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An investigation of the effect of drains on slope stability of homogeneous and non-homogeneous earth dams during rapid drawdown condition.

INTRODUCTION

In addition to its significant benefits, earth dams contain potential hazard that may threaten people and the environment [1]. The mechanism of the collapse of the earth dam is highly related to the fluctuation of water level and its interaction to the soil material of the dam body [2].

Hence, analyzing the effect of the rising and lowering of reservoir water level to the seepage on the earth dam by considering the water level fluctuation, dam slope inclination and the type of soil composing the dam body is necessary [3].

The drawdown condition is a classical scenario in slope stability, which arises when totally or partially submerged slopes experience a reduction of the external water level. Rapid drawdown conditions have been extensively analyzed in the field of dam engineering because reservoir water levels fluctuate widely due to operational reasons [4].

In case of rapid drawdown, which represents the most critical condition, it is assumed that the pore water pressure within the embankment continues to reflect the original water level [5].

The lag of the phreatic line depends on factors such as: permeability of soils, drawdown rate, drawdown ratio and slope gradient during rapid drawdown, the stabilizing effect of the water on the upstream face is lost, but the pore-water pressures within the embankment may remain high. As a result, the stability of the upstream face of the dam can be much reduced [6].

The dissipation of pore-water pressure in the embankment is largely influenced by the permeability and the storage characteristic of the embankment materials. Highly permeable materials drain quickly during rapid drawdown, but low permeability materials take a long time to drain [7].

When the countervailing upstream water pressure has disappeared, it causes a danger to the upstream slope. Soils inside the dam body remain saturated and seepage commences from it towards the upstream slope. Seepage and hydrodynamic pressures create downward forces acting on the upstream slope [8].

Those are adverse to the stability and create a critical condition to the upstream slope. While the development of deep seated failure surfaces is possible, the effect on earthen side slopes is most commonly seen in the form of relatively shallow slope failures, which if left unattended lead to the gradual deterioration of the whole dam One of the most effective methods of dissipation of excess pore pressure and improvement of stability is use of upstream drains [9].

These upstream drains are capable of draining the upstream slope and making the equipotential lines tend to become horizontal. They have a very significant effect on the stability of the upstream slope during rapid drawdown [10].

In order to investigate the effect of drains on upstream slope stability during rapid drawdown condition, three different zoned dams is used as the experimental models for this study. So those in the first phases of article the Effect of horizontal and diagonal drain and in the second phase the effect of horizontal and Chimney drains on stability of homogeneous and non-homogeneous earth dams was investigated. More over at the last part the optimum diagonal drain location during rapid drawdown condition was determined.

2. Effect of horizontal and diagonal drain on stability of homogeneous dam during rapid drawdown condition:

In order to investigate the effect of horizontal drain on the earth dam, a sample dam is analyzed once with a horizontal drain upstream and once again horizontal, non- drain for water rapid drawdown and shear stresses of these dams are compared.

Dam core is made of fine materials that are the clay and dam crust is silica sand that has low permeability. Drains include sand and gravel with uniform grain sizes, also dam foundation is made of clay sand and dam different zones' parameters are presented in Table 1.

Dam geometry for both horizontal and without drain is shown in Figure 1. ABAQUS software was used to analyze rapid drawdown of water in dams with intended features; this software is capable of dual-time function analysis such as consolidation or rapid drawdown.

For dam analysis, two-dimensional model is used assuming plane strain conditions. Dam height is 70 meters from surface and toe of dam width is 395 meters.

Width above dam crest above the crown equals to 10 meters and dam core width at base equals to 59 meters and above the core equal to 10 meters. Flat balance of horizontal drain is 30.5 meters from surface and drain thickness is 3 meters.

It is assumed in this analysis that dam structure is constructed entirely and dam reservoir is filled to the normal level. The vertical distance between normal level of water in the reservoir and the ground surface is considered 65 m. After full filling the reservoir, transient continues in dam until pore water pressure reaches steady state. After performing this analysis, the rapid drawdown analysis in the reservoir is performed after the end of previous steady state.

The results of the analyses show that shear stresses range for 36 meters drawdown of reservoir level in 18 days for horizontal non-drain upstream dam at distal area of toe of dam in upper crust is significantly more than the shear stresses in the same dam area with horizontal drain upstream. Shear stress values of this area in dam with horizontal drain upstream are between 600 Kpa to 700 KPa, and for non drain dam is 75 KPa to 900 KPa.

In central and near the core areas in the upper crust it can be seen that in horizontal drain upstream, shear stress of mentioned areas is more for dam horizontal non drain upstream. Shear stress in most border areas between the core and crust with drain is between 100KPa to 300 KPa while shear stress for dam horizontal non drain upstream in the upper two-thirds is 0KPa to 100 KPa and in the lower one-third of the area is 100KPa to 300KPa.

Diagonal downstream drain of two dams has the same shear stress range and stress range is between 0KPa and 750KPa. Shear stress distribution of diagonal downstream drain of two dams is approximately the same, however, greater shear stresses are observed for diagonal downstream drain than the adjacent points in downstream core and crust. This is due to the use of high quality materials with better resistance parameters in the construction of drain than drain adjacent materials in the core and crust. According to the high stiffness of drain materials than the adjacent materials in the crust and core of this area, it endures greater shear stresses caused by hydrostatic tension of water.

Range of shear stresses in upper crust diagonal drain is wide. This range is variable from 100KPa to 1100KPa. Range of shear stress in the lower one-third of diagonal drain is from 800KPa to 1100KPa, in the middle one- third is from 500KPa to 800 KPa and in top one-third is from 100Kpa to 500KPa.

Range of shear stresses in the lower crust is almost the same for both dams and both dams in downstream crust experience shear stress from 0KPa to 100KPa. However, the shear stress distribution in the downstream crust is different for both dams. In downstream crust with horizontal drain, farther from border with downstream diagonal drain, the more will increase shear stress, but in dam horizontal upstream non drain, the area near the outer surface of the downstream crust is exposed to higher shear stresses.

Horizontal downstream drains of two dams have the same range of shear stress from 0 KPa to 250 Kpa but the stress distribution is different for both dams. Horizontal downstream drain of dam with upstream drain has similar behavior of downstream crust, and the farther from border with diagonal drain toward the outer surface of the dam, the more will increase shear stress. Shear stresses on horizontal downstream drain of the dam non drain will decrease in areas relatively near the outer surface of drain of adjacent areas, but will increase again in areas adjacent to surface.

Shear stress distribution in foundation of two dams beneath downstream crust is almost identical. But there are differences in the range and distribution of shear stress under crust areas especially in the left half. Shear stress rate of 40 to 70 feet depth of this area at the dam with horizontal upstream drain is less than the dam non drain.

Shear stresses range in the core of dam with horizontal upstream drain increases to 300 KPa. The maximum amount of this stress in the core upstream non drain is 250KPa. The maximum shear stress is observed in the dam with drain in areas adjacent to diagonal upstream drain and in dam non drain in areas adjacent to diagonal downstream drain.

3. Effect of horizontal and Chimney drains on stability of non-homogeneous dam during rapid drawdown condition:

To evaluate the effectiveness of the drain system on the stability of non-homogeneous earth dam, discharging the water reservoir level is considered 15 m in a period of thirty days, and rapid discharge of reservoir event in non-homogeneous earth dam is simulated in three modes regardless of considering drain, with horizontal drain, with chimney drain using the simultaneous analysis of pore water pressure stress by finite element software of Geo-studio.

Modeled dams are the same in terms of type, height, slope, and materials and the only difference of them is in the number, thickness and type of upstream crust.

In this study, it is assumed that the width of valley is large and the size of two dimensions of width and height of dam is modeled smaller than third dimension that is length of dam. Dam height is 50 meters from foundation and toe of the dam width is 210 meters, also, the dam crest width is considered 10 meters.

Downstream and upstream slopes are identical and 1:2, in this model, we have tried that the most critical mode to be considered for slope. Clay dam core and gravel dam crust is considered with high permeability.

In this study, non-homogeneous earth dam parameters are modeled and are given in Table 2 that has been selected based on codes, considering crisis critical conditions.

The initial level of reservoir water height is 86 meters relative to the foundation floor, that is reduced 15 meters during 1 month, water level decrease is considered half a meter linearly and for every day.

Geo studio software is used to analyze the changes in pore water pressure, stresses and strains, from the beginning, in which the reservoir level is at steady state and until the transient flow continues until thirtieth day.

In double pore water stress- pressure analysis, pore water stress and pressure values are obtained based on the finite element method. In the latter case, the dam with chimney drain is investigated, which has a chimney upstream drain with 2 m thickness. For the third case, earth dams with three horizontal drains are investigated. Installed horizontal upstream drains have a length of 28 m, thickness of 2 m and the distance of 12 m. Finite element gridding is shown in Fig 2 for dams with horizontal and chimney drains

Safety Factor of upstream slope is obtained in three cases, after discharging the reservoir water level in four finite element methods of Sigma, Spencer, Bishop and Janbu, that is based on Limit Equilibrium Method. Much difference cannot be seen in the results obtained with different methods. The results obtained by the Sigma finite element method are more reliable because the compatibility of deformations are considered. Comparing three modes are shown in Figures 3 to 6.

In general, considering investigated cases, it can be said that reservoir rapid discharge leads to decreased stability of earth dam, especially in the upstream slope of the dam. Conducted studies, in three cases show that to what extent drain can lower pore water pressure and can increase the dam of safety factor.

By comparing the changes of safety factor in three modes including with chimney drain and horizontal drain and non drain, the effect of drain on upstream slope stability we can say that dam stability considering horizontal drains and chimney drains is about 20% higher than drain stability non drain.

While, the obtained safety factor from dam with chimney drain and horizontal drain does not differ and only the dam stability with chimney drain is about 20% better than horizontal drain. It should be noted that in using drains, with different layouts, different conclusions can be reached and optimal mode for drain location is obtained.

4. Effect of diagonal drain location on stability of earth dam during rapid drawdown condition:

In this section, the effect of diagonal drain location is investigated in discharging and reducing pore water pressure in a rapid discharge of the reservoir and its effect on the upstream slope stability. Three parts of earth dam is modeled in the case of non drain, drain near and drain near upstream crust by Slope/w software and stability analysis is done on them by this software and finally, according to the results the impact of diagonal drain location is discussed on stability number.

Dam height is about 78 meters and the dam crest length is 838 m and dam crest width is about 12 meters. Soil mechanical properties of the dam components are shown in Table 3.

In the first model, the drain location is near the dam core and in the second model; the drain is near the dam upstream crust. In another model, earth dam section non drain is analyzed and modeled in the conditions that the water level is in the maximum height of dam and in the maximum permitted depth.

In all of the above models, after simulation and analysis of program by Slop/ W program in Bishop Method, the confidence coefficient number of upstream slope stability is compared and evaluated that is calculated through the different placements of diagonal drain

4.1. Non drain earth dam:

This section is lack of diagonal drain and is modeled assuming flood conditions and the maximum water level of dam height that is depth of 54 meters above water level. Two cases are modeled for above section, in the first case; the water level is modeled in the maximum depth and in the second case, is modeled in the normal depth.

In all three models of this case, the slip surfaces cut foundation that are compound slips and have occurred in this model. According to the results, the obtained confidence coefficient for this case that is non drain section has the average of 1.954 and the average of pore water pressure is 851.87.

4.2. Diagonal drain near the core:

In this section, diagonal drain is placed near (adjacent) dam core. Drain has a thickness of 4.5 m and a slope of 72-degree and drain length equals to 78 m and at the lower part of dam is connected to horizontal drain for discharge.

Firstly, the water level is mapped assuming the flood conditions at maximum allowable depth and then is transferred during rapid discharge process from this level to normal level and other levels. The maximum allowable and normal depth of water equals to 50 and 46 m.

Now assuming the drawdown of water level to 2 m per day, the water level is lowered for next levels after analyzing in the normal depth. It can be stated that the of safety factor obtained for this case of diagonal drain placement is 2.263 averagely and pore water pressure is 344.678 averagely.

This mean relative to of safety factor value shows 14 percent increase in the dam stability of non drain model, however, the amount of pore water pressure shows 60% reduction and of safety factor of stability shows 45 percent increase compared to a model where the drain place is near to the upper crust and the amount of pore water pressure does not show significant changes.

4.3. Diagonal drain near the upstream crust:

In this model, the diagonal drain is located near the upper crust and thickness of drain is 4.5 m and slope and drain length is 128 meters. In this model, like the diagonal model near the core and with the same conditions of the water level, it was mapped; firstly assuming the flood conditions in the maximum allowable depth and then rapidly it was transferred and modeled from one level to another and other levels during rapid drawdown process.

It can be stated that the safety factor of this diagonal drain mode is averagely 1.256 and pore water pressure is 341.983 averagely. This mean shows 36% reduction relative to of safety factor of dam stability in non drain model and its pore water pressure also shows a 60% reduction and of safety factor in this case is reduced 45% compared to drain model near the core and the amount of pore water pressure does not show significant changes.

In the case of diagonal drain at upstream slopes, only the pore water pressure in the upper area of drain is appeared or in other words in the confined space between the drain and the upstream crust, and the water inside body upon reaching the drain area is discharged immediately, then drain is effective and enclosed in The space defined between the drain and upstream crust.

Although the amount of upstream slope stability in non drain cases is greater than amount of stability in the case of drain near upstream core and shows 36 per cent increase, but because of drain presence in all upstream areas of dam, the slip surfaces are very large, bulky and deeper in non drain cases so that dam will damage totally and irreparably due to slip, but in the cases where drain is located near the upstream range, since the drain is enclosed in the space between drain and upstream crust outer body and as soon as it reaches drain, is quickly discharged and has small, shallow slip surface and degradation is lower.

Conclusion:

Earth dam, as one of the most important earth structures is exposed to constant instability due to reservoir water level fluctuation. The rapid drawdown of reservoir level that is a special case of water level fluctuation in the reservoir, affects permeability, seepage and stability of dam because of sequential changes in saturation degree of earth dam core, and causes the dam failure. Studies show that drainage of earth dams has significant role in dam stability.

According to what was discussed in this article, in dams that crust materials have less permeability, using horizontal drains in upstream can enhance safety factor of the dam stability. These drains help stability increase during sudden changes in water level of the reservoirs by considerable decrease in shear stresses at toe of the dam in the upstream crust.

If the pore water pressure is decreased in the drains and their efficiency will improve and as a result safety factor will increase in the dam stability. According to studies when the reservoir water drawdown rate increases, drains impact on upstream crust pore water pressure reduction and finally, dam stability will increase.

Analyses indicate that Pore water pressures has the highest amount when the water level is in maximum height of dam has and decreasing water depth and water level drawdown will decrease the amount of pore water pressure.

The results of the models analysis indicate that in both cases of diagonal drain placement, while confining drain space in dam body, all drain and pore water pressure completely and simultaneously will be amortized by decrease of reservoir water level. Stability analyses in the core near drain mode show that safety factor of upstream slope stability is more efficient and slip surfaces have low density and damage compact is less.

ARTICLE INFO

Article history:

Received 25 September 2014

Received in revised form 26 October 2014

Accepted 25 November 2014

Available online 29 December 2014

REFERENCES

[1] Fathani, T.F., 2011. "Geotechnical Analysis of Earth Dam Failure." Prosiding Pertemuan Ilmiah Tahunan XIV (HATTI). Yogyakarta, pp: 485-491.

[2] Fathani, T.F. dan D. Legono, 2010. "Pengaruh Fluktuasi Muka Air Reservoir terhadap Stabilitas Bendungan Tanah Uji model di Laboratorium", Penelitian DPP/SPP Fakultas Teknik UGM, Yogyakarta.

[3] Legono, D., A.P. Rahardjo, T.F. Fathani, M. Fujita and I. Prabowo, 2009a. "Disaster Risk Reduction of Dam Failure through Development of Hydro-Geotechnical Monitoring Technique." Proc. of Asia-Pacific Symposium on New Technologies for Prediction and Mitigation of Sediment Disaster, Tokyo, 30-31.

[4] Alonso, E.E. and E. Romero, 2003. Collapse behaviour of sand. Proceedings of the 2nd Asian Conference on Unsaturated Soils. Osaka., pp: 325-334.

[5] Berilgen, M., 2007. Investigation of stability of slopes under drawdown condition. Computers & Geotechnics, 34: 81-91.

[6] Chardphoom, V., R. Michalowski, 2006.Limit analysis of submerged slopes subjected to water drawdown. Canadian geotechnical Journal, 43: 802-814.

[7] Duncan, J.M., S.G. Wright, 2005. Soil strength and slope stability. Hoboken (NJ): John Wiley & Sons.

[8] Corps of Engineers., 2003. Appendix G - Procedures and Examples for Rapid Drawdown. Engineering Manual, EM 1110-2-1902. Department of the U.S Army Corps of Engineers.

[9] Fredlund Murray and Feng Tiequn., 2011. Combined Seepage and Slope Stability Analysis of Rapid Drawdown Scenarios for Levees Design, Soil Vision System Ltd, Saskatoon, 640 Broadway Ave, Suite 202, 57N, SK, Canada.

[10] Novak, P.A. Moffat and C. Nalluri, 2007. Hydraulic Structures, School of Civil Engineering and Geosciences University Upon Tyne, UK and Formerly Department of Civil and Structural Engineering, UMIST, University of Manchester, UK (Fourth Edition).

(1) Abbas Zallakizadeh and (2) Hamid Samadi

(1) MSc in Civil Engineering-Hydraulic Structures, Graduated from Islamic Azad University (Dezful Branch), Islamic Azad University (Shoushtar Branch)

(2) Islamic Azad University (Shoushtar Branch)

Corresponding Author: Abbas Zallakizadeh, MSc in Civil Engineering - Hydraulic Structures, Graduated from Islamic Azad University (Dezful Branch), Islamic Azad University (Shoushtar Branch)

Table 1: Specification of Material Properties

Permeability   Dilatancy     Cohesion   Friction      Poisson
(m/day)        ([degrees])   (Kpa)      angel         Ratio
                                        ([degrees])

0.001          4             58         18.6          0.4
0.2            7             15         31.6          0.35
10000          8             0.1        32.8          0.3
0.4            6             32         24.4          0.33
0.0            6             35         27            0.3

Permeability   Young     Density   Dam
(m/day)        modulus   (Kg/m3)   segment
               (Mpa)

0.001          25        1800      Core
0.2            38        1850      Crust
10000          140       1900      Drain
0.4            1000      1950      Foundation
0.0            1200      2000      Cut off

Table 2: Specification of design parameters

Permeability   Cohesion   Friction      Poisson
(cm/s)         (Kpa)      angel         Ratio
                          ([degrees])

1.15x10-4      5          30            0.4
1.15x10-5      5          37            0.3
1.15           0.3        36            0.3
1.15x10-7      10         10            0.33

Permeability   Young     Density   Dam
(cm/s)         modulus   (KN/m3)   segment
               (Mpa)

1.15x10-4      50        20        Crust
1.15x10-5      400       23        Foundation
1.15           100       21        Drain
1.15x10-7      10        20        Core

Table 3: Material properties

Cohesion   Friction      Saturated      Unite     Dam
(Kpa)      angel         Unite weight   weight    segment
           ([degrees])   (KN/m3)        (KN/m3)

0          42            22             21        Crust
0          29            24             23        Foundation
0          15            20             19        Drain
11         17            20             19        Core
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Author:Zallakizadeh, Abbas; Samadi, Hamid
Publication:Advances in Environmental Biology
Article Type:Report
Date:Nov 1, 2014
Words:3834
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