HN10111511318
Evaluation of the Effect of Fly Ash on the Hydraulic Properties of the Coarse Soils Reinforced by Geotextiles in Earth Dams
Alireza mardookhpour
1, saman goolipour
21-Department of Civil and Water Engineering, Faculty of Engineering, Lahijan Branch, Islamic Azad University,Iran
2-Student Islamic Azad University of Zanjan [email protected]
Abstract
To determine the effect of fly ash on the hydraulics properties of the coarse soils, reinforced by geotextiles in earth dams, various mixtures of fly ash in reinforced sand have been considered and the role of fly ash in flow rate and hydraulics gradients investigated. The results of experiments show with increasing the fly ash ratio in coarse soils, the flow rate decreases significantly and may reach 0.006 of the flow rate in pure sand. Also from the hydraulics view points, the most suitable ratio in the base layer, reinforced by geotextiles, is proposed 20% at most.
Key words: Geotextiles, fly ash, earth dam, hydraulics gradient, reinforced sand
Introduction
Fly ash is one of the industrial byproducts, and has increasingly being used in dam construction applications as fill materials or grout mixes for embankments, as well as grout mixes for road bases [2].
Also Geotextiles are increasingly being used in dam constructions applications due their benefits economy over traditional methods. Geotextiles are often used in embankment constructions; Their two most important roles being the reinforcement of the foundation and separation of the embankment fill from the foundation soil [4]. In addition to these roles, geotextiles provide lateral drainage of percolation water and prevent the buildup of excess pore water pressure [5].
However; the hydraulic compatibility of a geotextile with the contact soil is an important issue and should be considered in design procedures [9]. The problem of fine particle clogging becomes more cumbersome when industrial byproducts are in contact with geotextiles in earth drainage systems [6].
The first main requirement for ensuring this hydraulic compatibility is that the drain should not be clogged throughout the life of the structure. The second requirement is that the soil piped through the geotextile should be minimal, so that the internal stability and modulus of the soil are not adversely affected [3].
In spite of ongoing efforts to use fly ash in dam construction, limited information is available about its hydraulic compatibility with geotextiles. Clogging of a geotextile drain by fly ash particles may cause significant reduction in permeability, thus reducing the flow capacity of the drain [7]. Even though, the fly ash is mixed with other aggregates (e.g sand), reduction of the permeability will be caused as a result of movement of fly ash particles through the drain [8].
Therefore, the fly ash should be hydraulically compatible with the adjacent geotextile. The problem of fine particle clogging becomes more cumbersome when industrial byproducts are in contact with geotextiles in dam drainage systems.
The nature of these geomaterials is different than regular soils, often consisting of significant amounts of fines [1]. The existing geotextile selection criteria may not be directly applicable to these materials and, in most cases, their filtration or drainage performance with geotextiles should be investigated by conducting laboratory tests. In this study Geosynthetics was tested with varying fly ash content with soil samples and their hydraulic conductivity is measured to check the viability of using Geosynthetics for dam project carrying clayey soils. The retention performance of these geotextiles was also investigated by analyzing the results obtained from the gradient ratio tests.
Materials and methods
To determine the hydraulic characteristic features of reinforced soil behaving with fly ash and
geotextile, two tests would be submitted in this research as follows: a- Long term flow test b. Gradient ratio test.
Long term flow test: Filtration was developed by Ecole Polytechnique of Montreal (EPM) in which flow rate of soil geotextile system is measured at a constant head. The US corps of Engineers established a direct measure of geotextile clogging potential with a gradient ratio apparatus. This long term filtration
determines the long term flow as described by EPM. Soil can be compacted to a specified density in this apparatus with a proctor hammer to represent the field density.
Gradient Ratio Test: It is estimated that the hydraulic properties of geotextiles along differ markedly from that of soil-geotextile system, the hydraulic behavior of combined soil geotextile influence the filtration ability of the geotextile in the long term flow situation, the flow rate of the soil geotextile system decreases as the pores of geotextiles get clogged. It is therefore, imperative that the clogging resistance of geotextiles be evaluated to ensure adequate long term filtration performance.
The US corps of engineers established a direct measure of geotextiles clogging potential with a gradient ratio apparatus.Materials used in these tests are: sand, fly ash,and gotextiles.
Geotextiles used in this research are from non-woven family of geosynthetics, having 50kN/m tensile strength, with drainage capacity of approximately 1700 L/day. The properties of sand and fly ash are given in Table 1.
Table 1. Characterization of fly ash, sand and other soils in the earth dam considered in research
Fly ash clay
silt sand
gravel parameter
1.9-2.2 2.6-2.8
2.5-2.7 2.25-2.55
2.55-2.67 Specific gravity
--- 18
1-7 ---
--- Plasticity Index
0.9-1.25 1.65-1.86
1.85-2.08 1.79-1.84
1.76-2.27 Maximum dry density
(g/cc)
22-27 18-23
17-20 9-12
10-13 Optimum moisture
content (%)
--- 8
6 ---
--- Cohesion (kN/m2)
34-38 0-3
24-29 25-30
30-35 Angle of internal
friction (degree)
3
10-
×
4 2 10-
×
3 2 10-
× 5 ---
--- Coefficient of
consolidation -Cv (cm2/sec)
0.1-0.4 0.1-0.6
0.1-0.15 0.01-0.05
--- Compression Index
4
10- 6– 10- 8
10- 7- 10- 7
10- 5- 10-
0.0015-0.01 1
Permeability (cm/sec)
5.1 ---
--- 6
4 Coefficient of
Uniformity-Cu
The soil sample is prepared as well graded with sieves; the thickness of the soil sample used for the study is as per ASTM standards. Mixed proportion of samples for testshave been chosen as Table 2.
Table 2. Sand Vs fly ash proportion Fly ash (%)
Sand (%)
100 0
80 20
60 40
40
60 20
80
0 100
Results and discussions
In long term flow test, a geotextile specimen of 150mm diameter has been cut and set in compacted soil in three layers with proctor hammer to give specified density, provided. The cylindrical metallic mould (15 X 30 cm) with compacted soil over geotextile specimen placed in constant –head instrument and water poured slowly on top of the soil specimen till a constant head is attained.
The flow rate measured at 15 min, 30 min, 60 min, 2 hrs, 4 hrs, and 6 hrs .Also peizometric levels H1, H2 and H3 would be read as a stabilized flow rate attained for gradient ratio test. The results of experiments have been demonstrated in Table3-4.
Table 3. Flow rate through soil mixed with fly ash in varying proportion
Sand 100%
Fly ash 0%
Sand 80%
Fly ash 20%
Sand 60%
Fly ash 40%
Sand 40%
Fly ash 60%
Sand 20%
Fly ash 80%
Time (hr)
Flow rate (Q) Cm3/min Collecting
water (V) (Cm3) Flow
rate (Q) Cm3/min Collecting
water (V) Cm3 Flow
rate (Q) Cm3/min Collecting
water (V) Cm3 Flow
rate (Q) Cm3/min Collecting
water (V) Cm3 Flow
rate (Q) Cm3/min Collecting
water (V) Cm3
570 8850
6.67 100
4.0 60
3.71 55
3.33 50
0.25
575 17250
6.47 200
3.33 100
2.92 90
2.66 80
0.50
563 33750
6.33 380
2.83 170
2.65 160
2.50 150
1
501 60100
6.16 740
2.67 320
2.45 300
2.08 250
2
472 84950
5.89 1060
2.33 420
2.12 380
1.88 340
3
440 105600
5.33 1280
2.22 550
2.06 490
1.79 430
4
420 128500
4.83 1450
2.16 650
1.96 600
1.70 510
5
399 143500
4.47 1610
2.08 750
1.85 705
1.61 580
6
Q= V×L / A×h×t , l=30cm
Table 4. Gradient ratio through soil mixed with fly ash in varying proportion
Sand 0%
Fly ash 100%
Sand 100%
Fly ash 0%
Sand 80%
Fly ash 20%
Sand 60%
Fly ash 40%
Sand 40%
Fly ash 60%
Sand 20%
Fly ash 80%
Time
(hr) piezometer i
(cm) i
piezometer (cm) i
pyrometer (cm) i
piezometer (cm) i
piezometer (cm) i
piezometer (cm)
L H3
H1
L H3
H1
L H3
H1
L H3
H1
L H3
H1
L H3
H1
1.46 30 52.8 9.0 0.70 30 32.3 11.3 0.83 30 39.0 14.0 0.86 30 42.5 17.0 0.96 30 49.3 20.5 1.23 30 46.8 9.7 0.25
1.44 30 54.4 11.3 0.66 30 34.8 15.0 0.80 30 41.0 17.0 0.84 30 43.1 17.8 0.96 30 50.9 22.0 1.23 30 47.0 10.0 0.50
1.42 30 56.6 14.0 0.65 30 37.5 18.7 0.78 30 42.8 19.2 0.84 30 43.7 18.5 0.94 30 52.5 24.1 1.19 30 47.2 11.3 1
1.41 30 59.6 17.2 0.63 30 39.5 20.6 0.75 30 44.0 21.5 0.82 30 44.0 19.3 0.93 30 54.2 25.2 1.17 30 47.5 12.4 2
1.39 30 60.7 19.0 0.63 30 41.5 22.6 0.72 30 45.5 23.8 0.82 30 44.5 19.9 0.93 30 55.1 27.2 1.11 30 47.9 14.6 3
1.38 30 61.9 20.5 0.61 30 42.4 24.1 0.70 30 46.6 25.6 0.81 30 45.0 20.6 0.92 30 55.2 27.6 1.01 30 48.3 18.0 4
1.38 30 62.4 21.0 0.60 30 43.3 25.3 0.69 30 47.6 26.9 0.80 30 46.2 22.2 0.91 30 55.9 28.2 0.92 30 49.0 21.4 5
1.38 30 62.9 21.5 0.60 30 44.2 26.2 0.69 30 48.0 27.3 0.80 30 46.5 22.5 0.90 30 56.0 29.0 0.91 30 49.5 22.2 6
Gradient= i= ([H3-H1]/ l , l=03cm
Figures 1-6 show the gradient values v.s fly ash percent in sample soil. According to figures, the flow rate for pure sand is very high and is equal to 570ml/min.The flow rate for pure sand is very high and is equal to 570ml/min.
When fly ash is mixed, even for 20% fly ash, the maximum flow rate at the beginning is only 6.67ml/min. This is a huge reduction in the flow rate. Hence addition of fly ash with sand reduces the flow rate largely and brings the flow rate to 0.01 of the flow are for pure sand. Also when fly ash mixes, reach 80%, it brings the flow rate to 3.33 ml/min and the flow rate is 0.006 of the flow are for pure sand.
From gradient ratio point of view, the preferable ratio of fly ash is 20% because the gradient ratio for this mix is the less than 1 and piping may not be occurred. For more fly ash mixes the gradient ratio may be beyond critical 1 and the stability of base is not safe. Figures 1-6 show the maximum and minimum gradient of combined sand and fly ash with different ratio.
Figure 1: Gradient in sand (20%) and fly ash (80%) soil
Figure 2: Gradient in sand (40%) and fly ash (60%) soil
Figure 3: Gradient in sand (60%) and fly ash (40%) soil
0.77 0.78 0.79 0.8 0.81 0.82 0.83 0.84 0.85 0.86 0.87
Sand 60% and Fly Ash 40%
0.87 0.88 0.89 0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97
Sand 40% and Fly Ash 60%
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Sand 20% and Fly Ash 80%
Gradient
Gradient
Gradient
Figure 4: Gradient in sand (80%) and fly ash (20%) soil
Figure 5: Gradient in sand (100%) and fly ash (0%) soil
Figure 6: Gradient in sand (0%) and fly ash (100%) soil
1.34 1.36 1.38 1.4 1.42 1.44 1.46 1.48
Sand 0% and Fly Ash 100%
0.54 0.56 0.58 0.6 0.62 0.64 0.66 0.68 0.7 0.72
Sand 100% and Fly Ash 0%
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Sand 80% and Fly Ash 20%
Gradient
Gradient
Gradient
Conclusion
The results of experiments show with increasing the fly ash ratio in the earth dams, the flow rate decreases significantly and may reach 0.006 of the flow rate in pure sand. Also from gradient ratio point of view, the suitable ratio of fly ash is 20% because the gradient ratio for this mix is the less than 1 and piping may not be occurred and the stability of dam would be in safe conditions.
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