Design of Dewatering Scheme.pdf
Transcript of Design of Dewatering Scheme.pdf
TAG ENGINEERING & CONTRACTING CO W.L.L. PUNJAB BARRAGE IMPROVEMENT PHASE-II (PBIP-II) PROJECT
Rehabilitation & Modernization of Jinnah Barrage
DESIGN OF DEWATERING SCHEME
January, 2012
Jahangir Akhtar & Associates 316-D, OPF Housing Colony, Raiwind Road, Lahore, Pakistan Tel: +92 42 35323313-5 Fax: +92 42 35323316 Email: [email protected]
Table of Contents
1 Introduction .................................................................................................................................... 3
2 Design Parameters and Analyses Model ................................................................................. 4
2.1 Permeability of Foundation Alluvium ................................................................................. 4
2.2 Seepage Analysis ................................................................................................................. 4
2.3 Software ................................................................................................................................. 4
3 Results ........................................................................................................................................... 5
4 Conclusions ................................................................................................................................... 6
Figures
Fig. 1 Plan showing Engineer’s proposal of arrangement of coffer dams
Fig. 2 Seepage Analysis Results based on Engineer’s proposal of coffer dams
Fig. 3 Plan showing alternative dewatering scheme
Fig. 4 Seepage analysis model based on alternative dewatering scheme
Fig. 5 Seepage analysis result based on alternative dewatering scheme
Fig. 6 Typical details of dewatering well proposed for alternative scheme
Annex-A
Cost Estimate for Alternative Dewatering Scheme
Punjab Barrage Improvement Phase‐II (PBIP‐II) Project Page 3 of 15 Rehabilitation & Modernization of Jinnah Barrage
1 Introduction
Government of Punjab has planned to rehabilitate Jinnah Barrage which was constructed
before independence of Pakistan. The project site is located at about 35 km in the north of
Mianwali Bannu Road. The rehabilitation of barrage comprises construction of a subsidiary
weir downstream of the main barrage and repairs of the main barrage.
Careful handling of water will be a key component in successful and timely completion of the
project. This matter has been discussed in the Tender documents in detail. Coffer dams will
be provided in order to barricade the construction area from water. Dewatering wells will
have to be installed to lower the ground water level within the enclosed area to sufficient
depths to facilitate the construction process by providing workable dry area.
M/S TAG Engineering & Construction (Pvt) Ltd. in Joint Venture with M/S C.W.E. and Izhar
Limited, one of the prospective bidders, approached M/S Jahangir Akhtar & Associates
(J&A) to analyse and design a dewatering scheme for the purpose of preparation of the bid.
The objectives of this analysis are to;
• Study the Engineer’s proposal provided in the Tender documents and check its
suitability
• Study various dewatering options through analyses and recommend the optimum
layout of the dewatering well system
• Propose the well design for the recommended option
This report prepared by M/S J&A covers the above objectives and includes the background
and results of all the analyses.
Punjab Barrage Improvement Phase‐II (PBIP‐II) Project Page 4 of 15 Rehabilitation & Modernization of Jinnah Barrage
2 Design Parameters and Analyses Model
This section describes the selection of design parameters for foundation material adopted for
the seepage analyses.
2.1 Permeability of Foundation Alluvium
During the geotechnical investigations carried out by Engineer in 2008 field permeability
tests were carried out in all the boreholes drilled through alluvium at the site. Coefficient of
horizontal permeability has been taken as 2.43 x 10-3 ft/sec (maximum value reported in the
geotechnical investigation report provided in the Tender documents for a rational analysis).
Anisotropy ratio for the alluvial deposits has been selected as 0.5 on the basis of past
experience.
2.2 Seepage Analysis
Using the coffer dam layout provided in the Tender documents at the proposed subsidiary
weir section (Fig.1), a seepage analysis was carried out. Dewatering wells have been
modelled on both sides of the excavation. The results of the analysis are shown on Fig. 2.
The design parameters for alluvial deposits used in seepage analysis are also marked in Fig.
2
An alternative arrangement of dewatering wells has also been studied. Plan showing this
arrangement of wells has been presented as Fig.3. In this arrangement coffer dams will be
at 164ft distance from the edge of excavation on both sides. Seepage analysis model and
results have been shown on Fig.4 and Fig.5 respectively.
Typical details of dewatering well proposed for alternative recommended scheme has been
shown on Fig.6.
2.3 Software
Seepage analysis has been carried out using a software Seep/w of Geo slope version 2004.
Punjab Barrage Improvement Phase‐II (PBIP‐II) Project Page 5 of 15 Rehabilitation & Modernization of Jinnah Barrage
3 Results
Seepage analysis based on the arrangement provided in the Tender documents has been
carried out using the model shown in Fig.2 using Seep/w. The results show day lighting
of water on the left hand side coffer dam.
Seepage analysis results based on the alternative arrangement have been shown on Fig.5.
These results indicate that after dewatering, the phreatic surface under steady state flow
conditions remains under the NSL and well below the required excavation levels. The analysis also indicates that; dewatering wells of 1.5 cusecs discharge capacity installed at 40ft c/c and both sides of the excavation ensure dry working area for construction.
Another analyses was carried out with dewatering wells of 1.0 cusecs discharge capacity
installed at 25ft c/c. On the basis of our analysis this arrangement proves expensive than the
recommended scheme.
Punjab Barrage Improvement Phase‐II (PBIP‐II) Project Page 6 of 15 Rehabilitation & Modernization of Jinnah Barrage
4 Conclusions
On the basis of the analyses, it is concluded that dewatering wells installed next to the
excavation with the coffer dam arrangement provided in the Tender documents does not
prevent water day lighting on left hand side of the excavation as shown on Fig.2.
Alternative dewatering scheme (on the basis of coffer dams shifted closer to the proposed
excavation limits) is recommended for implementation during the construction stage.
Plan of alternative dewatering scheme is shown on Fig.3 where as section is shown on
Fig.4.
Cost estimate for dewatering wells prepared for the recommended dewatering scheme is
also attached in Annexure-A.
FIGURES
665
670
675
680
Description: Sandy Gravel (Alluvial Material)Kx: 2.43 E-03 ft/secKy/Kx Ratio: 0.5
W/L @ El. 682 W/L @ El. 682
PUNJAB BARRAGE IMPROVEMENT PHASE-II (PBIP-II) PROJECT Rehabilitation & Modernization of Jinnah Barrage
Fig.2 Seepage Analysis Results based on Engineer's Proposal of Coffer Dams
Phreatic Surface
Equipotential Line Flow Line
37.
343
ft³/h
r
90.
059
ft³/h
r
Distance (ft)-950 -900 -850 -800 -750 -700 -650 -600 -550 -500 -450 -400 -350 -300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700
550
570
590
610
630
650
670
690
710
Ele
vatio
n (ft
)
550
570
590
610
630
650
670
690
710
Sandy GravelKx: 2.43E-03 ft/secKy/Kx Ratio: 0.5
W/L @ El. 682 W/L @ El. 682
Fig.4 Seepage Analysis Model based on Alternative Dewatering Scheme
PUNJAB BARRAGE IMPROVEMENT PHASE-II (PBIP-II) PROJECT Rehabilitation & Modernization of Jinnah Barrage
164ft164ft
NSL El. 66830ft30ft
Dewatering Well Dewatering Well
Sheet Pile Sheet Pile
Distance (ft)-325 -300 -275 -250 -225 -200 -175 -150 -125 -100 -75 -50 -25 0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 500 525 550 575 600 625 650 675
530
550
570
590
610
630
650
670
690
Ele
vatio
n (ft
)
530
550
570
590
610
630
650
670
690
665
665
675
675 Sandy GravelKx: 2.43E-03 ft/secKy/Kx Ratio: 0.5
W/L @ El. 682 W/L @ El. 682
Fig.5 Seepage Analysis Results
PUNJAB BARRAGE IMPROVEMENT PHASE-II (PBIP-II) PROJECT Rehabilitation & Modernication of Jinnah Barrage
Phreatic SurfaceEquipotential Line
NSL El. 668
Flow Line
91.
924
ft³/h
r
89.
253
ft³/h
r
Distance (ft)-325 -300 -275 -250 -225 -200 -175 -150 -125 -100 -75 -50 -25 0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 500 525 550 575 600 625 650 675
530
550
570
590
610
630
650
670
690
Ele
vatio
n (ft
)
530
550
570
590
610
630
650
670
690
ANNEXURE-A
PUNJAB BARRAGES IMPROVEMENT PHASE-II (PBIP-II) PROJECT
Rehabilitation & Modernization of Jinnah Barrage
Cost Estimate for Alternative Dewatering Scheme Sr. No.
Item Description Unit Qty Unit Rate (Rs.)
Amount (Rs.)
1 Cost of Installation of Dewatering Wells @ 40’ C/Chaving following specifications:
• Drilling of 130ft deep borehole • 1.5 cusec discharge submersible pump
with total head of 80ft • 30-40 hp motor • Mild Steel material shall be used (see Fig.2
for details)
No. 100 1,200,000 120,000,000
2 Genset rent day 3 Cost of Maintenance of Dewatering Operation
including diesel for gensets, maintenance cost of gensets and dewatering system, mobil oil and manpower
day
NOTE:
25% of dewatering wells should be shut off after 1 month. These wells will act as stand by
wells.