Underground Structures

Design and construction of under-ground structures require thought processes and procedures that are in many ways different from other design and construction projects, because the principal construction material is the rock mass itself rather than an engineered material. Uncer-tainties persist in the properties of the rock materials and in the way the rock mass and the groundwater will behave. These uncertainties  must  be  overcome  by  sound,  flexible  design and redundancies and safeguards during construction. More than for any other type of structure, the design of tunnels must involve selection or anticipation of methods of construction.

The underground structures or openings can broadly be classified into three categories; (i) Tunnels, (ii) Cavern and underground openings and (iii) Cavities and mines.

Tunnels

Tunnel is a horizontal or near horizontal excavation that is open to the ground surface at each end. Tunnel are constructed in soil or rock for transportation system water supply, sewerage and hydropower purpose.

Tunnels, other than mine tunnels are essentially an element of transportation. Tunnels are used to conduct the line under a natural obstacle, such as a hill or ridge. In cities, tunnels carry underground railroads and highways which because of traffic complexity, cannot be built over the ground. Tunnels are also used for water supply and sewage disposal. For hydropower generation projects tunnels are used as water conducting system.

Engineering Geological Studies For Tunnels

The extent of engineering geological studies depends on the stage of the project and on the importance of the work. The important investigations which have to be carried out for the tunnels are;

• Study of geological conditions of the area and of the properties of the rocks. The results and general findings/observations are taken into considerations in choosing the general alignment of the tunnel. 
• Hydrogeological conditions investigation of the area along the tunnel route, from which it may be estimated in which sections and in what quantities inflows of water into the tunnel can be expected.
• Hydrogeological conditions are of particular importance in planning tunnels for water supply and all tunnels crossing areas where corrosive waters are present. The occurrence of corrosive water is so undesirable that if possible, such sites should be avoided. In some cases, the presence of gas and the temperature of the environment need to be taken into account when assessing the suitability of the site.
• When the general alignment of the tunnel has been selected, features controlling its exact position are investigated.
  

The tunnel should not be located in rocks disturbed by weathering or disrupted by faulting. If the fault zone cannot be avoided, it is desirable to align the tunnel perpendicular to the fault plane. The major joints and fracture systems are also the controlling factors, particularly in underground openings of large diameters.

Location of tunnel entrances : Care must be taken not to disturb the stability of slopes by approach cuttings. Sliding movements and pressure could delay the working progress.

In order to estimate the cost of the tunnel construction and to elaborate the detailed project, pressure on tunnel lining must be determined and tunneling methods should be considered.

Assessment of drillability and excavability of rocks is necessary for providing the construction site with adequate equipment.

Co-operation in securing necessary aggregate material for tunnel lining, since its supply must be continuous as the work progresses.

Study of the effect of tunnel working on the surrounding area.

The drawdown of the ground water table may result in the disappearance of water in wells and springs. Subsidence of the ground surface may damage buildings, transportation routes and others.


In case where a geological map of at least 1:25000 scale is not available, reconnaissance investigation must involve detailed mapping of a wider area along the tunnel line. Mapping of a narrow belt would not give a complete picture of geological conditions. For deep mountain tunnels the whole drainage area of the designed tunnel should be mapped. During reconnaissance investigation, dips and strikes of beds cropping out in all valleys and gourges transecting the mountain rage are measured, particularly in those running parallel to the tunnel route. Deep valleys and gorges in the proximity of the tunnel are the best source of information for structure and general geological setup likely to be encountered in the tunnel alignment. Where there is lack of natural outcrop or the locations where thick soil cover is present subsurface explorations are necessary. If the boundaries of individual strata is covered by superficial deposits, the use of test pits is advisable. On the basis of geological map, supplemented by the results of field and subsurface exploration, longitudinal profiles are constructed. However, prediction of geological conditions for deep, long tunnels is a difficult task. Because of the lack of well exposed boundaries between geological formations and minor inaccuracies of the topographical maps, geological methods do not always furnish actual conditions likely to be present in the tunnel alignment.

Engineering Geological Explorations For Tunnels

Geological, geomechanical and hydrogeological factors determine the degree of difficulty and cost of construction for any underground structure. For the design and construction of any underground structure following data/ information is required;

• Geologic profile covering data on stratigraphy, structure and identification of principal rock types and their general characteristic. 
• Rock mass characteristics and geomechanical properties.
• Hydrogeological data and information.
• Exposure to construction risk
  

The underground project is conceived, defined and broadly scoped out during the reconnaissance phase. Engineering geological information required during this phase is obtained mostly from secondary data. At reconnaissance stage emphasis is on defining regional geology and the basic issues of design and construction. At this stage data acquisition includes;

• Availability of data on geology, hydrogeology. 
• Remote sensing data.
• Preliminary geologic field mapping.
• Selected exploratory boring in critical location
  

Field Mapping

Initial field studies should start with a careful reconnaissance over the tunnel alignment, paying particular attention to the potential portal location. Features identified on maps and air photos should be verified. Rock outcrops, exposed in road cuts or along the banks of the stream provides a information on lithology faults, joints and contacts of various lithologies. The field survey must account for features which may pose difficulty during excavation for underground structure, such features may be;

• Slides particularly in portal areas. 
• Major faults
• Sinkholes and Karstic terrain
• Hot springs
• Volcanic activity
• Anhydrite, gypsum or swelling shales
• Caves
• Stress relief cracks
• Zones of deep weathering
  

Onnce alignment and portal site alternatives have been finalized, a detailed geological mapping should be carried out. Data pertaining to joints, faults and bedding planes should be collected and mapped. Stereographic analysis to workout the preferred orientation of joint planes should be done. The major predominant joint system and their variation along the alignment must be determined, by projecting them over the proposed alignment. Thus, based on the surface mapping the projections must be made on proposed alignment so that the geological conditions in the proposed tunnel alignment are well predicted in advance.

Ground water has the potential to cause great difficulties for underground work, efforts should be made to define the groundwater regime. Data/ information on aquifers, sources of water, water quality and depth to groundwater should be collected. Mapping of perennial streams and other water bodies should be carried out. Proximity of the ground water table may judged by the type of vegetation growth on the site. As a part of the hydrogeological survey, all existing water wells in the area should be located and the ground water levels should be taken. Additional hydrogeological work to be carried out at a later stage including measurements of groundwater levels or pressure in boreholes, permeability testing using  packers etc.

Geophysical methods of exploration are useful at the early stage of a project. Because the geophysical explorations are relatively inexpensive and can cover relatively large volumes of geologic material in a short time. Most common geophysical explorations carried out for underground works are seismic refraction or reflection and electrical resistivity surveys. Seismic explorations can measure the seismic velocity of underground materials and discover areas of velocity contrasts, such as between different kinds of rocks or at fault zones. They are also useful in determining the elevation of the ground water table. Seismic velocity can also describe the quality of rock likely to be encountered along the tunnel alignment.


It is appropriate to conduct initial field explorations in the form of borings or trenching at the early stage. This is primarily to verify the presence or location of critical geologic features that could affect the feasibility of the project or have a great effect on the selection of tunnel portals.

Explorations For Preconstruction Planning

During the engineering design phases, explorations must be carried out to acquire data not only for the design of the underground structure but also for its construction. For this reason, exploration programme for underground works must be planned by engineering geologist in close cooperation with designers and construction engineers. Most geotechnical data for design are obtained during preconstruction planning. Supplemental explorations must be carried out in the later design stage.

Environmental And Geologic Data Requirements

The specific environmental data needs for a particular underground project very much depend on the geologic and geographic environment. The factors which effects the underground exploration program are;

• Existing infrastructure, obstacles underground and above. 
• Surface structures within area of influence
• Land ownership 
• Contaminated ground and ground water
• Natural gassy ground or ground water with deleterious chemistry
• Access constraints for potential work sites and transport routes
• Sites for much transport and disposal
• Legal and environmental constraints
  

During the reconnaissance and preliminary stage the required geological data including geological profile, rock and rock mass properties, hydrogeology and exposure to geologic hazard is collected. With this data it is possible to divide the tunnel alignment into zones of consistent rock mass condition. Criteria for zonation would be site specific but the factors could be intact rock, rock mass and excavation system characteristic. Each zone should be characterized in terms of average expected condition as well as extreme conditions likely to be encountered. The stratigraphy and geological structures, as determined during preliminary study, forms the framework for zonation of the alignment and the planning of the exploration. Particular attention should be given to the following types of information;

• Top of rock, depth of weathered rock 
• Water bearing zones, aquifer, fault zones and caves
• Karstic ground conditions
• Very strong and very abrasive material that may affect tunneling equipment. 
• High stressed material with potential for over stress
• Potential for gases
• Corrosive groundwater  
• Slake susceptible material and material for potential to swell. 
• Material affected by water
• Zones of weak rock (low intact strength, altered materials, faulted and sheared material)
  

Strategies For Exploration

Because of complexities of geology and the variety of functional demands, no two tunnels are alike. It is therefore difficult to give hard and fast rules about the required intensity of explorations. However, some rules which may help in the planning of explorations are;

• Plan exploration to define best, worst and average conditions for the construction of the underground works. Locate and define conditions that can pose difficult during construction. 
• Use expert geological knowledge to produce the most accurate geologic interpretation so as to form a geologic model that can be used as a framework to organize data.
• Determine and use the most cost effective methods to discover the information sought. 
• Anticipate methods of construction and obtained data required to select construction method and estimate cost.
• Anticipate potential failure mode for the complete structures and required types of analysis and obtain the necessary data to analyse them. 
• Drill at least one boring at each shaft and at each portal.
• Special problems may require additional explorations.