Related to Base and Subbase
Case Study: Seattle Asphalt Subbase Contamination (2021)
Project Overview
- Name: SR-99 Tunnel Approach Roadway
- Location: Seattle, Washington
- Year: 2021
- Project Size: $1.5 billion
- Scope: Roadway construction including subbase and pavement on tunnel approaches
- Lead Agencies/Contractors: Washington State DOT
Category of the Issue, Problem, or Challenge
- Subbase Contamination
- Material Quality
Summary of the Issue, Problem, or Challenge
During subbase placement, contractor discovered pockets of organic material mixed into the subbase fill, compromising its load-bearing capacity and requiring removal and replacement.
Root Cause Analysis
| Factor | Details |
| Improper Site Clearing | Organic topsoil was not fully stripped before subbase placement. |
| Inadequate Material Verification | Insufficient field testing for organic content during subbase placement. |
| Communication Breakdown | Lack of coordination between earthwork and paving subcontractors. |
Impacts Due to the Issue, Problem, or Challenge
- Re-excavation and removal of contaminated subbase sections
- Delay of 4 weeks and cost increase due to additional excavation and material procurement
- Strengthened site clearing and inspection protocols
Corrective Actions Taken
- Revised stripping limits and inspection responsibilities clearly documented
- Increased frequency of organic content testing on subbase materials
- Enhanced communication protocols between subcontractors during earthwork
Lessons Learned
- Complete removal of unsuitable materials before subbase placement is essential
- Frequent testing prevents placement of contaminated materials
- Coordination between earthwork and paving teams avoids material quality issues
Case Study: Minneapolis Pavement Base Segregation (2017)
Project Overview
- Name: I-94 Reconstruction
- Location: Minneapolis, Minnesota
- Year: 2017
- Project Size: $250 million
- Scope: Interstate reconstruction with full-depth base and pavement replacement
- Lead Agencies/Contractors: Minnesota DOT
Category of the Issue, Problem, or Challenge
- Base Course Quality
- Material Segregation
Summary of the Issue, Problem, or Challenge
Compaction tests showed variable density and strength across the base course. Further analysis found segregation of coarse and fine aggregates during placement, leading to weak zones prone to deformation.
Root Cause Analysis
| Factor | Details |
| Placement Practices | Inadequate control of aggregate feed and spreading methods. |
| Equipment Issues | Use of improperly calibrated equipment causing uneven material distribution. |
| Insufficient QC Checks | Lack of frequent density and gradation tests during base construction. |
Impacts Due to the Issue, Problem, or Challenge
- Partial removal and re-compaction of base course
- Delays of 3 weeks with increased costs for additional quality control and materials
- Implementation of stricter placement controls
Corrective Actions Taken
- Improved placement procedures with calibrated equipment and operator training
- Frequent in-process QC testing for segregation and density
- Updated specifications to mandate segregation control measures
Lessons Learned
- Segregation during base placement can severely affect pavement performance
- Operator training and equipment calibration are critical controls
- Regular QC sampling is necessary to detect and correct segregation early
Case Study: Charlotte Base Course Settlement (2019)
Project Overview
- Name: I-485 Widening Project
- Location: Charlotte, North Carolina
- Year: 2019
- Project Size: $320 million
- Scope: Interstate widening with new base course and pavement layers
- Lead Agencies/Contractors: North Carolina DOT, Private Contractor
Category of the Issue, Problem, or Challenge
- Base Course Settlement
- Geotechnical / Materials
Summary of the Issue, Problem, or Challenge
Following placement of the base course, uneven settlement was observed in multiple sections, causing surface irregularities and cracking in subsequently placed asphalt layers.
Root Cause Analysis
| Factor | Details |
| Weak Subgrade | Subgrade soils had low bearing capacity and moisture issues. |
| Insufficient Compaction | Compaction of subgrade and base course below specifications. |
| Lack of Geotechnical Monitoring | Inadequate monitoring of subgrade moisture and density during construction. |
Impacts Due to the Issue, Problem, or Challenge
- Removal and reconstruction of base course in affected areas
- Repair of early pavement distress, delaying opening by 5 weeks
- Additional costs incurred for rework and geotechnical consulting
Corrective Actions Taken
- Conducted thorough geotechnical investigations to identify weak zones before construction
- Implemented improved moisture control and compaction procedures for subgrade and base layers
- Enhanced QC with daily field density tests and moisture monitoring
Lessons Learned
- Early detection and mitigation of weak subgrade soils reduce settlement risk
- Continuous geotechnical monitoring during construction is vital
- Coordinated QC efforts between geotechnical and paving teams improve outcomes