Related to Earthwork Construction

Case Study: SR 520 Floating Bridge – Borrow Source Dispute & Fill Rejection (2016)

Project Overview
• Name: SR 520 Floating Bridge and Landings Project
• Location: Seattle, Washington
• Year: 2016
• Project Size: $4.5 billion
• Scope: Floating bridge construction with major embankment approaches
• Lead Agencies/Contractors: WSDOT /

Category of the Issue, Problem, or Challenge
• Earthwork – Borrow Source Conflict
• Change Orders & Testing

Summary of the Issue, Problem, or Challenge
Contractor-provided borrow material for fill embankments was rejected by WSDOT after failing durability and abrasion loss criteria. The contractor disputed the rejection, claiming the specs had not been clearly defined regarding borrow gradation and fines. A protracted dispute led to supply delays and multiple design adjustments to accommodate available material.

Root Cause Analysis

Impacts Due to the Issue, Problem, or Challenge
• $6.8 million in change orders and material procurement costs
• Bridge embankment completion delayed, cascading to structural scope
• Arbitration process triggered between contractor and DOT
• Updated borrow material guidelines across DOT standard specs

Corrective Actions Taken

1. Revised specs issued with explicit gradation and durability thresholds
2. Third-party testing lab brought in to resolve test result disputes
3. Advance approval process required for all future fill sources
4. Inclusion of backup borrow sources in bid stage submittals

Lessons Learned

• Earthwork specs must be precise and enforceable, especially for critical structures
• Fill source testing should be collaborative, with agreed protocols
• Regional borrow source constraints must be factored into design schedule
• Dispute resolution clauses should be clear in contracts

Case Study: I-70 Mountain Corridor Cut-Slope Failure (2022)

Project Overview

• Name: I-70 Mountain Corridor Expansion
• Location: Eagle County, Colorado
• Year: 2022
• Project Size: $327 million
• Scope: Widening of mountainous I-70 highway including extensive cut-and-fill sections.
• Lead Agencies/Contractors: Colorado Department of Transportation (CDOT) /

Category of the Issue, Problem, or Challenge

• Geotechnical
• Excavation & Earthwork

Summary of the Issue, Problem, or Challenge
During cut operations on a steep rock slope, a major failure occurred when a large portion of excavated slope collapsed onto the construction bench below. The failure damaged heavy equipment, paused site activity, and necessitated emergency stabilization work.

Root Cause Analysis

Impacts Due to the Issue, Problem, or Challenge

• Project delay of approximately 3 months
• Emergency stabilization costs estimated at $2.2 million
• Additional $1.5 million spent on redesign and slope reinforcement
• OSHA investigation due to equipment exposure in failure zone

Corrective Actions Taken

1. Emergency slope scaling and catchment wall installation
2. Redesign of cut slopes with more conservative angles
3. Deployment of slope inclinometers and real-time monitoring
4. Enhanced groundwater mapping and dewatering protocol

Lessons Learned

• Jointed rock in mountainous terrain demands conservative slope design.
• Geotechnical instrumentation must be deployed during active excavation.
• Water behavior in fractured rock is difficult to predict and must be monitored continuously.
• Bench width and safety buffers should err on the side of caution in high-risk zones.

Case Study: MBTA Green Line Soldier Pile Wall Deflection (2018)

Project Overview

• Name: MBTA Green Line Extension
• Location: Somerville, Massachusetts
• Year: 2018
• Project Size: $2.3 billion
• Scope: Light rail extension requiring deep excavations and utility relocations.
• Lead Agencies/Contractors: Massachusetts Bay Transportation Authority /

Category of the Issue, Problem, or Challenge

• Geotechnical
• Quality Control
• Excavation Support

Summary of the Issue, Problem, or Challenge
Excessive lateral deflection was observed in a soldier pile and lagging retaining wall supporting a 30-foot deep excavation for a station box. The movement exceeded design tolerances, triggering emergency measures.

Root Cause Analysis

Impacts Due to the Issue, Problem, or Challenge

• 2-month project delay to redesign tieback system
• $4.2 million cost increase due to wall reinforcement and new monitoring
• Localized settlement affecting adjacent sidewalk and utility lines

Corrective Actions Taken

1. Additional tiebacks installed and tested
2. Active dewatering system added behind wall
3. Independent third-party QA on all support system installations
4. Wall monitoring program expanded to include tilt meters and vibration sensors

Lessons Learned

• Excavation support systems must include contingency load paths.
• Dewatering must be integrated with shoring design.
• Tieback tensioning should be independently verified and logged.
• Real-time deflection data is critical in urban excavation zones.

Case Study: SR 520 Floating Bridge Borrow Dispute (2016)

Project Overview

• Name: SR 520 Floating Bridge Replacement
• Location: Seattle, Washington
• Year: 2016
• Project Size: $4.56 billion
• Scope: Replacement of aging floating bridge with updated pontoons and approaches.
• Lead Agencies/Contractors: Washington State Department of Transportation (WSDOT) /

Category of the Issue, Problem, or Challenge

• Contractual
• Materials Engineering
• Earthwork

Summary of the Issue, Problem, or Challenge
A major borrow source was rejected during fill operations due to failing grain size and plasticity specifications. This created a shortage of acceptable fill material for approach embankments.

Root Cause Analysis

Impacts Due to the Issue, Problem, or Challenge

• 5-month delay in fill operations
• $6.8 million in additional material and hauling costs
• Need to re-source material from out-of-state site
• Legal dispute between contractor and WSDOT (later settled)

Corrective Actions Taken

1. Revised specs to clarify gradation and compaction targets
2. Appointed neutral third-party lab for dispute resolution
3. Required contractor pre-approval of all borrow zones
4. New fill placed in test sections prior to full delivery

Lessons Learned

• Borrow site specs must account for field variability and seasonal changes.
• Dispute resolution mechanisms should be built into QA processes.
• Test sections are critical to verifying compaction under real conditions.
• Contractor-proposed materials need early, formal acceptance.

Case Study: Seattle Deep Cut Slope Instability (2018)

Project Overview

• Name: SR 99 Tunnel Access Roadway
• Location: Seattle, Washington
• Year: 2018
• Project Size: $90 million (access roadway portion)
• Scope: Construction of a deep cut-and-cover connector to the main tunnel approach
• Lead Agencies/Contractors: Washington State DOT /

Category of the Issue, Problem, or Challenge

• Geotechnical
• Excavation & Fill

Summary of the Issue, Problem, or Challenge
A deep cut slope supporting the tunnel connector collapsed following heavy rainfall, causing significant delay and requiring a complete redesign of the slope retention system.

Root Cause Analysis

Impacts Due to the Issue, Problem, or Challenge

• Three-month delay due to slope failure and redesign
• Increased costs for additional support walls and dewatering systems
• Safety incident with minor injuries to two workers
• Public concern over project safety and environmental risks

Corrective Actions Taken

1. Temporary shoring and regrading
2. Installation of deep horizontal drains
3. Independent review of geotechnical design and field modifications
4. Enforcement of cut slope monitoring and excavation limits

Lessons Learned

• Conservative assumptions in slope design are critical in wet climates
• Temporary excavation conditions must be engineered with the same rigor as permanent systems
• Stormwater must be proactively managed in excavation zones

Case Study: Salt Lake City Stormwater Infiltration Undermines Fill (2020)

Project Overview

• Name: SLC North Interchange
• Location: Salt Lake City, Utah
• Year: 2020
• Project Size: $120 million
• Scope: New interchange and fill embankments over variable soils
• Lead Agencies/Contractors: Utah DOT /

Category of the Issue, Problem, or Challenge

• Earthwork
• Water Infiltration / Drainage

Summary of the Issue, Problem, or Challenge
A newly placed fill embankment over a stormwater detention basin failed partially due to infiltration and piping beneath the toe.

Root Cause Analysis

Impacts Due to the Issue, Problem, or Challenge

• Partial embankment failure requiring reconstruction
• Redesign of stormwater infrastructure
• Schedule delays and additional geotechnical mitigation costs

Corrective Actions Taken

1. Removal and recompaction of fill with subgrade improvement
2. Cutoff wall installed between embankment and detention basin
3. Long-term monitoring wells installed for groundwater levels

Lessons Learned

• Fill placed over variable soils requires subgrade prep and water control
• Hydrologic connections near embankments must be fully considered
• Geotechnical review should include drainage and long-term infiltration effects