with the passage of the 2nd TBM under the river, some 20m away and did not cease after the depression was filled; this also suggests that the depression was not the primary cause of the fluctuations
• The tunnel is only 17-20m below ground level; ground movements in the range of 20-30mm were expected, though the maximum observed was 3.32mm (wrt what datum ? 20 microns is impressively accurate) (4.1.9)
• Data from the Align gauges was not available before Feb 2022 due to equipment failure (2.1.2). Does the March - April data (not shown) show sudden drops ? Presumably not, as this would be strong evidence for a pre-existing condition.

It would appear that the hydrodynamics of the river & aquifer are poorly understood, despite the monitoring equipment put in place. The cause of the fluctuations has not been identified, and it is not clear if the remedial works carried out were necessary or effective. Preparations for the TBMs passing under the Misbourne (bridge inspection, equipment failure) were inadequate, and, as subsequent experience has shown, had a significant dissolution feature been encountered then there would have been a serious impact on the river.

Request B - Subsidence at an unmapped landfill

This incident isn’t related to HS2 activities. It was noted in the report as an indication of potential contaminant sources that we detected in our monitoring boreholes.

Request C - Unreported ground movements

Just the two we know about - Shardeloes & Hyde Heath

Request D - Geological Faults

44 faults identified, 9 identified by the TBMs of which 2 resulted in ground movements.

Conclusions

Would this route and tunnel design have been selected, if the ground investigation data had been available ? The presence of 44 faults in approx. 12km of tunnel suggests that the Misbourne was at high risk, particularly since the river tends to flow above pre-existing fault lines.

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