Getting the Job Done How to Successfully Design, Permit, and Construct
a Challenging Highway Crossing
Northern California Pipe Users Group 27th Annual Sharing Technologies Seminar Concord, CA February 21, 2019
Presented by:
Colin Dudley Brown and Caldwell David Mathy DCM Consulting Additional Co-Authors:
Peter Kambel City of Modesto Chris Peters Brown and Caldwell
Agenda
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1. Project Background and Overview
2. Crossing Design Challenges
3. Permitting Process
4. Construction Observations
Project Overview
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Project Location
Project Overview
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• 3,100 ft of 24-in diameter RCP
• Identified in the City’s 2007 Collection System Master Plan as a high priority project due to severe corrosion of concrete and steel
• Single barrel Highway 99 crossing dating from 1950s
Project Overview
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Project Alternatives Rehabilitation • Existing alignment difficult to access
and within flood plain • Would require continuous bypass
pumping
Replacement • Relocate to Zeff Road
• Out of flood plain • Easier access for maintenance
• Reduce amount of bypass pumping
New highway crossing included in both alternatives
Crossing Design Challenges - Geotechnical
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Geologic Setting
Crossing Design Challenges - Geotechnical
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Test Borings
Crossing Design Challenges - Geotechnical
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Test Boring B-3
Tunnel Zone
Silty Sand (SM) Loose 46% fines
Poorly Graded Sand (SP) Loose 2% fines
Crossing Design Challenges - Geotechnical
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Tunnelmans Ground Classification
Granular materials without cohesion … run like granulated sugar or dune sand
Modesto “sugar sands”
B-3, SP loose sands = running behavior
Crossing Design Challenges - Geotechnical
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Test Boring Profiles
B-3 B-2
Crossing Design Challenges - Geotechnical
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Very Optimistic Interpreted Tunnel Profile
Crossing Design Challenges - Geotechnical
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Reasonable Interpreted Tunnel Profile
Crossing Design Challenges - Geotechnical
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Very Pessimistic Interpreted Tunnel Profile
Permitting – Tunnel Description
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• 36-inch steel casing • 3/8-inch overcut banding (3/4-inch overcut on diameter) • 344 foot length • Minimum cover = 5 feet under northbound Hwy 99 • Maximum cover = 13 feet under highway berm • Separation from highway storm drain = 16 inches
Permitting – CalTrans Coordination
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Encroachment Permit Manual, Section 600 • Requirements at time of Design (2013 Permit Manual)
• Caltrans initial interpretation was 15 feet min. cover per the HDD requirements (Section 623.2), not bore and jack as proposed
• CalTrans concern with shallow bore and jack stemmed from excessive settlement on a previous crossing
• Tunneling (30-inch or larger bore and jack) under access-controlled right-of-way allowed only if studies establish soil structure is sufficiently stable (Table 6.1)
• Permit Manual Revised in 2018 Met with CalTrans at the Stockton District office
• Reviewed Project design in detail including: • Geotechnical conditions • Plan and profile for the crossing • Mitigation measures to address systemic settlement of
Highway 99 and impacts on 18-inch diameter storm drain
Permitting – Proposed Mitigations
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• Geotechnical instrumentation (SMPs, SSMPs and UMPs) • 100 feet of tunnel before going under Highway 99 travel lanes
• allowed for instrumentation to check for settlement (3 SSMPs) • allowed for Contractor to dial in methods • allowed for baselining tunnel spoils weights
Permitting – Proposed Mitigations (Cont.)
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• Permeation grouted storm drain bedding and pipe embedment
• Limited overcut to 3/8-inch banding • Required full and continuous lubrication of overcut • Monitored tunnel spoils weights • Soil plug in lead casing (no free bore) • No grade checks under travel lanes • Monitored instrumentation • Immediate contact grouting (36-inch casing) Calculated systemic settlement with 5 feet of cover and all
mitigations = 0.35”
Permitting – Project Approval
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With all mitigations carefully included in the project plans and specifications CalTrans approved the project with 5 feet of tunnel cover.
Construction Observations – Weight of Spoils
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Crane scale used to weigh every bucket of spoils removed
Running average monitored for significant change
Weight of spoils impacted by: • Buildup in jacking shaft • Incomplete removal by crew • Build up in casing if augers not
removed for grade checks
Construction Observations – Spoils Observation
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Cohesiveness and fines content of spoils used to estimate behavior of tunnel face and changes in soil composition
Construction Observations – Tunnel Face Obs.
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Tunnel face observed when line and grade was checked by laser
Note teeth marks from cutter head in face of tunnel. Typical of tunnel face at grade checks.
Construction Observations
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Very Optimistic Interpreted Tunnel Profile
Construction Observations
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Settlement monitors • Surveyed every 2 hours during
boring operations • Showed little to no settlement
Contact Grouting
• Total injected was less than theoretical void space
• Indicative of tight void space and use of lubrication
Project Team and Costs
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Owner: City of Modesto Designer: Brown and Caldwell Trenchless Consultant: DCM Consulting Geotechnical Engineer: Crawford and Associates General Contractor: Rolfe Construction Boring Contractor: Pacific Boring Total Project Cost: $5,744,000 Bore and Jack Cost (including shafts): $613,000 (~$50/in-dia/ft)
Thank you.
Questions?