Arun J. Valsangkar

Centrifuge testing and numerical analysis of box culverts installed in induced trenches

Induced trench construction is routinely used for circular conduits, but its application for box culverts is less common. To understand the complex soil–structure interaction issues related to the design of induced trench box culverts, centrifuge tests were performed to measure earth pressures on a model box culvert installed in several induced trench configurations. These tests were modelled with FLAC and good agreement was achieved. A parametric study performed with FLAC identified a preferred compressible zone geometry having a width of 1.2 times the culvert width and a thickness of 0.5 times the culvert height. For this geometry, the earth pressure on the top was 0.28 times the overburden, the lateral earth pressure on the sides was 0.47 times the mid-height overburden, and the contact pressure at the base was 0.73 times the overburden plus the pressure from the dead load of the culvert. The average base contact pressures for the induced trench geometry were 35% lower than those for the corresponding ...

Field performance, centrifuge testing, and numerical modelling of an induced trench installation

The field performance of an induced trench installation is compared to the results of centrifuge testing and numerical modelling. The measured vertical pressure at the crown of the pipe in the field ranged from 0.24 to 0.36 times the overburden pressure. The horizontal earth pressures measured in the field at the springline level determined a coefficient of lateral earth pressure between 0.39 and 0.49. The culvert was monitored over a period of 2 years following completion of embankment construction indicating no measurable changes in earth pressures and deformations. A model box culvert simulating the prototype height of soil cover, the pipe width, and the thickness of the compressible layer was tested using a geotechnical centrifuge. The prototype structure was also evaluated using numerical modelling to predict full earth pressure distribution and deformations. A comparison of field data, centrifuge testing, and numerical modelling shows that the Marston–Spangler theory used in designing induced trench...

Earth pressures on twin positive projecting and induced trench box culverts under high embankments

Although twin culverts are often preferred for watercourse crossing, little is known about the earth pressures they experience under high embankments. Centrifuge test results have been used in this research to verify a numerical model used to evaluate culvert spacing and compressible zone geometry for twin positive projecting and induced trench box culverts. Lower pressures were calculated for twin positive projecting culverts than for a single culvert. For the induced trench case, one compressible zone spanning both culverts was the preferred geometry for culverts spaced at 0.5Bc and 1.0Bc (where Bc is the culvert width), while two zones 1.2Bc wide were found to be optimal for 1.5Bc spacing. The twin configurations generally resulted in slightly higher vertical and lower lateral earth pressures than a single culvert configuration. The base contact pressures were 25%–76% greater than the top pressure plus dead load because of shear stresses mobilized along the sidewalls; however, they were 41%–47% lower t...

Unpaved geosynthetic-built resource access roads: Stiffness rather than rut depth as the key design criterion

Abstract The economic significance of unpaved, resource access roads is enormous, contrary to widespread opinion, and their unique behaviour and requirements are in need of further study. It is pointed out that overall transportation costs and efficiencies are inextricably linked to the relationships between roads and vehicles. Because of the impact that road stiffness has on fuel consumption and therefore vehicle operating costs, it is contended that stiffness rather than permanent rut depth should be adopted as the key design criterion for resource access roads. In addition, carrying the rut depth criterion used in the design of sealed roads into the design of unsealed access roads is inappropriate, because for these roads, ruts can be eradicated by periodic maintenance operations. Because it is reasonable to expect that the stiffness of a low-standard access road structure could be significantly increased by the inclusion of a geogrid or geotextile, the range of road stiffness to be expected, and how it is affected by geosynthetic inclusions, was investigated by cycled-load testing of large-scale, model pavement structures, consisting of granular bases provided with various geosynthetics placed on peat subgrades. Surprisingly, the improvement in model pavement stiffness over that for the subgrade itself was not great.

Performance of an Induced Trench Installation

An induced trench installation was instrumented to measure earth pressures both within the embankment and on the pipe. Deformations of the pipe and settlements within the embankment also were monitored. At the full height of the embankment, the average vertical and horizontal stress on the culvert remained relatively constant at 0.32 and 0.48 times the overburden stress, respectively. The pressures measured directly on the pipe compare favorably with other case histories presented within the literature and are in good agreement with the observed vertical elongation of the culvert. Pressure cells positioned within the embankment measured stresses roughly equal to the overburden stresses and indicate no significant redistribution of stresses other than those observed immediately around the pipe. The relative settlements within the embankment, above the induced trench installation, correspond with the design assumption that the fill within the prism of soil over the pipe settles downward relative to the adja...

Soil-Structure Interaction of Very Flexible Pipes: Centrifuge and Numerical Investigations

AbstractThe unprecedented increasing cost of steel coupled with the superior mechanical and anticorrosion properties of fiber-reinforced plastic (FRP) creates a huge opportunity for FRP pipes to take the market share of steel and be the pipe of choice for geotechnical engineers for a variety of applications ranging from potable-water systems to feed lines and penstock for hydroelectric power plants. Recently, very flexible large-diameter FRP pipes have been installed in eastern Canada and the United States to replace old wood-stave pipes to convey water to drive the turbines of small hydroelectric plants. The behavior of buried very flexible pipes is one of the complex soil-structure interaction (SSI) problems in geotechnical engineering and has not been studied thoroughly until now. The flexibility classification of pipes is determined based on their effective stiffness. Accordingly, flexible pipes will have an effective flexibility of 10 kPa/m or less. In this paper, the results of centrifuge modeling o...

Field Performance and Analysis of 3-m-Diameter Induced Trench Culvert Under a 19.4-m Soil Cover

An induced trench installation was instrumented to monitor earth pressures and settlements during construction. Some of the unique features of this case study are as follows: (a) both contact and earth pressure cells were used; (b) part of the culvert is under a new embankment and part was installed in a wide trench within an existing embankment; (c) a large stockpile was temporarily placed over the induced trench; and (d) the compressible material was placed in two stages. The maximum vertical pressure measured in the field at the crown of the culvert was 0.24 times the overburden pressure. The maximum horizontal pressure measured on the side of the culvert at the springline was 0.45 times the overburden pressure. The column of soil directly above the compressible zone settled approximately 40% more than did the adjacent fill. The field results at the crown and springline compared reasonably with those observed with numerical modeling. However, the overall pressure distribution on the pipe was expected to be nonuniform, the average vertical pressure calculated by using numerical analysis on top of the culvert over its full width was 0.61 times the overburden pressure, and the average horizontal pressure calculated on the side of the culvert over its full height was 0.44 times the overburden pressure. When the full pressure distribution on the pipe is considered, the recommended design loads from the Marston–Spangler theory slightly underpredict the maximum loads, and the vertical loads control the design.

Earth Pressure Reduction System Using Geogrid-Reinforced Platform Bridging for Buried Utilities

AbstractConstruction of new highway embankments in urban areas often requires building over existing underground utilities and lifelines. Embankments constructed over such utilities often induce substantial additional earth pressures causing overstressing and/or unacceptable deformations of the buried pipes, resulting in interruption of service for both the utility and highway. There are several techniques that can be used to mitigate this problem. Geogrid-reinforced bridge platforms are one of the newer techniques to deal with this problem. In order to gain a better understanding of the earth pressure reduction that may be achieved by geogrid bridging, the Ontario Ministry of Transportation (MTO) constructed an instrumented full-scale test embankment over a geogrid-reinforced bridged trench. In this paper, the results of this full-scale test along with a numerical study to evaluate the stress reductions achieved by the use of geogrid bridging platform installation are reported. In addition, results of a ...

Model tests on peat — Geotextile — Lightweight aggregate system

Abstract Considerable research has been done in recent years dealing with the interaction of normal weight aggregate and geotextiles overlying soft compressible soils. In some instances, lightweight aggregate is used instead of normal weight aggregate to reduce settlements, and in the bridge abutment areas, to minimize lateral forces and to reduce drag loads on piles. However when used with geotextiles, it is not known whether the overall roadbed stiffness is affected when lightweight aggregate is used in place of normal weight fill. This paper reports the results of experimental research dealing with interaction of lightweight aggregate and geotextiles overlying peat subgrades. Variables investigated in the present study are: differing aggregate types and densities, thickness of the aggregate layer and geotextile types. The results indicate that the overall roadbed stiffness is unaffected when lightweight aggregate is used instead of normal weight aggregate, for small deflections and initial load application.

Explicit Frequency-Dependent Equations for Vertical Vibration of Piles

AbstractPracticing engineers generally obtain stiffness and damping coefficients for dynamic design of pile foundations from design charts. The design charts require multiple interpolations and produce approximate frequency-independent coefficients. Explicit expressions consistent with the underlying theory for the vertical vibration of single piles are presented in this technical note. These expressions are easily implemented in a computer program, such as a spreadsheet, for design use. The proposed method preserves the frequency-dependent nature of the dynamic coefficients and enables one to account for the true condition of the pile tip. The effectiveness of the proposed method is illustrated with numerous examples.