civil Medium

Seepage is one of the common problems in building basements. Structures like retaining walls, ceilings, water tanks, earth dams & dam foundations where water contact is available often show traces of seepage. Technically, seepage is the slow infiltration of any fluid through a porous medium. In most cases, this fluid is water, and it drives by the hydrostatic pressure acting on one side of the structure.

Seepage is significant in the designing of dams. Significantly high water pressure on one side of the dam can push the water to flow through the foundation and dam body. The seepage action is unrealistic to eliminate 100%, but it can reduce to a satisfactory value. Familiar places that you can witness seepage are,

• Water retaining structures and their foundations (dams & weirs)
• Basements & retaining walls
  • Through expansion, construction joints
  • Cracks and cold joints
  • washed or poorly compacted concrete

In almost every case, seepage is harmful to the structure in many ways. The moving water can erode the soil or the foundation. Earth dams mostly suffer from this situation. Also, seepage causes a constant water loss from the reservoir or the tank. Even the water flow underneath can cause uplift pressure on the dam, making it unstable.

Not only that, but seepage water can also transmit harmful substances deteriorating the concrete. In most cases, chloride & sulfate ions present in the soil can react with the reinforcements and reduce the life span of the structure.

Water tanks, tunnels, basement walls, and retaining walls are some familiar places where seepage is visible easily. Cracks and different joints in the structure can transmit seeping water. Also, weak or poorly compacted concrete can show wet patches. This is dampness in the concrete.

Dampness, leakage, and seepage

Even all these caused by soaking water through a porous medium; each word has a slight difference.

Dampness

Wet damp patches are one of the prevailing situations you may encounter with walls and ceilings. Void areas in the brick or concrete construction will soak water and appear as wet patches. This wetness is not an immediate risk for the structure, however, but it can grow unpleasant molds.

Wet patches can appear on walls and ceilings due to two reasons. The first one is porous construction materials. Poorly compacted concrete and brick walls can transmit wetness from outside. Similarly, rising damp in walls caused by the capillary action of the tiny pores in bricks.

The second reason for the dampness is excess humidity. When the air-bone moisture content is high, it will subject to condensation. You can separate condensation from normal dampness using mold patterns.

Mold growth due to general dampness will be one or two large patches. In most cases, the outline of the patch will be clearly visible. However, condensation will cause a series of mold patches separate from each other.

Common solutions for dampness

There are several industrial solutions to treat molds or mildew. Scrubbing the area with a steel wire brush and applying bleach will remove the fungus and the stain. However, this is not a long term solution. After a few months, the fungus will grow again. The recommendation is to treat the leakage or the porous wall permanently. Injecting sealants or epoxy is one of the ways to fill out the pore structure. Inserting dry rods is another convenient and straightforward method available.

Another effective method is installing a damp-proof course. This will act as a barrier to the rising damp in the walls. Concrete with 1:1.5:3 usually applied at least 150 mm above the ground level as the proofing membrane. The height of 150 mm is to ensure the splashing rainwater on the wall will not cause any rising damp.

Some alternate ways of undertaking dampness issue

• A tightly packed material structure will have less permeability. So, try to use a more impermeable concrete mix. It will reduce the dampness
• The use of waterproof coatings, additives, and plasters can threaten dampness.
• Separated external and internal walls show less dampness because of the cavity between them. Use of cavity wall system is one of the helpful ways of treating dampness
• There are colorless water repellents available in the market. These will seal the pore holes on the surface and repel the water contacting
• bitumen coating as well as the use of tar sheets is commonly applicable on soil back-filling side of concrete basement walls 
• Sometimes dampness can observe due to water pipe leakages. Ceilings are most vulnerable to this. You may have to remove some concrete to reach the leakage and repair it. After the repair, it is mandatory to re-cast the area with a non-shrinkage concrete mix.Open the windows and improve ventilation if the outside is dry
• Use dehumidifiers to dry out the air manually. It will reduce condensation

Leakage

Leakages are common with cracks and joints. Complete section separation due to thermal or shrinkage effect is the common reason for leakages. Sealing the crack using epoxy material is effective in treating these situations.

Leakages through the joints are a bit problematic than cracks. In most cases treating the joint while protecting the service of the joint is hard to cover. Obviously, construction joints can consider as cracks and treat the same way for leakages. However, treating other joints, such as expansion, contraction, isolation, and seismic, are hard. It is essential to maintain discontinuity while treating the leakage in these joints. Installing rubber or copper water stops is one way of handling such cases. The flexibility of rubber and copper will ensure the separate movement without any leakages.

Seepage

In most cases, the word seepage is using in situations where water flowing through the soil. Unbalanced pressure head, as well as the flow rate, are the must-have factors for seepage to take place. Layers of sand, silty sand, gravel, and rock joints are common features in soil that supports seepage.

Water retaining structures and their foundations are frequent victims of seepage. It’s critical to calculate seepage at the design stage as well as in the operation stage to minimize the water loss and improve stability.

Seepage analysis can be done in many methods. The flow net diagram is one of the simplest and most effective methods available. However, there are different and advanced methods available at the moment. Some of the other seepage analyzing methods are,

• Models
  • Gel models with an electrical analogy
  • Sand models
  • Viscous flow models
• Analytical methods
  • Mapping or transformation
  • Semi-empirical method of fragments
  • Closed-form methods
• Numerical and computer methods
  • Finite difference method
  • Finite element method

Flow net diagram

This is a 2D sectional view of water flow through soil or porous medium. Flow net diagram consists of flow lines and equipotential lines representing possible flow path and pressure variation.

The effective hydrostatic pressure, permeability, and geometrical properties of the foundation controls the seepage flow. Curvilinear equipotential lines define the pressure stages inside the soil. This emphasizes the equivalent pressure stages of the flow.

Flow lines are parallel lines similar to equipotential lines. These connect the waterway to the downstream side while intersecting equipotential lines at right angles.

How to draw a flow net diagram

• First, define boundary conditions of the flow
  • The flow will be zero though impermeable boundaries
• Flow lines will be parallel to the impermeable layer
  • Flow lines are parallel to each other and perpendicular to equipotential lines
• Then, consider the slope angle of the entrance face and exit face in developing flow lines.
• Draw equipotential and flow lines in similar spacing so that they make curvilinear squares.
• Develop the flow net diagram using a rough sketch until you satisfy with the size and shape.
• Distribute equipotential lines evenly so that number of pressure drops is a whole number. It’s easier for calculations.
• Finalize the soft sketch and finish it with permanent lining
• Calculate the effective head
• Use interpolation technique to calculate the pressure head at an intermediate point
• Use Darcy’s law to calculate the flow (Q = k × i × A ; Q = flow rate, k = Permeability, i = pressure gradient, A = cross sectional area )

Monitoring the seepage flow rate is essential during the operation because a higher flow rate can washout the material in the dam as well as the foundation. Locating a toe drain at the downstream side can collect the seepage water. V notch prepared at the end of the toe drain to measure the flow.

Control the seepage with grouting

When the seepage is higher than the permissible level, designers can introduce curtain and consolidation grouting as the treatment. Both of these will reduce the permeability.

Curtain grouting

Developing a cement grout barrier to the foundation is known as the curtain grouting treatment. Pressurizing one or several grouting lines below the dam foundation will make an impermeable zone. Additionally, it will increase the length of the flow line and reduce the seepage flow.

Consolidation grouting

As mention in the name, consolidation grouting will fill up the void structure and increase the soundness of the foundation. This treatment will improve the foundation bearing capacity as well as the water tightness.

The effectiveness of the grouting can monitor using water pressure tests. Permeability of the soil strata is measured by the lugeon value. It’s the liter per minute flow rate of under 10 bar pressure along a linear meter of the test hole.

While the gravel calming highest permeability, the variation of permeability of a few types of soils is as follows.

Permeability of coarser soils can be determined using the constant head permeability test. ASTM D2434 is the recognized standard of this test. When the grain size reduces, permeability also decreases due to smaller void structures. As a result of that, using a constant head permeability test on fine-grained soil will take considerable time. To avoid the difficulties, we use falling head permeability test according to the ASTM D5084 standard.

Conclusion

All the seepage, leakage, and dampness cases consider as defects in buildings. Using the right material and following the right procedures will help to avoid these situations. Waterproofing methods, DPC constructions, and epoxy injection are common ways of facing these issues. The use of dehumidifiers and improving ventilation to invite dry airflow into the building can reduce condensation.

Seepage in water retaining structures is hard to stop. Consolidation and curtain grouting techniques are popular methods to reduce the permeability to an acceptable level.