Houston‘s Reservoirs Seepage Flow Control

Pipe resistance is usually determined by the soil’s gradation, plasticity, and degree of compactness. The most powerful method to avoid piping is to lengthen the water flow paths within the dam’s walls. As previously said, cutoff barriers, impermeable blankets, and impervious cores are the three alternatives that will be explored in this project as methods of avoiding seepage in the Addicks and Barker reservoirs. - of these preferred methods necessitates a distinct installation method and offers distinct advantages.

Cut-off walls are essentially impermeable barriers that are built which infiltrate the permeable underlying base soil. They come in categories namely: Concrete, Slurry trench, Plastic concrete and Bentonite-cement cut-off wall. Although they are distinct categories, their difference only occurs in the material used in constructing them. This technique is associated with the installation of physical barrier of low permeability to prevent water from seeping into an undesired area. Usually, the cutoff will consist of compacted earth fill or sheet steel piling. The method is more successful if the permeable stratum is relatively thin than a thick layer. Its construction will slurry material since the excavated trench will be backfilled with slurry which solidifies into the desired impermeable barrier along the direction of the earthen dam. If sheet pile wall is preferred instead, sheet piles will be required, which will be interlocked and driven through the embankment and into the depths of the soil. (Cedergren, 1989) For a cut-off wall to be effective, it needs to penetrate deep in the soil stratum which is permeable. In case of a porous foundation, then penetration will be more tedious but more economical.

Any cut off wall requires materials which have a very high mechanical resistance which can withstand erosion. It is essential to have a high deformability which will resist any induced strains of alternative hydraulic or mechanical sources without cracking or failing. Using concrete cutoff may be disadvantageous due to cracking. Sheet steel piling is susceptible to leaks that results when the interlocks open between the piling. Otherwise, cutoff walls are considered the best and most convenient way of doing away with both seepage and subsurface erosion.

Impermeable cores

The core section of an earthen dam serves to minimize or stop water seepage from upstream to the downstream side. Therefore, selecting a proper core material is essential if a reservoir is to function correctly. Historically, core materials that are of glacial origin tend to have more piping accidents as compared to those of alluvial origin. An internal core is made of compacted clay material that has back drains which can remove water on the downstream side. The core may be developed from locally available asphaltic concrete or the ordinary concrete.

In our case, both Addicks and Barker reservoirs are built on a foundation of sandy clay soil. Although this is fundamental impermeable core, it is still susceptible to seepage (Xiao & Barreto, 2015). Thus, for this project, there is need to construct an impervious core to boost the one that exists naturally to minimize the chances of erosion or seepage entirely.

The installation of an impermeable core will involve excavation into the foundation soil. The process will be more costly and tedious depending on the depth of the reservoir. Nonetheless, the construction does not need as much excavation space as compared to the construction of a slurry cutoff trench. If the supply of soil for the impervious core is limited but, there is a lot of permeable material, it is necessary to construct a thin core. Also, in this case, a thin core is more economical to construct because the cost of pervious materials is cheaper. The impervious core maybe preferred to a thin core because a thick core may consume a lot of construction time especially when the weather conditions do not allow it. Moreover, the thickness of the impermeable core depends on how much seepage loss could be tolerated in the reservoirs (Xiao & Barreto, 2015). A thinner core, from a stability point of view, is usually preferred because it is more stable. A thicker core is, however, more resistant to cracking.

Impermeable blankets

The impermeable blankets refer to a relatively thin layer of clay that is compact at the bottom of a reservoir. They are mainly constructed when the reservoir has a permeable base to increase the length of the flow line. It is important to note that a vertical and horizontal impermeable blanket can be merged to form a cutoff trench. An impervious coverage will prevent any water from leaking into the foundation of soil which is permeable. The use of impermeable coverage is how it stops and or reduces seepage. It is essential that after constriction, the blankets are protected against damages minimizing the chances of infiltration or erosion.

Rocks such as shale, sandstones, gneisses, and schist are very suitable for not only crushing but also compaction, and they can serve as a part of dam embankment. However, even in their compacted states, they are still more permeable than clay having a plasticity that is low. Therefore, they cannot be applied as impermeable blankets. As such, laterite, which contains the same re-cementing features as those of coquina, is used as a material for a waterproof barrier. Laterite is scarce and expensive to transport to the desired area. Luckily, the foundation soil of Addicks and Barker reservoirs are reasonably impermeable. (Cedergren, 1989) This feature suggests that other materials other than laterite can be used as material to construct the waterproof blanket barrier.

This technique has been applied before in Forth-Randal over River Missouri and even in Columbia River and has proven to be a long-term solution to seepage in dam reservoirs. Therefore, it is a convenient method to be used in Addicks and Barker reservoirs in the prevention of seepage and piping that might happen even far in the future (Xiao & Barreto, 2015).

Comparative assessment of the solutions wall

Table 1.

Cutoff walls

Impermeable Core

Impermeable Blanket

Construction technique

Well understood, practiced and accepted

Involves zoning the embankments

Well used for long-term solution

Power consumption

Moderate

High

High

Cost

Varies according to site condition

Fairly expensive

Expensive if materials are to be transported

Ease of installation

Easy

Moderately difficult

Tedious

Pollution control

Effective

Minimal

Moderately Effective

References

Cedergren, H. R. (1989). Seepage, drainage, and flow nets. Chichester: John Wiley.

Xiao, M., & Barreto, D. (2015). Geotechnical engineering design.