MIDWEST STATES POOLED FUND PROGRAM
Phase IIA Vehicle Dynamics Testing, Validation of Vehicle Models, and Computer Simulation of Rock Ditch Liners
Sponsoring Agency Code
TPF-5(193) Supplement 77
In recent years, greater attention has been placed on controlling erosion and improving the quality of water runoff that leaves the right of way found along roads. As a result, riprap, rock check dams, ditch liners, and other devices have been more frequently installed within and/or adjacent to the roadside. Unfortunately, very little to no research has been performed in order to determine the crashworthiness of the erosion control features noted above.
The American Association of State Highway and Transportation Officials (AASHTO) A Policy on Geometric Design of Highways and Streets contains general guidance regarding the safe use of roadside hardware, drainage features (i.e., drainage channels, ditches, and curbs), and roadside geometries, such as side slopes. However, the AASHTO document provides no specific guidance regarding the crashworthiness of many common erosion control features, such as ditch liners and check dams. The 2006 AASHTO Roadside Design Guide also addresses the crashworthiness of selected drainage features, such as drainage channels, ditches, curbs, and culvert openings. Unfortunately, no significant guidance exists regarding the safe design and placement of roadside channels and ditches incorporating erosion control features, such as ditch liners and check dams.
State Departments of Transportation (DOTs) often expend significant financial resources on highway and road construction projects addressing safety concerns associated with the design and layout of clear zones, especially when considering roadside drainage and water quality. In these situations, agencies must balance safety and environmental concerns. As such, ditch liners and check dams are occasionally used within clear zones to mitigate environmental concerns associated to degraded water quality even though there are no clear guidelines regarding their safety performance. There exists a need to investigate the crashworthiness of selected erosion control features, such as ditch liners and check dams, when used in combination with sloped drainage features. In addition, preliminary safety guidelines are needed to assist engineers with the design and placement of erosion control features.
Erosion control measures have been used extensively throughout the nationâ€™s road infrastructure to protect the integrity of the roadway and its corresponding roadside cross sections. Factors that encourage the use of these features include, but are not limited to, the type of soil and the natural grade of the land itself. Roadsides composed of silty soils with relatively steep grades are highly susceptible to the effects of water runoff and bear the consequence of an increased propensity for soil erosion. In most cases, it would not be economically feasible to drastically alter the configuration of the landscape itself. Instead, an erosion control feature is incorporated into the roadside terrain to increase its resistance to the effects of water runoff. Although these features work extremely well in slowing the flow of water and in protecting the surface of the roadside, caution should be taken when their placement falls within the clear zone, including rock check dams and rock ditch liners.
Very little study has been directed toward the safety of vehicular impacts with common erosion control features, such as rock ditch liners and rock check dams. Most designers recognize that large stones, such as those shown in Figure 1, represent a serious hazard to errant motorists. However, engineers have been given little guidance regarding how to safely design these features. Ditch checks dams can also pose a serious hazard to errant motorists. Sloped, stone ditch checks are not substantially different than a median crossover or roadside driveway. When struck at speeds in excess of 50 mph, these features may pose a serious rollover risk to an errant vehicle.
In the initial phase of this project (Phase I), a review of the design practices utilized by 14 State Departments of Transportation (DOTs) was made regarding rock ditch liners and rock check dams. In addition, national guidelines set forth by various federal agencies for similar structures (i.e., ditches, parallel-drainage structures, cross-drainage structures, and curbs) were reviewed and identified a variety of design specifications. During this investigation, a series of key safety considerations were noted, which led to the development of preliminary guidelines for the safe design and placement of rock check dams and rock ditch liners within the clear zone. For example, a maximum rock size ranging from 4 to 8 in. should be considered in the construction of check dams, and the approach slope face of the dam should be lined with a thin layer of rocks no larger than 1 to 2 in. To minimize the size of the hazard and decrease the vehicle trajectory resulting from potential collisions, a check dam should be constructed with a 2-foot maximum height along high-speed roadways. Although rock check dams shall be constructed with a 6H:1V: or flatter approach slope along high-speed roadways, a 10H:1V or flatter approach slope is recommended. Additionally, ditch side slopes should be no steeper than the corresponding approach slope of the check dam.
Rock ditch liners should be sufficiently entrenched within the soil ditch so that the final upper rock surface
is approximately flush with the non-lined, adjacent soil terrain. The side slopes of a trapezoidal ditch
should be no steeper than 1V:6H near high-speed roadways when constructed with a generally-smooth,
rock lining surface. The liner should be constructed of a gradation composed of a
D50 rock size ranging from 6 to 8 in. and a maximum rock size (i.e., D100) ranging from 10 to 12 in.
However, the actual size of rock used and placement configuration (i.e., dumped, wire-enclosed, or
grouted) is dependent upon the volume of flow expected for the ditch.
The primary research objective for this study includes the continued development of safety guidelines for
use in the design and placement of ditch liners and check dams along highways and roadways. During
the Phase I effort and as part of the initial research funding, preliminary safety guidelines were proposed
along with a preparation of a research plan for use in their future evaluation. At this time, the Wisconsin
Department of Transportation has deemed the preliminary guidelines viable and has requested that
continued research be performed to further evaluate and modify the guidelines using computer simulation
and full-scale vehicle crash testing.
Due to the significant scope of this ongoing research program, the study has been split into multiple phases. The objective for each specific phase is listed below:
1. Develop preliminary guidelines for the safe construction of rock ditch liners and rock check dams â€“ Phase I (Completed)
2. Conduct LS-DYNA computer simulation to develop critical crash testing matrix for evaluating vehicular impacts into rock ditch liners and rock ditch checks - Phase II (Reduced scope with partial funding of $110,000 only)
3. Perform a series of full-scale crash tests on rock ditch liners and check dams placed in a traversable ditch section - Phase III
Phase I Preliminary Guidelines (Completed)
As mentioned previously, the first phase of this project was devoted to reviewing existing guidance established by other states and federal agencies for the safe placement of rock ditch liners and rock check dams within the clear zone. Also, potential vehicle instabilities were analyzed from previously performed full-scale crash tests involving features similar to check liners or check dams (i.e., vehicles traversing slopes or impacting curbs). Preliminary guidelines for the construction of rock ditch liners and rock check dams were developed.
Phase IIA Computer Simulation and Test matrix (Current Proposal â€“ Ditch Liners Only & Reduced Scope)
The second phase of this research study will be devoted to simulating vehicular impacts with rock ditch liners to verify and further enhance the preliminary guidelines proposed in Phase I. A vehicle dynamics computer code capable of accurately simulating ground impacts with vehicle tires and suspension assemblies (e.g., LS-DYNA, HVOSM, ADAMS, etc.) will be used in this second phase of the project. It should be noted that only rock ditch liners will be studied in this Phase IIA effort.
To obtain useful data necessary for the calibration of the numerical models, a series of full-scale crash tests will be needed to assess the kinematics of selected vehicles traversing uneven surfaces. In particular, dynamic testing will be used to evaluate the stability of a vehicle traversing a rock ditch liner that is installed on level terrain.
Once numerical models are calibrated, computer simulation will be performed to identify critical impact scenarios for various rock-ditch liners. In particular, the numerical models will be used to investigate vehicle stability while varying parameters, such as initial speed and angle, rock size, and ditch side slopes and configuration. Full-scale vehicle crash testing will be performed for later use in validating Ls-Dyna vehicle and terrain computer models. Once the initial crash testing of level terrain features and validation computer simulations have been performed, several rock ditch liner configurations will be further developed and evaluated via additional simulations and crash testing.
Phase III Full-Scale Crash Testing (Future Research)
If the Wisconsin DOT determines that the recommended Phase I & IIA guidelines meet their needs for controlling erosion along high-speed facilities, MwRSF will prepare a Phase III research proposal for the full-scale vehicle crash testing and evaluation of the Phase I & IIA design guidelines.
It should be noted that a Phase IIB effort could be initiated on check dams as well.
Guidelines for the design and placement of rock ditch liners will enable engineers to provide a safe roadside and minimize roadside erosion. Further, the numerical calibration of vehicle suspension systems will provide a significant reference for additional studies regarding vehicle kinematics in the future.
Snapshot of Recent Developments
130 Whittier Research Center
2200 Vine Street
Lincoln, Nebraska 68583-0853
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