INTRODUCTION

Tunnel and underpass projects beneath operating highways and railroads have become popular solutions in busy urban environments and are among the most sensitive and complicated underground projects in terms of design, construction, monitoring, and, most importantly, safety.

The level of public exposure to construction risks on such projects is much greater compared to other typical tunnelling projects and therefore requires exceptional due diligence in planning and execution of the work while maintaining seamless communication with stakeholders and the public to ensure the least impact on commuters and residents.

This article is Part 2 of a two-part-paper that discusses lessons learned through experiences from several shallow underpass projects and offers potential measures and techniques to mitigate relevant risks.

CONSTRUCTION CONSIDERATIONS

This section addresses construction considerations when using a typical earth pressure balance (EPB) type of tunnel boring machine (TBM) for excavations on a project.

Evaluation of TBM Operating Parameters and Optimising Face Pressures

To ensure the ground below the live road/rail is secure from the effects of tunnelling, the EPB operating parameters pertaining to the expected ground conditions under the highway/rail crossing need to be designed ahead of time to provide the operator with suitable operating targets and tolerances that would ensure no impact to the ground surface while keeping the excavation and tunnelling process safe and efficient.

The information available in TBM data acquisition systems can show early signs of system malfunctions and indicate which components require attention or which control settings need to be modified.

Successful EPB operation below highways and tracks requires tight control of face pressure and monitoring. The pressure of different elevations in the excavation chamber could be assessed by information coming from the EPB cells.

TBM operators closely monitor excavated muck and adjust the type and amount of water, bentonite, polymers, and foam to ensure the material is properly forming a plug to resist piezometric and ground pressures.

Depending on many factors, including ground conditions, groundwater, tunnel geometry, etc., different face pressures will be assigned to different areas along the tunnel alignment. The actual EPB pressures need to be logged during tunnelling to confirm correspondence to the values estimated by the project team before mining reaches the sensitive area below the railroad or highway.

If the face pressures are generally low, there is a risk of ground failure, ground loss, or groundwater drained or lowered down, causing consolidation settlement. On the other hand, if the operator uses excessive face pressures, there is always a risk of heave, blowouts, or frac outs.

Control of Excavation Weights and Volumes

Over excavation is a critical issue for low-cover tunnels, as there is little opportunity for ground loss to attenuate over a larger surface area. This means one cubic metre of ground loss, for example, can become a similar size hole on the surface.

Most EPB machines today are equipped with weight sensors and laser scanners to estimate the weight and volume of excavated ground. The theoretical weight and volume that a TBM data acquisition system is expected to show are usually estimated manually based on the TBM dimensions, ground properties, and advancing distances. These estimated figures are compared with the quantities shown on TBM graphs to check for overexcavation.

This information is also useful in the analysis of excessive volume loss and settlement. TBM advance with over-excavation can generally be recognised on TBM data graphs by a higher-than-normal grade in the excavation weight or volume line.

Slurry Frac Out or Blow Out of Conditioning Agents to Surface of Road or Rail

Fluid material in the pressurised excavation chamber, by nature, has the potential to blow out to the surface.

The risk of frac out of slurry/grout or blow out of conditioning agent to the surface needs to be mitigated by careful study of ground and optimisation of TBM operating parameters for all tunnel projects. However, this risk is much more critical when tunnelling is underway below transit corridors with low cover.

The risk of grout or other fluids to the highway surface, for example, is not just the environmental clean-up and wasted time for the commuters. The slippery conditions they create on the road could cause devastating multi-vehicle accidents and serious injuries.

Another common issue that does not help this situation is that the chance of the tunnel hitting old monitoring wells or abandoned boreholes increases in the areas that were under construction years ago.

These old wells and boreholes can act as a ‘path of least resistance’ for the pressurised fluids in the chamber and allow the frac out to surface to take place at much lower pressures than initially estimated and anticipated.

Ground Movements and Instabilities

In general, most concerns of tunnelling-induced ground movements around shallow crossings of transport corridors are related to the settlement (or vertical downward movement of the ground) due to the effect of gravity combined with over-excavation and volume loss. Other reasons, such as consolidation (due to drainage) and vibration (due to movement of heavy machinery and equipment), could also be the reasons for downward ground movements.

However, settlement is not the only type of ground movement to cause concerns; ground convergence, lateral movements, or ground heave (e.g., due to over pressurising the face or pre-excavation grouting (PEG)) could also have serious impact on railroads or highways. Settlement and ground movements can be controlled through a range of mitigation measures such as grouting, controlling the advanced lengths, timely installation of ground support, response monitoring, and appropriate mitigation to adverse trends.

Initial ground movements (generally happening between the tunnel and surface) are not the end of the story for the impacts that could be problematic for public safety. For example, settlements that are built up to the surface usually end up creating a void below the road or track structure (as the rigid structure of road/ rail bridges them for some time) until it is large enough to fail, creating a relatively large hole with possible sharp edges that could cause serious safety issues on busy transit corridors.

Even if the road structure is flexible enough to change shape with the ground below it in real time, it is important to remember that the effects of excavation will take some time to be developed to the surface and also take some more to come to a halt gradually. This time span depends on many factors, including geology, depth, mining method, etc.

Therefore, when the focus is on monitoring the more sensitive areas around the face of excavation as it moves forward, the time that the mining operation is in progress (by TBM or other methods) should not be the decisive factor in specifying the time and frequency of surface monitoring. For example, even if the TBM has stopped mining for a few days with no surface movements observed, the effects of any ground loss from days before could just show up on the surface.

Punching Loads and Induced Vibration

A specific complexity that needs to be studied in the design of support systems under live loads is the constant punching load and induced vibration applied by moving vehicles (movement of heavy trucks and equipment, in particular).

These loads and vibrations can weaken the ground during construction, induce additional settlement, and destabilise the tunnel support system in the long term.

An experienced design team can help mitigate these serious risks.

Depressions, Cracks and Sink Holes

Depressions are localised pavement areas where some elevations are slightly lower than those on the surrounding surface. In many instances, light depressions are not noticeable by the naked eye until after rain, when ponding water creates ‘birdbath’ areas.

Depressions can be caused by the settlement of the foundation soil due to tunnelling and ground movement impacts. Depending on their magnitude and location, such depressions can become a safety risk if developed on highway pavements or rail tracks. Therefore, in-person visual monitoring of the surfaces area is important while shallow underpass excavation is underway.

Cracks on the road or track slabs refer to various types of distresses that occur on the surface. The type of cracking depends on many factors but, generally, cracks that develop due to tunnelling are a result of general settlement or particular surface movements that have affected the road/track base beyond the limits of plastic deformation.

Sinkholes are surface depressions that result from the collapse of a submerged cavity in soil. Such cavities when a consequence of construction activities and tunnelling impacts usually develop gradually but won’t be detected until collapse, due to the rigid or semirigid pavement or track base acting to bridge, to some extent and time, over the weak zone. The formation of sinkholes is often sudden, therefore, and can lead to extensive damage and loss of life, especially in urban areas.

INSTRUMENTATION AND MONITORING

Due to the environmentally sensitive nature of tunnelling projects that cross under rails or roads, there needs to be development of a strict and thorough instrumentation and monitoring plan (IMP) to ensure public safety and convenience. Generally, the objectives of the IMP include the following:

  • Obtain information on the ground response during tunnelling to assess the project and public safety by comparing the observed ground response with allowable disturbance levels
  • Monitor the impact on the surrounding environment and adjacent structures
  • Evaluate critical design assumptions and verify design parameters and models
  • Provide construction control and assess construction Performance
  • Measure performance of the ground support as well as mitigation measures
  • Provide information and performance data to capture lessons learned

A typical geotechnical monitoring system usually consists of surface monitoring points, inclinometers, piezometers, shape arrays, tunnel convergence points, robotic theodolites, wireless data loggers, and target points along the alignment to measure any deformation in and outside of the tunnel. It would also be important to monitor changes in the groundwater table at critical sections along the alignment. Such data are also used to characterise site conditions, verify design assumptions, monitor construction effects, assess pre-support systems’ selection, and determine the requirements for ground stabilisation.

Monitoring Frequency

The monitoring points within the zone of influence of the tunnel need to be read daily or hourly and then, later, with reduced frequency after the lining is installed and the excavation face has advanced from the area.

The frequency and intensity of geotechnical monitoring vary depending on the importance of the parameter that is being monitored and the construction stage. Usually, the highest frequencies are during active tunnelling or excavations. After completion of a section, the frequencies can be reduced and continued for a defined period as a confirmatory measure.

While some parameters, such as ground surface or building movements, may require at least one round of reading during the tunnelling shift, some parameters may require more frequent or even continuous monitoring using automatic data loggers or remote data acquisition systems. These data are typically uploaded to an online web-based system with remote access that visualises the data and provides general insight and notifications if the review or alert levels are reached.

Generally, the engineer specifies the monitoring location, type, and frequency while the contractor implements a geotechnical instrumentation programme, including installing, reading, and reporting the data collected by installed instruments. The engineer, contractor, and technical groups will then analyse monitoring data to assess the tunnel’s stability and the structures above.

Baseline Survey

Establishing a well-examined baseline and continuous monitoring of road or rail surface and subsurface movements is essential to obtain a technical understanding of ground response and optimise tunnelling parameters. Having a baseline also helps in managing claims and distinguishing between the ground movements resulting from tunnelling from other unrelated reasons to the mining activity.

For example, certain areas have ground conditions sensitive to rain that would show some heave when saturated and then settle to the original levels after drying or draining out. Another example is the effect of temperature. Certain ground types could show considerable different values between cold and hot days in the year. That is why it is important to take a baseline well before construction to help filter out the ground movements that are irrelevant to the tunnelling and project activities.

Settlement Study

The assessment of the volume of ground loss during tunnelling under transit corridors is critical and is usually related to the magnitude of ground movements expected at the surface.

Generally, for a typical tunnel in soft ground conditions, the assumption is that the surface settlement volume equals the volume of lost ground, and the distribution of surface settlement associated with tunnelling activity in a uniform ground is assumed to follow an inverted Gaussian normal probability curve.

A commonly proposed approach to estimate surface settlements includes evaluating the volume of ground loss, evaluating the proportion of ground loss reaching the ground surface, and determining the settlement trough. The depth of the settlement trough is normally considered the maximum value of the surface settlement.

Pre- and Post-Construction Condition Survey

Pre-construction condition surveys need to be conducted within the tunnelling zone of influence to document the condition of highways, railway tracks, retaining structures, nearby buildings, surface facilities, and subsurface utilities. A supplementary condition survey may be performed with the same standards and requirements when structure movements or deformation levels have been exceeded or alleged damage claims have been received.

As part of the highway/railway pre-construction survey, tools such as ground penetration radar (GPR) will be required to assess the presence of voids beneath the pavement or track structure and to fill and maintain them before tunnelling commences. Developing a detailed stress map of pavement structures will also be required using a falling weight deflectometer (FWD).

The FWD is used to determine the structural strength of pavement structures. This involves a dynamic, nondestructive measuring process that can be deployed to identify portions of pavements where additional loading or stress relief can induce cracking and deformation.

Upon substantial completion of the tunnel and other required structures, post-construction condition surveys are carried out where the pre-construction surveys were conducted. Like those surveys, the post-construction survey will comprise field notes, digital photos, video, sketches, and other records that will assist to accurately and appropriately document the post-construction condition of structures within the zone of influence of the excavations.

Automated Motorised Total Station Versus Manual Survey

Automated motorised total station (AMTS) or, as sometimes called when used for highway and road monitoring, elevated total stations, can transmit highprecision measurements from a safe distance to the project’s instrumentation and monitoring database/ software. They provide automated optical monitoring of displacements and deformation. These systems can combine surveying, imaging, and high-speed scanning and can provide continuous data at various accuracies and frequencies, depending on the equipment, targeting system, and other factors.

The total stations required to monitor ground movements along the tunnel alignment are usually mounted on a high structure close to the lanes (in the median in most highway cases) or tracks for the project’s duration. However, monitoring directions can be adjusted remotely, or if needed, the location of AMTS can change to adapt to the changing project conditions.

Prism versus Reflectorless Method

There are mainly two ways the total station measures: using the traditional and still more common prism or through reflectorless technology.

With the prism method, the total station sends out invisible infrared waves that are reflected by the prism By measuring the prism’s position and knowing the precise angle and distance to that prism, the total station calculates the prism’s location or coordinates.

In many projects, surface areas around highway lanes or tracks are inaccessible or deemed too dangerous to enter; or the cost and public impact of closing lanes/ tracks to allow the installation and maintenance of targets cannot be justified.

In other cases, the targets that were planned to be installed won’t be able to withstand typical or particular road activities for a long time, such as the weight of heavy machinery passing by, road clean up, or snow removal operations. Other targets like more durable nails or pins could withstand road activities to some extent, but those usually need to be surveyed manually.

The alternative in these situations is to utilise a reflectorless pavement surface monitoring system, which has the potential to mitigate the concerns mentioned by making use of elevated AMTS and reflectorless EDM (Electronic Distance Measurer) technology. The reflectorless method does not use a prism. Instead, it uses a visible red laser beam that allows the user to lay out or measure points from any surface able to reflect the beam. This means the system could eliminate the risk of targets being negatively impacted by road or track activities.

Additionally, such a system could reduce the need for lane closures to install targets and manual survey operations, in general, to help with the safety of the monitoring crew.

However, surveying without any reflectors or target points has some issues. Such systems are usually installed on high poles close to the lanes or tracks to establish a good line of sight for all areas that require monitoring. Depending on how they are supported, they could swing a little with any strong wind or vibration, causing even larger errors on surface readings.

Other weather impacts like high summer temperatures or ‘black ice’ on asphalt or tracks could also affect the readings. Similar issues could happen with heavy traffic, rain, snow or something as simple as garbage or dried mud on the ground.

Typical mitigation is to implement a code into the software that filters out the data it detects as an anomaly or significantly inconsistent with other readings, but sometimes it filters too much or too little data. Sometimes the EDM system beam hits the lower part of vehicle tires very close to the highway surface, so inconsistencies are not large enough to be detected by the software thresholds, and another round of manual survey or additional confirmation readings need to be provided so the project team is assured about the output results.

Another challenge with a reflectorless system is that as soon as some surface settlement or heave occurs, it can be hard to be sure what happens to the laser as it travels to and from the initial target. In many cases, when the surface experiences some local settlement or heave, the point that ATS shoots at the next round might not be the same point that was measured in the previous round (e.g., due to formed curvature or damage around that point), which could result in measurements that won’t initially seem to make sense unless the data are cleaned up or overwritten by manual survey data.

Surface Damage and In-Person Visual Inspections

For many owners of transit corridors, the possibility of damage to the road surface is not about the exact amount of ground or surface movements or reaching established review or alert levels. It is understood that the road or track rigid base structure could possibly bridge any local settlement or even sink holes below it.

Therefore, while surveying the surface movements provides valuable data, the project team cannot conclude that there is (or will be) no ground movement only because the surface hasn’t shown considerable movements yet. Ground movements less than review or alert levels (especially if the corridor is aged) could damage or fracture the asphalt or track in a way that causes accidents for moving vehicles or trains. For example, a tyre could burst due to asphalt fracture, creating a sharp edge opposite the direction of traffic.

Specialised Monitoring Instruments for Highways and Tracks

Due to the sensitivity and logistics of setting up the instruments on live transit corridors, specific instrumentation and monitoring technologies have been introduced to the market in the last two to three decades that will help the projects by eliminating the need for maintenance, avoiding any structural damage to pavement or tracks, and reducing the safety risks for project crew and commuters in general.

Examples of such tools include: 1) track settlement and twist sensors; 2) specialised track target and non-destructive types; and, 3) horizontal shape accelerometer array.

1) Track Settlement and Twist Sensors

 A track settlement sensor system is intended for monitoring settlement and the twist of railroad tracks that may be affected by nearby construction activity, such as tunnelling or adjacent excavation. Settlement sensors are mounted longitudinally along the track alignment, typically with a mount spacing of a few metres and one or several track twist sensors mounted perpendicularly to the settlement sensors.

2) Specialised Track Target and Non-destructive Types

The targets installed along the railroads need to be customised to be easily visible but also not in any way hinder the regular operation or maintenance of tracks. Specialty instruments have been introduced in the industry for some time. In some cases, a railroad owner is more sensitive to any possible impact of installing targets on their infrastructure, so there are also types of targets that are less destructive or non-destructive in any way.

3) Horizontal Shape Accelerometer Array

The shape accelerometer array (SAA) consists of a chain of rigid segments connected by flexible joints containing micro-electro-mechanical sensor accelerometers, which measure the segment’s movement or tilt. An SAA installed horizontally can measure movement in the vertical direction. During installation, the SAA and communication cables will be inserted into a PVC conduit, which will be pushed into a horizontal borehole drilled through the whole length of monitoring underneath the highway or railroad structure. Data cables will be attached to the data loggers, which consist of a data transmission unit table to be powered by a solar panel and housed in a weatherproof cabinet.

FINAL TIPS FOR SUCCESSFUL TUNNEL CROSSINGS

The following recommendations and lessons learned that could make a difference in the success of a projects if done properly and in a timely manner:

  • Use a traffic or track operations engineer to perform a specific risk analysis for the area.
  • Perform regular in-person visual inspections of the surface and record the daily conditions by taking photos.
  • Have more instruments and information rather than less. The cost of installing additional monitoring instruments is negligible compared to the cost of a project halt due to insufficient data.
  • Get familiar with the process and obtain lane and track closer permits ahead of time.
  • Use vibration monitoring to provide insight into whether vibration compaction and soil consolidation were factors in any ground movement.
  • Develop plans for mitigation and restarts at early stages of a project, and therefore before the review or alert levels can be reached.
  • Baseline any patches, cracks, or issues on the surface rail or roadway before construction starts.
  • When speculating about causes of surface movements, exclude and disregard any issues that could have happened many times since the track or highway was built. For example, a significant rain event or vibration from moving vehicles, unless the highway has been built recently, should be excluded.
  • Use automated instruments (with consideration given to their pros and cons) to increase the frequency of readings above what is possible manually.
  • For EPB TBM applications, monitor the muck volume more accurately with a pair of laser scanners and a pair of weight scales so there is no doubt, at any point, if any ground movements have been caused by over-excavation. Consider including extra EPB cells at the TBM design stage both for better accuracy and redundancy.
  • Consult with TBM manufacturer specialists and operators for training and advice on mining operations.
  • Manage and review excavation data regularly so there is no doubt at any point if any ground movements have been caused by over-excavation.
  • Be mindful about the construction of other elements of the project that could impact the nearby highway or railroad. Depending on geology, the construction of shafts or working platforms can settle or heave considerable area around their locations.
  • Provide monitoring so that owners of transit facilities are sure that their property won’t be affected by the tunnelling project. Provisions for long-term monitoring after the contractor has demobilised from the site need to be put in place as needed.