The art of Tunnelling
Tunnelling is the art of dealing with “Geologic Uncertainties”. Different methods of tunnel construction are considered and their details deliberated upon as construction requires large excavations of soil, rock etc. However, modern equipment has made excavation and backfilling easier.
Tunnels are being constructed for roadways, railways, and even as waterways. In cities like Mumbai, Metro Rail works predominantly underground. There is a tendency in the development of public underground infrastructure projects to fund projects either on bi- or multi-lateral bases. It is in the interest of owners to achieve contractually well-structured and balanced contracts in accordance with international standards to increase transparency, as underground infrastructure projects are vulnerable to corruption, and mostly blamed on geologic uncertainties.
Through extensive training programs, backed by best business practices, there is an impact of added engineering value reflected in the life cycle cost and project success. The procurement phase has the greatest impact on the life cycle cost of projects, yet it is the least costly component. Contracts shall render “Geotechnology” coherent with essential clauses, detailed definitions, and a consistent structure. Buildings and engineering works are designed by, or on behalf of, the employer.
Risk shall be allocated to the party, which is best placed to control it, to bear it, and to deal with it. Acting as third party, contracts shall be drafted by consulting engineers who are experienced in the design and management of projects. Contracts should be complete and flexible, ranging over most needs and readily adaptable to fit requirements. The contract shall be administered by the engineer, appointed by the employer.
The stakeholders in the contract are the owner/employer, the contractor, and the engineer being the employer’s representative, whereas contractual clear relationships shall be established between employer and engineer as well as between employer and contractor. There shall be only an administrational relation between contractor and engineer. For the settlement of disputes, a dispute adjudication board shall be established above employer and contractor.
Contracts shall be featured by balanced terms and clauses, allowing for an application of common laws including civil laws. They shall be widely applicable under various project delivery and contracting concepts. More specific provisions shall be included regarding obligations of parties and their rights.
Structural & Contractual Tunnelling
The ground, forming a geostatic load resulting from excavation of a tunnel, can be turned into mass material providing support to any just excavated cavity. To achieve structural understanding, the engineering model had to be developed in an uncountable number of different geo-mechanical conditions.
The basic idea of an advanced Tunnel Construction Concept was born, whereas the ground itself, when properly treated and carefully controlled by monitoring, forms the most essential part of the lining. When looking into the interaction of lining and ground between stress, strain and time, the goal to be achieved is to be seen in the transfer of primary stress of the original equilibrium (prior to excavation) into secondary stress of the new equilibrium, after excavation and lining installation have been completed.
Due to the infinite variety of conditions and when compared with defined materials like concrete, shotcrete, steel and others, and in comparison with other civil engineering structures, e.g. in bridge construction, the use of universal applicable mathematic formulas for definition of the design of an underground structure is still to be seen in the context of geologic investigations and interpretations, latest state-of-the-art recommendations, guidelines, and, most important, on site construction experiences. It all results in modelling, respective design, and construction of the very tunnel structure.
Underground Structures in urban areas are almost universally applicable, and there has never been more experience available than today. Where the ground does not provide the required strength, there are means and methods for ground improvement available. The client along with his design consultant should decide (based on risk, safety, environmental impact, cost, and time), whether the implication caused by ground improvement does justify the choice of ground improvement measures.
In TBM Tunnelling as well as in Conventional Tunnelling, ground reactions e.g., surface settlements, deflections of tunnel linings, tunnel face reactions, and any impact to existing neighboring structures, need to be monitored and controlled by giving flexible responses via toolbox of additional support measures, as required.
Management procedures shall correspond with the latest developments. Late payment problems shall be tackled, and the role of the engineer updated. To have the best control of projects, the responsibility of the client must include proper organization in various design phases and his involvement in main project aspects in order to make good choices of most suitable conditions of contract. Upon project analysis, subject, and type of contract (construction only, design & build, etc), risk sharing (construct or be involved in design), intended management of contract/project, type/method of payment, shall be decided.
Objectives of infrastructure construction contracts are an increase of supply of e.g., energy through addition of renewable, low-carbon energy, strengthen capacity regarding preparation/implementation of economically, environmentally, and socially sustainable projects. There are two early project objectives, construction of Project itself and support for Capacity Building and Institutional Strengthening of developer.
Financing institution e.g., TWB, want to engage Consultants in support of the owner in the review and improvement of project management systems and process, especially with respect to schedule, contract and risk management. Consultant’s main task, envisaged to be carried out, is reporting.
Tunnel projects shall start with a comprehensive investigation of ground conditions by collecting samples from boreholes and other geophysical techniques. An informed choice can then be made of machinery and methods for excavation and ground support, which will reduce the risk of encountering unforeseen ground conditions. In planning routes, horizontal and vertical alignments optimization of ground and water conditions shall be achieved.
Independent from choice of tunnelling technology, geotechnical key factors are “stand-up time”, being the amount of time, a newly excavated cavern can support itself prior to any added structural support. Knowing this parameter allows the engineers to determine how far an excavation can proceed before support is needed, which in turn affects the speed, efficiency, and cost of construction.
The Curtailed Genesis of Tunnelling
Bochum Metro Section A3/A5. First Single & Double Track Shallow Soft Ground Tunnels using NATM Design. Geomechanic know how from “Mountain Tunnelling” got transferred into “Urban Tunnelling”. Experiences with Finite Element Calculations for the design of the Waldeck II Cavern Powerhouse have been used for the design of metro tunnels and metro stations to simulate ground behaviour and to design sprayed concrete support.
Munich Metro, Section 5/9-5. Single Track Tunnels. Previous TBM experiences with one pass precast concrete lining segments have been published following Frankfurt Metro Projects. In Munich, precast single lined designed tunnel segments with unified, auto-connected and double converging segments in both joint types have been used first time. Station cross section has been expanded from 120 m2 (Bochum) to 180 m2 (Munich). Findings from the Geomechanic Technology of NATM design and construction have been transferred on a global scale into TBM Technology with single lined precast concrete segments.
About 40 years ago there have been intense discussions when it came to evaluation, choice and decision making between NATM/CTM (Conventional Tunnelling Method) and TBM (Tunnel Boring Machine Method), called Mechanized Tunnelling. Since then, a gradual approximation between the two Tunnelling Techniques, accompanied by in situ Observation & Monitoring has taken place.
Mechanized Tunnelling Technology
Various Types of TBMs are used to highly automate the entire tunnelling process, reducing tunnelling costs. In certain predominantly urban applications, tunnel boring is viewed as quick and cost-effective alternative to laying surface rails and roads. Disadvantages of mechanized tunnelling arise from usually large size equipment - difficulty of transporting the large machine to the site, or (alternatively) high cost of assembling on-site.
As both Techniques have Pro’s and Con’s, it is fair to state, that both Tunnelling Techniques have benefitted from each other. The fields of competition have shifted from Geomechanics towards Contract, Schedule, Cost, Geotechnical Baselines and Risk Management, with Innovation in Tender Document Preparation.
Conventional Tunnelling Method (CTM)
Also named “The New Austrian Tunnelling Method” (NATM) was developed in the 1960s, and is the best known of a number of engineering solutions that use calculated and empirical real-time measurements to provide optimized safe support to the tunnel lining. The main idea is to use geological stress of surrounding rock mass to stabilize the tunnel itself, by allowing a measured relaxation and stress reassignment into surrounding rock.
With Conventional Tunnelling, design parameter designation shall be carried out by the clients engineering team, by the consultant’s design offices, or by the construction contractors design team, where the consultant is cooperating closely with the client being finally responsible for design in all phases of the tunnel respectively the cavern project.
Various types of tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the ground water conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and shape of the tunnel and appropriate risk management.
Tunnel Boring Method
Bored tunnel method is a modern technology, where tunnel boring machines are used which automatically work and makes the entire tunnelling process easier. It is also quicker process and good method to build tunnel in high traffic areas. Tunnels boring machines (TBM’s) are available in different types suitable for different ground conditions. These machines can be used in difficult conditions such as below the water table etc. A special pressurized compartment is provided for TBM to work in below water table conditions. The workers should not enter that compartment except for repair works. Care should be taken while TBM is in working conditions.
Cut & Cover, Immersed, Other Hybrid Technologies
These technologies of tunnel construction are generally used to build shallow tunnels. In this method, a trench is cut in the soil and it is covered by some support which can be capable of bearing load on it. The cutting can be done by two methods. One is bottom-up method in which a tunnel is excavated under the surface using ground support. Another method is top-down method in which side support walls are constructed first by slurry walling method or contiguous bored piling. Then roof is located on the top of the walls and excavation is carried out. Finally, base slab is constructed.
This report on “Infrastructure Changing the World” by Harald Wagner c/o DHW Consulting Engineers, Bangkok/Vienna has been supported by TAI (Tunnelling Association of India), associated with CBIP (Central Board of Irrigation and Power) of India. Underground infrastructure engineering is one of the most challenging fields of advanced technology as it requires the capability to combine structural engineering with geotechnical and environmental engineering. The report has been derived from the book “Usable Grounds to Unite Sustainable Above and Below Grounds”, which will be launched by the end of 2022. It is written to guide academics and practitioners of the underground construction industry, project owners, consultants, contractors as well as young engineers into the world of underground infrastructures. Since, every underground infrastructure project is unique, the book attributes special emphasis to the satisfaction of engineers resulting from innovative solutions.