Geotechnical Engineering: Slope Stability

1 Geotechnical Engineering: Slope Stability Course No: G06-001. Credit: 6 PDH. Yun Zhou, PhD, PE. Continuing Education and Development, Inc. 9 Greyridge Farm Court Stony Point, NY 10980. P: (877) 322-5800. F: (877) 322-4774. Department of Transportation Publication No. FHWA NHI-06-088. Federal Highway Administration December 2006. NHI Course No. 132012_____. SOILS AND FOUNDATIONS. Reference Manual Volume I. Testing Theory Experience National Highway Institute Technical Report Documentation Page 1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. FHWA-NHI 06-088. 4. Title and Subtitle 5. Report Date December 2006. SOILS AND FOUNDATIONS 6. Performing Organization Code REFERENCE MANUAL Volume I. 7. Author(s) 8. Performing Organization Report No. Naresh C. Samtani*, PE, PhD and Edward A.

2 Nowatzki*, PE, PhD. 9. Performing Organization Name and Address 10. Work Unit No. (TRAIS). Ryan R. Berg and Associates, Inc. 2190 Leyland Alcove, Woodbury, MN 55125. 11. Contract or Grant No. * NCS GeoResources, LLC. 640 W Paseo Rio Grande, Tucson, AZ 85737 DTFH-61-02-T-63016. 12. Sponsoring Agency Name and Address 13. Type of Report and Period Covered National Highway Institute Department of Transportation 14. Sponsoring Agency Code Federal Highway Administration, Washington, 20590. 15. Supplementary Notes FHWA COTR Larry Jones FHWA Technical Review Jerry A. DiMaggio, PE; Silas Nichols, PE; Richard Cheney, PE;. Benjamin Rivers, PE; Justin Henwood, PE. Contractor Technical Review Ryan R. Berg, PE; Robert C. Bachus, PhD, PE;. Barry R. Christopher, PhD, PE. This manual is an update of the 3rd Edition prepared by Parsons Brinckerhoff Quade & Douglas, Inc, in 2000.

3 Author: Richard Cheney, PE. The authors of the 1st and 2nd editions prepared by the FHWA in 1982 and 1993, respectively, were Richard Cheney, PE and Ronald Chassie, PE. 16. Abstract The Reference Manual for Soils and Foundations course is intended for design and construction professionals involved with the selection, design and construction of Geotechnical features for surface transportation facilities. The manual is geared towards practitioners who routinely deal with soils and foundations issues but who may have little theoretical background in soil mechanics or foundation engineering. The manual's content follows a project-oriented approach where the Geotechnical aspects of a project are traced from preparation of the boring request through design computation of settlement, allowable footing pressure, etc.

4 , to the construction of approach embankments and foundations. Appendix A includes an example bridge project where such an approach is demonstrated. Recommendations are presented on how to layout borings efficiently, how to minimize approach embankment settlement, how to design the most cost-effective pier and abutment foundations, and how to transmit design information properly through plans, specifications, and/or contact with the project engineer so that the project can be constructed efficiently. The objective of this manual is to present recommended methods for the safe, cost-effective design and construction of Geotechnical features. Coordination between Geotechnical specialists and project team members at all phases of a project is stressed. Readers are encouraged to develop an appreciation of Geotechnical activities in all project phases that influence or are influenced by their work.

5 17. Key Words 18. Distribution Statement Subsurface exploration, testing, Slope Stability , embankments, cut slopes, shallow No restrictions. foundations, driven piles, drilled shafts, earth retaining structures, construction. 19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price UNCLASSIFIED UNCLASSIFIED 462. Form DOT F (8-72) Reproduction of completed page authorized CHAPTER Slope Stability . Ground Stability must be assured prior to consideration of other foundation related items. Embankment foundation problems involve the support of the embankment by natural soil. Problems with embankments and structures occasionally occur that could be prevented by initial recognition of the problem and appropriate design . Stability problems most often occur when the embankment is to be built over soft soils such as low strength clays, silts, or peats.

6 Once the soil profile, soil strengths, and depth of ground water table have been determined by field explorations and/or field and laboratory testing, the Stability of the embankment can be analyzed and a factor of safety estimated. If the embankment is found to be unstable, measures can then be taken to stabilize the foundation soils. As illustrated in Figure 6-1, there are four major types of instability that should be considered in the design of embankments over weak foundation soils. Recommendations on how to recognize, analyze, and solve each of the first three problems are presented in this chapter. Lateral squeeze is more closely related to the evaluation of foundation deformation and is discussed in Chapter 7 (Approach roadway Deformations). The Stability problems illustrated in Figure 6-1 can be classified as internal or "external.

7 ". "Internal" embankment Stability problems generally result from the selection of poor quality embankment materials and/or improper placement of the embankment fills and/or improper placement requirements. The infinite Slope failure mode is an example of an internal . Stability problem; often such a failure is manifested as sloughing of the surface of the Slope . Internal Stability can be assured through project specifications by requiring granular materials with minimum gradation and compaction requirements. An example of a typical specification for approach roadway construction is presented in Chapter 7. The failure modes shown in Figure 6-1b, c and d, can be classified as external Stability problems. Primary Reference The primary reference for this chapter is as follows: FHWA (2001a).

8 Soil Slope and Embankment design Reference Manual. Report No. FHWA. NHI-01-026, Authors: Collin, J. G., Hung, J. C., Lee, W. S., Munfakh, G., Federal Highway Administration, Department of Transportation.. FHWA NHI-06-088 6 Slope Stability Soils and Foundations Volume I 6-1 December 2006. Shallow translational failure (Infinite Slope condition). Embankment Fill Firm Soil (a). (b). (c). (d). Figure 6-1. Embankment failures: (a) Infinite Slope failure in embankment fill, (b). Circular arc failure in embankment fill and foundation soil, (c) Sliding block failure in embankment fill and foundation soil, and (d) Lateral squeeze of foundation soil. FHWA NHI-06-088 6 Slope Stability Soils and Foundations Volume I 6-2 December 2006. EFFECTS OF WATER ON Slope Stability . Very soft, saturated foundation soils or ground water generally play a prominent role in Geotechnical failures in general.

9 They are certainly major factors in cut Slope Stability and in the Stability of fill slopes involving both internal and external Slope failures. The effect of water on cut and fill Slope Stability is briefly discussed below. Importance of Water Next to gravity, water is the most important factor in Slope Stability . The effect of gravity is known, therefore, water is the key factor in assessing Slope Stability . Effect of Water on Cohesionless Soils In cohesionless soils, water does not affect the angle of internal friction ( ). The effect of water on cohesionless soils below the water table is to decrease the intergranular (effective) stress between soil grains ( 'n), which decreases the frictional shearing resistance ( '). Effect of Water on Cohesive Soils Routine seasonal fluctuations in the ground water table do not usually influence either the amount of water in the pore spaces between soil grains or the cohesion.

10 The attractive forces between soil particles prevent water absorption unless external forces such as pile driving, disrupt the grain structure. However, certain clay minerals do react to the presence of water and cause volume changes of the clay mass. An increase in absorbed moisture is a major factor in the decrease in strength of cohesive soils as shown schematically in Figure 6-2. Water absorbed by clay minerals causes increased water contents that decrease the cohesion of clayey soils. These effects are amplified if the clay mineral happens to be expansive, , montmorillonite. FHWA NHI-06-088 6 Slope Stability Soils and Foundations Volume I 6-3 December 2006. Cohesive Strength Water Content Figure 6-2. Effect of water content on cohesive strength of clay. Fills on Clays Excess pore water pressures are created when fills are placed on clay or silt.