Timothy J McGrath

Calculating Loads on Buried Culverts Based on Pipe Hoop Stiffness

Evaluating the hoop compression capacity of buried pipe, whether for total stress, local buckling capacity, or general buckling capacity, requires an accurate design model to compute the compressive thrust in the pipe wall. Flexible pipe has traditionally been designed based on the assumption that vertical soil load is the weight of soil directly over the pipe, known as the “soil prism load.” Field experience and research have shown that some pipe with low cross-sectional area and low modulus of elasticity can be buried at depths much greater than calculated by using the soil prism load, indicating that this load assumption is too conservative under some conditions. Investigation using the Burns and Richard elasticity solution for a circular tube embedded in an elastic medium shows that the ratio of the soil stiffness to the pipe hoop stiffness (EA/R) is often the controlling factor in determining the load on a buried pipe instead of the flexural stiffness of the pipe. The significance of the hoop stiffness factor for determining load on flexible and rigid buried pipe is explored here and development of a simplified design model for predicting loads based on the Burns and Richard theory is presented. The proposed equations are consistent with past practice and with tests showing that pipe with low hoop stiffness can carry far greater depths of fill than predicted by past practice.

Management of Utah Highway Culverts

More than 47,000 culverts have been installed under the highways of Utah. The Utah Department of Transportation (DOT) maintains these culverts but has no comprehensive system for assessing condition and planning maintenance activities. Utah DOT initiated a study to determine the condition of its culverts by field surveys. The objective was to develop a system of qualitative and quantitative performance measures to assess both the long- and short-term behavior of highway culverts and to support the Utah DOT effort to modify and populate a computerized data-base designed to store culvert inspection data that can be used for statewide culvert asset management. Culvert management practices currently used by Utah DOT and other agencies are described. A total of 272 culvert inspections conducted during this project showed the inventory to be aging but not generally in need of immediate maintenance. The Utah DOT database, developed to track culvert condition, is effective but could be improved. Improvements woul...

Field Tests of a Large-Span Metal Culvert

Full-scale field tests were conducted on a 9.5-m (31-ft 2-in.) span low-profile arch culvert to provide input for development of improved large-span culvert load and resistance factor design specifications. The test structure was constructed of 5.5-mm (0.215-in.) thick black steel plate with 152 × 51 mm (6 × 2 in.) corrugations. No supplemental circumferential or longitudinal stiffeners were added. Two tests were conducted with SW backfill material, one each compacted to 87 and 92 percent of maximum standard Proctor density. Live-load tests were conducted with a truck with tandem axles loaded to 156 kN (35,000 lb) per axle at depths of fill of 0.3, 0.6, and 0.9 m (1, 2, and 3 ft). Measurements were made of culvert deformation, culvert strain, culvert-soil interface pressure, soil density, soil strain, foundation movement, and temperature. Thrusts and moments were determined from the culvert strain.

Field Tests of Buried Pipe Installation Procedures

Full-scale field tests to evaluate pipe-soil interaction during backfilling were conducted on the campus of the University of Massachusetts at Amherst. The program consisted of 14 tests, with each test including concrete, plastic, and metal sections. In addition to pipe type, installation variables included in situ soil conditions, trench width, backfill material including controlled low-strength flowable material, haunching effort, and compaction methods. Test trenches were excavated under undisturbed in situ soil conditions. Eleven installations were conducted with 900-mm (inside diameter) pipe, and three installations were conducted with 1500-mm (inside diameter) pipe. Pipe and soil behavior was monitored during backfilling. Measurements included pipe shape, pipe strains, pipe-soil interface pressures, soil density, soil stresses, and soil strains. The pipes were buried to a cover depth of 1.2 m and were then excavated to inspect the pipe bedding and haunches. The results indicated significant variations in pipe behavior caused by installation practices. The rammer compactor produced greater backfill density than did the vibratory plate compactor with the same number of coverages and produced higher residual lateral soil stresses that contributed to better overall pipe performance during backfilling. The silty sand backfill tripled the peaking deflection during sidefill compaction compared with that with angular crushed rock backfill, but the former was more sensitive to deviations from good installation practice. Backfill compaction in the region from the spring line to 30 degrees below the spring line has a significant positive effect in mitigating poor bedding and haunching conditions. Pipe tests with controlled low-strength material for backfill performed very well.

Recommended Design Specifications for Live Load Distribution to Buried Structures

This report provides recommendations to revise the AASHTO LRFD Bridge Design Specifications relating to the distribution of live load to buried structures. The report details the development of simplified design equations (SDEs) for structural response based on three-dimensional (3D) analysis of 830 buried culverts. In addition, the report provides guidelines for conducting 2D and 3D modeling for design situations with conditions not covered by the SDEs. The material in this report will be of immediate interest to roadway and bridge designers.

Modernize and Upgrade CANDE for Analysis and LRFD Design of Buried Structures

This report documents research performed to develop, modernize, and upgrade CANDE (Culvert ANalysis and DEsign). The new version is called CANDE-2007. The report details the research performed to update the program. CANDE installation files are included on a CD-ROM (CRP-CD-69) with this report. The installed program includes integrated help files and 14 tutorial examples. The report and software will be of immediate interest to culvert designers.

Evaluation of Simplified Design Methods for Buried Thermoplastic Pipe

Simplified equations to calculate deflections and circumferential strains in buried thermoplastic pipe have been developed for AASHTO based on soil-structure interaction solutions. These equations are evaluated here using measurements of HDPE and PVC pipe responses in full-scale laboratory tests. The comparison indicates that the equations are suitable design tools for standard flexible pipe installations. Structural stiffness of the pipes may have an influence on the performance of the deflection equation, since the method overestimated the deflections for a ribbed PVC pipe with high hoop stiffness. The methods for estimating hoop strain and vertical arching factor worked very well for both pipes, and the equation for bending strains provided somewhat conservative values. Recommended design values for constrained modulus of the soil lead to conservative but reasonable predictions in all cases.

INSTRUMENTATION FOR MONITORING LARGE-SPAN CULVERTS

This paper describes experience gained from an extensive instrumentation plan devised to monitor the behavior of 2 large-span culverts, soil behavior, and culvert-soil interaction during backfilling and live load testing. The aims of the plan were to help control construction deformations, including ensuring safe working conditions, and to gather sufficient data to advance the state of the art in the design and construction of large-span culverts. The instrumentation plan and sample results from the test program are included and discussed.

Structural Behavior of Three-Sided Arch Span Bridge

A study was undertaken to evaluate the methodology used for the structural design of three-sided culverts with arched top slabs. An 11 -m span by 3.4-m rise bridge was instrumented and monitored during installation, under an HS-25 + 30 percent live load and at 6-month intervals for 2 years after installation. The bridge consisted of ten 1.6-m-wide precast segments. Three of the interior segments were instrumented with soil stress cells mounted on the legs of the bridge and with anchor pins for use with a tape extensometer to determine change in shape of the bridge. Survey data were taken on the same three segments and the two adjacent segments. Visual observations were also made to monitor cracking. The live load test was conducted with 0.3 m of cover. Final cover was 0.9 m. The bridge showed less movement under the live load than under the 0.9 m of earth load. The 2-year data show that the shape of the bridge and the soil stresses at the sides of the bridge cycle on an annual basis and that the spans hav...

Designing Stormwater Chambers to Meet AASHTO Specifications

Thermoplastic stormwater chambers have been successfully used on many projects to meet current federal regulations controlling discharges from newly developed sites. These structures are commonly located under parking lots or roadways and subjected to vehicular loads on a regular basis. Given this application, it is logical to design these structures for loads specified in the AASHTO LRFD Specifications for Highway Bridges, which provide design procedures for thermoplastic culvert pipe for use under roadways. However, experience has shown that many drainage engineers are not familiar with the AASHTO Specifications and have difficulty evaluating product manufacturer’s claims relative to AASHTO requirements. This paper presents the key aspects of the AASHTO specifications that should be applied to stormwater chamber design, such as determining applicable loads, load factors, design of profile sections, and consideration of time dependent properties. Also addressed are design matters for chambers not directly addressed by AASHTO. Of particular importance in developing unique structures is the validation of designs and chamber safety levels through full scale testing.