Rj Mair

Monitoring Twin Tunnel Interaction Using Distributed Optical Fiber Strain Measurements

In this field trial, a new monitoring technique using distributed strain sensing known as Brillouin optical time-domain reflectometry (BOTDR) was introduced to monitor the behavior of bolted-concrete linings of a recently completed tunnel when a second bored tunnel was constructed side by side at a distance less than one tunnel diameter apart. This was done by measuring circumferential strains in 12 rings using optical fiber that was installed using the point-fixing method. The strain distributions around the circumference of the rings show a generally similar profile. Maximum compressive strains measured below the tunnel springline nearest to the excavated tunnel were larger than the maximum tensile strains measured at the tunnel crown, distorting the circular tunnel into an oval/ellipsoid that was about symmetrical to the horizontal axis. Several methods were introduced to compare strain measurements made by BOTDR and diameter changes recorded by tape extensometer. This involves the use of a symmetrical tunnel distortion model and the basic differential equation for a circular arch. The calculated results showed some degree of similarity between the two methods. The analysis highlighted the importance of measuring the axial strain in the tunnel ring and subtracting the axial strain component to calculate the actual deflection of the lining as a result of bending.

The use of fibre optic sensors to monitor pipeline response to tunnelling

This paper describes the use of fibre optic sensing with Brillouin Optical Time-Domain Reflectometry (BOTDR) for near-continuous (distributed) strain monitoring of a large diameter pipeline, buried in predominantly granular material, subjected to a pipe jack tunnelling operation in London Clay. The pipeline, buried at shallow depth, comprises 4.6 m long sections connected with standard bell and spigot type joints, which connect to a continuous steel pipeline. In this paper the suitability of fibre optic sensing with BOTDR for monitoring pipeline behaviour is illustrated. The ability of the fibre optic sensor to detect local strain changes at joints and the subsequent impact on the overall strain profile is shown. The BOTDR strain profile was also used to infer pipe settlement through a process of double-integration and was compared to pipe settlement measurements. The close approximation of the measured pipe settlement provides further confidence in fibre optic strain sensing with BOTDR to investigate the intricacies of pipeline behaviour, pipe-soil interaction and interaction between pipe sections when subjected to ground movement.

Pressuremeter measurement of total horizontal stress in stiff clay

Lift-off provides the most rational method for estimation of in situ total horizontal stress from pressuremeter tests in clay. An alternative method assumes elastic initial response of the clay and identifies a yield point from the cavity expansion information. Two examples are given of the application of both methods. At a London clay site the two methods produce similar results which agree reasonably well with estimates of in situ stress deduced from laboratory suction measurements. At a Barton clay site the two methods are used with results from both self-boring pressuremeter and high pressure dilatometer tests. Reasonable agreement is again obtained. (Author/TRRL)

Pile-Soil Interaction and Settlement Effects Induced by Deep Excavations

AbstractDeep excavations may cause settlement and damage to adjacent buildings, even if they are found on piles. The corresponding pile deformations are determined by axial and lateral effects. This paper describes an analytical model relating axial pile deformation to the vertical soil displacement resulting from the deep excavation and also suggests ways to determine the pile response to lateral displacements. The axial pile-soil interaction is clearly different for end-bearing and friction piles. Common generalizations that end-bearing piles settle the same as the soil settlement at the base level and friction piles with the ground surface settlement present lower and upper bounds, which are only valid for certain idealized cases. The settlement of piles with a large component of shaft friction is determined mainly by the actual load on the pile relative to the pile ultimate capacity. The lateral pile response is governed mainly by the relative stiffness of the pile to the soil. The proposed model was ...

Centrifuge Modeling of Deep Excavations and Their Interaction With Adjacent Buildings

Major cities in the world are experiencing a rapid growth in population while becoming increasingly overcrowded and congested. In recent years, this has created a huge demand for underground infrastructure, which often involves the design of major mass transit tunnel systems; these tunnel systems (underground tunnels and metro stations) are becoming increasingly necessary to construct in very close proximity to existing buildings. The prediction of excavation-induced deformations, therefore, becomes a key issue in the planning and design process for these schemes. However, current design approaches are conservative and often lead to unnecessary concern and expenditure in the design and provision of protective measures. A better understanding of the mechanisms involved in the excavation soil–structure interaction could reduce costs and help avoid potential problems. A series of small-scale model tests was carried out in the geotechnical centrifuge at Cambridge University to investigate the interaction between excavations and model buildings. Excavations (simulated by adopting a novel two-fluid technique) in a “free field” were also undertaken to assess the difference between free-field ground movements and those affected by a stiff model building. A detailed description of the centrifuge models and test procedures is presented in this paper, followed by the presentation of test results that demonstrate the effect of the stiffness of the model building on the excavation-induced displacements.

Ground movements above tunnels: a method for calculating volume loss

Excavation of a tunnel induces small movements at the surface of the ground above. Those movements are often estimated by empirical methods. This paper presents a simple derivation of formulas for elastic ground, based on the reciprocal theorem. It confirms that the effect of removal of the weight of the soil in the tunnel is not negligible.

Centrifuge Modelling of Diaphragm Wall Construction Adjacent to Piled Foundations

Diaphragm walls are often implemented to support the sides of deep excavations and in urban environments situations increasingly occur where it is necessary to install them close to existing piled structures. There is a lack of good quality field monitoring case studies to help understand this complex three-dimensional soil–structure interaction problem. It can be investigated using numerical analysis or an alternative approach is to perform small-scale model tests in a geotechnical centrifuge. This paper describes a sophisticated model construction system that was developed to simulate the construction sequence (excavation and casting of concrete) of a single or series of three diaphragm wall panels in sand as part of a study to investigate the influence of constructing diaphragm walls adjacent to piled foundations. Prior to wall construction an adjacent instrumented model pile was driven in-flight and a constant force maintained at the pile head to simulate axially loaded piles. The system response was captured through the use of miniature soil stress cells, LVDTs, and laser displacement sensors. A detailed description of the centrifuge model and test procedures developed is presented and the necessary simplifications and associated errors discussed. The effectiveness of the small-scale physical model is demonstrated by presenting some selected test results (soil stresses and deformations).

Tunnelling close beneath an existing tunnel in clay – perpendicular undercrossing

A series of centrifuge model tests in clay was carried out to investigate the response of an existing tunnel at different clear distances to new tunnelling. A three-dimensional (3D) staged tunnelling model was adopted to simulate a wide range of tail void volume losses for the new tunnel construction while monitoring detailed 3D soil surface settlements and tunnelling-induced strains in the existing tunnel lining. This paper also presents a detailed case study of a similar scenario in the London Underground redevelopment of Bond Street station; various state-of-the-art instrumentation methods, including fibre optic Brillouin optical time domain reflectometry, instrumented tunnel bolts and photogrammetry, were deployed to monitor the response of the existing Royal Mail tunnel due to the new tunnelling works close beneath. The combination of field and centrifuge modelling data provides important new insights into the deformation mechanisms encountered in such complex tunnelling scenarios.

Modelling the 3D brittle response of masonry buildings to tunnelling

Raw data supporting figures in the publication: G. Giardina, S. Ritter, M.J. DeJong, R.J. Mair, Modelling the 3D brittle response of masonry buildings to tunnelling, Proceedings of the International Conference on Structural Analysis of Historical Constructions, SAHC 2016

Monitoring the effects of tunnelling under an existing tunnel-fibre optics

Underground tunnel networks are at the heart of United Kingdoms infrastructure, carrying more than 1,100 million passengers each year along its 249 miles long network via 11 underground lines, serving 270 stations.With a complex existing underground rail network already in place; it is inevitable that new tunnels will be constructed within close proximity to existing tunnels. At London Liverpool Street Station, the new eastbound Crossrail platformtunnel was constructed underneath the existing Royal MailTunnel at a parallel alignment over a length of over 100m with a clear distance of less than 2m separating the two. Fibre optic strain sensing system based on Brillouin Optical Time Domain Reflectometry (BOTDR) was installed to measure continuous strain profiles of the cast iron linings of Royal Mail Tunnel. This has provided valuable insights to the deformation mechanisms both during the pilot and final tunnel enlargement.