Tooting Broadway Station Geological Issues – Background information

At the beginning of 2015 Transport for London (TfL) engaged design consultants to refine the Crossrail 2 scheme and in so doing validate some of the early assumptions through more in-depth analysis.

This exercise raised concerns relating to the ground conditions at Tooting Broadway. TfL considered this advice and carried out further research leading to the production of a report (‘Tooting Broadway Station Geological Issues’). This report and associated research data was subsequently independently peer reviewed by a recognised industry expert.

The report identified the potential methods of mitigation for constructing in such conditions and an early indication of cost, programme and risk impacts. Engineering activity is ongoing to develop the alternative Balham scheme and this activity will be supported by targeted ground investigation. This will enable a full comparative assessment between Tooting Broadway and Balham to be prepared. This work cannot be concluded until the ground investigations are completed early in 2016.

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Executive Summary Recent geological desk studies have been carried out around the Tooting Broadway area, in connection with planning for Crossrail 2 tunnelling works. These studies have shown that there are significant faulting zones in the horizon where Crossrail 2 plan to construct both TBM (Tunnel Boring Machine) and SCL (Sprayed Concrete Lining) driven tunnels.

The desk study has also indicated that along with faulting, there are likely to be high water levels and pressures, implying that SCL constructing will attract more risk and be more costly to the project.

The provenance of the geological information that we currently have for the area around Tooting Broadway, comes from historical ground investigations obtained from the British Geological Survey Onshore Geoindex.

The historical data indicates that the depth to the base of the London Clay varies from 8.5 mbgl (metres below ground level) (+6.5 m AOD) (metres Above Ordnance Datum) to 30.48 mbgl (-14.48 m AOD) in the Tooting Broadway area. Northwest of the Wimbledon Fault the base of the London Clay varies from 70.41 mbgl (-62.91 m AOD) to 97 mbgl (-74.44 m AOD). The top of the Chalk appears to be relatively shallow in the region of Tooting Broadway with depths between 33.00 and 57.00 mbgl. Where levels relative to OD were able to be calculated the top of the Chalk varies from -19.51 to -39.78 m AOD at Tooting Broadway. To the northwest of the Wimbledon Fault the top of the Chalk is at greater depths of 101 (-92.81 m AOD) to 109 mbgl (-86.44 m AOD). It should be noted that the existing ground levels at Tooting are approximately +16 m AOD. The crown of the Crossrail 2 station tunnels are approximately 28 mbgl.

A number of construction options for the station tunnels have been looked at. Two potential options are favoured:

- Sprayed Concrete Lining (SCL) in combination with ground freezing

- Large diameter Earth Pressure Balance (EPB) Tunnel Boring Machine (TBM), utilising specialist face conditioning

Both approaches would require large boxes to be constructed at each end of the station.

In terms of programme, the following has been determined:

- Using SCL with ground freezing will add approximately 74 weeks on to the base programme, including 20 weeks on critical path plus risk

- Using a large diameter would add approximately 120 weeks on to the base programme plus risk (121 weeks delay on 1st TBM run through; 76 weeks on 2nd)

The base programme assumes a station constructed using SCL in favourable ground conditions (favourable ground conditions would be for SCL tunnels constructed predominately in London Clay).

In terms of cost, the following has been determined:

- Using SCL with ground freezing will add approximately £250m to the base cost.

- Using a large diameter would add approximately £330m to the base cost.

Both approaches would see risk and contingency increased. This is assumed as: risk=50%, contingency=30%.

In round numbers, either solution approximately doubles cost and adds significant risk and contingency.

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Contents Executive Summary ............................................................................................................. 2 Contents .............................................................................................................................. 3 1 Introduction ................................................................................................................... 4 2 Crossrail 2 Route .......................................................................................................... 5 3 Geology through Wandsworth ....................................................................................... 8 4 Tooting Broadway ....................................................................................................... 11

4.1 Why is the fault a problem? ......................................................................... 11 4.2 Could the Base Case be constructed without mitigations? .......................... 11 4.3 Could the Base Case be constructed with mitigations? ............................... 12 4.4 Implications for Cost, Programme, Environmental Impact and Risk ............ 13

5 Alternative Alignments ................................................................................................. 14 6 Conclusions and Recommendations ........................................................................... 17 7 References .................................................................................................................. 18 Appendix A: Technical Memo ............................................................................... 19 Appendix B: Decision Tree .............................................................................................. 20 Appendix C: Programme ................................................................................................. 22

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1 Introduction This report has been written to highlight the geological issues that exist in the Tooting Broadway area of London, with respect to the proposed Crossrail 2 alignment and poses the question, should Crossrail 2 be concerned with these geological issues?

The report also looks at whether an alternative alignment via Balham could be feasible.

The scope of the report therefore covers:

- Route alignment

- Geology

- Construction methodology

- Risk

- Cost

- Environmental impact

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especially during peak hours. Wimbledon and Clapham Junction are effectively, ‘fixed points’ on the existing Crossrail 2 alignment. From inspection of Figure 2 it therefore leaves only Tooting Broadway and Balham as viable stations stops on the Northern Line in this location.

Consideration was also given to taking the Crossrail 2 alignment to Tooting Bec. Unfortunately, as with Tooting Broadway, Tooting Bec also appears to be on the south eastern side of the fault. Tooting Bec also has fewer connection opportunities which would have a negative effect on BCR and thus the project business case.

The Base Case (Figure 1) envisages that the running tunnels between Wimbledon and Clapham Junction would be constructed using EPB TBMs. Station platforms, passageways and escalator barrels would be constructed using SCL (Sprayed Concrete Lining). The SCL work assumes that the station platform tunnels are constructed in favourable ground conditions (i.e. London Clay).

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3 Geology through Wandsworth The Crossrail 2 route section between Wimbledon and Clapham Junction predominately passes through the London Borough of Wandsworth.

During the Base Case route development, had determined the anticipated the geology through Wandsworth as generally being:

- Superficial deposits and Made Ground

- London Clay

- Lambeth Group

- Thanet Sand Formation

- Upnor Formation

- Chalk Group

The interpretation of the geology was based only on published geological maps and s own knowledge of the ground conditions in the London area. This interpretation was

plotted on geological plan and profile drawings.

At Tooting Broadway station, the interpretation did show that there was a thinning of the London Clay, when moving northwards along the alignment (approx. thickness at the south end of 40m; north end 20m). It also picked up that the top surface levels of the Lambeth Group and Thanet Sand were also increasing. Table 1 lists the top surface levels: Strata Level at south end of Tooting

Broadway station Level at north end of Tooting Broadway station

Ground Level 119 115

London Clay 115 111

Lambeth Group 75 85

Thanet Sand 52 63

Table 1 Approximate surface levels for London Clay, Lambeth Group and Thanet Sand at Tooting Broadway (Levels in metres Tunnel Datum (mTD) = Ordnance Datum +100m)

It should be noted that the length of the Crossrail 2 station is currently assumed as 250m.

didn’t highlight any particular issue of faulting through the Wandsworth area.

London Underground carried out a peer review in late 2014. This review drew attention to change in strata level at Tooting (Table 1), as well as the severe inundation that was experienced during the construction of Stage 1A of the Thames Water Ring Main (TWRM), as particular risks to tunnelling in the Tooting Bec area.

have since undertaken their interpretation of the geology around Tooting (Appendix A: ). They have used a range of resources including The British Geological Survey (BGS) Onshore Geoindex and papers by Newman (2009) and Clarke & Mackenzie (1994).

have superimposed their geological interpretation on to the plan and profile drawings. These are referred to Appendix A: .

It is also noted that there are very few tall buildings in this area (implying few buildings with deep foundations). Most of the buildings are low rise. The LU Northern Line tunnels are relatively shallow, with the crown levels at Tooting Broadway Station being only 7-8m below existing ground level. These tunnels were built predominately through the London Clay.

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would intercept the chalk. It would be however, more likely that the tunnels would be in the Thanet Sand and Lambeth Group.

Figure 4 shows a sample of the geology experienced during the construction of the TWRM. This illustrates similar, sudden changes in the interfaces between strata, which can, to some extent, also be inferred from Figure 3 along the Wimbledon Fault. When tunnelling through this type of strata, this would lead to sudden changes in ground conditions, groundwater levels and pressures. Figure 4 also depicts high water levels associated with the Streatham Fault. Available information suggests that water pressure of around 2 Bar could be experienced at Crossrail 2 tunnel level.

At this time Crossrail 2 has not yet committed to undertaking any ground investigation. This means that there is still some uncertainty and risk as to where we believe the Streatham and Wimbledon Faults are. From published information on the faults, we are therefore allowing for a 200m envelope either side of the Wimbledon Fault.

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4 Tooting Broadway 4.1 Why is the fault a problem?

- The fault is a serious risk to the project in terms of construction (short term) and maintenance (long term).

- Design risk: tunnels might have to be designed for seismic loading (how active are the faults?), not enough sumps/pump locations, under specified pumps, joint details in linings underspecified, Lining thickness under/over specified.

- Construction risk: high groundwater pressures, mixed ground conditions, running water (inundation) and sand, excessive ground movement.

- Maintenance risk: continued water and sand/silt ingress from the fault, additional maintenance to the tunnels, tunnels act as a conduit through faults – continuous water ingress, ongoing movement of the fault could damage tunnels.

- It is not visible from ground level. There a few if any clues at all.

- Position has only been inferred from historic deep wells and ground investigations for other reasons (e.g. TWRM) – there hasn’t been a focused effort to locate the fault.

The principal concern is that there will be running sand in the face during construction. This is incompatible with SCL tunnelling.

4.2 Could the Base Case be constructed without mitigations? Given, assumptions of:

- Best estimate ground conditions (i.e. a mixed face Lambeth Group, Upnor Formation, Thanet Sand and Chalk (possibly))

- High water pressure

- A very high likelihood of running water/sand construction of SCL station tunnels

In these ground conditions, the construction of SCL station tunnels (assume ‘diameters’ > 11m) is not considered feasible unless significant additional mitigations are employed.

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4.3 Could the Base Case be constructed with mitigations? Appendix B: contains a decision tree that is initial appraisal of a range of construction approaches. This appraisal is summarised in Table 2: Excavation System Lining Issues

(1) Open face tunnelling

(1A) No Ground Treatment SCL / Segmental Stability of excavation

(1B) Grouting / Surface ground improvement SCL Stability of untreated pockets, Surface intrusion

Programme

(1C) Dewatering SCL / Segments Surface Intrusion, Magnitude of drawdown, Some water trapped

Stability, Where to Discharge to? Programme

(1D) Freeze SCL / Segments Surface intrusion, Tunnel Safety Heave, Programme

(2) Closed face TBM

(2A) EPB Segments Mixed face management, Impact running tunnel progress, Programme, Spoil Conditioning, Water pressure

Headwall, Power, Cutterhead retrieval, Programme, Cross passages

(2B) Slurry Segments Spoil tanks - big space – Landtake, Water pressure, Cross passages, Settlement control, Power

Programme, Running tunnel impact, Headwall, Cutterhead retrieval

(2C) Multiple Small Drives Cast in Place Programme, Running tunnel impact Headwall, Cross passages

(3) In tunnel ground treatment

Open face SCL / Segments Programme Uncertainty of success

(4) Open cut Diaphragm Wall Property take, Base heave EI, Space for spoil and Bentonite farm

Table 2: Summary of likely construction methodologies

To build SCL tunnels in the ground conditions described above, major mitigation measures would be required. Mitigations could take the form of:

- Ground freezing

- Compressed air

We are currently assuming that the possibility of high permeability in the chalk and high water pressure at the base of the shafts/boxes. This situation would overwhelm a dewatering system. Additionally, there isn't a suitable water course adjacent to the site for extracted water to be drained into3. However, in real terms, this would mean that Crossrail 2 would be depressurising the chalk aquifer. During the construction of Stage 1A of the TWRM, an attempt was made to depressurise the chalk aquifer. This pumping test was ultimately unsuccessful. We are also currently assuming that the variable nature of the soil and an assumed large area at ground level for surface access would be too great for a grouting solution.

3 It might be possible to take the extracted water, if it is potable and sell it back to Thames Water. However, setting up a system to do this could be very disruptive and expensive.

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4.4 Implications for Cost, Programme, Environmental Impact and Risk Table 3 provides a crude estimate for ground freezing and the use of large diameter TBM, when compared to SCL.

The key points are an increase in cost and programme duration. The key environmental impact would be that the excavated material removed from site would be very wet. See Appendix C: for a summary programme.

Cost Programme Environmental Impact Risk

SCL Base case £400 Million Base case excavated material removed by tunnel

Not buildable4

2 x boxes/shafts & TBM Base case + £330 Million + risk + contingency Say £600 Million extra

Base case + 120 weeks (+121 weeks to overall critical path)

Wet excavated material removed from site by lorry

Wet box/shafts digs Settlement (buildings, Northern Line Tunnels) Long term sand and water leaks

2 x boxes/shafts & Freeze Ground

Base case + £250 Million + risk + contingency Say £450 Million extra

Base case + 74 weeks (+20 weeks to overall critical path)

Wet excavated material removed from site by lorry

Wet box/shafts digs Heave Long term sand and water leaks

Table 3: Summary of estimated differentials to Cost, Programme, Environmental Impact and Risk

4 Not buildable unless major mitigation measures are put in place.

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5 Alternative Alignments As mentioned previously in Section 2, an exercise to restate the business case for Crossrail 2 has been recently undertaken by the project team.

As part of this exercise, additional route options were considered. A route that avoided Tooting Broadway and went via Balham was looked at. These alignments are depicted in Figure 5. It should be noted that both Tooting Broadway and Bec are located on the south east side of the faults. Balham appears not to be. Thoughts of possible alignments to Colliers Wood or Clapham South were ruled out, as a feasible alignment from Wimbledon to Clapham Junction would not be possible (i.e. Colliers Wood is too far south and Clapham South is too far north). See Figure 6 below.

From Figure 5, four additional route options were investigated. All four routes used Wimbledon and Clapham Junction as ‘fixed points’ from the Base Case alignment string. The orientation of these stations was fixed, relative to the Base Case. This was to maintain the chosen worksites. The only nuance to this was the red route option, which changed the orientation of Clapham Junction Station. The other three route options maintained the Base Case orientation of Clapham Junction.

At Balham, two route options (red and orange) saw the station tunnels oriented in a north east – south west direction. The yellow and blue route options oriented the station tunnels in a north west – south east direction.

The absolute minimum radii for horizontal curvature, was set at 505m.

The red route was rejected outright as its orientation did not coincide with the selected worksites.

The other three routes were assessed and were found not to provide as much Passenger Transport benefit, as the route via Tooting Broadway.

Figure 2 is a CAD version of the sketch in Figure 5.

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6 Conclusions and Recommendations Conclusions:

1. Available information indicates geological issues in the Tooting area of London.

2. The geological issues take the form of faulting, potentially unstable soil and high water levels and pressures.

3. These issues would present severe difficulties for the construction of the proposed SCL tunnels for Crossrail 2, without mitigations.

4. Constructing Crossrail 2 through this geology with mitigations could be possible. However, these mitigations would add significant cost and programme duration. Some of the risk would not be entirely alleviated (environmental impact would be significant and whole life asset risk of tunnel in wet Thanet Sand).

5. Re-routing Crossrail 2 from Tooting Broadway to Balham likely to have little BCR impact and would still provide key benefit of a Northern Line interchange.

Recommendations:

1. Carry out further desk study into the geological issues at Tooting.

2. Undertake targeted ground investigation in the Tooting and Balham areas. Try and identify where the faults are. Report factually and interpretively.

3. Carry out ongoing groundwater monitoring of the ground investigation area.

4. Seek review of construction methods.

.

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7 References - The British Geological Survey (BGS) Onshore Geoindex

(http://www.bgs.ac.uk/geoindex/)

- Newman, T. 2009. The impact of adverse geological conditions on the design and construction of the Thames Water Ring Main in Greater London, UK.

- Clarke, R.PJ. & Mackenzie, C.N.P. 1994. Overcoming ground difficulties at Tooting Bec.

- Tooting and Balham scanned source material

- Management of the London Basin Chalk Aquifer 2015, Environment Agency

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Appendix A: Technical Memo

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Figure 2. Extract of Geological Model of the Ring Main Tunnel beneath Tooting Bec (Ref. 2).

In conclusion, it may be expected that the ground conditions at a proposed Balham Station station will be

better than at Tooting Broadway as the surface of the Chalk and groundwater levels are deeper.

Notwithstanding, the exact location of the Wimbledon Fault is unknown but expected to lie just south of a

station at Balham depending on its location. There is likely to be a significant groundwater variation across

the fault making tunnelling conditions variable.

References:

1. British Geological Survey. 1998. South London. England and Wales Sheet 270. Solid and Drift

Geology. 1: 50 000.

2. Newman, T. 2009. The impact of adverse geological conditions on the design and construction of the

Thames Water Ring Main in Greater London, UK.

3. British Geological Survey, 2015. Onshore Geoindex. Available at: http://www.bgs.ac.uk/geoindex/.

4. Environment Agency, 2015. Management of the London Basin Chalk Aquifer. Status Report 2015.

Page 4

Appendix A

Balham and Tooting Broadway Top of Chalk Plan

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Appendix B: Decision Tree

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Appendix C: Programme