Tunnelling in sand in Farringdon Crossrail Station

Document type: Technical Paper
Author: Angelos Gakis Dr. Dipl-Ing, MSc DIC, CEng MICE, Michael Orsario, Noel Cooper, Borja Trashorras
Publication Date: 31/08/2016

Abstract

The sprayed concrete lined (SCL) tunnels in Crossrail Farringdon Station have been mainly excavated in the Lambeth group, a highly variable formation comprising generally very stiff, over-consolidated clays with interbedded sand lenses. The platform tunnels, approximately 11.4m wide by 10.6m high, were enlarged from the existing TBM tunnels using SCL techniques following extensive probing from the TBM tunnels to investigate the presence of pressurised sand lenses.

In two occasions, extensive, thick sand lenses with water under pressure were encountered that required significant mitigation measures to be deployed.

At first, a 2 to 3m thick sand lens, with an extent of approximately 100m along the tunnel, was encountered during the enlargement of the westbound platform tunnel (PTW) near tunnel axis level. A systematic in-tunnel depressurisation scheme was successfully utilised by the contractor reducing sufficiently the pore-water pressures in the sand.

The second case was of significantly increased difficulty as a sand lens of similar thickness was encountered in a very unfavourable position, that being in and above the crown of the eastbound platform tunnel (PTE) for approximately 25m along the tunnel. Additionally to the dewatering scheme that was prescribed by the temporary works design, the contractor decided to reduce the risk further by injecting chemical grout. These mitigation measures were combined with the application of toolbox items that enhanced the roof stability through the installation of grouted spiles and the face stability through the sub-division of the top heading into smaller pockets.

In both cases, the implementation of the mitigation measures was successful, enabling the safe completion of the platform tunnels in Farringdon.

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Notation

BFK                 BAM, Ferrovial, Kier Joint Venture, the main contractor
DSP                 Dr. Sauer & Partners, BFK’s specialist SCL designer
LSB                 Lower Shelly Beds
LTB                 Laminated Beds
LMB                 Lower Mottled Beds
MLGH              Mid Lambeth Group Hiatus
PTE, PTW       Platform Tunnel east, west
RES(S)            Required Excavation and Support (Sheet)
SCL                 Sprayed Concrete Lining
TAM                Tube-a-Manchette
TBM                Tunnel Boring Machine
UF                   Upnor Formation
UMB                Upper Mottled Beds

Introduction

Farringdon is one of Crossrail’s major underground stations in central London, set in between the future Liverpool Street and Tottenham Court Road stations. The layout of the station includes two main platform tunnels, eight cross passages, two escalator tunnels with their lower chambers, two ventilation tunnels and four stub tunnels (see Figure 1). The platform tunnels, approximately 300m long each and 10.6m high and 11.4m wide, were enlarged from the previously built TBM tunnels using sprayed concrete lining (SCL) methods. The excavation works for the SCL tunnels were completed in September 2015. BFK joint venture (BAM, Ferrovial Kier) was the main contractor and Dr. Sauer & Partners their SCL specialist designer, responsible for the SCL works prior to ring closure.

The main risks with relation to the SCL tunnelling arose from the interbedded and randomly distributed sand lenses in the Lambeth Group. Although the clayey units of the Lambeth proved to be an ideal tunnelling medium, the sand lenses, water-bearing in their majority and 3m thick in most encounters, required extensive treatment prior to the safe advance of the excavation.

The following sections present two specific cases where sand lenses were encountered during the course of works in both platform tunnel west (PTW) and east (PTE). The implemented mitigation measures that enabled the safe execution of the tunnelling works as well as the considerations that led to the specific solutions are described.

Geology in Farringdon

The tunnelling works in Farringdon were executed mostly in the Lambeth Group formations (ca. 85%) and to a lesser extent in London Clay and Thanet Sand units. The Lambeth Group comprises variable, over-consolidated sediments, ranging from stiff to very stiff clays and silts to dense sands and gravels as well as interbedded sand lenses. The typical units of the Lambeth group encountered in Farringdon are shown in Figure 2.

Five faults were encountered during the tunnelling works (shown in Figure 1 at platform level):

Farringdon Fault – dipping to the west with a vertical throw of 1 to 2m.
Smithfield Fault – dipping to the east with a vertical throw of 7 to 9m.
John’s Street Fault – dipping to the west with a vertical throw of 0.8 to 1.2m.
Lindsey Street Fault – dipping to the west with a vertical throw of 0.8 to 1.2m.
Charterhouse Fault – dipping to the east with a vertical throw of 4 to 5m.

Although some deterioration of the soil properties was expected in the faulted zones, no actual effects were visible. In fact, the exposed tunnel faces were as stable inside the zones of influence of faulting as they were outside and the measured convergence of the SCL lining was comparable.

Due to the complexity of the geological conditions, a “real-time” 3D geological model was developed for the area of Farringdon station providing a continuously updated conceptual ground model ^[1].

The variability and uncertainty of the geological conditions as well as the geotechnical risk arising from the potentially water-bearing sand units rendered the decision-making during the tunnelling works critical.

Sand Lenses

Lambeth Group clay units have been ideal for open face excavation tunnelling, exhibiting significant stand-up time in the exposed faces and only minor, localised instabilities. Interbedded sand lenses were however encountered in both Upper Mottled Beds (UMB) and Lower Mottled Beds (LMB). The sand lenses of the LMB were dry to moist due to the under-drained pore pressure profile of the area, which is attributed to the historical water abstraction from the lower aquifer. The sand lenses in the UMB on the other hand were water-bearing in most of the encounters with a maximum measured pore-water pressure equal to 100kPa (1bar).

Figure 1 – Sand lenses in the Upper Mottled Beds (UMB) assessed after the end of the SCL works.

The distribution of the sand lenses in the UMB as assessed after the completion of the SCL works is shown in Figure 1. Two major sand lenses were encountered during the tunnelling works in Farringdon, shown as “Lens A” and “Lens B”. Both required extensive treatment to allow a safe execution of the SCL works.

Lens A, which was in total 235 running metres long and approximately 3m thick, was mainly encountered in the lower part of the top heading and in the bench excavations in PTW, between Smithfield Fault and St John’s Street Fault. Lens B, which was 83 running metres long and approximately 3m thick, was encountered in and above the crown of the enlargement, at the east end of PTE. Both lenses dipped from north-west to south-east.

Samples from these lenses were collected and grading analyses were carried out. The results indicated that the sand had generally higher fine content at the top of the lens (fine silty sand) becoming clean, medium sand towards its base as shown in the particle size distribution curves of Figure 3.

Figure 2 – Sub-units of the Lambeth Group encountered in Farringdon.

Figure 3 – Particle size distribution curves from samples collected in several positions inside the sand lens in PTW and PTE.

Mitigation measures

Several pre-defined toolbox items were deployed to respond to the unforeseen ground conditions encountered during the excavation of the SCL tunnels inside the two sand lenses described in the previous section. These items had either the form of ground treatment (depressurisation and chemical grouting), implemented ahead of the commencement of the excavation or of roof / face support (spiles, pocket excavation), applied right before or during the actual excavation.

The implementation of the toolbox items was managed through the required excavation and support (RES) process in accordance with the BFK Construction Management Plan. The decision-making process included the evaluation of the latest geotechnical information such as face maps of previous advances, probing results, pumping records, monitoring data, the review of the 3D geological model, latest shift reports, sprayed concrete early strength results, compensation grouting data. The decision was detailed and formalised on the Required Excavation and Support Sheet (RESS).

In-tunnel depressurisation

In-tunnel depressurisation included the installation of wellpoints at specific spacing through the existing TBM tunnel lining and in certain cases through the completed primary lining, in order to target the sand lenses. The wellpoints were connected to a vacuum pump which enabled water pumping under constant pressure or flow rate. In Farringdon the decision was made to maintain a constant vacuum pressure of approximately 0.6 bar following small scale trials. All the drillings were performed with appropriate means for the control of ground water in case of significant seepage (stuffing boxes and inflatable packers).

Chemical Grouting

Chemical grouting involved the injection of a sodium silicate-accelerator mix through tube-a-manchette (TAM) pipes under low pressure, targeting the water-bearing sand units. The objective was to reduce the permeability and increase the strength of the granular material.

Pocket Excavation

Pocket excavation refers to the division of the face excavation into smaller sections. The objective was to control the stability of the ground with reduced stand-up time and to allow a more rapid initial support installation. In general it may range from a vertical division into two pockets (split-face) to numerous small pockets.

Grouted Spiles

Refers to perforated steel pipes, rammed or installed in pre-drilled holes or self-drilling pipes with a lost drill bit. They were installed above the crown of the tunnel and extended ahead of the face to enhance roof stability. The injection of suitable grout should improve the stability of the ground and reduce the permeability of non-cohesive material. The distance between the spiles may generally be adjusted to the ground conditions.

Case A: Westbound platform tunnel

Problem description

Based on the in-tunnel probing results (see Figure 4) and the updated 3D geological model in the area of the platform tunnel west, a succession of sand lenses was expected to be encountered between TM 131 and TM 265. Although there was no encounter of the sand lens during the enlargement works west of Smithfield Fault, the approximately 7.5m down-throw of the fault moved these lenses in the lower part of the top heading and bench excavations of the tunnel. A certain level of hydraulic continuity was implied by the significant horizontal extent and thickness of the lenses.

Preliminary testing during the in-tunnel probing revealed a maximum water pressure of 0.7bar (70kPa), which dropped significantly (to 0.2-0.1bar) once free gravitational flow was allowed. Maximum free flow rates of 7l/min were measured through the hose of the packers used to safely seal the open holes.

Considerations

In order to facilitate the enlargement of PTW, the design requirement was to reduce the residual pore-water pressure in the sand to less than 10kPa and any flow rate from drilled holes (typically 100mm diameter) to less than 3l/min prior to the advance of the excavation. The sand lenses were expected to be encountered in the lower part of the top heading and in the bench excavations, positions that were considered as moderately unfavourable.

Two alternative ground treatment solutions were investigated by the contractor: an in-tunnel depressurisation scheme utilising vacuum-aided depressurisation through wellpoints and two grouting schemes, introducing either cementitious grout or chemical grout injections. These solutions were compared based on the total programme impacts, their cost, the subsequent construction impacts and health and safety considerations. Small-scale trials were also performed to assess the efficiency of each method in the site-specific conditions. After the comparative assessment, in-tunnel depressurisation qualified as the optimal solution.

Figure 4 – Expected ground conditions in PTW TM 149.2 based on in-tunnel probing results. At the top is the London Clay followed by the Upper Mottled Clay with an interbedded sand lens, 3 to 3.5m thick. At the bottom are the Laminated Beds, the Lower Shelly Beds and the Lower Mottled Clay.

Ground treatment

The in-tunnel depressurisation scheme for PTW included the installation of 106 wellpoint, from the TBM tunnel, distributed in both sides with every 10^th wellpoint being used as a piezometer. Pumping was performed under constant vacuum pressure (0.6 bar).

The details of the in-tunnel depressurisation installations are summarised in Table 1 and the locations of the installed wellpoints are shown in Figure 5. Figure 6 shows the installed wellpoints inside the TBM tunnel.

Installation location	Both sides of the TBM pilot tunnel. Typically installed through the lining or just below the tunnel axis, inclined downwards to intercept the Sand/Clay interface, approximately 3.5 to 4.0m beyond the enlargement envelope.
Spacing	3m.
Quantity	80No wellpoints in total installed along both sides in the TBM tunnel with a standpipe piezometer every 10^th wellpoint.
Length	6 to 8m.
Bore Size	76mm internal diameter (lost drill bit) or 90mm (open hole).
Completion	38mm internal diameter PVC liner (lost drill bit) or 25mm (open hole).
Screen Length/Type	2m length, slotted or resin-bonded filter sand.
Annulus Backfill	Grout seal through tunnel lining only.
Drilling Equipment	Track mounted rotary drilling rig (5T or 10T depending on the level and the inclination)
Pumping Equipment	Duty and standby vacuum pumps.

Table 1 – Details of in-tunnel depressurisation installations in PTW.

Figure 5 – Location of the wellpoints installed in PTW.

After vacuum pumping under constant pressure of 0.6bar was maintained for a period of approximately 2 weeks, the pumping rate dropped from an initial value of 15l/min to a stable value of 4l/min as shown in Figure 7. The additional drop of the pumped flow rate that is observed in the graph is due to the advancing of the tunnel works and the decommissioning of the wellpoints behind the excavated face.

Figure 6 – Wellpoints installed in PTW.

Figure 7 – Pumping rate (l/min) under constant pressure with time in PTW.

Excavation and support

Once a steady flow rate was achieved, it was considered safe to proceed with the excavation works inside the area of the sand lens with vacuum pumping operational. In order to achieve enhanced stand-up time of the exposed sand, it was decided that the excavation would be performed in four pockets. Each pocket was excavated and subsequently sealed with an initial, 75mm thick shotcrete lining. The exact division of the face was decided in the daily RES meetings and detailed on the corresponding RESS after an examination of the depressurisation and ground investigation data and the agreement between all the signatories.

Figure 8 shows the face map from a top heading excavation (TM 156 in PTW) and Figure 9 shows a close up of the excavated sand. The depressurisation and the implementation of the pocket excavation resulted in a stable excavated face. The treated sand itself exhibited a significant stand-up time with only minor, superficial ravelling.

An advance rate of approximately 1.8m/day was achieved inside the treated area.

Figure 8 – Face map of top heading excavation in PTW TM 156.2 after the depressurisation implementation.

Figure 9 – Exposed sand lens in PTW TM 156. The sand has been depressurised and exhibited adequate stand-up time with minor superficial ravelling.