Buttressing Of Ash Dyke Lagoon V-1 at NTPC VSTPS: A Case Study · Vindhyachal Super Thermal Power...
Transcript of 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
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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
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
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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
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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)
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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
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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
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
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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.
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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
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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 .
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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.