Buried Structures and CANDE Finite Element Analysis

CANDE (Culvert ANalysis and DEsign) is a finite element program developed for the Federal Highway Administration specifically for buried structure analysis. It models the nonlinear interaction between a buried structure and the surrounding soil, which simplified design methods cannot capture accurately for flexible pipes, large-span culverts, or complex installation conditions.

What CANDE Analysis Adds

Simplified buried structure design methods (such as those in AASHTO LRFD or ASCE/ANSI standards) assume idealized installation conditions and linear soil behavior. For routine installations these are adequate. For anything outside that envelope, a more detailed analysis is warranted:

• High fill heights where soil arching is significant
• Unusual cover depths or surface loading that doesn't fit the standard tables
• Existing structures being evaluated for load rating or increased cover
• Relining projects where the structural capacity of the composite system must be demonstrated
• Custom section shapes or materials not covered by standard design procedures

CANDE handles all of these cases by modeling the full soil-structure system in finite elements, including the staged construction sequence and the nonlinear stress-strain behavior of the soil.

Flexible Pipe and Culverts

For flexible pipe (HDPE, CPVC, corrugated metal), deflection control and ring compression are the critical limit states. CANDE captures the load-sharing between the pipe and the surrounding embedment soil, which is central to how flexible pipe systems actually perform.

Common project types: large-diameter HDPE culverts with high fill, corrugated metal pipe under highway loads, structural plate culverts, and precast arch systems. I can provide design or review services for installations from initial design through construction observation documentation.

Culvert Relining and Rehabilitation

Structural relining of deteriorated culverts, whether with cured-in-place pipe, slip-lining, or formed-in-place systems, requires demonstrating that the composite system (existing pipe plus liner) has adequate capacity for the remaining service life and load conditions.

CANDE models the two-ring composite system explicitly, including the interface conditions between the existing structure and the liner. This allows direct calculation of the load distribution between the two rings, which cannot be reliably estimated from simplified hand methods.

Precast Concrete Vaults, Boxes, and Manholes

Precast concrete buried structures (box culverts, three-sided spans, large vaults, junction structures) are typically designed by precast manufacturers for standard loading. When site conditions deviate from those standard assumptions, an independent analysis is needed.

I use CANDE for these evaluations, supplemented by hand calculations for joint and connection details. Deliverables include a full calculation package with model documentation, load cases, and a summary of governing limit states with code references.

If you have an existing culvert, drainage structure, or buried vault that needs analysis, contact me with the available drawings and site information.