Digging a foundation is not always as easy as building a simple slab foundation. When the foundation becomes complex, digging it also becomes critical.
The influence of the water table must be taken into account when planning and building the foundation.
The geotechnical parameters used for the project are influenced by the presence of groundwater. Therefore, special attention should be paid to the impact of groundwater levels during planning.
There are special cases where we cannot lower the water table as this could affect the surrounding structures. The depth of the cellar can vary, especially when building a cellar. 3m – 6m .
This depth may be even greater than these values. As the water level continues to fall, the water level in the surrounding area will also gradually fall.
Figure 01 shows the reduction in the water level.
As the water table drops, irrigation water pressure decreases.
If there are buildings nearby that are not on poles and in the ground, they may settle. Firstly, cracks appear in the brick walls and, depending on the cause of the settlement in the foundation, structural element failures may subsequently occur.
Therefore, care is taken to ensure that the groundwater level does not fall, especially in deep basements. Different techniques are used to support the excavation and to seal out water penetrating the excavation area.
Some of them are as follows.
- Cross piles
- Diaphragm wall constructions
- Sheet pile walls with lateral support systems
Secant pile foundation: most popular excavation method for foundations
First of all, we recommend that you read articles like you Type of foundation There you will find more information about types of foundation.
A secant pile is a retaining wall built to hold earth in place during deep excavations.
Secant cuttings consist of male and female cuttings or are called primary and secondary cuttings.
The secondary pile consists of a reinforcement cage and the primary pile is cast without a reinforcement cage.
First the primary stacks are launched and then the secondary stack is created between the primary stacks to overlap the primary stacks, as shown in Figure 02.
Typically, these piles are driven into the rock to provide a rocking effect to prevent tipping and prevent water from entering the excavated areas.
When used as a retaining wall, the reinforcement design must take into account all loads, such as soil pressure, water pressure, surcharge, etc.
If it is expensive to use large diameter piles to support piles as free cantilevers, lateral bracing is provided in deeper excavations, as shown in Figure 03.
Diaphragm walls – excavation for foundation
A diaphragm wall is a concrete wall constructed by excavating a deep trench.
It can be constructed as an in-situ concrete wall or as a prefabricated wall. If there is a lack of space, diaphragm walls are constructed as they do not require much space. Excavation work for the foundation is now often carried out using this method.
Furthermore, in some constructions these walls are used as part of the permanent structure.
Currently there are machines that have up to 50 meters deep diaphragm walls.
However, depending on the project requirements and based on the underlying level, termination may occur.
The wall is usually anchored to the rock to cushion overturning moments.
Excavation of sheet piles for foundation
The most commonly used method for securing excavations is the sheet piling system as it is simple and economical compared to other methods. With this method, foundation excavation work can be carried out easily.
However, when protecting deep excavation pits, this method becomes more complex for the construction site as more lateral supports are required.
Sheet pile walls must be supported at regular intervals to balance the load. Therefore, more and more supports need to be installed. At some point this will become an obstacle to the works.
There are methods to avoid internal side bracing if there is sufficient space around the excavation pit. To avoid obstructions, anchoring systems can be used as shown in Figure 04.
Influence of groundwater levels on shallow foundations
The availability of groundwater above the foundation or base level always poses a problem for the construction team.
Water table effects must be carefully considered without violating assumptions or considerations made during design.
The influence of the water table must be taken into account until works are carried out above the water table level.
Drainage systems are typically installed to prevent water from penetrating beyond the excavation level.
The number of drains and wells is calculated based on the height of the water and taking into account the permeability of the soil.
Drains are connected together to efficiently direct water to wells. Water will be pumped continuously until construction is completed or as planning requires.
If pumping is stopped after construction of part of the walls and bottom of a tank, it may rise due to rising groundwater levels. Therefore, completion of pumping work must be carried out in accordance with the design instructions.
Furthermore, pumping must always be done without objection.
Especially when installing the waterproofing membrane, stopping or preventing pumping increases the water pressure on the waterproofing membrane, which can lead to expansion and failure of the membrane.
Groundwater recharge is an important factor that must be taken into consideration, even if the excavation depth is only about 3 m if there are structures nearby.
To reduce the drop in the water table, it is recommended to replenish the water table through wells around the excavation. This will reduce the impact on other structures due to settlement resulting from reduced irrigation water pressure.
The Foundation (Engineering) Wikipedia article is worth reading to get started with the designs.
