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: jakhtar47@gmail.com

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.