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Pile settlement due to tunnelling in London Clay:a case study

COOLING PRIZE PAPER

Introduction Improvements in tunnelling technology have made the construction of metro systems in problematic urban environments a viable option but there are still challenges, such as cases where tunnels have to be constructed close to piled foundations.

The interaction mechanisms between tunnelling, the ground and piled foundations are not well understood and few case studies exist in the literature. As a result engineers are forced to adopt very conservative and hence costly solutions when assessing the potential impact of tunnel excavation on piled structures. Construction of the Channel Tunnel Rail Link (CTRL) provided a unique opportunity to monitor the response of full-scale piles to tunnelling-induced movements.

This paper presents the results of a full-scale trial, which took place during the construction of CTRL Contract 250 in Dagenham, Essex. The study involved the installation, loading and monitoring of four instrumented piles along the route of two, 8m diameter twin tunnels and the comparison of the resulting pile settlements with surrounding ground movements.

Based on the results of this study three zones of influence were identified where pile settlement was correlated to surface movements. The pile-ground surface settlement correlations presented seek to improve knowledge for predicting pile settlement due to tunnelling.

Background The subject of tunnelling near piled foundations has received particular attention lately.

The North/South Line in Amsterdam, the CTRL project and the proposed Crossrail scheme are examples where there is a great need to gain better insight into the response of piled structures to tunnelling.

However, there is a scarcity of well-documented field studies on this subject. Two notable exceptions include the Heinenoord fullscale trial near Rotterdam (Adviesbureau, 1999) and the study by Coutts and Wang (2000), which presents field results from instrumented piles subjected to tunnelling-induced movements, as part of the North-East Line project in Singapore.

In the Heinenoord study the response of 38 timber piles and 18 concrete piles was monitored during construction of 8.3m diameter twin tunnels through both Holocene deposits (layers of soft clay and peat) and Pleistocene dense sand. Field observations showed that pile settlement could be classified into three categories, depending on the position of the pile toe relative to the tunnel axis (Figure 1a).

Jacobsz et al (2001) describe results from a number of centrifuge model tests investigating the form of these zones of influence for the case of tunnelling near axially loaded piles driven in dry sand. Similar zones of influence in which pile settlements could be correlated with surface movements were identified from these tests (Figure 1b). Other noteworthy model studies include the work of Bezuijen and Van der Schrier (1994) and Loganathan et al (2000). The subject has also been studied numerically (Vermeer and Bonnier, 1991; Mroueh and Shahrour, 2002), as well as analytically (Chen et al, 1999).

Project context CTRL Contract 250 involved construction of 5.2km of twin 8m diameter bored tunnels between Ripple Lane, Dagenham and Barrington Road, Newham, using two Lovat earth pressure balance tunnel boring machines with tail-skin grouting.

Four instrumented piles and numerous instruments for monitoring ground response were installed at a site about 1km from the TBM launch area, well in advance of tunnelling activities. The piles were installed seven months before tunnel excavation and all ground instrumentation was in place at least two months before construction.

The twin tunnels were excavated at separate times.The first TBM (up-line) passed though the monitoring section one month before the second TBM (down-line). At the instrumented site tunnelling took place through London Clay.

Ground conditions The ground conditions comprise 3m of made ground overlying about 4.5m of soft alluvium of peat and clay. A succession of 3.7m of dense Terrace Gravels is beneath, which overlies about 14.8m of London Clay and the Harwich Formation. Groundwater is 4m below ground level. Table 1 shows a detailed description of the soil strata.

Instrumentation The plan of instrumentation on Figure 2 shows the surface positions of the four driven piles and ground instruments installed relative to the twin tunnel alignment. Instrumentation consisted of 21 surface settlement points (SSPs) and numerous boreholes for measuring the surrounding ground response using vibrating wire piezometers, rod extensometers and in-place electrolevel inclinometers. This paper concentrates on the short-term settlement response of the piles and SSPs.

The SSPs comprised an extended BRE socket (BRE Digest 386, 1993) embedded into a 1m deep concrete column (about 150mm in diameter). SSPs were placed at 2.5m intervals in a line perpendicular to both tunnel axes (Figure 2). The SSPs were positioned to cover the full extent of the surface settlement trough from the up-line tunnel and most of that for the down-line. The spacing of the two tunnel centre lines at this section was 16m (two tunnel diameters) and the depth to the tunnel axes was 18.9m. The vertical displacement of the SSPs was measured by precise levelling using a digital Leica NA3003 precise level with a 2m invar bar-coded staff, giving an accuracy of -0.15mm.

Four driven cast insitu piles with a nominal diameter of 480mm were installed at the surface positions shown in Figure 2.Two piles were end bearing (BC and BO) in the Terrace Gravels and were 8.5m long; the other two (friction piles, FC and FO) were 13m long and founded in London Clay (Figures 3 and 4). Piles BC and FC were installed directly above the up-line tunnel centre line and piles BO and FO at an offset of 9m from it. Pile locations were strategically selected to investigate the existence of zones of influence (Jacobsz et al, 2001).

One month before the first TBM arrived, all four piles were loaded to about 50% of their ultimate capacity using kentledge reaction platforms (Figure 5). Four automatically controlled hydraulic pumps were used to maintain constant pile loading during tunnel construction.

The end bearing piles were loaded with 650kN and the friction piles with 240kN.

The displacement of each pile relative to the ground surface was monitored using four potentiometric displacement transducers (PDTs) set up at the head of each pile. The PDTs were mounted on reference beams supported on stakes driven a minimum of five pile diameters from each pile. Changes in the level of the reference beams were monitored by precise levelling on to bar-coded strips attached to the stakes.

Monitoring results Figure 6 shows the profiles of transverse vertical displacement of the SSPs due to construction of the up-line and down-line tunnels. In both cases the settlement profiles can be well described by the form of a Gaussian distribution curve. The construction of the up-line and down-line tunnels caused immediate maximum vertical displacements of 5.2mm and 12.7mm relating to volume losses of 0.2% and 0.5% respectively.

Figure 7 shows the development with time of vertical displacement of the friction piles FC and FO (at centre line and offset positions respectively) compared with the corresponding ground movements during construction of the up-line tunnel.

In the case of pile FC the curves of pile head and ground surface settlement initially show a heave of 2mm due to TBM arrival, probably resulting from the face pressure. This is followed by a settlement of 3.5mm and 1.7mm respectively, due to the passage of the TBM face beneath this section and subsequent heave due to tail grouting.

Results indicate that pile FC experienced more settlement than the ground after the TBM had passed. The ratio of pile head to ground surface settlement R gradually reduces as the ground consolidates with time from a maximum value of 2.3 (at five days) to a value of 1.6 (at 15 days). In contrast, the offset pile FO follows the pattern of surface displacement during and following passage of the up-line tunnel, ie R = 1.The magnitudes of initial heave and final settlement for pile FO are considerably less than those of pile FC.

Figure 8 shows the corresponding development of vertical displacement for the end bearing piles BC and BO compared with ground movements. Similar to the response of centre line pile FC, pile BC experienced greater settlement than the ground following the passage of the TBM.

Pile BC showed initially a slight heave of 0.5mm due to arrival of the TBM (less than the heave of pile FC because of the greater distance between pile toe and tunnel crown). The ratio R is seen to reduce with time from a maximum value of 2.5 (at 5.5 days) to a value of 1.8 (at 15 days). The sudden heave shown by pile BC after 13.5 days (Figure 8a) corresponds to a temporary reduction in the load at the top of the pile due to a breakdown of the load control system.

Pile BC settled more than pile FC immediately after the TBM had passed (6mm compared to 4mm), probably because of the greater load acting on it. Pile BO followed closely the pattern of ground movements seen in pile FO.

Vertical displacement profiles for the piles due to the down-line tunnel are shown in Figures 9 and 10. Piles FO and BO are now at an offset of 7m from the centre-line of the down-line tunnel (a similar offset from the up-line tunnel - 9m), while piles FC and BC are at an offset of 16m.

The response of piles FO and BO is identical to that of the surrounding ground during and following passage of the second TBM. This correlates well with their response to the up-line tunnel construction from which they were at a similar offset. Both piles showed a short-term settlement of about 4mm. In contrast, piles FC and BC, at a much greater offset, showed zero or slight settlement due to tunnelling.The ground surface settlement for these piles was rather small initially but gradually increased due to consolidation.

The results of this trial indicated the existence of three zones of influence, similar to those presented in previous studies (Adviesbureau, 1999; Jacobsz et al, 2001), in which pile head and ground surface settlements can be correlated, (Figure 11).

Piles with their bases in Zone A settled more than the surface. This is associated with a reduction in pile base load due to tunnelling which was compensated by mobilisation of reserve shaft capacity, hence resulting in differential pile settlement of the order of 2-4mm relative to the ground.

Piles with their bases in Zone B (defined by an angle b between zones A and C) settled by the same amount as the surface. The value of b is probably a function of the shearing resistance of the soil and the tunnelling volume loss. For London Clay b was found to be 45infinity, however for soft clays it would probably be less. Piles in zone B also experienced reduction in their base loads, although of smaller magnitude. Finally, piles with their bases in Zone C settled less than the surface. The base loads of these piles increased with time indicating the development of some negative shaft friction along the length of these piles.

Conclusions Three zones of influence were identified in which pile head settlements were correlated to surface ground movements:

l Piles in Zone A settled more than the surface l Piles in Zone B settled by the same amount as the surface l Piles in Zone C settled less than the surface l Piles experienced a load redistribution along their length depending on the position of the pile toe relative to these zones of influence.

Acknowledgements The author is indebted to several researchers at the Cambridge Geotechnical Group and Imperial College, London for their assistance in taking some of the field measurements described in this paper.

The author would also like to acknowledge the support provided by EPSRC, Mott MacDonald, the Cambridge European Trust, and the CTRL Contract 250 consortium joint venture: Edmund Nuttall/Wayss and Freytag/Kier.

References Adviesbureau Noord/Zuidlijn (1999). Evaluatie van de meetresultaten van het proefpalenprojek tpv de tweede Heinenoordtunnel (in Dutch). Report no R981382, Amsterdam.

Bezuijen A and Van der Schrier JS (1994). The influence of a bored tunnel on pile foundations. Proc of Int Conf Centrifuge 94, Singapore, Rotterdam, Balkema, 681-686.

BRE (1993). Digest 386: Monitoring building and ground movement by precise levelling.

Chen LT, Poulos HG and Loganathan N (1999). Pile responses caused by tunnelling. ASCE J Geotech and Geo-environmental Engng Vol 125 (3), 207-215.

Coutts DR and Wang J (2000). Monitoring of reinforced concrete piles under horizontal and vertical loads due to tunnelling. Tunnels and underground structures, (eds Zhao, Shirlaw and Krishnan), Rotterdam, Balkema, 541-546.

Jacobsz SW, Standing JR, Mair RJ, Soga K, Hagiwara T and Sugiyama T (2001). Tunnelling effects on driven piles. Proc of Int conf on response of buildings to excavation-induced ground movements. Ciria, London, 337-348.

Loganathan N, Poulos HG and Stewart DP (2000). Centrifuge model testing of tunnelling-induced ground and pile deformations.

Geotechnique 50, No. 3, 283-294.

Mroueh H and Shahrour I (2002). Three-dimensional finite element analysis of the interaction between tunnelling and pile foundations. Int J Numer Anal Meth Geomech. Vol 26, 217-230.

Vermeer PA and Bonnier PG (1991). Pile settlements due to tunnelling. Proc 10th European Conference on Soil Mechanics and Foundation Engineering, Florence, Balkema, Vol 2, 869-872.

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