Underground Space & Tunnelling - Challenges & Technology.

The geological and geotechnical investigation program is required be adequately accomplished in order to provide realistic information for economic and safe design and construction of the underground structures, writes MM Madan, former Executive Director, NHPC.

In India construction of tunnels in the form of underground passages and water carrying conduits dates back to Mahabharata era. The famous lakshagriha escape route planned by Pandavas was through secret tunnel under the lakshagriha. Many more tunnels have been found in the old forts and palaces which were used as secret escape routes. In British era many railroad tunnels were made. However, tunnelling for hydro power took a lead and most of them were constructed mostly in Himalayan terrain.

In the modern era, tunnelling and underground works constitute a major portion of development of rail tunnels, road tunnels, hydro power, mining, strategic oils reserves, water supply & sewerage, coal mines, nuclear power projects and urban area metro rail network as well as strategic secret escape routes etc.

Around the world tunneling has become a part of life. Many large and long tunnels have been completed successfully in road & rail sector. 50km long tunnels are very common today. For example, 137 km long water supply tunnel was made in 1945 in New York. Similarly, in Finland also 120 km long tunnel has been made for water supply. In rail sector China and Finland have 123 km and 100 km long tunnel respectively. In road sector the famous Gothard Base Tunnel of Switzerland is 151.84km long and has two single tunnels of about 57km length each.

The total road tunnels constructed in China are more than 15285km (2017) and followed by Japan which has more than 4026km (2012) of road tunnels and Norway 1338km (2017). India's tunneling and underground segment has witnessed significant development in recent past. For example, Metro rail network itself has executed about 500km of tunnels in India. Presently tunnels are being constructed at a pace of over 100 km per year.

India has also constructed longest 11.21 km PirPanjal railway tunnel and 9.2 km long Chenani-Nashri and Rohtang 8.9km long road tunnels in the fragile Himalayas besides 373 kms of tunneling for Delhi Metro only in the country, to name a few, completed and commissioned recently. In the Indian context, as per available data, the longest road and rail tunnels are mostly constructed in J&K, followed by Himachal. In almost all hilly areas road and rail tunnels have been planned and execution is going on at fast pace.

In addition, major road tunnel projects like 6.5 km long Z-Morh, 14.2 km long Zojilla, having strategic importance, are in hand for implementation. Sikkim is constructing rail tunnel of about 42km length besides 25km long road tunnel is also under construction. The longest power tunnel for hydro project under construction is at Parbati-stage -II project having a length of 31.5 km besides to name a few; Kishangangatunnel (14km single face), Dulhasti tunnel (10.6km), Uri tunnel (10.5km), BSL project tunnel (13.12km), Nathpa Jhakari (28km), Teesta St-3 (13.8km) & Teesta St-5 tunnel (17.2km) for hydro projects have been completed successfully.

In spite of availability of the state-of-the-art technology and equipment, the timely delivery of such projects for creating underground space has always been a matter of concern. At present, many tunneling projects are under construction in all the sectors. It is estimated that in the future, the infrastructure development will take place underground considering the environment, scarcity of land, and for the purpose of security. It has been planned to construct about 6000km of road tunnel and 1000km of rail tunnels in addition to hydro and strategic area tunnels. However, in India there are no records available for all the tunnels constructed in different sectors.

Challenges

It has been observed that tunneling projects in India faces many challenges. The tunnels which are located in Himalayas are faced with complex geological conditions and then contractual disputed override and the work gets affected. Even EPC contracts and turnkey contracts enter into unending disputes.

In execution of hydropower projects, it has been observed that, unforeseen geological conditions and associated geotechnical problems are major contributor to the cost and time overruns. Geological conditions alone cannot be blamed for all the cost and time overruns. Most of the time, the time and cost overrun are the result of inadequate investigations, inadequate geological data, inappropriate interpretation of available data and incompetency in dealing with the problems during execution and poor decision making to deal with the issues once they have arisen. The basic problem faced in attempting to predict the prevailing geological and geotechnical situations in the construction of surface structures and underground works is the inadequacy of the information obtained from the site investigations in squeezed investigation program.

The complex geological regions like the Himalayas pose major challenges for construction of hydroelectric projects. Projects located in the region have faced many geological and geo-hydrological issues while executing surface and subsurface works. The surface problems broadly include existence of inhomogeneous deep overburden in the river bed or buried valley, abrupt change in bed rock profile, occurrences of fault zones / shear zones / fractured rock in the dam foundation and slope instability in the abutments as well as rock fall /slide and failure of back slope of surface power house, etc. The major subsurface problems in tunneling and underground works include issues such as squeezing ground conditions, upheavals, tectonic stresses, popping, rock bursting, high temperature gradient, heavy ingress of water during execution, existence of large shear zones, flowing ground conditions, abrupt change in strata conditions, low cover zones or very high cover/ overburden and emission of methane and other gases etc.

Surface Structures

The occurrence of weak zones in the form of fault, shear zone etc. and buried channels is a hazard in relevance to dam foundation and other surface structures of a hydropower project. Their presence in the foundation of dams poses problems of leakage creating uplift pressures, settlement and sliding, etc. Thus the dam designers have to live with these adversities, analyzing and treating them adequately to make the structure safe beyond doubt. Treatment of these zones of weaknesses is mainly dependent on their disposition, dimensions, nature of material in the zone, its location with respect to a structure and the type of structure, etc.

Underground Works

In tunneling and underground works, the complex geological setting has caused considerable stability problems. The geological uncertainties for underground openings are related to two major factors: non-geological and geological.

-    The non-geological factors are connected to the level of skill and expertise gained by experience and the interpretation and decision making skills during the planning and construction phases of tunneling projects. The ability to evaluate and tackle the stability issues during planning and construction and the tools, methods and technologies used in that process have great significance, since erroneous interpretation may result in considerable loss.

-    The geological factors are related to the geological complexity of the region. The complexity is represented mainly by four engineering geological characteristics that have caused major stability problems during tunnelling. These are: (a) weak rock mass quality (b) high degree of weathering and fracturing (c) rock stresses and (d) groundwater effect.

It is a fact that the key to success or failure of any tunneling project is the quality of the rock mass that the tunnel passes through and rock support measures that are applied during tunnel excavation as well as the speed of providing supports so minimize the deformation of the freshly excavated rock mass. In this respect, accurate evaluation, analysis and interpretation of the rock mass quality play significant roles. A major challenge therefore is to address the geological uncertainties so that cost effectiveness and safer tunneling may be achieved.

Inaccessibility to the investigation sites of tunnels and caverns, particularly in the Himalayan region and other hilly areas, is the major hindrance in carrying out geological / geotechnical investigations. Thick vegetation, rugged topography, river crossing, obscured geological evidences along the tunnel alignments, etc., make the task of geological / geotechnical investigation more difficult. As the tunnels are very long, variations in rock types and rock mass characteristics are imminent. Due to poor accessibility it is difficult to carry out detailed explorations by drilling at the identified locations and these issues delays the investigation program.

While tunneling in complex and varied geological settings, prediction of rock mass quality, analyzing stress induced problems, squeezing  and prognostication of inflow and leakage have often been found difficult during the planning stage. Considerable discrepancies have been found between predicted and actual rock mass conditions, resulting in significant cost and time overrun for many of the tunneling projects. Finding innovative solutions for quantifying geological uncertainties is a key factor for cost effective and optimum tunneling through Himalayan rock mass.

Prior knowledge of impending geological complexities is always helpful in giving pre-emptive engineering solutions. Hence, need for adequate engineering geological investigations is necessity in order to minimize the risk of encountering unknown or unanticipated adverse geological conditions. The geological and geotechnical investigation program, which is planned in four stages viz., Pre-feasibility stage(PFR), Feasibility stage(FR), Detailed Project Report (DPR) and Construction stage, is required be adequately accomplished in order to provide realistic information for economic and safe design and construction of the underground structures. The extent of work to be undertaken at FR and DPR stages generally depend on the complexity of geological conditions at project sites. Sometimes the critical factors /problems are missed in pre-construction phase and they only come to light during construction stage, these lead to extensive delays and cost overruns. Sometimes these adverse geological conditions continue to remain unsolved even during operation phase making situation highly risky.

The experience of huge cost and time overruns has resulted in the concept of ‘Bankable DPR' supported with adequate geological / geotechnical investigations. DPR is expected to provide a roadmap of the project so that the project can be assessed techno-economically and constructed with minimum time and cost overruns. A bankable DPR is a document which can give enough confidence to the lending agencies that each component of the proposed project has been adequately explored in such a way that there will be very limited scope of deviation and the nature of work would not undergo major changes during construction stage. The constraints and uncertainties are well understood and suitable provisions are made in design so that these are not met as surprises. The main requirement of a DPR being bankable is the assurance of the project being constructed within the estimated cost and time schedule.

The problems especially while executing hydropower projects in complex Himalayan region may be regarded as challenge and opportunity for generating new knowledge base and thereby increasing self-reliance, particularly in tunneling and underground works: The construction of hydroelectric projects involving surface as well as tunnels and underground civil structures is a demanding task when these structures are located in rugged terrain under complex geological setup.

It is observed that execution of a number of hydroelectric projects in India have become hazardous, time consuming and costly affair on encountering adverse geological conditions. The key to accelerated implementation of hydropower project under such circumstance, therefore, lies in expeditiously managing these adverse occurrences.

In the recent time, development of various advanced investigation techniques and instruments aimed towards comprehensive site investigation have proved very helpful in minimizing the occurrence of geological surprises.  Besides, there has been many fold improvement in construction methodologies and construction equipment as well as modern construction chemicals, etc. that are suitable for effectively tackling the adverse geological occurrences during surface and underground construction works and mitigating the detrimental issues, thereby boosting the level of confidence in project execution.

Recommendations

The construction of hydropower projects, particularly in Himalayan settings, requires thorough investigations of geological and geotechnical features. The objective of geological /geotechnical investigations is to get the basic data for the economic, safe design and construction of surface and underground structures.

The aim should be to obtain as complete information of the subsurface geological conditions to arrive at a techno-economic evaluation of the project.

Appropriate investigations during the initial planning stage may ultimately lead to a most economic design of the structures.

Geological mapping, subsurface exploration and rock mechanical (in-situ / lab) investigations are helpful in freezing the alignment of tunnels, sitting of underground structures and fixing of dam alignment etc to ensure stability and safety of dams and as well as other surface and underground structures.

At times, situation might demand further investigations, therefore, time and quantum of site investigation should commensurate with the size and complexity of the project.

The investigations are needed to be carried out in four stages: Pre-feasibility stage (PFR), Feasibility stage (FR), Detailed Project Report (DPR) and Construction stage. Results of each stage should be carefully reviewed and a conscious decision taken for the scope of work in the next stage.

The ultimate goal should be to determine, with reasonable accuracy, the subsurface rock mass condition both for surface and underground works:

During underground excavation rock mass behavior around the opening or tunnel/cavern undergoes significant changes which results into deformation or displacement and it is important to understand how it will react to or behave during underground excavation.

Besides, the investigations would also to be value addition for mechanized tunneling equipment like tunnel boring machine (TBM), to be used as an alternative to the conventional drill-blast method (DBM).

Each underground project is unique and vast uncertainty / risk exists in these projects. The program of geological and geotechnical investigations should be framed to collect, generate and analyze engineering geological and rock mechanics data for site characterization, in different stages of investigation and also to identify potential geo-hazards that may exist at project sites.

The investigation program should effectively cater to all stages of the sites evaluation process. Based on comprehensive investigations of the various project components using advanced tools and techniques a Bankable DPR should be prepared, deliberating possible problems associated with various project components. Investigations need not cease when construction or execution has started.

Inadequate engineering geological investigation can substantially increase the risk of encountering unexpected adverse conditions that can seriously delay or even stop construction with costly consequences.

One of the difficult and intricate aspects of any geological/geotechnical investigation is deciding how much exploration to do. The amount of exploration to be done on any given project site should be determined by experience / budgetary provision. It must also be known that how detailed may be one exploration / investigation program for surface and underground works in particular, it is not that all problem areas can be duly predicted. Hence, there is a possibility of encountering adverse geological occurrences in the form of geological uncertainties, during construction stage which obviously involve physical and financial risks.

In view of the above, it should be a practice to establish a base line of geological/geotechnical data and prepare a ‘Geotechnical Baseline Report' (GBR) for defining and allocating the risk and associated cost, which may be shared with the EPC contractor. In this way the executing agency will have adequate resources and expertise in handling such adverse conditions timely, as prompt decision making by the implementing agency is the essence of the matter. This risk sharing mechanism also optimizes the cost as the contractor knows well in advance the risk and its sharing mechanism.

For efficient execution of the project, attention is required to be given towards utilization of state-of-the-art investigation instruments & techniques. With the recent technological advance, the practice of investigation has undergone radical changes. For comprehensive site investigation and successful construction of surface and underground structures in complex geological situations and in dealing with the challenging issues, critical attention must therefore be given towards prospective usage of advanced tools and techniques: 

Adoption of suitable modern construction equipment and methodologies would unravel the geological complexities and adversities well in advance: Now-a-days, with the availability of efficient construction techniques, equipment and modern chemicals, etc. it has become possible to advantageously work through the adverse geological conditions.

While other subsurface structures like surge shafts, desilting chambers and powerhouse caverns are constructed in limited spaces with lesser variations in ground conditions, the long tunnels encounter frequently changing ground conditions and face serious constraints in investigations due to inaccessibility and high superincumbent cover that pose constraints in dependable estimations. Besides application of standard investigation techniques, possible solution lies in systematic advance probing during construction: As the Himalayan geology in tunnels changes frequently, therefore, whatever amount of exploration is done on ground cannot predict exactly the type of strata that will be encountered. Therefore investigation ahead of tunnel face should become a part of excavation cycle in the form of probe drilling and underground geophysical study, etc. in order to get the geological foreknowledge of what lies ahead of tunnel face.

Preparedness at project site should always be in the form of readiness in dealing with the worst condition by keeping alternate equipment and material at the work location. Continuous geological monitoring and inputs from expert committees important in facilitating the designer to go in for mid-way correction  in the designs, in case essentially required. Prompt decision making by the implementing agency is vital in managing adverse geological occurrences.

It is important to understand that in hydroelectric project investigation, applications of various techniques at sites for surface structures and underground works would primarily be governed by the necessity as well as the adequacy of information needed and selecting the appropriate tools and techniques according.

As each project is unique and the geological issues are diverse, it is challenging task to execute in geologically complex conditions:

The geotechnical applications cannot be generalized and implemented, as a routine, at sites for similar civil structures and it is prudent that utilization of state-of-the-art techniques and instruments should in consideration of the relevance, applicability as well as the stage of the project development. Also, in the project construction stage, during execution of surface and underground works, the efficiency of project execution can be greatly enhanced by the judicious adoption of pragmatic approach, involving suitable advanced methodologies and appropriate construction equipment.

Concluding Points

-    Importance of investigation of the geological conditions; how much to investigate

-    Risk sharing by developer vis a vis contractor

-    Adopting latest execution techniques in difficult ground like ground freezing, 5 P method of excavation, DRESS methodology, pre-grouting

-    Adopting latest supporting techniques

-    Adopting latest construction equipment

-    latest construction chemicals

-    Modifying designs as per the Himalayas geology

-    Adopting various latest methods of ventilation in tunnels

-    Shortage of skilled manpower and skill development at various stages

-    Respecting dispute resolution mechanism

-    To avoid delays in struck projects, quick decision making to be inculcated- respective ministry to make strict rules- approved panel of third party consisting of experienced engineers may be created

-    NATM to be implemented in right earnest based on Himalayan geology.

-    Future tunnels to be planned with using tunnel boring machines (TBMs) depending on applicability in various regions.

-    Adoption of long term use of Instrumentation for monitoring stress and deformity after commissioning of projects

-    Provisions of geological baseline report in the bidding process

-    Importance of geophysical investigations by advance methods.

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