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?