Subsurface water is often a critical factor in various engineering works. It may pose problems not only during the construction stage but it has a potential to affect the safe functioning of an engineering project. Almost every engineering project like, buildings, highway pavements, airport runways, dams, underground structures, structures on rock and soil slopes may be affected in one way or other by the subsurface water, which may lead to failure of such projects, thus causing loss of life and property. Therefore it is an important aspect of any engineering geological investigation to assess the possible adverse effects of the subsurface water on the proposed engineering projects. In fact it is not only the subsurface water which will be affecting the engineering structures but in many cases the construction of engineering structures will be responsible in changing the subsurface water regimes. Particularly the underground structures and the reservoir projects have a potential to change the subsurface water regimes considerably. Therefore, it becomes important that engineering geological investigation of a proposed site must address and evaluate the possible impact of construction on existing subsurface water regime conditions and of existing subsurface water regimes on the proposed construction. Another purpose is to anticipate what can be expected during construction besides to develop criteria for design and construction.

Engineering Significance

Subsurface water affects site selection, cost, durability and even the safety of the structures. Subsurface water plays a prominent part in slope movement, volume changes by shrinkage and swelling and collapse of loose soils. It may also erode the foundation of the structures, Rocks and soil properties, which forms the foundation of the structures, are often changed by groundwater. Subsurface water may influence excavation and construction methods by flowing into excavations, by producing seepage forces and uplift pressures and also by its corrosive action. Subsurface water conditions may also affect underground waste disposal.

Natural subsurface water regimes may be directly influenced by hydraulic structures. Therefore, one aim of engineering geological investigation is to facilitate by the provision of subsurface water data/ information, is the prediction of undesirable changes in the subsurface water regime and the recommendation of procedures to avoid them. In engineering geological investigation the following important information on subsurface water conditions should be evaluated;

• the distribution of sub-surface water
• water content
• direction and velocity of subsurface water flow
• springs and seepages from individual water bearing horizons
• depth to water table and its range of fluctuation
• regions of confined water and piezometric levels
• hydrochemical properties such as pH, salinity, corrosiveness
• presence of bacterial or other pollutants.
  

Subsurface water conditions are significant for three major aspects of civil engineering works;

• subsurface water may pose a problem to construction, 
• it may be an erosive agent that degrades the foundation of the structure and 
• subsurface water may be critical to the functioning of the structure.

Problems Created in Constructions

Construction of engineering structures may be made difficult or even impossible by subsurface water. Neither excavations nor constructions can be properly carried out with the presence of excessive water. This fact is true not only for major projects like dams and big underground cavities but equally important for small constructions like building foundations.

Tunnel and other underground structures

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

The most serious problem encountered during excavation is the sudden inflow of large quantities of water. Ground water and seepage pressures are major factors for flowing and squeezing ground. The possible high corrosive effect of water on lining of the opening should also be considered.

On the other hand construction of a tunnel may vitally change the water regime of a locality. The concept “water regime” involves location of water within the rock, direction and velocity of its movement and changes in both location and motion in time, seasonal and otherwise. The tunnel introduces changes into the water regime in the same way as it changes the local stress conditions. Generally, speaking a tunnel act as a drain. Tunnel driven under lakes, rivers and other surface water bodies may tap a considerable volume of flow.

Water Tables and Tunnels

The ground water table in the rock may be either local or bounding a limited water accumulation or continuous over a certain distance. It is obvious that a tunnel located above the water table is safe from water invasion, hence it is desirable to locate the tunnel above the water table. However, this is not possible always. Construction of tunnel not only modifies the sub-surface water regime, but also influences the surface flow of water in the locality with a corresponding lowering of the water table. Even in some cases the water in wells and spring drained dry by the excavation of a tunnel.

As a rule, the amount of water flowing in the tunnel decreases as the construction progresses. This phenomenon is caused by the gradual exhaustion of water at the source of flow and especially by the decrease in hydraulic gradient and hence in velocity of flow. As the flow progresses, the level of water at the source drops with a consequent decrease of hydraulic head. Correct estimate of water inflow into the tunnel to be constructed is of great importance, as it influence the construction works.

Engineering Significance of Subsurface Water in Dam Projects

The subsurface water has a great significance in the dam projects. The dam foundations are laid down at great depths depending upon the height of the dam and the foundation conditions. In most of the cases the deepest foundation level of the dams are below the natural subsurface water levels. Thus, there will always be an inflow of water into the excavation for foundation, which may hinder the construction activities. In most of the cases this inflow of water may be checked by pumping out this water however, in exceptional cases the inflow of water may be more than what is being pumped out. Such conditions may create great problems in excavation and concreting.

The major engineering significance of subsurface water conditions in dam projects is summarized below;

• The retention of water in the reservoir basin is determined by the piezometric level and subsurface water divide position with respect to the reservoir level. 

• The presence of subsurface water in rock mass causes water pressure within the discontinuities, which results in reduction of cohesion and may cause problem of instability of reservoir rim slopes and the abutments.

• Subsurface water may pose problems in excavation and construction activities.

• Subsurface water may be an erosive agent that may damage the structure.

• Subsurface water fluctuations may cause uplift problems in the dam foundation area.
  

Dams are generally constructed across the river valleys to impound water behind it to form reservoirs. An ideal site for a large impounding reservoir is a gorge with steep banks upstream so that a small dam can impound a large volume of water with a minimum extent of water spread. Leakage from a reservoir takes the form of sudden increases in stream flow downstream of the dam site with boils in the river and the appearance of springs on the valley sides. It may be associated with major defects in the geological structure such as solution channels, fault zones or buried channels through which large and essentially localized flows takes place. Seepage is generally more difficult to trace especially in connection with the investigation of solution channels in limestone.

In some cases leakage from reservoirs can be a potential source of trouble. When a dam impounds a body of water behind it, the rocks forming the floor of the reservoir are subjected to considerable hydraulic pressure. The water under hydraulic head will find its way through interconnected joints, bedding planes and porous rocks. If these joints and bedding planes dips towards the adjoining valleys this leakage from the reservoir will be considerable.

Piezometric Conditions

The water from the reservoir may seep through the flanks of the reservoir and the river bed and may recharge the subsurface water regime of the area. However, it is not always true that the reservoir water will recharge the subsurface water of the region, the flow in some conditions may be from the subsurface water towards the reservoir. For this the factors which influence the water leakage from the reservoir are; (i)    Piezometric conditions in the reservoir’s floor & flanks and (ii) Natural permeability of the floor and flanks

Subsurface Water Conditions Existing on the Flanks of a Reservoir

Knill (1971) explained that four subsurface water conditions exist on the flanks of the reservoir these are;

• Condition First - The subsurface water divide and piezometric level are at a higher elevation than that of the proposed top water level. In this situation no significatent water loss takes place. Seepage through a reservoir margin is determined not by permeability but by pre-existing ground water behaviour. Permeability with depth decreases and at extreme depth the flow rate is negligible.

TWL  -  Top water LevelP       -  Groundwater Pressure Fig.1    Reservoir conditions in relation to differing water table and piezometric  situations.

• Condition Two -The sub surface water divide, but not the piezometric level, is in excess of the top water level of the reservoir. In these circumstances seepage can take place through the separating ridge into the adjoining valley. Deep seepage can take place but the rate of flow is determined by the in situ permeability.

• Condition Three - Both the subsurface water divide and piezo-metric conditions are at a lower elevation than the top water level but higher than that of the reservoir floor. In this case the increase in subsurface water head is low and the flow from the reservoir is not increased significantly. Deep seepage from the bed of the reservoir may be initiated under conditions of low piezometric pressure in the reservoir flanks. 

• Condition Four- The water table is depressed below the base of the reservoir floor. This indicates deep drainage of the rock mass or very limited recharge. A depressed water table does not necessarily mean that reservoir construction is out of question but subsurface water recharge will take place on filling, which will give rise to a changed hydrogeological environment as the water table rises. In such cases impermeability of the reservoir floor is important.
  

Building Foundations and Failure of  Pavements

The subsurface water has a potential to change the engineering properties of the rocks and soils. Thus, the bearing capacity of soils may be considerably reduced because of the subsurface water. The strength of the rocks is also affected when they are saturated with water. Some of the soils possess expensive nature and when these soils are subjected to change in moisture level they may expand thus any structure constructed over these soils may be damaged because of the uplift pressures. Similarly, during the seasonal fluctuation of subsurface water the soils may be unsaturated and thus the soils may shrink which may result into settlement of the structures constructed over them. The deformation of the expensive soils, as foundation material, may result into uneven settlement or upheaval of the structures, this may result into cracks in the walls and the floor of the structure, however in extreme condition the entire structure can fail.

The role of subsurface water in pavement design has an important consideration. The fluctuations in subsurface water conditions are responsible in changing the properties of the subgrade. Repeated wetting and drying of the subgrade may considerably reduce the bearing capacity of the soil. Generally, it happens that the wet soil along the shoulders dry faster than the crown area of the pavement this results in variation of the strength of the subgrade material. Such variations in the strength property of the subgrade material results into longitudinal cracks in the pavement.  

Engineering Significance of Subsurface Water in stability conditions of slopes

The important factors which influence the stability of the rock or soil slopes are; slope angle, dip of the failure plane or plunge of the line of intersection of two wedge forming planes, shear strength parameters, cohesion and angle of internal friction and finally the water saturation condition.

For the stability condition either of the above mentioned factors may have comparatively high or low influence. Some of the factors may be responsible in inducing the forces for sliding where as, the others may contributes in providing the resistance to the sliding. The net result of these factors will defines whether the slope is going to slide, under the given conditions, or it will be stable. The relationship of the factors responsible for inducing sliding and the factors responsible to provide resistance to the sliding is defined by the limit equilibrium. Factor of safety can be defined as the ratio of the total forces available to resist sliding to the total forces available to induce sliding. If the factor of safety is less than unity, it implies that the resistive forces are less than that of the forces, which induce sliding. Thus, for the condition when the FOS is less than 1.0 the slope may fail, provided it is subjected to a triggering factor, natural or man made. The factor of safety of a potential unstable slope may be improved by providing several methods such as, safe slope design by altering the geometry of the slope, reinforcement by providing rock bolts, retaining structures, shotcreating etc. The role of subsurface water in stability condition of a slope is very crucial. As it has a potential to saturate the soil mass and thus increases the overall weight of the soil mass.