ANALYSIS OF GEOTECHNICAL PROPERTIES EFFECTS ON DAM SLOPE FAILURES
INTRODUCTION
Mairuwa dam was constructed for water supply to Funtua town and its environs and it is the major source of water supply to Funtua town for domestic and irrigation purposes. However, the water supply has been grossly inadequate. The amount of water pumped is insufficient to meet the increasing water demand. The residents of Funtua town rely mostly on hand dug wells, boreholes and rain harvesting. Yields from irrigated farms are often poor as a result of inadequate water supply.
Mairuwa dam which was constructed in 1970 had at inception a storage capacity of 5.5 million cubic meters. It has a catchment area of 120 km2, crest length of 457 m and the maximum height of 12 m. Since the construction of the dam in 1970, no study has been done aimed at appraising its design storage capacity in view of the widespread poor land-use practices over the impounding reservoir’s drainage basin which accelerates soil erosion.
It is, therefore, possible that the initial storage capacity of the impounding reservoir may no longer be obtainable at present due to siltation. The study focuses on the measurement of the remaining storage in Mairuwa impounding reservoir, rate of siltation in the reservoir, and rate of erosion in its drainage basin.
Statement of Problem
Slope stability is important in the design and construction of earth dam because exposed to dangerous conditions for the end of construction that mean no water level in reservoir (dry condition for upstream side slope)and rapid drawdown condition when the removal upstream water pressure that supported the slope for earth dam, it causes a danger to the upstream slope .
There are many methods for slope stability analysis to assess factors of safety such as (limit equilibrium and Finite element) methods by computer software, the limit equilibrium including different methods (Ordinary, Bishop, Janbu, Morgenstern-price and Spencer).
These methods applying the computer program SLOPE/W is applied to define the potential slip surface and calculate the factor of safety of homogenous earth dam under change water level condition and rapid drawdown for the reservoir with time. The minimum required of factors of safety for earth dams equal (1.3) for upstream slope (1)… (Scroll down for the link to get the Complete Chapter One to Five Project Material)
LITERATURE REVIEW
Development of Dams in Nigeria
During the past three decades, Nigeria has witnessed an upsurge in dam construction between 1970 and 1995, 246 dams were constructed in the country (Federal Ministry of Water Resources and Rural Development’s Register of Dams 1995).
The effect of the Sahelian drought of 1972-1975 aggravated the already stressed food situation in the country, prompting the various levels of government to embark on a rigorous policy to increase food production. To achieve this, the impoundment of rivers was seen as necessary to provide sufficient water for year-round irrigation which led to the construction of over 246 dams (Imervbore et al, 1986).
According to Ofoezie (2002), a total of 323 dams have been constructed and many more are under construction. Of the 323 dams 106 are large dams (dams with walls higher than 15m, or 10-15 m high with crest lengths of over 500 m, or having a reservoir capacity of 1million m3); 27 are medium-sized dams (with walls 8-10 m high); and 192 are small dams (walls less than 8 m) Figure 3.
The Nigeria Register of Dam published by the Federal Ministry of Water Resources and Rural Development in 1995 has a record of 145 dams among which is Mairuwa Dam. (Table I). In the Compendium of Nigerian Dams, 2007 there are 201 dams listed in alphabetical order indicating type, uses, location and other relevant statistics with visuals of the dam. Mairuwa Dam is one of these dams (Table 2)
The volume of water contained in all Nigerian dams was calculated by Schoeneich, (2003) to be 40.3 km3. However, the capacity of all dams in Nigeria is estimated to be 44.2 km3 (FAO, 2008).
Table 1: Mairuwa dam statistics (Nigeria Register of Dams, 1995)
| Name | MAIRUWA |
| Hydrological Area | 1 |
| Owner | KATSINA SW. B |
| Longitude | 70 15’ |
| Latitude | 110 30’ |
| River | SOKOTO |
| Nearest town | FUNTUA |
| State | KATSINA |
| Type of Dam | EARTHFILL |
| Purpose | WS, IR |
| Engineering | KIRK GRUNDY |
| Construction by | DELBATO |
| Year of Completion | 1970 |
| Crest Length (m) | 457 |
| Maximum Height (m) | 12 |
| Catchment Area (km2) | 120 |
| Reservoir Storage Capacity (mcm) | 5.5 |
| Dead Storage Capacity (mcm) | 0.2 |
| Active Storage Capacity (mcm) | 5.3 |
| Spillway Type | OGEE |
Constraints of Construction and Management of Dams in Nigeria: Dam Slope Failures
Akanmu et al., (2007) recognized the following as part of the problems of reservoir water operation in Nigeria: (Scroll down for the link to get the Complete Chapter One to Five Project Material)
RESEARCH METHODOLOGY
Materials properties of the earth dam
In this study take the values of the soil strength parameters of the earth dam as shown in table (1). The earth dam (homogenous) which its dimensions are: the width of the crest for the dam(B)=constant=10 m,(b) =constant=4m as shown in Figure(1), the water level are taken as a ratio from the height of earth dam (H=10m) of (h=0, 0.2H, 0.4H, 0.6H, 0.8H).
Slope stability analyses methods
Limit equilibrium methods are important in slopes stability analyses. These methods calculate the factor of safety (F) by dividing a potential sliding mass into several vertical slices. There are several methods to evaluate the static stability of earth AL-Wand dam these methods including:
- The Ordinary or Fellenouis method: Fellenouis (1936) was developed this method and is sometimes referred to as “Fellenouis method.” The Ordinary method are satisfies the moment equilibrium for a circular slip surface, but neglects both the inter slice normal and shear forces. The advantage of this method is its simplicity in solving the (F), since the equation does not require an iteration process. The (F) is based on moment equilibrium and computed as: Abramson et al. and Nash as cited in.
Where c’ and ϕ’ = cohesion and internal friction angle respectively in effective stress terms.
l = the length of the slice base (m).
W=weight of each slice (kN). u = pore water pressure (kN/m2).
α = inclination of slip surface in the middle of the slice.
- Bishop Simplified Method: (Bishop, 1955), advanced this method is very common in practice for circular shear surface (SS), this method considers the interslice normal forces but neglects the interslice shear forces, this method satisfies moment equilibrium for (F).
The trial value is assumed for the factor of safety and the quantity, mα, is computed on shown below
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DATA FINDINGS AND DISCUSSION
In this study, the factor of safety calculates the upstream slope of earth dam by (Morgenstern-price (M1), Spencer(M2), Bishop(M3), Janbu (M4), Ordinary(M5)) and finite element method(FEM). The minimum value factor of safety (F) (critical slip surface) for the upstream slope with various parameters such as the soil strength parameters (cohesion, angle of internal friction and unite weight of soil) are study and the values of (F) calculate by (M1) because this method satisfies all the static equilibrium conditions which consist of moment and force equations and thus will produce a more validation for the factor of safety.
Table (1): Material properties of earth dam model (10, 11)
| Permeability(m/sec) | Modulus of Elasticity(kN/m2) | Poisson’s ratio | Unit weight(kN/m3) | Cohesion (kN/m2) | Angle of internal
friction(deg.) |
|
10-9 |
10000 |
0.334 |
16, 21 |
20 | 22 |
| 24 | 23 | ||||
| 28 | 25 | ||||
| 30 | 26 | ||||
| 33 | 28 | ||||
| 35 | 30 |
In the cases (1) the model built to end of construction (dry condition), there is no water table present in the reservoir and in the embankment dam body. After that take many cases with change the gradual rise of the water level (h) for the reservoir and then the gradual drawdown of the water level (h) for the reservoir with time. Another case used rapid drawdown for the water level in the reservoir of the earth dam.
Table (2): Minimum values of factor of safety for increase progressively (h) at (γ=16kN/m3)
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CONCLUSION AND RECOMMENDATION
Conclusion
From the results of this study, the researchers placed the following conclusion: The slope stability of the upstream slope for earth dam increasing when the soil strength parameters (cohesion, angle of internal friction) increase and storage of the water, the slope stability of the upstream slope for earth dam decreasing when the soil strength parameter (unite weight of soil) increase. The earth dam may be exposed to the collapse in the case of the rapid drawdown of water level… (Scroll down for the link to get the Complete Chapter One to Five Project Material)
Recommendations
There are several actions that could be taken to reduce the rate of siltation of the Mairuwa dam thereby prolonging its lifespan. Such actions include:
Construction sediment traps or weirs upstream of the dam will help in trapping of sediment and also serve as sources of water for cattle and for irrigation. The sediment traps will intercept much of the 38,880m3 of sediment being deposited yearly into Mairuwa Impounding Reservoir. This sediment captured in these weirs will be excavated annually… (Scroll down for the link to get the Complete Chapter One to Five Project Material)
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