SCL / NATM tunnel lining displacement monitoring - Case Study

The observational method is widely used in tunnelling works. For a successful implementation of observational method in tunnelling, an efficient monitoring, data evaluation and interpretation on site is of prime importance. Reviewing the common practice on sites in Singapore, it could be said that, there is a potential for improvement in the data collection and interpretation techniques employed for mined tunnelling works in Singapore. Although there are several advanced tools available in the market for data handling, evaluation and interpretation; there is a scope for development of simple and easy to use tools, to properly handle the spatial displacement measurements. With the adoption of Eurocode 7 in Singapore, it is further more important to evaluate the mined tunnel movements, continuously update the geotechnical model & adjust the excavation/support system to suit the actual ground conditions.

This post presents simple implementation of absolute tunnel displacements visualization in the ubiquitous spreadsheet software. A practical procedure for predicting and comparing displacement behind the face in relation to face advance and time is presented with a case study of mined tunnel in shallow overburden condition. The case study considers a mined tunnel which is in overburden ranging from 6m to 8m under a road with significant traffic as well as major utilities and services. The design included temporary excavation support using staggered piperoof,  sprayed concrete with lattice girder and wire mesh. Observations & insights provided by spatial visualization of the tunnel displacements are presented.

A custom-built spreadsheet macro was prepared for this case study project to visualize the displacement vectors at each monitoring array and deflection curve along the tunnel alignment at any point in time based on the latest raw data obtained from the instrumentation and monitoring contractor. Below figure shows the monitored displacement characteristic of the mined tunnel as vector plot in a plane perpendicular to the tunnel axis. The displacement plot in this figure is for the lattice girder location “S3-5” (which could be selected from the option button on left side. List indicates the location of all monitoring array locations). The displacement pattern can be described as almost symmetrical in the cross section with slightly increased displacement on the right side. In a homogenous, isotropic ground, the crown prism movement is expected to be vertically down however the movement is towards left side which could indicate a possibility of singularities near the face. In case there was any fault / singularities on the right side of the tunnel face, the readings on the right side (instrument P3 and P5) would register larger movements. In this manner, features outside the excavation area (faults, slickensides etc), change in stress situation and kinematics could be predicted based on these displacement vector plots. 

Custom built spreadsheet VB script to visualize displacement vector

Below figure shows the monitored displacements vectors in a plane parallel to the tunnel axis. The plot in longitudinal direction shows displacement vectors tending against the excavation direction. Influence of foliation modifies the longitudinal settlements. Using the knowledge about the influence of the anisotropy orientation on the displacement development, the geological structure encountered on site can be used to predict the expected spatial displacement vector orientation. Any significant deviation from this “normal” behaviour will thus reflect an abnormal behaviour. In case the reason for the abnormality does not originate from a failed support system, the type of deviation can be used to predict changes in the ground condition ahead of the tunnel.

Custom built spreadsheet VB script to visualize 3D prism movements

Ground settlements at different stages of excavation

Monitoring methods along with above interpretation methods could improve short term prediction in tunnelling. The information, which can be extracted from displacement monitoring data, is enormous. This insight plays a crucial role in difficult geotechnical conditions or in sensitive environments. In order to meet the requirements of a Eurocode 7 compliant “observational method” design, dedicated design preparation work prior to construction and organized monitoring system is required. This case study proves that this could be achieved easily on site without the need for any special software. 

Reference:

[1] Senthilnath G T, Mined tunnel monitoring as a part of observational approach - A case study, Tunnelling and Underground Construction Society (Singapore), 2017