Ohio Department of Transportation Division of Production Management Office of Geotechnical Engineering. Geotechnical Bulletin

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1 Ohio Department of Transportation Division of Production Management Office of Geotechnical Engineering Geotechnical Bulletin GB 6 Shear Strength of Proposed Embankments January 15, 2016 Geotechnical Bulletin GB6 was developed by the Office of Geotechnical Engineering based on research performed by Ohio University. This is the second edition. Earthen embankments are a feature of many, if not most, transportation projects. Their cost and performance are critical. On projects that propose new embankments, since the embankments have yet to be constructed, it is impossible to test for the shear strength of the embankment material. The designer, in order to determine both short term and long term stability, must estimate the shear strength parameters of the embankment material prior to construction. The primary purpose of this bulletin is to provide a sound and consistent methodology for determination of shear strength parameters for proposed embankments that are not yet constructed. This bulletin is therefore not applicable to in-situ soils that have not been moisture conditioned and compacted per ODOT onstruction and Material Specifications (MS) Item 203 Embankment. Estimated shear strength parameters should not be used to analyze critical applications, such as stability analyses of existing embankments or evaluation of existing embankments supporting a structure. This bulletin does not negate the need to perform strength testing as part of the geotechnical exploration when appropriate. In most cases, the material used to construct a new embankment is obtained by excavating existing soil on or near the project site. In the geotechnical exploration performed for the project, the existing soils within the proposed right-of-way have been sampled and undergone laboratory testing. A research project has been conducted by Ohio University, sponsored by ODOT, to ascertain the shear strengths that are being achieved in our roadway embankments across the state. As the more prevalent soil types in Ohio, embankments constructed of silts and clays (A-4a, A-4b, A-6a, A-6b, and A-7-6) were the targeted research sites. Shear strength parameters for these soil types, in spatially distributed embankment locations across Ohio, were obtained. Statistical analyses were performed in order to correlate these strength values to standard soil properties which would be available from testing prior to embankment construction, such as Atterberg limits and gradation values. Knowing these standard soil properties for the probable fill material then allows

2 Page 2 of 7 for shear strengths to be predicted prior to the actual construction of the embankment. This bulletin presents the Office of Geotechnical Engineering s recommended shear strengths for use in designing embankments, based on the classification and properties of the expected fill material. Limitations and guidelines are provided to assist in the proper use of the estimated values. This bulletin may be obtained from the Office of Geotechnical Engineering s web site ( This web site also contains other ODOT geotechnical documents and bulletins. A. Research The research project determined both short term (undrained) and long term (drained) shear strengths for the sampled embankment materials by conducting unconfined compression (U) tests and consolidated-undrained (U) triaxial tests on relatively undisturbed samples. Both single variable and multi-variable equations were then utilized to correlate the individual shear strength parameters ( c,, c, ) to a variety of general soil properties for the tested samples. Formulas were developed for each specific soil classification as well as formulas covering all soil types together. Any formula that had a correlation with the coefficient of determination (R 2 ) value equal to 0.8 or above was considered statistically strong. Based on the completed statistical analyses, the research provided guidelines for estimating shear strength parameters at three levels of sophistication, ranging from default values at Level 1 to complex multi-variable equations at Level 3. While statistically strong correlations were found for the four strength parameters and various soil properties for most of the five soil classifications, there were some limitations: As A-4b material is not typically used in ODOT embankments, not enough A-4b samples were obtained to yield statistical significance. The range of the independent soil properties was limited to the tested samples. The full range of a given soil property for a particular soil classification was generally not encountered, thereby limiting the strength correlations to only a part of the soil property range for that particular soil classification. Some of the strongest correlations were produced by utilizing complex and sensitive formulas which may not translate well to standardized roadway design procedures.

3 Page 3 of 7 Based on these limitations and the desire for straightforward methods for estimating shear strength values, the Office of Geotechnical Engineering simplified the research recommendations to those contained in the next section. The final report for the research project may be found at the following link: otechnicalreportsdetail.aspx# B. Recommended Shear Strengths (Based on Research) Roadway embankments are often constructed of material found very near the site of the proposed work. During the geotechnical exploration for the project, basic soil properties and ODOT classification of these existing materials will be determined. Based on the research conducted, shear strengths for the future embankments can be estimated from these soil properties and classifications. In Table 1, ODOT s recommended shear strengths for the design of embankments based on the research are presented. Table 1 Estimated Shear Strengths (Based on Research) lassification A-4a A-4b A-6a * A-6b * A * Not enough A-4b samples were obtained in the conducted research to determine any statistically significant shear strength estimates. * These listed values for are maximums. Determine the value of from the following equation: = (PI) PI Eqn. 1 where PI is the Plasticity Index for the existing material to be used in the embankment construction. Use the value calculated from the formula, but not exceeding the maximum shown in the table.

4 Page 4 of 7 Utilizing the values in Table 1 will require the designer to have knowledge of the project layout and a good approximation of what locations will be utilized as the sources of fill for the proposed embankment. In addition to knowing the probable soil types to be used as fill, the following guidelines should be utilized to determine the final design shear strengths: Estimating shear strengths for granular material and non-durable shale was beyond the scope of the research project, thus strength values are not provided in Table 1. Not enough A-4b samples were obtained in the conducted research to determine any statistically significant shear strength estimates. If it is likely that there will be enough of these materials used in the construction of the embankment to influence the overall makeup of the fill, then other means should be utilized to estimate shear strengths. Refer to Section for guidance. When the analysis of the probable embankment fill material shows that only one of the listed soil types will be used, then the long term friction angle to be determined from Equation 1 should use the average PI for all the locations expected to be used for fill. When it is likely that the embankment will be constructed of two or more soil classifications, the final estimated design strengths should be determined by weighted average strengths from each soil type based on the percent of each classification present in the expected borrow locations. Determine the estimated strength values for each classification from Table 1, then weight them based on the percentage occurrence, and combine the weighted values to get the final estimated strength.. Recommended Shear Strengths (Based on Experience) Granular material, A-4b soil, and non-durable shale are all acceptable embankment materials. The research does not address a methodology for estimating the shear strengths of these materials. In Table 2, ODOT s recommended shear strengths for the design of embankments based on experience are presented.

5 Page 5 of 7 lassification Granular and A-4b Non-plastic * Table 2 Estimated Shear Strengths (Based on Experience) 0 * 0 * A-4b Plastic Non-durable shale 30** Granular material includes ODOT lassifications A-1-a, A-1-b, A-3, A-3a, A-2-4, A-2-5, A-2-6, and A-2-7. Significant quantities of granular or non-plastic A-4b embankment from on-site or near-site sources are rare. Due to the variability in the strength properties of these materials, judgment is required in estimating appropriate design strengths for proposed embankments. ** The listed value for is a maximum. Determine the value of from the following equation: = (PI) PI Eqn. 1 where PI is the Plasticity Index for the existing material to be used in the embankment construction. Use the value calculated from the formula, but not exceeding the maximum shown in the table. D. Example Determination of Estimated Shear Strength A roadway project requires the construction of two 30 high bridge approach embankments with 2:1 side slopes. In the area where the ontractor will most likely get fill material there are a total of 7 borings. It is assumed that, from this area, the top 7 feet of material found below the 6 inches of topsoil will be utilized as the embankment borrow source. SPT samples were obtained at 1.0, 3.5, and 6.0 feet in each of the 7 borings. Of the 21 SPT samples, 7 (33%) were in A-4a material, 10 (48%) were in A- 6a, and 4 (19%) were in A-6b. The PI s for the samples are as follows: A-6a 11, 12, 12, 11, 14, 14, 12, 15, 13, 11 A-6b 18, 22, 23, 25 Per Table 1, the PI of the A-4a soil is not utilized in the shear strength estimation. The average PI for the A-6a and A-6b soils are 12.5 and 22 respectively.

6 Page 6 of 7 Based on this information, the estimated shear strengths for the 3 soil types on this project can be determined. These values are shown in Table 3. Table 3 Project 1 Estimated Shear Strengths lassification A-4a A-6a * A-6b * * Values based on Equation 1 and do not exceed the maximum value per Table 1. To weight the classifications, multiply the A-4a values by 0.33, the A-6a values by 0.48, and the A-6b values by 0.19, yielding the values shown in Table 4. Table 4 Project 1 Weighted Estimated Shear Strengths lassification A-4a A-6a A-6b Total Use Note that the recommended values to use for c and exceed the values shown as maximum for A-6b in Table 1. This is acceptable as the influence of the stronger A-4a and A-6a materials allow for the higher strength estimates. It is recognized that the above weighted average method would logically be correct with the weighted average computed on tan, rather than. However, with the somewhat narrow range of values that are used, the result of the procedure is nearly the same

7 Page 7 of 7 whether using tan or, and the simplified method using, as illustrated above, can be used. E. Summary The strength values determined by the methods in this document can be utilized in certain typical situations. When embankments are proposed and there is adequate information on the soil to be used as borrow, shear strength values estimated by this method can be utilized for stability verification. When an existing embankment is going to be widened and strength testing is not part of the proposed geotechnical exploration, these estimated shear strength values will be useful in examining the stability of the proposed slope configuration. When analyzing existing embankment failures, a method other than that presented herein should be utilized to determine strengths. Do not use estimated shear strength values from this document when the embankment strengths are critical design parameters and therefore strength testing should be performed, or when strength tests are available for an existing embankment soil. Embankment design and construction will continue to be an important feature of transportation projects. It is important that designers have as much information about the proposed embankments as possible. Based on the research conducted, this document presents a more reliable methodology for estimating shear strengths of proposed embankments. It is anticipated that, as more data becomes available in the future, this methodology can be further refined, designers will utilize more consistent shear strength parameters in their designs, and more reliable embankment designs can be developed.