Therefore, in order to find the best alternative, well-organized structured decision-making process below is necessary like Gregory and Keeney (1994) method or rational planning (Lund, 2008).
- Problem definition
- Objectives
- Alternatives
- Consequences and evaluation
- Tradeoffs
- Uncertainty and Risk Tolerance
- Making decisions
Prior to suggestion of practical dam remediation methods, remediation case history review is helpful to understand major considering alternatives.
Seepage and Piping
Excessive seepage or piping can cause sudden devastating failure of dams. From my experience, a good exemplary symptom of this is increase of leakage flow or turbidity, or sometimes sinkhole on the crest. A very good remediation measures are described by “White paper for the ASDSO Seepage Workshop, Oct., 2000, Talbot, Poulos, and Hirschfeld” and personally it is preferred to use this methods.
Remedial measures for preventing piping are aimed at controlling seepage so that the seepage does not cause internal erosion of soil from the embankment, foundation, or abutments of a dam. Remedial measures for preventing piping may not reduce the rate of seepage and, in fact, often increase the rate of seepage.
Remedial measures for reducing water loss are aimed at reducing the quantity of seepage through the embankment, foundation, and abutments. Although such measures may reduce the pressures and the rate of water flow through a dam, its foundation, or abutments, it is nevertheless vital to install proper drainage systems on the downstream side of the dam as the primary line of defense against piping.
The addition of downstream drainage is usually the best solution for controlling seepage in embankment dams. Control of seepage can be accomplished by:
- adding a drainage zone by removing a portion of the downstream slope and constructing a new filter-drain covered by a reconstructed downstream slope;
- adding a drainage zone by constructing a filter-drain on the existing downstream slope and covering the drain with a new downstream shell zone;
- adding an embankment chimney drain to the dam by trenching into the dam and backfilling the trench with appropriate filter material;
- installing a toe drain extending into the foundation at the toe of the dam;
- installing a downstream, weighted, blanket drain;
- installing downstream relief wells;
- cleaning existing clogged drains; and
- cleaning existing relief wells.
The solution that is best-suited to a particular dam will depend on a variety of factors. Some of the more important factors to be considered are a) the embankment zoning and foundation stratigraphy, b) the seepage patterns and quantities, c) seepage pressures, d) the ability to lower the reservoir for construction, e) the availability of construction materials, and f) property constraints and construction access.
Also, methods available to reduce the amount of seepage include:
- an upstream blanket constructed with low permeability materials (e.g., soil, asphalt, soil cement, roller compacted concrete, concrete, or a geomembrane);
- a “cutoff” or facing on the upstream slope of the dam constructed with low permeability material (e.g., soil, soil and bentonite mixtures, soil cement, roller compacted concrete, concrete, asphalt, metal, masonry, or a geomembrane); and
- an internal “cutoff” within the dam and foundation constructed of low permeability material (e.g., concrete, soil-bentonite mixtures, soil-cement mixtures, sheet piling, or grout) – sometimes called diaphragm walls and constructed with such methods as slurry trench excavations, deep soil mixing, or jet grouting.
Main Resource: James R. Talbot, Steve J. Poulos, and Ronald C. Hirschfeld, White paper for the ASDSO Seepage Workshop, Oct., 2000, Denver
Seismic mitigation
According to Marcuson et al. (1996, JGGE), among 52 dams having problems, 22 dams had operational fixes (pool level restrictions), and 30 dams had engineering fixes. Also,
Of 38 researched dams:
• 11 cases: remove and replace
• 8 cases: upstream and/or downstream
• 6 cases: Increase freeboard (raise crest; lower pool)
• 5 cases: Remove dam from service, or replace with new dam
• 8 cases: Soil improvement
• Stone columns: Hinkley dam in NY; John Hart dam in B.C.; Mormon Island dam in CA.
• Dynamic compaction: Jackson Lake dam; Mormon Island dam in CA; Steinaker in Utah.
• Drains: Gravel piles at Kingsley in Nebraska.
• Compaction grouting: Pinopolis West dam in SC Piles MS
• Piles: Sardis dam in MS.
Of course, right methods depend on various factors in that particular dam site and conditions.
In California, major dam safety threatening factor is much oriented toward seismic mitigation because there are so many active faults and always have some potential hazard.
In view of many researches, my opinion is firstly, we should select some possible methods for liquefaction mitigation through smart decision making process, and then, the most recommended method should be considered based on the reasonable safety standard and costs.
There are some key considerations when we think about ground improvement for a dam safety.
- Design ground motions
- Will “liquefaction” be triggered?
- Consequences of liquefaction?
- Ground deformations (e.g., flow, lateral spreading, settlement)
- Effect on the dam (Safe? Functional? Repairable?)
- Cost of damage (direct repair costs & loss-of-use costs)
- What level of performance is required?
- Life Safety?
- Functionality?
- Time for repairs?
When we firstly search for multiple alternatives for seismic remediation or mitigation, the following charts is helpful at the very beginning of stage.
Earthquake induced Liquefaction Mitigation (Jim Mitchell 2008)
Detail description of each method is introduced in the webpage. Some typical ground improvement methods to reduce consequences of liquefaction in dams are as follows:• Vibro-methods:
- Vibro-rod methods -- vertical vibration of a penetrating probe
- Vibroflotation -- a horizontally vibrating probe
- Vibro-replacement: the probe cavity is filled with imported material
(e.g., crushed stone, gravel, sand, or even concrete)
• Deep dynamic compaction (DDC)
• Deep soil mixing (DSM)
• Grouting
• Compaction grouting
• Jet grouting
• Permeation grouting
• Drains (area coverage, perimeter curtains)
• Blasting
• Biological treatment – waste products of bacteria may stabilize soil
In addition to this, other types of alternatives to ground improvement should be considered simultaneously.
- Relocate to a better site?
- Modify the dam to withstand effects of liquefaction?
e.g., Tie footings together with grade beams?
- Manage the risk.
Repair damage versus prevent it (cost of lost functionality)?
Different performance levels for different parts of the system?
Overtopping
For the overtopping due to heavy rainfall and lack of free board, the best method is to increase reservoir capacity or spillway capacity. From my personal experience, if a dam has no spillway or has a weir with no flood control function, recent extraordinary weather pattern can cause increase of PMP or PMF. Thus, spillway discharge rate should be increased for the assumption of emergency.
In addition to that, downstream river should be fixed for modified design flood of the stream. Sometimes we can find that dam flood capacity is enough, but a downstream flow capacity when spill of water is remarkable insufficient. Subsidiary spillway installation in type of tunnel or open cut, morning glory can be considerable to prevent overtopping. Or raising a dam height and get more freeboard is another good method to think about.
However, it causes a lot of costs to construct both cases. Therefore, reasonable feasibility study should be proceeded.
Slope Stability
Slope failure problem in dam body is correlated to the happening of earthquake or static instability of dam body. Especially, for the cases of the following time stage is very crucial for slope stability and proper analysis should be taken.
- End of construction (short-term)
- Steady state seepage (long-term)
- First filling (many case histories)
- Earthquake
- Sudden draw-down
Analysis can be available using limit equilibrium analysis, FEM, FDM programs. For the detail factor of safety of each item, USACE slope manual is preferred to use.
For the slope stabilization, various kinds of alternatives are recommended in terms of conditions of dam. Dam owners and engineers should decide the optimum method considering proper level of safety and economic costs.
- upstream and downstream berm or upstream blanket
- upstream slope protection, riprap, etc.
- upstream impermeable barrier (membrane)
- grouting into dam body
- put some horizontal ramp
- make decent slope angle
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