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 technique and exclusion technique as shown in figure-1.
Fig.1: Ground Water Control in Excavations by Exclusion
Fig.2: Controlling Ground Water in Shafts
Methods of Ground Water Control in Excavations
- Motivations or reasons that encourage the utilization of exclusion methods to control ground water in excavations
- Exclusion methods used to control ground water in excavations
Motivations or Reasons that Encourage the Utilization of Exclusion Methods to Control Ground Water
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 methods in the aforementioned situations and alike cases.
Exclusion Methods to Control Ground Water in Excavations
There are number of techniques by which ground water exclusion are obtained:
- Forming impervious barriers by grouting with cement, clay suspension
- Chemical consolidation for controlling ground water in excavation
- Ground water control by compressed air
- Freezing ground water control
Forming Impervious Barriers by Grouting with Cement, Clay Suspension or Bitumen
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 material or fluids into the fissures of rocks or pore spaces. Fineness of fissures in rocks or soil particle size distribution would control types of materials used for grouting.
This means that the grout material particle size should be considerably smaller than the pore spaces. Figure-3 illustrates limiting particles sizes of materials 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 material 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 materials 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 technique is considered, it is required 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 provided in Table-1 along their application conditions and procedures.
Table-1: Principle Methods for Grout Injection
|Grouting Methods||Suitable Conditions||Grouting Procedure|
|Open hole||Very coarsely graded soils or rocks with broad fissures||Firstly, grouting pipe, which its lower end closed by an expandable plug and upper end is sealed on the surface, is driven into the soil. Secondly, grout is forced into the pipe and driven out the plug and wide fissures will be filled with grout.|
|Stage grouting||Not specified||In this method, a hole is drilled in advance then a lance is inserted after that grouting is carried out by either bottom up method or top down method. In the former, the hole is dug and a lance is inserted into the hole then grouting is proceeded. The latter approach, the upper part of the hole is grouted and after its setting, the lower part would be grouted.|
|Sleeve grouting||Suitable for grouting soils||It makes use of Tube-a-Manchette as shown in Figure-5. After a hole is drilled to a determined depth and cased a sleeve tube is inserted and surrounded by partially plastic grout. Then, the case is pulled up and perforated injecting pipe is inserted into the sleeve pipe. finally, the grouting material is injected and plastic grout would be broken and the grout material will spread through the ground.|
Fig.5: Tube-a-Manchette Used for Grouting in Soils
Chemical Consolidation for Controlling Ground Water in Excavation
Chemical consolidation method 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 material 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 important consideration in this technique is to postpone the formation of grout gel. This is because grout penetration would be easier and more efficient 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 example, resins and lignins and acrylic polymers.
Fig.7: Acrylic Polymers
Control Ground Water in Excavation by Compressed Air Method
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 should be nearly impermeable. 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. Thirdly, air static pressure throughout the entire dry hollow space of the structure is should be constant
In order to achieve the above conditions, the following construction procedures should be considered.
- Construct diaphragm wall as shown in Figure-8 along the side of the planned structure such as tunnel. The depth of the wall should extend below the final bottom slab of the structure.
Fig.8: Preparation for Compressed Air Technique to Control Groundwater, Diaphragm Wall Construction
- After that, construct a lid for the tunnel and the joint between the lid and diaphragm wall should be compressed airtight. In this stage, preliminary drainage may be employed provided that the conditions do not pose obstacles. It should be bore in mind that both diaphragm wall and the lid should be airtight as well.
- Construct a dividing wall or bulkhead with locks for workers and materials at one portal of the tunnel to avoid the escape of air toward the other end of the tunnel. Complementary momentary diaphragm wall might be placed at specified spacing along longitudinal axis of the tunnel and at its far end portal.
- Configuring mechanical and electrical machineries for waste material disposal as shown in Figure-9, compressed air provision and supporting plant.
Fig.9: Removing Waste Materials from Excavations
- Finally, the excavation processes and compressed air utilization will be started under the lid at the tunnel portal.
In this technique, substantial care should be practiced to prevent undesired events since compressed air techniques involves high level of risk that could lead to human loss.
Fig.10: Excavation and compressed air application, PL: air pressure height in the tunnel, Dtt: distance between artesian ground water and invert WK: artesian Tithonian water pressure
Control of Ground Water in Excavations by Freezing
Controlling excavation ground water by freezing is not recommended to use unless all other methods fails to provide 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.
However, there cases in which freezing is the only practical method to control ground water for example 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 should be known that the most outstanding benefits of freezing technique is the effective controlling of ground water which other methods 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 Method
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.