Pond ash is a

Sub theme- Innovation of foundation, rail and highway under extreme condition

Design and stability analysis of pond ash railway embankment V G Havanagi

A K Sinha

S Mathur

Scientists, GTE Division, Central Road Research Institute, Mathura Road, New Delhi 110020, India E-Mail - [email protected], [email protected], [email protected]

ABSTRACT - Pond ash is a suitable embankment material for the construction of road. However, this material is not in practice for the construction of railway embankment in India. Very limited basic study is available in the literature. Due to this, study the behavior of pond ash railway embankment under dynamic condition, a pilot project on „Design of pond ash railway embankment‟ is carried out by Central Road Research Institute (CRRI) New Delhi, India. Pond ash is procured from National Thermal Power Station (NTPC), Kahalgaon, Bihar, India. In this paper, geotechnical properties of pond ash and local soil are presented. Design and stability analysis of embankment with different height is carried out considering the uniformly distributed load due to train loading under the water logged and sudden drawdown conditions with earthquake factor. It is observed that critical factor of safety is coming for lower height of embankment in comparison to the higher embankment. Berm is also considered for higher height of embankment.

1.0 INTRODUCTION About 100 million tons of pond ash is produced in India and only about 20% is utilized for Roads, Buildings and other Civil engineering applications. Bulk utilization of Pond ash is being carried out for road embankment construction in the massive road development programs taken up by the Indian government viz. National Highway Development Program (NHDP) and Pradhan Mantri Gram Sadak Yojana (PMGSY). India has the second largest railway network in the world. The utilization of pond ash in railway embankment would not only increase substantially the per cent utilization, but also provides an alternative to costly good earth and protects the environment. In this context, a pilot project has been taken up to construct 10 km of pond ash railway embankment within the National Thermal Power Corporation (NTPC) plant area, Kahalgaon, Bihar, India. In this this paper, geotechnical characterization of materials are presentenced. Design and stability analysis of pond ash railway embankment is also presented under different conditions. 2.0 MATERIALS Pond ash samples were collected from the pond area at different locations, from NTPC, Kahalgaon. Soil samples were collected along the alignment of proposed railway embankment. i.e. Hurra satiation to NTPC Kahalgaon, Bihar, India.

1

3.0 GEOTECHNICAL CHARACTERISTICS OF POND ASH To study the geotechnical characteristics of collected pond ash and soil, different laboratory experiments were carried out which include:(a) Grain size analysis (b) Atterberg limit tests (c) Specific gravity test (d) Proctor compaction test (e) Direct shear test, and (f) permeability test. The results have been discussed in sections 3.1 to 3.6. 3.1 Grain size analysis Grain size analysis was carried out as per IS: 2720 (Part 4) – 1985. Pond ash was observed to be a coarse grained material. About 67 % of the material is in the sand size while remaining 33% of the material is in the silt size. The soil was observed to be fine grained soil. The particle size distribution curves for pond ash andPage soil 4are Format No.QSP/GTE/05.F.02 of 4 shown in Fig.1. 100

% Finer

80 Pond ash Soil

60 40 20 0 0.001

0.01

0.1 1 Particle size (mm)

10

Fig.1 Grain size distribution curves for pond ash and soil Fig.Pond Grainash size analysis of coal ash sample Proctor curve for 3.2 Atterberg limit test The plasticity characteristics of pond ash and soil were carried out as per IS: 2720(Part 5) – 1985. It is observed that pond ash is a non-plastic material. The liquid limit and plastic limit of soil were observed to be 45% and 23% respectively. The value of Plasticity Index (P.I.) was estimated as 22 % which indicates medium plastic nature of local soil. According to BIS-1498, pond ash is classified as SM i.e. silty sand mixture and local soil was classified as CI i.e. inorganic clay and silts of medium plasticity. 3.3. Specific gravity test (G) Specific gravity test was carried out as per IS 2720(part 3). The value of specific gravity was obtained as 2.29 and 2.67 for pond ash and soil respectively. Pond ash has low specific gravity as compared to soil.

2

3.4 Proctor compaction test Modified Proctor test was carried out as per IS: 2720 (Part 8) -1983. The compaction curves for pond ash and local soil are shown in Fig.2. The value of Maximum Dry Density (MDD) for pond ash is observed to be 13.5 kN/m3 while the Optimum Moisture Content (OMC) is 18%. Compaction curve of pond ash was found to be flat indicating that variation in moisture content does not predominantly affect the dry density. The value of Maximum Dry Density for soil is obtained as 19.5 kN/m3 and Optimum Moisture Content is 10%.

25

Dry density, kN/m3

Soil Pond ash

Zero air void lies

20

15

10 5

15

25

Moisture content, % Fig.2 Proctor compaction curves for Pond ash and soil 3.5 Direct shear test Direct shear test was carried out on compacted sample of size 6 cm x 6 cm x 2.5 cm as per IS: 2720 (Part 13)-1986. The test was carried out on samples compacted at 95% of its modified Proctor dry density. All the samples were tested under the saturated condition. The normal stress varied in the range of 50 to 150 kN/m 2. The sample was sheared at a rate of 0.25 mm/min to simulate the drained condition. All the samples bulged before failure. The cohesion(c) and angle of internal friction (ɸ) are obtained as 0 kN/m2 and 310 for pond ash and 51 kN/m2 and 160 for soil respectively. The value of angle of internal friction for pond ash as 31 0 indicates its usefulness in embankment fill applications. 3.6 Permeability test Permeability test was carried out on compacted pond ash as per IS: 2720 (Part 17) 1986. Remolded samples were prepared at modified dry density. The coefficient of permeability is determined as 2.6x10-6 m/sec for pond ash and 2.4x10-8 m/sec for soil respectively. This value of the permeability of pond ash indicates that it is a free draining material and has the potential for its utilization as an embankment fill. The value of the permeability for soil indicates that it has poor drainage property. Summary of geotechnical properties of coal ash and soil are given in Table 1.

3

Table 1. Geotechnical properties of pond ash and soil Parameter Specific gravity test

Particle size analysis

Atterberg limits test

Soil classification Proctor Compaction test Permeability test Direct shear test

Gravel, % Sand, % Silt, % Clay, % D10,mm D60,mm Cu Cc Liquid limit, % Plastic limit, % Plasticity Index, % 3

MDD,kN/m OMC, % Coefficient of Permeability, m/s Cohesion, kN/m2 Friction angle, degrees

Pond ash 2.29 0 67 33 0 0.02 0.20 12.5 0.72 NP -

Soil 2.67 3 26 31 40 -NA0.01 -NA-NA45 23 22

SM 13.5 18 2.6 x10-6

CI 19.5 10 2.4x108

0 31

51 16

4.0 DESIGN OF POND ASH RAILWAY EMBANKMENT Railway embankment is different from highway embankment especially for drainage and loading condition on the embankment. The cross section of railway embankment have to be designed in such a way that differential settlement of the embankment shall be minimum. This would ensure proper level of sleepers/rails which is very much needed for smooth movement of train traffic. Pond ash embankment of different heights 3m, 5m, 7m and 11m were designed considering different material specifications and sub soil characteristics as discussed below in section 4.1 to 4.9. 4.1 Fill material: Pond ash was used as an embankment fill material. 4.2 Cover/Intermediate soil layer/Sub grade layer: Local soil having a PI value of 22 % was proposed as cover/intermediate soil layer for the proposed pond ash embankment as per IRC: SP: 58-200. Considering its poor drainage characteristics and high PI value, extra provisions were made for proper drainage of railway embankment. Cover soil thickness and slope was adopted as 2.24m (horizontal) and 1V:2H respectively. Pond ash embankments having height more than 3m, intermediate layers of thickness 0.4m are provided for better confinement and compaction.

4

4.3 Blanket: A blanket of sand layer of thickness 1m may be provided on the top of soil cover. The gradation and other properties of blanket material as per specification as per Guideline No. GE: G-1(2003). 4.4 Ballast: A 0.3 m thickness of stone aggregate (ballast) is provided on the blanket layer at a distance of 1m from the slope of the embankment. Properties of the ballast are as per Guideline No. GE: G-1(2003). 4.5 Berm: Berm is provided when the height of the embankment more than 6m. A 2m top thickness having slope 2H: 1V is provided of local soil in addition to cover soil on the slope. An intermediate berm may be also provided for 11m height embankment. Berm soil may be used as same as cover soil. This would increase the stability of higher height pond ash embankments. 4.6 Toe wall: A toe wall made of fly ash bricks is provided at the toe of the pond ash embankment of different heights. This will be helpful to reduce the possible scour of embankment at the toe. The height of wall proposed is 1m and having a weep hole covered with geotextile. Weep hole drains the accumulated water in the sand layer provided at the bottom of the embankment. 4.7 Drainage measures: There is every possibility of heavy inflow of water into the railway pond ash embankment. The ingresses water should be able to drain out from the embankment to avoid development of pore water pressure. Geotextile wrapped 0.4m thick sand layer is provided over each intermediate soil layers. This starts from slope of the embankment and end at 2m inside the pond ash embankment. This would ensure proper drainage of each confined cell of pond ash embankment, compensating for the poor drainage characteristics of local cover soil. A 0.3 m layer of sand is provided at the bottom of pond ash embankment on the virgin in situ compacted soil. This layer would further facilitate the dissipation of pore water pressure due to infiltration of water from the top of the embankment. 4.8 Load on the embankment: The safe load due to traffic on the railway embankment was considered as 250 kPa on the top of blanket surface of railway embankment. 4.9 Design cross sections: A typical design cross sections of railway embankment proposed for construction are shown in Fig.3.

Fig. 3 Cross-section of 7 m height5of pond ash railway embankment

5.0 STABILITY ANALYSIS OF EMBANKMENT Stability analysis was carried out by a computer software considering the pond ash embankment to be under (a) Partially saturated (b) Fully saturated and (c) and sudden draw down conditions. Analysis was carried out with and without seismic factors for above conditions. Analysis was carried out considering the seismic forces in the horizontal and vertical direction. The basic seismic coefficients considered in the analysis are h= 0.05 (horizontal) and v=0.025 (vertical) as per BIS code. Factor of safety values for different heights of embankments are shown in Table 2 and 3. Typical stability analysis typical 7m height coal ash embankments is shown in Fig. 4. Table 2 Factor of safety values for 3m and 5m railway embankment Compacted Embankment height condition of 3m 5m embankment Without With Without With seismic seismic seismic seismic Partially saturated 1.26 1.19 1.31 1.22 Fully saturated

1.09

1.02

1.14

1.04

Sudden draw down

1.11

1.05

1.11

1.02

Table 3 Factor of safety values for 7m and 11m railway embankment Compacted Embankment height condition of 7m 11m embankment Without With Without With seismic seismic seismic seismic Partially saturated 1.50 1.40 1.72 1.57 Fully saturated 1.35 1.23 1.69 1.46 Sudden draw down 1.20 1.11 1.36 1.24

Fig. 4 Stability analysis for 7m height pond ash railway embankment 6

Factor of safety values from the stability analysis indicated that different heights of railway embankments are safe under specified loading, saturated and seismic conditions. From the analysis, it is observed that the effect of load is significant on lower height embankments as compared to higher height embankments. The factor of safety values for 3m and 5m height embankments are lower and observed to be in the range of 1.03-1.31 whereas these values ranged between 1.11-1.72 for 7m and 11m height embankments for different conditions. The saturation condition of the pond ash railway embankment also had an effect on stability values. The sudden draw down conditions resulted in least factor of safety values. 6.0 CONCLUSIONS Pond ash and local soil were collected from Kahelgaon, Bihar, India, the place of proposed construction of railway embankment and were investigated for their geotechnical characteristics. Different cross sections of railway embankment of heights 3m, 5m, 7m and 11m were designed considering the requirements of drainage and based on results of stability analysis. Some of the conclusions drawn from the study are as follows: The pond ash which is used as a fill material is classified as SM i.e. silty sand mixtures. Proctor compaction curves were found to be flat indicating that variation in moisture content does not predominantly affect the dry density. Pond ash was found to have angle of internal friction (ɸ‟) of 310 and coefficient of permeability of 2.6x10-6 m/sec. These values indicate its usefulness in railway embankment fill applications. The local soil is classified as CI type of soil, which indicates inorganic clay and silts of medium plasticity. The permeability value is observed to be 2.4x10 -8 m/sec. Minimum thickness of 1m (perpendicular) local soil is proposed as cover soil for different heights of pond ash embankment. Considering the poor drainage characteristics of local soil, geotextile wrapped sand layers are proposed for proper drainage of water from the embankment. For pond ash embankments having height more than 3m, intermediate soil layers of thickness 0.4m are provided for better confinement and compaction. Provision of Berms for embankments exceeding 5m, improves the stability of the embankment. A toe wall made of fly ash bricks is provided at the toe of the pond ash embankment of different heights. This will be helpful to reduce the possible scour of embankment at the toe. The height of wall proposed is 1m and having a weep hole covered with geotextile. Stability analysis of pond ash railway embankment carried out under specified loading, saturation and seismic conditions indicated safe factor of safety values. Effect of rail load is significant on lower height embankments as compared to higher height embankments. The factor of safety values for 3m and 5m height embankments are observed to be less as compared to 7m and 11m height embankments, the most critical being the 3m height embankment.

7

7.0 NEED FOR FURTHER STUDY Construction of pilot experimental test section of limited length (10 km) using pond ash is likely to progress at Kahalgaon, Bihar, India. The performance of the constructed railway embankment will be monitored over a period of at least two monsoon seasons before recommending for large scale field applications. ACKNOWLEDGEMENT The authors are thankful to the Director, Central Road Research Institute, New Delhi, India for giving permission to publish this paper. We are thankful to Sh. V K Mathur, AGM, NTPC, India for sponsoring this research project. The support provided by Sh. R.K.Swami, Sh. P S Prasad, Mrs. Uma Arun and other laboratory staffs of the GTE division is also acknowledged. REFERENCES IS 2720(Part3). Methods of test for soils: Determination of specific gravity of soils. Published by Bureau of Indian standard, New Delhi, India; 1980.

IS 2720(Part4). Methods of test for soils: Grain size analysis of soils. Published by Bureau of Indian standard, New Delhi, India; 1980. IS 2720(Part5). Methods of test for soils: Determination of Liquid limit and Plastic limit. Published by Bureau of Indian standard, New Delhi, India; 1985. IS 1498. Classification and identification of soils for general engineering purposes. Published by Bureau of Indian standard, New Delhi, India; 1970. IS 2720(Part8). Methods of test for soils: Determination of water content dry density relation using heavy compaction. Published by Bureau of Indian standard, New Delhi, India; 1980. IS 2720(Part 39). Methods of test for soils: Direct shear test for soils. Published by Bureau of Indian standard, New Delhi, India; 1977. IRC: SP: 58. Guidelines for use of fly ash in road embankments. Published by Indian Road Congress, New Delhi, India; 2001. Guideline No. GE: G – 1(2003), “Guidelines for earthwork in railway projects” Geotechnical engineering directorate. Published by RDSO, Manak Nagar Lucknow – 11.

8