TECHNICAL PAPER Short- and long-term tunnelling- induced settlements at Station Eyre Hover, Sotiris Psomas and Colin Eddie, UNPS

ABSTRACT In a forever changing world, urban environments are becoming gradually more congested – both by increasing populations and increasing infrastructure. e underground space is no exception, with basements, foundations, pipes, utilities and tunnels among the structures competing for room. While the eect of any new construction on its vicinity can be reasonably predicted in the short-term, methods for doing so for the long-term are less well dened. is paper presents an investigation into the tunnelling- induced short- and long-term settlements measured over the Whitechapel Station sprayed concrete lining (SCL) tunnelling works between 2012 and 2014, and a tentative prediction of the future ground behaviour. A novel approach to analysis has been used owing to the long-term interaction between the two parallel station tunnels and this method is proposed for predicting such behaviour at similar sites in the future. It was found that settlements and slopes increase over time at a logarithmically decreasing rate, and rapidly can FIGURE 1: Whitechapel Station no longer be described by a Gaussian curve. e long- term eects are dependent on the number of tunnels and their spacing, the permeability of the lining, and the A107 material surrounding the excavation.

Vallance 1.0 INTRODUCTION Road Vallance 1.1 THE PROJECT Gardens Construction works for Crossrail’s new Whitechapel Whitechapel Station and Tunnels (C510) started in 100 m A11 April 2011, with the aim of easing commuter congestion on the underground network and providing additional interchanges to existing lines by 2018.

30 GROUND ENGINEERING December 2015 FIGURE 2: Theoretical additional settlements above a At Whitechapel, two parallel platform tunnels were excavated using SCL techniques as were the associated single tunnel caused by an increase in the trough works, including tunnel boring machine launch chambers, width parameter and the maximum settlement cross passages and shas ( gure 1). A pilot-enlargement Offset from tunnel axis (m) sequence was used for most of the tunnels, whereby -50 -40 -30 -20 -10 01020304050 a smaller 6.5m diameter tunnel was excavated and 0 lined rst, followed by the full excavation of the 11m 4 diameter tunnels. is had the aim of reducing the overall 8 settlement caused by the excavation works. Short-term trough 12 An extensive surface and in-tunnel monitoring Long-term trough programme was put in place, including manual and Settlement (mm) 16 Difference automated readings of road studs, extensometers, 20 inclinometers, prisms and targets on buildings. Q

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Q Data was collected at regular intervals (weekly to FIGURE 3: Theoretical additional settlements for fortnightly) prior to the works to determine the baseline twin tunnels at a spacing of (a) 15m, (b) 36m and (c) 55m ground movement at the start of tunnelling works in uence. e frequency was increased to daily during (a) 15m Offset from 1st tunnel axis (m) SCL works, and reduced to lower frequencies thereaer. -50 -40 -30 -20 -10 0102030405060708090 100 e predicted short-term volume losses at Whitechapel 0 were applied to a model of the site in Oasys Xdisp and 10 the expected settlements were assessed. One zone of Short-term trough compensation grouting was planned as a result, to the 20 Long-term trough West of Brady Street. Settlement (mm) Additional settlement 30 e C510 project has involved four main contractors Tunnel location (not to scale) – , BeMo Tunnelling, Morgan Sindall and Vinci Construction, known as the BBMV joint venture. (b) 36m -50 -40 -30 -20 -10 0102030405060708090 100 1.2 SETTLEMENTS ABOVE TUNNELS 0 Short-term tunnelling-induced ground movements in 10 soils are well understood and typically describe a Gaussian curve which can extend dozens of metres away from the 20

tunnel alignment (Peck, 1969). e magnitude of the Settlement (mm) settlements depends on the type of ground around and 30 above the tunnel, the tunnelling method and a number of other factors. (c) 55m Long-term ground movements have been observed in -50 -40 -30 -20 -10 0102030405060708090 100 many tunnelling projects in clays, including the Jubilee 0 Line Extension (Burland et al, 2001), and while these may 10 be due to a number of issues, they are generally caused by tunnels acting as drains. Concrete is a material with 20

a low hydraulic conductivity, but it oen has a mass Settlement (mm) 30 permeability several orders of magnitude higher than its material permeability. Its relative permeability (RP) to the surrounding ground can be described by equation 1, and governs the extent to which consolidation settlements may occur. FIGURE 4: Components of long-term volume loss curve-fit EQUATION 1 Offset from 1 axis (m) -50 -40 -30 -20 -10 0102030405060708090 100 RP = k1Cclay Solution t1ksoil not valid where: Solution T1US T2US not valid k1 mass permeability of tunnel lining (m/s) Tunnel 1 -3i Valid P+3i Cclay depth of clay cover above tunnel crown (m) 1 solution 2

Settlement (mm) Tunnel 2 t1 thickness of tunnel lining (m) kh kv Total ksoil soil equivalent isotropic permeability = kh soil horizontal permeability kv soil vertical permeability Wongsaroj et al (2013) concluded from a series of For a single tunnel in the long-term, the trough nite element tests that if the RP was inferior to 0.1 width and the peak settlement are expected to increase. (approximately equating to the lining permeability being Even if the short- and long-term settlements are three orders of magnitude lower than the soil), the tunnel approximately Gaussian curves, the dierence between lining could be considered impermeable, while if the RP these – the additional settlement – is not ( gure 2). A was superior to 100, the lining would be fully permeable. zone surrounding the centreline of the tunnel experiences It is generally accepted that in the long-term, the uniform additional settlement, while the slopes at the edge settlement trough widens and attens (Mair and Taylor, of the curve increase due to dierential settlements. In 1997). Neither the total nor the additional long-term twin tunnel situations, the spacing between the tunnels settlements describe a Gaussian curve. In this paper, the will in uence the shape of the combined additional short-term is dened as the period between the start of settlement trough, from an almost Gaussian curve for in uence of the pilot tunnel on a given monitoring array, tunnels in close proximity, through a uniform settlement to the end of in uence of the enlargement. e in uence similar to that in gure 2, to independent individual of the tunnel is taken as the position of the face ±1.5 curves as in gure 2, for tunnels with a large spacing in tunnel axis depths to the tunnel alignment. between ( gure 3). Q

32 GROUND ENGINEERING December 2015 TECHNICAL PAPER

Q 2.0 ANALYSIS OF SHORT- AND LONG-TERM SETTLEMENT MEASUREMENTS FIGURE 5: Monitoring arrays (road studs) at Whitechapel 2.1 CURVE FITTING FOR SHORT-TERM Brady St Monitoring array SETTLEMENTS Combined array Gaussian curve tting can only be applied to data Kempton Court collected in the short term, for a single tunnel not aected by neighbouring excavation works. At Whitechapel, both single and double tunnel sections are being constructed. In the latter case, the individual contributions of each Vallance Road tunnel to the overall settlements measured along the array must be determined. For this reason, a modi ed method Castlemain St for double-Gaussian tting in the long-term is proposed. 100 m In the short term a double Gaussian curve – superimposing the single Gaussian curves of both tunnels – was tted to the settlement measurements (equation 2), FIGURE 6: Settlement trough at Vallance Road array resulting in a high degree of t (R2>0.99). Each parameter in the equation was either pre-set as a known value Offset from tunnel axis y (m) (prede ned constant, eg the known depth of the tunnel) or -50-60 -40 -30 -20 -10 0102030405060 0 le for the soware to nd the best t (varied constant, eg 5 the individual trough widths). 10 It was not assumed that the tunnels would produce 15 identical responses despite having the same depth and 20 diameter (same trough width, same maximum settlement, Settlement (mm) End of short term, curve fit 25 etc), so both sets of parameters were set as independent End of short term, measured from each other. 30 is type of curve tting would have been extremely inecient in spreadsheet format, so this was done using the Matlab curve tting toolbox. underestimated the measurements by up to 4mm. e points where the settlement trough had tended back to EQUATION 2 zero in the short-term had increased in settlement, and further points did not tend back to zero in a Gaussian 2 2 Sy = Smax,1 x exp (y–P1) + Smax,2 x exp (y–P2) manner (see section 3.1). - 2 - 2 (2(K1z1)) (2(K2z2)) e equation was therefore modi ed to represent a double Gaussian with a uniform oset (Δ) from the original ground surface (equation 3). e t was good Where (R2>0.99), with points at a distance just beyond ±3i Sy settlement at y (dependent variable) overestimated by less than 0.5mm. Smax, n maximum settlement of nth tunnel (varied constant) is solution de nes the long-term settlements as y transverse distance from comprising two elements: one describing a Gaussian, rst tunnel axis (independent variable) added to one applying a uniform settlement to the entire Kn trough width parameter zone within ±3i (i=Kz) of the tunnel axes. of nth tunnel (varied constant) While this may not be the exact mode of settlement, th zn depth to n tunnel level (prede ned constant) it describes the long-term behaviour within ±3i of the Pn distance between peak of Gaussian of nth tunnel tunnel axis much more adequately than a Gaussian and rst tunnel axis (varied constant) solution, and this solution is only valid within ±3i of the tunnel axes. Since this is the zone of maximum slope, this rough equation 2’s parameters, it is possible to separate area may be sucient to quantify the long-term behaviour the individual contributions of each tunnel. of the ground. It was observed that over time, the maximum settlement associated with each tunnel migrates towards EQUATION 3 the other, hence the decision to leave P as a varied, 2 2 rather than as a prede ned constant. is suggests an Sy = Smax,1 x exp ( y–P1) + Smax,2 x exp (y–P2) + Δ - 2 - 2 amount of interaction between the tunnels which results ( 2(K1z1) ) (2(K2z2)) in a settlement trough which is more than a simple superposition of two Gaussian curves. Where 2.2 CURVE FITTING FOR LONG-TERM Sy settlement at y (dependent variable) SETTLEMENTS Smax, n maximum settlement of In the long-term it was observed that while the behaviour nth tunnel (varied constant) was still mostly Gaussian, the t to equation 2 was y transverse distance from poor at the edges of the trough, where the tted curve rst tunnel axis (independent variable) Q 34 GROUND ENGINEERING December 2015 TECHNICAL PAPER

Q FIGURE 7: Settlement trough at Castlemain Street array Kn trough width parameter of nth tunnel (varied constant) Offset from tunnel axis y (m) zn depth to nth tunnel level (prede ned constant) -80 -60 -40 -20020 40 60 0 Pn distance between peak of Gaussian of nth tunnel 5 and rst tunnel axis (varied constant) 10 Δ uniform settlement (varied constant) 15 End of short term, curve fit 20 Long term (79 days), curve fit Settlement (mm) End of short term, measured For the long-term, each tunnel’s contribution (C) to the 25 Long term, measured uniform oset Δ cannot be known, but can be assumed by 30 the ratio of its trough width parameter to the sum of both, since the tunnels are similar (equation 4). FIGURE 8: Combined and individual settlement troughs EQUATION 4 at Kempton Court/Brady Street array with data zeroed at:

Cn (%) = 100 x Kn K1 + K2 Total settlement - short term PTE contribution - short term Total settlement - long term PTE contribution - long term With Short term, measured PTW contribution - short term th Kn trough width parameter associated to n tunnel Long term, measured PTW contribution - long term Non offset Gaussian fit - long term

2.3 A MODIFIED USE OF XDISP FOR (a) Start of influence from PTE Offset from PTW, y (m) CALCULATING LONG-TERM SETTLEMENTS 0--40-5 30 -20 -10 0103020 40 50 60 70 80 90 100 During the design phase, volume losses and associated 0 trough width parameters can be applied to a model of 5 10 the ground surface above the tunnel in soware such as 15 Xdisp, to assess the predicted short-term settlements and 20 the eect on the buildings above. is soware works 25 by applying purely Gaussian distributions to the model Settlement (mm) 30 35 geometry, and cannot process non-Gaussian solutions. 40 e following method has been devised to combine 45 Xdisp analyses with long-term settlement trends to 50 surmount this limitation, for a double tunnel situation. By separating the non-Gaussian combined settlement (b) Start of influence from PTW Offset from PTW, y (m) trough area into four components: 0--40-5 30 -20 -10 0103020 40 50 60 70 80 90 100 0 5 G 10 Tunnel 1 uniform settlement (T1US), contributing an area of : 15 20 ( ) C1 Δ x ( P2 - P1 + 3i1 + 3i2) 25

Settlement (mm) 30 G Tunnel 2 uniform settlement (T2US), contributing an 35 area of : 40 45 C2 Δ x ((P2 - P1) + 3i1 + 3i2) 50 G Tunnel 1 Gaussian settlement, contributing an area of (red, in gure 4): e additional settlements due to long-term eects may

Smax,1 x i1 2π be assessed by: a) Applying the appropriate short-term volume losses G Tunnel 2 Gaussian settlement, contributing an area of to each of the tunnels in Xdisp to obtain the short-term

Smax,2 x i2 2π settlement contours b) Calculating the long-term volume losses and applying these to the Xdisp model to obtain the long-term contours, e Gaussian volume losses and associated trough width to which the combined vertical osets must be added parameters may be applied to an Xdisp model, while the c) Calculating (b)-(a) uniform settlements may be added to the entirety of the points using Matlab or a spreadsheet. It must be noted that 3.0 RESULTS this solution is only valid for a zone within -3i1 of the axis 3.1 MEASURED SETTLEMENTS AND VOLUME of tunnel one and +3i2 of the axis of tunnel two. Beyond LOSSES this, the settlements will eventually tend to zero, but the e curve tting method outlined in the previous section rate and distance over which this occurs is unknown. was applied to the settlement data taken from road Q

36 GROUND ENGINEERING December 2015 TECHNICAL PAPER

Q stud measurements (expected accuracy: ±1mm) along FIGURE 9: Logarithmic long-term increases in: two monitoring arrays and a combined monitoring array running along Kempton Court and Brady Street ( gure 5). Tunnel 1, VL Tunnel 1, Gaussian only e southern end of Brady Street array overlies an area Tunnel 2, VL Tunnel 2, Gaussian only aected by compensation grouting and would therefore not (a) Volume loss measure the virgin ground response. 0.6 (%) A small discontinuity can be seen between both arrays, 0.5 but this does not signicantly aect the curve tting. Curve tting was carried out at the end of short-term 0.4 and at a number of points during the year following the 0.3 end of construction, in the case of the combined array. e results are shown in gures 6 to 8 and the calculated volume 0.2 losses are included in table 1. Where long-term values 0.1 are provided, their individual Gaussian-only (GVL) and end of construction, Δ V, 0

uniform (ΔVL) components are given. For the array over Additional volume loss since the -0.1 both tunnels the data was zeroed at the start of short-term 0 50 100 150 200 250300 350 400 inuence for each tunnel separately since the tunnels were Time since end of construction (days) not driven simultaneously. Tunnel 1 Tunnel 1, Gaussian only 3.2 EXTENDING THE DATA: PREDICTING FUTURE Tunnel 2 Tunnel 2, Gaussian only SETTLEMENTS (b) Maximum settlement for both total and e changes in K, Vl, Δ and Smax were assessed over Gaussian component of volume loss time at the combined array, and agreed with the logarithmic 12 decrease observed in the literature (Wongsaroj et al, 10 2013). For both tunnels underlying the combined array,

the increases were very similar, despite the dierences in Δ Smax (mm) 8 the short-term response. e increases in volume loss and maximum settlement are presented in gure 9. 6 e dierence between the total volume loss and the 4 GVL is caused by the vertical oset and encompasses the rectangular areas T1US and T2US in gure 4. e evolution 2 of Δ was therefore back-analysed mathematically through Additional maximum settlement 0 known values of i. e increase in Δ was found to be 0 50 100 150 200 250300 350 400 logarithmic, as would be expected from the dierence of Time since end of construction (days) two logarithmic curves.

Table 1 – Volume losses and trough width parameters determined by curve fitting

Combined Array, tunnel Combined Array, tunnel Castlemain Street Vallance Road 1 (PTW) 2 (PTE)

S S S S Vl (%) K (GVL) max Vl (%) K max Vl (%) K (GVL) max Vl (%) K (GVL) max (mm) (mm) (mm) (mm)

End of short 1.04 0.491 20 1.13 0.540 24 1.09 0.496 34 1.29 0.555 36 term

1.28 1.37 1.48 End of study (GVL) (GVL) (GVL) 0.548 24 - - - 0.534 45 0.560 45 latest data + 0.14 + 0.25 + 0.28 (ΔVL) (ΔVL) (Δ VL)

Date of latest +79 days n/a +376 days +359 days data

38 GROUND ENGINEERING December 2015 FIGURE 10: Logarithmic increase in: e rate of increase in the maximum settlement was also derived from these relationships (in gure 11, the (a) Δ By subtraction Measured continuous line represents the portion of the curve 6 illustrated in gures 9 and 10, while the dashed line shows the extrapolation to 120 years). , (mm)

Δ 5 3.3 APPLYING THE PREDICTED SETTLEMENTS 4 TO THE WHITECHAPEL TUNNELS – IMAGINARY 3 SCENARIO WITH NO SECONDARY LINING OR WATERPROOFING 2 e results of this analysis were used to predict the 1 future settlements at Whitechapel. is assumes that no waterproong measures are put in place, which obviously

Uniform settlement offset, Uniform settlement offset, 0 is not the case in reality as waterproong forms part of the 0 50 100 150200 250300 350400 full lining design. e logarithmic trendlines were used Time since end of construction (days) to determine the trough widths and volume losses at 10 and 120 years, and the settlement contours were generated (b) Determined by two independent methods 0.565 using Xdisp and post-processed using Matlab. e results of the analysis are plotted as contours in 0.560 gures 12 and 13, for settlements predicted to occur if no secondary lining is put in place. As can be expected, the 0.555 majority of settlements occur in the short and medium 0.550 term.

0.545 ough width parameter, K ough width parameter, 4.0 APPLICATION Tr 0.540 e behaviour observed at Whitechapel depends on a number of factors, including the depth of the tunnel, the 0.535 ground conditions (strata, materials etc), the method of 0 50 100 150200 250300 350400 tunnelling and the construction geometry. e trends Time since end of construction (days) derived in this study cannot be directly applied elsewhere. e general method, however, is proposed as a starting point for a study into long-term consolidation settlements e values of Δ which were previously determined by in the Clay, and highlights the need for collecting curve-tting showed a good degree of t to the trendline long-term data. ( gure 10a). K was also found by back-analysis and despite It is important to stress that data trends must be noise in the measured data, the values determined by both extrapolated with a lot of caution: this method uses methods were similar ( gure 10b). logarithmic trendlines to dene the long-term ground e logarithmic trendlines for K, Vl, Δ and Smax were behaviour around tunnels. projected 120 years into the future, to speculate about the While this is based on observations published in the additional settlements within the tunnels’ lifetime, if no literature, no extensive long-term (100 years+) study of additional waterproong measures were to be put in place. ground movements exists to support this assumption. Q

FIGURE 11: Extrapolation of consolidation behaviour observed at Whitechapel after end of short term

0.9 5.0 0.07 18

0.8 4.5 16 0.06 0.7 4.0 14 Increase in K Increase 3.5 0.05 0.6 12 Δ per tunnel (mm) 3.0 0.5 0.04 10 2.5 0.4 0.03 8 2.0 0.3 Rate of settlement (mm/year) 6 1.5 0.02

Additional GVL (percentage points) Additional GVL (percentage 0.2 1.0 4 0.01 0.1 0.5 2 0 0 0 0 0 40 80 120 0 40 80 120 0 40 80 120 0 510 Time since end of short term (years) Time since end of short term (years) Time since end of short term (years) Time since end of short term (years)

December 2015 GROUND ENGINEERING 39 TECHNICAL PAPER

FIGURE 12: Long-term (10 years) additional assessed using Xdisp, while a uniform component was added settlement contours at Whitechapel using MATLAB. An increase in settlements and slopes is expected over the entire site. Settlement (mm) is study has highlighted the need for a good 10 18 26 12 20 28 understanding of the long-term eects of tunnelling in clay, 14 22 30 since these can be expected to occur for a number of years 16 24 aer the end of construction. Depending on the tunnel layout, type of ground and the tunnel lining properties, the additional settlements may have consequences ranging from zero eect, to an increase in building damage if these additional settlements occur non-uniformly over short distances and short time periods.

ACKNOWLEDGEMENTS anks are due to BBMV for providing the settlement data FIGURE 13: Long-term (10-120 years) and to UNPS and Crossrail for granting permission to further settlement contours at Whitechapel publish this paper.

Settlement (mm) 4 10 REFERENCES 6 12 Burland, J B, Standing, J R and Jardine, F M (2001) Building 8 14 response to tunnelling. Case studies from the Extension, London, Volume 2, Case studies, Ciria Special Publication 200. Divall, S, Goodey, R J and Taylor, R N (2014) e inuence of a time delay between sequential tunnel constructions. In Proceedings of the 8th International Conference on Physical Modelling in Geotechnics 2014. Mair, R J (2008). Tunnelling and geotechnics: new horizons. Géotechnique, 58(9). Mair, R J and Taylor, R N (1997) eme lecture: Bored Q Adding a secondary lining or waterproof membrane tunnelling in the urban environment. Proceedings of the 14th to the tunnel will also reduce the rate of settlement. International Conference on Soil Mechanics & Foundation Engineering 4. Hamburg. 5.0 CONCLUSIONS Nyren, R J, Standing, J R and Burland, J B, (2001) Surface e short- and long-term tunnelling-induced settlements displacement at St James’s Park greeneld reference site above at three arrays at Whitechapel were assessed. twin tunnels in the London Clay, in: Burland, J B, Standing, J R In the twin tunnel situation, the individual settlement and Jardine, F M (2001) Building response to tunnelling. Case troughs of both tunnels were identied and assessed in studies from the , London, Volume 2, both the short and long-term. It was found that the short- Case studies, Ciria Special Publication 200. term volume loss of the second tunnel was higher than Peck, R B (1969) Deep excavations and tunnelling in so that of the rst, at 1.09 and 1.29% respectively. For single ground. In Proceedings of the 7th international conference on tunnels unaected by adjacent construction works, the soil mechanics and foundation engineering. State of the art measured short-term volume losses were 1.04 and 1.13%. Volume, Data was analysed at discrete increments over a Standing, J R (2001) Elizabeth House, Waterloo, case study year aer the end of construction, and the evolution in: Burland, J B, Standing, J R & Jardine, F M (2001) Building of the settlement parameters was studied. e increase response to tunnelling. Case studies from the Jubilee Line in settlements and volume loss was found to follow a Extension, London, Volume 2, Case studies, Ciria Special logarithmic trend, with the rate of settlement over time Publication 200. decreasing logarithmically. e long-term behaviour Standing, J R and Burland, J B, (2006). Unexpected tunnelling observed at Whitechapel was consistent with that expected volume losses in the Westminster area, London, Géotechnique from a review of case studies of tunnelling in the London 56(1). Clay. e settlement trough widened and continued Wongsaroj, J, Soga, K and Mair R, (2013) Tunnelling-induced to develop aer the end of construction. Parts of the consolidation settlements in London Clay, Géotechnique, site experienced uniform settlement, while others saw 63(13). increases in slope. e additional settlements at any point depended on the location and proximity of additional excavation or tunnelling works. is medium-term data was extrapolated to tentatively predict future settlements at Whitechapel, in 10 years’ time, if no waterproong measures are put in place. e Gaussian component of the long-term behaviour was

40 GROUND ENGINEERING December 2015