buttressing of ash dyke lagoon v-1 at ntpc vstps: a case study · vindhyachal super thermal power...

International Journal of Civil and Structural Engineering Research ISSN 2348-7607 (Online) Vol. 5, Issue 2, pp: (63-71), Month: October 2017 - March 2018, Available at: www.researchpublish.com

Page | 63 Research Publish Journals

Buttressing Of Ash Dyke Lagoon V-1 at NTPC

VSTPS: A Case Study

Prabhat Kumar1, Raghvendra Narayan

2, M.K. Mangla

3

Vindhyachal Super Thermal Power Station, NTPC Limited, Singrauli, Madhya Pradesh-486885, India

Abstract: Utilization and management of fly ash generated from the power generating process is one of the biggest

challenge being faced by pit head coal based power plants like NTPC Vindhyachal. Fly ash is being supplied to ash

based industries for its utilization and the balance fly ash and bottom ash is being disposed off in the captive ash

ponds. Capacity addition of ash dyke by acquiring additional land is also a mammoth task in present

circumstances. Hence, buttressing of existing ash dyke lagoons is not only an avenue of ash utilization on

sustainable basis, but it also provides an opportunity to expand the capacity of ash storage in dyke without

acquiring any additional land. This paper discusses the details of viability, site conditions and method of

buttressing of ash dykes with use of pond ash and bottom ash as filter materials and issues associated thereof.

Keywords: NTPC Vindhyachal, VSTPS, BUTTRESSING OF ASH DYKE LAGOON.

1. INTRODUCTION

Coal based thermal power plants share larger pie of Indian power sector with approx 187 GW of coal based thermal out of

total installed capacity of 308 GW. Indian coal has 30-40% ash content which translates into approx 300 million

MT/annum ash generation in India, 65% of generated ash is disposed off in ash ponds, which works out to be 1625

hectares of additional land every year for ash pond construction.

As per current design practice, ash dyke is abandoned after four height raisings, buttressing is a concept of lateral

strengthening of ash dyke using pond ash. Evacuation of pond ash for lateral filling enhances the dyke life and creates

avenue for successive height raisings thus significantly increasing the capacity for ash disposal. Most noteworthy point is

that it does not necessarily require additional land.

NTPC Vindhyachal is the largest power plant of the country with installed capacity of 4760MW, annual coal consumption

is approx 240 Lac MT and annual ash generation is approx 84 Lac MT. Station has six ash ponds for ash disposal viz. V-1

dyke, V-2 dyke, V-3A dyke, V-3B dyke, V-4A dyke and V-4B dyke. V-1 ash is the oldest dyke and all four height raisings

have been completed and dyke is exhausted.

2. VIABILITY

V-1 ash dyke of NTPC VSTPS is located at Shahpur village at a distance of 07 Km from plant. Base area of starter dyke is

474 Acres and after four successive upstream raisings, area of ash pond has reduced to 272 Acres. Total design capacity of

ash dyke V-1 is 294 Lac Cum, as detailed in table 2.1 below:

Table 2.1 Designed capacity data of ash dyke V-1

Starter dyke 1st raising 2

nd raising 3

rd raising 4

th raising

Area of dyke (Acres) 474 405 331 308 272

Height of Embankment (M) 7.0 3.0 3.0 3.0 3.0

Dyke capacity (Lac Cum) 135 49 40 37 33



Table 2.2 Capacity of dyke after buttressing

As detailed in table 2.2, after buttressing and two subsequent height raising of 05 M each, dyke capacity will be enhanced

by 134 Lac Cum which is 45.6% of original designed capacity. Reverse calculation indicates that approximately 216 Acre

of new land would have been required to create ash disposal capacity of 134 Lac Cum. Hence buttressing of existing ash

dyke lagoon V-1 will save this much of new land.

3. SITE CONDITIONS

Starter dyke of V-1 ash dyke of NTPC VSTPS was constructed in 1987 with earth/soil. So far 04 upstream raisings have

been done each of effective height 03M. Feature of the dyke can be detailed as follows:

1. Starter Dyke:

i. Starter Dyke is made of earth and maximum height of dyke is 14M.

ii. Total area of starter dyke is 474 Acres.

iii. Upstream and downstream slope of dyke is 1 (V):2.5 (H)

iv. Top of the dyke is 06 M and top level is 277.00 M

v. Starter dyke is provided with internal drainage system which consists of sand blanket and sand chimney.

vi. Downstream side of dyke embankment is provided with 200mm sand 200mm aggregate and 600mm of boulder.

vii. About 1.7 Km of dyke embankment is in the submergence area of Rihand reservoir.

viii. About 1.8 Km of length of dyke is filled with ash up to top on downstream side and some portion of dyke is common

with the over flow lagoon of S-1.

ix. There is not much area available in other parts as well. Hence additional land availability beyond toe of dyke is a site

constraint.

2. Raisings:

i. Over starter dyke 04 height raisings have been done each of net height 03M with pond ash as main fill material and soil

as top and slope protection.

ii. Upstream and downstream slope of ash dyke raising is 1(V):3(H)

iii. Each height raising has been provided with drainage system which consists of horizontal blanket and vertical sand

chimney.

iv. Past performance of the dyke has been satisfactory so far and no breach has occurred.

3. Site constraints:

i. Towards Rihand reservoir side: There is no land available beyond the toe of the dyke and hence it restricts the outward

buttressing of the dyke. Peripheral raising will be done from top of starter dyke in this region

ii. Towards S-1 OFL side: The over flow lagoon of S-1 dyke is adjacent to the V-1 dyke. This restricts the buttressing of

dyke in this region, this OFL is not in use and hence minimum 15 M of area from existing OFL can be spared for this

purpose.

iii. Towards Hydro power plant of SSTPS: Starter dyke embankment is buried towards this side of dyke due to ash filling.

iv. Towards Spillway: Dyke embankments form a “U” towards spillway of dyke. Buttressing is not possible in its current

shape and hence the existing spillway will be dismantled and new water escape structure of buttressed dyke will be

connected to water escape structure of starter dyke through concrete Hume pipes.

Buttressing of dyke 5th

raising 6th

raising

Area of dyke (Acres) Lateral filling 272 272

Height of Embankment (M) Lateral strengthening by filling pond ash as

fill material

5.0 5.0

Dyke capacity (Lac Cum) 24 55 55



4. FEASIBILITY STUDY

After conceptualization of idea of buttressing for ash dyke lagoon V-1 at NTPC VSTPS, consultancy was obtained from

NIT Rourkela based on the detailed topographical survey and geo-technical investigation of V-1 ash dyke conducted at

site.

4.1. Detailed topographical survey:

Detailed topographical survey of the area was done to assess the land availability for lateral strengthening of ash dyke

lagoon V-1. Depending upon the land availability and topography of area dyke embankment has been divided into 08

different sections.

4.2. Geotechnical Investigation:

Geotechnical investigation was carried out to determine the geo-strata and pertinent physical properties of soil beneath so

that safe and economical foundation for the proposed structure can be designed. Boring and other field tests facilitate

determination of geo-strata and collection of soil sample for laboratory testing. Geotechnical work broadly involved:

i. Making borehole at specified location, conducting SPT and collecting samples from there.

ii. Ground water observation in bore holes

iii. Conducting various laboratory tests to determine engineering properties viz. grain sixe distribution, consistency tests,

compaction and density test, permeability tests, consolidation tests and shear strength test.

Analysis of results, substantiate the fact that clay soil exists at starter dyke and beyond toe of starter dyke a thin layer ash

exists. A reasonably thick layer of ash deposition is found in the 1st 2

nd 3

rd and 4

th raising of ash dyke. The average RL of

ground water table varies from 276.110 M to 277.35 M. Sub soil strata of a particular location of V-1 ash dyke is

represented below in figure 4.1.

Fig. 4.1 sub soil strata of existing V-1 ash dyke

5. DETAILED DESIGN

All along the periphery of existing dyke downstream buttressing will be constructed within available space and new

internal drainage system will be developed and connected with the existing drainage system. A minimum 15 of space is

required beyond toe of starter dyke to construct buttressing of ash dyke but due to site constraints and non availability of

land in certain portions, partial buttressing will be adopted. It is to be noted that outward extension of dyke toe provides

additional space for ash disposal and it can be raised further in future, but space constraint around existing toe of starter

dyke restrict its extension towards Rihand reservoir side. Accordingly in this region buttressing has been proposed from

top of the starter dyke. In certain locations starter dyke is high and toe of the dyke is buried, in this region buttressing will

start from 1st raising onwards. Plan of ash dyke lagoon V-1 is represented in figure 5.1 and phase wise buttressing is

represented from fig 5.2 to fig 5.5, construction details can be summarised as follows:

1. Construction of dry ash embankment around periphery of starter dyke from existing ground level/top of starter

dyke/top of first raising to the top of 4th

raising i.e. up to level of 289.00 M

2. Construction of fifth raising from EL 289.00 M to 294.00 M (Fig 5.4)



3. Construction of sixth raising from EL 294.00M to 299.00M (Fig 5.5)

5.1. Embankment design:

Existing dyke embankment will be stripped off to remove the vegetation, thereafter it will be compacted to 95 % of MDD.

The pond ash obtained from the lagoon shall be used in forming the complete cross-section of the fill. The materials shall

be placed in the fill in continuous layers, stretching right across the whole section, not more than 30 cm in compacted

thickness and rolled by minimum 10 tons vibratory rollers of minimum 6 passes. The compaction should be done up to a

sufficient depth to provide a satisfactory bonding surface before the next layer of fill material is placed.

Fig 5.1 Plan of Buttressing of ash dyke lagoon V-1

5.1.1. Pond ash as Fill material: Samples of bottom ash were collected and sent to CSMRS, New Delhi for test of

suitability as fill material for embankment construction. Grain size analysis of tested pond ash sample shows that clay sizes

vary from 2% to 4.3%, silt sizes vary from 27.6% to 83.4% fine sand vary from 13.6% to 60.2%, medium sand sizes vary

from 0.6% to 21.8% and coarse sand sizes vary from 0% to 1.1%, gravel sizes were absent. Liquid Limit of pond ash vary

from 38.9 to 42.1, thus it is non-plastic in nature. The value of MDD and OMC vary from 1.201 g/cc to 1.218 g/cc and

22.1% to 25.1% respectively. Based on these results pond ash was recommended to be used as fill material for lateral

filling of ash embankment.As per estimates approximately 19, 20,000 cum of pond ash shall be utilized in lateral filling of

ash dyke embankment.

5.1.2. Earth cover over ash fill: It includes placement and compaction of at least 50cm thick earth cover over the entire

ash fill as a protection layer. The cover should be provided such that it should meet the slope of dyke formed with ash as

per drawings.

Fig 5.2 Existing ash dyke lagoon V-1 after 4th raising



Fig 5.3 Phase-I Buttressing of ash dyke lagoon V-1

Fig 5.4 Phase-II 5th raising of ash dyke lagoon V-1 from EL 289.00M to 294.00 M

Fig 5.5 Phase-III 6th raising of ash dyke lagoon V-1 from 299.00 M

5.2. New Internal drainage system:

Existing starter dyke and raising have well placed internal drainage system which consists of sand blanket, sand chimney

and rock toe at the toe of the dyke. Internal drainage system of buttressed dyke and subsequent raisings is designed in such

a way that it is connected with the old drainage system of the dyke. Bottom ash blanket will be laid over existing



downstream slope of starter dyke and subsequent raisings up to RL 289.00M horizontal blanket with 2% slope will be laid

on horizontal embankment of the dyke and on horizontal ground beyond the toe of starter dyke. The thickness of the

blanket should be 750mm as detailed in figure 5.3.

Peripheral finer drain shall be laid at all levels along the dyke length. Perforated pipes shall be laid along the toe drain

which will be covered by geo-textiles and minimum 900mm of bottom ash shall be provided over the perforated pipe to

allow safe drainage of water. Over this filling compacted ash shall be filled up to the berm of the dyke, thereafter bottom

ash blanket shall be laid. Over this bottom ash layer normal ash filling of pond ash starts. The scheme is detailed below in

fig 5.6.

Fig 5.6 Internal drainage system in buttressing of ash dyke V-1

Seepage water from existing rock toe shall be collected by perforated peripheral drains provided along toe drain and then

same shall be discharged in to the side finger drain through T joint for passage of water to lower toe drain. The side finger

drain made of PVC shall be covered with bentonite mixed clay soil to make it impervious as detailed in fig 5.7.

Fig 5.7 Cross section of finger drain

Fig 5.8 Cross section of peripheral drain

Vertical sand chimney shall be provided from existing 4th

raising to proposed 6th

raising in two phases from E.L. 289.00 M

to 294.00M and from 294.00 M to 299.00M.The thickness of the vertical chimney shall be 750mm and it shall be

connected to the horizontal blanket as detailed in fig 5.4 and fig 5.5. Horizontal sand blanket shall be laid on the ground

beyond the toe of existing ash dyke and shall be connected to newly constructed peripheral rock toe drain. Rock toe shall

be provided at NGL, EL 277M, 283M, 289M and 299M, internal drainage made of bottom ash blanket and chimney shall

be connected to the rock toe. Height of the rock toe shall be 1.5M at NGL and it shall be kept 1.0M at all other levels, fig

5.9.



Fig 5.9 Section of outer/peripheral rock toe

5.2.1. Bottom ash as filter material: Bottom ash is coarser in nature and hence it has potential to be used as filter material

in place of sand. To establish the facts detailed study was conducted at IIT BHU and CSMRS, New Delhi. Based on the

particle size analysis carried out at these labs it was established that bottom ash is suitable to be used as filter media with

respect to the pond ash as fill material. Laboratory tests conducted over bottom ash sample can be summarised as below:

Grain size analysis shows that bottom ash predominantly contains fine sand sizes followed by medium sand sizes, silt sizes

and coarser sand sizes. Clay sizes contains of 2-3%, silt sizes 8.8% to 13.9%, fine sand 58.8% to 59.4% medium sand sizes

from 20.8% to 27.9% coarse sand sizes 2.5% to 2.9%. PI value of bottom ash in 68.2 to 69.9, hence non plastic in nature.

The MDD and OMC value ranges between 1.047g/cc to 1.055 g/cc and 33.5% to 36.4% respectively.

Filter material suitability analysis was also carried out and bottom ash was found suitable to be used as filter media with

respect to pond ash. Out of 19 samples 17 samples of bottom ash having D15 (BA)>0.1 mm, corresponding ratio of

D15(BA)/D15(PA) is more than 5 for all samples thus meeting the filter criteria. Out of 19 samples, 17 samples of bottom

ash are having percentage of material passing 75µ<7% thus meeting the criteria.

Based on above results bottom ash was recommended to be used as filter material with respect to pond ash. Approximately

4, 87,000 cum of bottom ash is proposed to be used as filter media in buttressing of ash dyke lagoon V-1.

5.3. Decantation System and Water escape structure:

Existing spillway of V-1 dyke shall be dismantled and new wells will be constructed inside the V-1 ash dyke. Since these

wells are proposed to be constructed over ash fill, proper foundation shall be prepared by placing geo-grid and ash in

layers of 300mm and thereby constructing a footing to erect the well. Concrete Hume pipes of diameter 1 M shall collect

the water from these wells and it will be discharged into the existing open channel connected to V-1 OFL. Scheme is

detailed below in fig 5.10

Fig 5.10 Decantation well and its connection to existing well and further to over flow lagoon



Fig 5.11 Construction of new water escape structure over ash filling

6. CONCLUSION

Ash disposal is the most challenging area of coal based thermal power plant operation, with increasing scarcity of land and

emphasis on 100% ash utilization; new avenues of ash utilization are being explored. But avenues for large scale ash

utilization are limited, particularly for remotely located coal based power plants. Buttressing of existing ash dyke is a rare

ray of hope, as it serve dual purpose, it utilizes huge volume of ash and further creates avenue for further height raising to

create additional space for ash disposal without going for additional land. In the present case of buttressing of ash dyke

lagoon V-1 total 19, 20,000 cum of pond ash and 4, 87,000 cum of bottom ash will be utilized. Utilization of bottom ash in

place of sand is a sustainable practice, as it saves equal amount of naturally occurring sand which is a scarce resource in

itself.

REFERENCES

[1] NIT Rourkela (2014, Dec). Report on design of buttressing of ash dyke lagoon V-1 at NTPC VSTPS by Dr. Umesh

Dayal and Dr. C R Patra

[2] Arkitechno Consultants Pvt. Ltd. (2015, Feb). Report on Geotechnical investigation of ash dyke lagoon V-1 by Dr. P.

K. Dash

[3] CSMR, New Delhi (2016, May). Report on laboratory investigation of bottom ash and pond ash collected from

NTPC VSTPS

[4] NTPC. (2008, April). Guidlines for Ash Disposal Management in NTPC Stations. Operation Guidance Note . Noida.

[5] NTPC Limited. (2009, May). Technical Specification for Ash Dyke Package. Noida, U.P., India.

[6] S.R., G. (2005). Design and Maintainance of Ash Pond for Fly Ash Disposal. Indian Geotechnical Society .



AUTHORS’S PROFILE:

Author - 1 Prabhat Kumar

Dy. Manager (O&M - Civil)

NTPC Vindhyachal

B. Tech. (Civil Engg.)

NIT - Srinagar

Author - 2 Raghvendra Narayan

Dy. Manager (AUD)

NTPC Vindhyachal

M.Sc. (Env. Science)

Worked earlier in

Ultratech Cement Ltd

Author - 3 M.K. Mangla

AGM (AUD/AHM/Civil)

NTPC Vindhyachal

BE. (Mechanical Engg.)

University of Roorkee.

buttressing of ash dyke lagoon v-1 at ntpc vstps: a case study · vindhyachal super thermal power...

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