Groundwater causes extreme geotechnical problems in excavations such as sand running for most of construction projects such as tunneling. So, issues caused by ground water would increase construction budget and extend construction time unless the ground water is properly controlled which is the root cause of the problems.
There are two major methods for controlling ground water including pumping method and exclusion method as shown in figure-1.
Fig.1: Ground Water Control in Excavations by Exclusion
Fig.2: Controlling Ground Water in Shafts
There are cases in which the application of pumping techniques to control ground water is not recommended, for example, in water bearing rock formation and high permeability ground.
This is because utilization of large capacity pumps, which are required for high permeability ground and formation of well points in rock formation would be considerably costly.
So, it would be economical to consider exclusion approaches in the aforementioned situations and alike cases.
There are number of methods by which ground water exclusion are obtained:
This strategy is considered in water bearing rock formation or high permeability ground where the use of high pump capacity or digging well point is expensive.
In this technique, the permeability is reduced by creating an impervious barrier by injecting suspension substance or fluids into the fissures of rocks or pore spaces. Fineness of fissures in rocks or soil particle size distribution would control types of substances used for grouting.
This means that the grout substance particle size must be considerably smaller than the pore spaces. Figure-3 illustrates limiting particles sizes of substances which may be grouted by different types of grout.
Fig.3: Various Types of Soils Which can be Grouted with Different Types of Grouting Material
Additionally, groutability ratio, which is the ration between D.15 size of soil to the D.85 size of grouting substance particle, is also used to determine suitability of suspension grouts.
So, suspension grouts would not be appropriate choice for the soil under consideration unless the groutability ratio is higher than 5:15 for clay grouts and 11:25 for cement grouts.
Furthermore, it is necessary to pay attention to the quantity of substances used for grouting since it could be costly if the excessive amount is employed. This concern might be dealt with by considering chemical grouts even though its cost is higher than clay and cement for the same quantity.
As far as fluid gout is concerned, it is more effective than suspension grout since it fills all pores and spaces in soil whereas small size pores would be left empty in the case of suspension grout.
When grouting approach is considered, it is needed to practice great care regarding structures and facilities such as sewer sanitary system around the grouting area. This is because grouting is conducted under great pressure, so it might impair considerable damage to these facilities.
Fig.4: Distribution of Grouting Pipes Around Excavation Area
Finally, there are three major methods for injecting grouts which are offered in Table-1 along their application conditions and procedures.
Table-1: Principle Methods for Grout Injection
Fig.5: Tube-a-Manchette Used for Grouting in Soils
Chemical consolidation approach is suitable for sandy gravels and fine grading sands. The most usual chemical material used for chemical consolidation is the sodium silicate. If the sodium silicate is mixed with other chemicals, moderately strong and insoluble silica gel can be produced.
Two approaches have been practiced to conduct chemical consolidation, namely, two shot process and one-shot process. By and large, the latter process which is the most common one has replaced the former process.
In two shot process, two pipes with spacing of 50cm are forced into the ground, then sodium silicate are driven to one pipe and calcium silicate injected into the other while they are pulled up gradually.
Alternatively, one chemical is injected while the pipe is driven into the ground, the other chemical substance is driven though the pipe as it is withdrawn.
As far as one-shot process is concerned, chemical grouts are usually created prior the injection process. So, the most crucial consideration in this technique is to postpone the formation of grout gel. This is because grout penetration would be easier and more effective when its viscosity is low.
Fig.6: Chemical Grout Formation prior to Injection
Therefore, it is desired to have low viscosity grout during injection and the increase in grout viscosity occurred after the completion of injection process.
Finally, several attempts have been made to achieve gouts with such favored property, for instance, resins and lignins and acrylic polymers.
Fig.7: Acrylic Polymers
There are several factors that motivate the application of compressed air to control ground water in excavations. For example, the use of other ground water control methods is not possible due to hydrological conditions.
The use of compressed air is advised in the case where environmental concerns are encountered specifically when ground water employed as a reservoir for drinkable water, consequently the use of solid materials like cement is prevented.
Compressed air technique is commonly employed for controlling ground water in excavations of tunnels and shafts.
Controlling ground water by compressed air cannot be carried out unless certain conditions are met. Firstly, the side walls and lid of the structure in which air is kept must be nearly impermeable. Thirdly, air static pressure throughout the entire dry hollow space of the structure is must be constant Secondly, compressed air static pressure shall be equal to the hydrostatic pressure of ground water at the lowest point of the to be maintained dry.
In order to achieve the above conditions, the following construction processes must be considered.
Fig.8: Preparation for Compressed Air method to Control Groundwater, Diaphragm Wall building
Fig.9: Removing Waste Materials from Excavations
In this method, substantial care must be practiced to prevent undesired events since compressed air methods involves high level of risk that could lead to human loss.
Fig.10: Excavation and compressed air application, P L : air pressure height in the tunnel, D tt : distance between artesian ground water and invert W K : artesian Tithonian water pressure
Controlling excavation ground water by freezing is not recommended to use unless all other approaches fails to offer desired result or inappropriate to choose due to certain factors. This is because the cost of controlling ground water by freezing is significantly high due to large number of boreholes required to be drilled around the excavation area.
on the other hand, there cases in which freezing is the only practical method to control ground water for instance in extremely deep shaft excavation where the pressure of ground water is seriously high.
To prevent the formation of unfrozen spaces in the frozen area, boreholes shall be exactly vertical and errors must be kept as minimum as possible in addition to provide small spacing between boreholes.
Regarding disadvantages of freezing method, considerable time needed for the completion of drilling boreholes, installing plants, freezing grounds and certain types of soils might experience heaving.
Added to that, compressed air operation is possible to hinder due to low temperature of excavation and construction activities such as concreting will face difficulties.
Nonetheless, it must be known that the most outstanding benefits of freezing approach is the effective controlling of ground water which other approaches are lacking.
Freezing procedure involves drilling boreholes around excavation area, then inserting an outer plastic or steel tube with diameter of 10-15cm and an inner tube of 3.8-7.5cm into the boreholes, the outer tube end is closed whereas the inner tube end is opened.
The upper end of inner tube is connected to refrigeration plant from which cooled brine is pushed into the inner tube and after that returns to the refrigerator plant. The time during which the ground is frozen ranges between 1 to 4 months.
Fig.11: Controlling Ground Water in Excavation by Freezing approach
Finally, it is recommended to use liquid nitrogen rather than brine because freezing time would be reduced considerably. One might argue that the liquid nitrogen is expensive but its low construction cost may offset that and it freezes the ground five times faster than case where brine is used.