Final85214-Suresh Parashar

Complex Geotechnical Aspects of Mega Projects and Their Effective Management – Changi WRP Project Singapore Experience Dr. Suresh Parashar, Ph.D., P.E., MASCE Senior Technologist, CH2MHILL International, UAE

&

Edward Sloan, CCM, PMP Senior Construction Manager, CH2MHILL International, USA

DFIMC 2012

Scope of Presentation

DFIMC 2012

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Introduction to DTSS & CWRP

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Geotechnical Aspects of CWRP

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Ground Water Cut-off Wall

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Deep Excavation and Slope Stability

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Supplementary Site Investigation

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Pumping Station Deep Shafts

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Deep Foundations

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Summary & Lessons Learnt

Introduction to DTSS & CWRP

DEEP TUNNEL

Phase 1 capacity : 176 MGD (800,000 m3/day) Ultimate capacity : 528 MGD (2,400,000

m3/day)

Construction period: 2001 to 2008 Estimated cost : SGD$2.2 Billion (~US$ 1.5 Billion)

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CHANGI WATER RECLAMATION PLANT

Project Statistics • Planned Project Duration: Apr 2001 - Dec 2007, 81 months. • Peak construction expenditure (in 2005) S$45 Million/month (~US$ 35 Million).

• Construction team – 400,000 man-months, peak staff of 6000 - 7000. •

Workers living on site - 3000.

•

Total value of contracts - S$ 2.2 billion.

• 13 Construction packages and 8 equipment procurement contracts.

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Project Statistics

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Geotechnical Aspects of CWRP • Ground Water Cut-off Wall • Deep Open Excavation & slope Stability • Supplementary Site Investigation • CBP Retaining Wall

• Ground Improvement (Jet Grouting) • Temporary Excn. Support - Sheet Piles, H Piles / Timber Lagging, DWall, Soil Nails, Walers & Struts •Pumping Station Deep Shafts – Diaphragm Walls • Interconnecting Tunnels (NATM) • Bored Piles Design, Construction & Pile Load Tests • Geotechnical Instrumentation & Monitoring • Backfilling

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Ground Water Cut-off Wall • 2700 m long perimeter cement

bentonite ground water cut-off wall (Est. Area = 73,400 m2) to keep the ground water away from entering the open excavation during construction.

Total Length

 2700 m

Thickness

600 mm

Depth

21 – 30 m BGL (RL 105)

Primary Panels

6 m to 7.7 m long (3 Bites of Grab)

Secondary Panels

1.9 m to 2.2 m (1 Bite of Grab)

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Ground Water Cut-off Wall • Design and Construction Challenges • The specialist contractor, based on his previous experience, proposed a cement

bentonite slurry cut-off wall consisting of 200 kg PBFC Cement, 30 kg GTC 4 Bentonite and 925 kg of Water in one cubic meter of slurry. • In the CB method, self-hardening slurry typically contains (by wt.) - 4 to 7% Bentonite, 8 to 25 % Cement and 65 to 88 % Water. •The CB Proportions (by wt.) of Contractor’s Proposal: 2.6 % Bentonite, 17.3 % Cement and 80.1 % Water. •Design mix that has too little bentonite leads to cement setting out and excessive water bleeding. • Design Mix will be UNSTABLE & thus NOT SUITABLE (GTC4#1 – 21% Bleed).

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Design and Construction Challenges (Cont.) • Finalized Design Mix •Material Non-Marine Clay Area

Marine Clay Area

Cement

200 kg

155 kg

Bentonite

50 kg

55 kg

Water

917 kg

929 kg

4.5%

4.5%

36 sec

34 sec

Bleed (2 Hr) Viscosity

• Lessons Learnt: • Verify the relevance of previous experiences (applicability & performance) • Have a reasonable questioning attitude, seek details to avoid surprises • Be part of solution & follow-up •Team work approach – seek win win solutions • Manage stakeholders expectations

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• Design and Construction Challenges (Cont.) • CB wall was excavated using Mechanical & Hydraulic Grabs. As excavation progresses, the CB slurry becomes increasingly difficult to work and slows down the productivity. • Contractor proposed to use the replacement method - to excavate the trench under bentonite slurry, to be subsequently displaced by cement bentonite slurry. • Mock-up demonstration tests were carried out in a Perspex tank, showed promising results. A cut-off wall primary panel was constructed using this technique and tests were conducted to verify performance as hydraulic barrier. • Result - The constructed panel could not hold water in the cored space at the middle of the panel. • Observation - Inadequate density difference between the bentonite slurry in the trench and the CB slurry for replacement method produced unsatisfactory cut-off wall. • In a recent project, replacement method has been used satisfactorily where CB wall is constructed using Hydromills. At the end of excavation, the bentonite slurry in trench is replaced by fresh bentonite slurry just prior to its replacement with Cement Bentonite slurry using special delivery tremie pipe. • Lesson Learnt – Construction method and equipment used have to be compatible as they govern the final performance . Verify the expected DFIMC 2012 performance to avoid unexpected surprises.

Deep Open Excavation and Slope Stability • Open Excavation up to 22 m Deep

•(Mass excavation of approximately 4.5 Million m3).

• Slopes designed with adequate safety factors based on subsurface conditions identified at the design stage. • During additional investigation along cut-off wall perimeter, significantly thick deposits of soft marine clay encountered along the eastern & northern sections. • Based on revised sub-surface profiles & estimated shear strength of marine clay, slopes were deemed to be unstable (PLAXIS & FLAC Analysis). • Slope stabilization measures would be required!!

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Deep Open Excavation and Slope Stability • The proposed slope stabilization – 50m

wide Jet grouting at the center of slopes • Est. Cost : US$ 10 to 15 Million • Est. Project Delay: 6 months to 1 year • The Critical Questions: • What is the exact extent of the soft marine clay and its thickness? • What is its in-situ shear strength? • How does it really impacts the stability of slopes? • Where are most critical sections? • Put simply, Do we understand the real problem and its actual extent?

• Without understanding the real problem, how could we find the optimum fix for it?

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• To seek answers to critical issues: • Supplementary SI planned with following critical requirements & approach : • Ensure highest quality investigation possible, award the investigation work based on quality rather than cost basis. • Cone Penetration Tests (CPTU) to determine the profile of marine clay along the proposed cut-slopes as well as their estimated engineering properties. • Based on CPT profiles, identify the locations for boreholes. • The SI layout was continuously reviewed by the Engineer’s geotechnical staff as the site investigation work progressed and adjusted accordingly, if required. • Boreholes at close proximity at select locations to allow collection of an adequate number of samples for laboratory testing. • Boreholes were drilled to different depths in the soft clay, with installation of a rapid response piezometer near the bottom of each borehole to provide a groundwater pressure profile through the soft clay. Each piezometer was calibrated in an open deep well prior to installation in the borehole. • Field Vane Tests were carried out to determine the undisturbed and remolded in-situ shear strength of soft soils. • The undisturbed samples were X-rayed in the tubes to determine the quality of the samples and natural variation within the samples to guide test specimen selection.

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• Deep Open Excavation and Slope Stability (Cont.) • Supplementary SI critical requirements & approach (Cont.): Lab Testing • The advanced laboratory testing by NTU consisted of K0 Consolidated Undrained Tri-axial Compression (CK0UC) tests, K0 Consolidated Undrained Tri-axial Extension (CK0UE) tests, K0 Consolidated Undrained Direct Simple Shear (CK0UDSS) tests and OneDimensional Consolidation tests. • In addition, some physical property tests such as liquid limit, plastic limit and specific gravity were also conducted. • Additional laboratory tests such as index tests, grain size distribution, UU, CIU and consolidation tests were carried out at contractor’s laboratory. Samples at the contractor’s laboratory were extruded in the Engineer’s presence who selected the samples for different tests. • Total Cost of SI = SG$ 184 K (US$115K)

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• Slope Stability (Cont.) • Critical slope sections were re-analyzed using PLAXIS FEM Program & PCSTABLE slope stability program. • Slopes had adequate safety factors.

Slope Section

Est. Factor of Safety

Undrained Analysis

Drained Analysis

Remarks

I

PCSTABL 6

1.39

1.51

Northern Slope

PLAXIS

1.45

1.53

Northern Slope

PCSTABL 6

1.25

1.42

North-East Corner

PLAXIS

1.26

1.42

North-East Corner

PCSTABL 6

1.40

1.77

Eastern Slopes

PLAXIS

1.44

1.83

Eastern Slopes

II

III

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• Slope Stability (Cont.) • • • •

Contiguous Bored Pile Wall at Effluent Junction Chamber (12 m Cantilever CBP Wall) had required ground improvement works using Jet Grouting based on original designs CB Wall reanalyzed without Jet-Grouting based on data from supplementary SI and better quality MC properties. Result - Jet Grouting Not required Omitted Jet Grouting saving S$2.9M, after compensating contractor for design work, test trials and loss of profit.

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Potential Risks of Slope Instability & CB Cut-off wall interaction

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The sub-soil conditions could still be worse than those discovered by site investigation at some unknown location Any slope failure could result in breach of cut-off wall. This could result in flooding of deep excavation endangering lives of workers, property damage & project delays

• • •

This risk would be unacceptable

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• Risk Monitoring Approach • Monitor the performance of cut-slopes, CB wall, plan contingency measures in advance to be adopted if any slope section shows signs of instability • Adjust the monitoring frequency depending on construction progress and behavior of cut slopes • Final Results – All slope sections and CB wall performed satisfactorily.

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Lessons Learnt •

The quality of SI data is most critical. It’s quality rather than quantity that matters. The “additional” cost to ensure “highest quality possible” for SI investigation pays huge dividends in reliable data & designs and minimized project risks of cost overruns and schedule delays etc.

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Designs are only as reliable as the design input irrespective of sophisticated computer programs.

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Understanding of actual extent of problems, design assumptions, correlations for estimating soil properties etc needs to be critically reviewed and verified.

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Comprehensive risk monitoring programs including contingency measures need to be in place.

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Teamwork approach, stakeholder management is critical.

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Finally, use the language of “Decision Makers” while conveying the potential risks, available options to manage them, associated costs and benefits while seeking senior management / client endorsements.

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Pumping Station Deep Shafts • Influent Pumping Station Shafts – 40 m in diameter and up to 72 m deep shafts built using circular diaphragm walls. • Coarse Screen Shaft (CSS) receives raw / treated effluent from DTSS (T01) & screen out larger particle, guides flows into Influent Pumping Stations (IPS) that pumps the flow into Influent or Bypass pipelines.

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Pumping Station Deep Shafts (cont.) •

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The IPS shafts were designed to be constructed by first installing 1.2 m thick circular diaphragm walls. Inner RCC Ring Walls, 1.6 m thick, were designed to carry all the loads from the base and intermediate equipment levels, as well as lateral ground loading following a top-down staged construction sequence in 10 stages. The diaphragm wall shafts were constructed using Bauer BG50 crane-mounted hydromills. The nature, availability and cost of this equipment meant that only two sets would be available for the IPS work. A risk analysis identified that the construction of these large, deep shafts not only lay on the critical path, but had the potential for significant cost and schedule overruns. It became clear to the project team that other measures would be necessary if the project schedule was to be achieved.

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Pumping Station Deep Shafts (Cont.) •

Alternative sequences, taking into account the level of intermediate structural slabs and shaft wall penetrations were considered and analyzed using the finite element method software, PLAXIS.

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It was concluded that a reduced number of 8 stages would keep the compressive loading due to hoop stress, as well as bending moments of the panels to acceptable levels while providing considerable time benefits and necessary comfort level to all stakeholders.

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A comprehensive system of instrumentation was then developed to allow verification of site performance with the theoretical analysis at all stages of construction, and contingency plans were also developed to revert to the more conservative excavation sequence should the need arise.

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The performance monitoring of the diaphragm wall panels for actual behavior during construction provided the tool for risk management and the necessary comfort level to approve and adopt the revised excavation sequence up to EL 64 (i.e. top 38 m of shafts). Below this depth, the original excavation sequence with minor adjustments for the construction requirements was adopted in the construction of IPS shafts at CWRP.

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The IPS shafts were completed on time and Pumping Station Facility was commissioned ahead of schedule.

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Lessons Learnt •

An informal partnering arrangement resulted in an open teamwork approach to the works which provided the framework for alternative methods to be successfully adopted for the benefit of the project and all stakeholders.

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Use of extensive instrumentation to verify, confirm and validate assumptions that had been made in the theoretical analysis, and to empower the site staff with the tools to demonstrate the performance of the shafts at all stages of construction.

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The final result was that the shafts were safely constructed below budget, allowing the Influent Pumping Station to be commissioned and operational 4 months ahead of schedule.

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Bored Piles Design, Construction & Load Testing • The CWRP project involves more than 3300 bored piles of diameters ranging from 700 mm to 2200 mm.

• Piles were designed with Full Length Reinforcement. Estimated total length of bored piles at CWRP = 160,000 m. Pile Dia. mm

No. of Piles

No. of Pile Tests

700

142

1300

Working Load Comp Tons

Tension Tons

2

307

-

1010

10 + 3 (ULT)

1060

300

1500

140

1

1410

470

1800

335

3

2035

680

2000

1332

13

2510

700

2200

170

2

3040

700

Total

3129

31 + 3 (ULT)

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Bored Piles Design, Construction & Load Testing • Ultimate Load Tests on instrumented piles using Kentledge method (2) and Multi-stage O-cell method (1) conducted from existing surface with de-bonded casing and strain gauges to obtain site specific design parameters ahead of completion of excavation due to critical schedule. LOAD TRANSFER CURVES

0

500

1000

1500

2000

2500

3000

3500

105

ELEVATION, (mRL)

95

85

570 75

1135 1668 2178

65

2469 2715 2950 3192

55

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Bored Piles Design, Construction & Load Testing • Site Specific Design Correlations • Shaft Friction Depth

SPT N

Measured fs - I Cycle

Measured fs 2nd Cycle

Fs Corre. Ist Cycle

Fs Corre. 2nd Cycle

21.233.2

50

200

130

4.0 * N

2.6*N

33.239.2

85

170

170

2.0 * N

2.0*N

39.248.2

87

80

220

1.0 * N

2.5*N

2.56 *N

2.43 * N

Avg.

Design Recommendation Fs =

2.4 * N

• End Bearing Resistance At 1st Cycle, qb = 3,000 kPa At 2nd Cycle, qb = 10,069 kPa Average qb = 6500 kPa Correlation factor for qb = 75* N

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Redesign of Pile Rebar Cage Lengths •

The average pile lengths at CWRP were in excess of 50m and maximum pile lengths exceeded 65 m.

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Full length reinforcement required rebar cage fabrication in multiple sections and then splicing over the drilled boreholes. This activity could take up to 6 to 8 hours depending on the pile length and size of rebar cage.

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This has significant slowing down effect on piling production while increasing the potential for borehole collapse impacting the quality of bored piles.

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The tension load on piles varied from about 23% to 33% of compression loading as such it was concluded that the full length reinforcement was not required to resist the design loading on piles.

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Reinforcement cage length were redesigned such that it provided minimum factor of safety against tension loading in excess of 3, were in multiples of 6 m length (i.e., 18 m, 24 m, 30 m and so on) thus eliminating wastage of reinforcing bars during rebar cage fabrication.

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The Piling at CWRP completed on time and redesign of reinforcement lengths resulted in about 6 months of schedule saving along with approximately S$5 Million in cost savings.

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Summary & Overall Lessons Learnt •

The Megaprojects present unique geotechnical challenges due to their complexity and potential impact on the overall risk to the project schedule and budgets.

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Effective management of these challenges requires empowered experienced professionals familiar with design and construction aspects of the project.

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Teamwork approach between Owner, Engineer and Contractor is critical for effective management of geotechnical challenges and design optimizations as well as managing design changes arising due to the variable subsurface conditions etc .

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Comprehensive risk monitoring programs using performance monitoring approach and contingency measures need to be in place for effective management of geotechnical aspects .

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Use of “Language of Decision Makers” while discussing the potential risks, available options to manage them, associated costs and benefits while seeking senior management / client endorsements is critical.

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Finally, a “culture of challenges and trust among the team members ” that allows issues like these to be entrusted to the team to solve is critical. As an expert one can solve issues that enhance the delivery but if the culture of the project does not allow it…all of these great solutions would not have been possible.

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CWRP presented multiple geotechnical challenges that were successfully managed without adverse impact to the project schedule and budget.

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References: 1. Parashar, S.P. & Sanmuganathan. D. (2005) Quality. Cost and Benefit Aspects of Site Investigation Works – A Changi water Reclamation Plant Project Experience. Proc. of Underground Singapore 2005. 2. Parashar, S.P, Wong, I.H. & Sanmuganathan. D. (2005). Ground water Cut-off Wall at Changi Water Reclamation Plant Project. Proc. of Underground Singapore 2005. 3. J. Chu, Parashar, S.P. and Sanmuganathan D.(2007). Comparison of Undrained Shear Strength of Singapore Marine Clay determined by Laboratory and In-situ tests. Proceedings of Underground Singapore. November 29-30, 2007. 4. Suresh Parashar, Roger Mitchell, Moh Wung Hee, Devaraj Sanmuganathan and Gordon Nicholson (2007). Performance monitoring of deep shafts at Changi WRP Project, Singapore. Proceedings of 7th International Symposium on Field Measurements in Geomechanics, Boston, September 2007.

5. Suresh Parashar, Roger Mitchell, Moh Wung Hee, Devaraj Sanmuganathan, Ed Sloan and Gordon Nicholson (2007). Performance monitoring of deep excavation at Changi WRP Project, Singapore. Proceedings of 7th International Symposium on Field Measurements in Geomechanics, Boston, September 2007. 6. Parashar, S.P., Chu J. and Sanmuganathan, D. (2008). Characterization of the undrained shear strength of marine clay at Changi WRP Project, Singapore. Proceedings of the 3rd International Conference on Site Characterization, April 1-4, 2008, Taipei, Taiwan.

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