SLOPE FAILURES: REPAIR OPTIONS. Sean Yoon, Ph.D., P.E. Bridge Division Geotechnical Branch

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1 SLOPE FAILURES: REPAIR OPTIONS Sean Yoon, Ph.D., P.E. Bridge Division Geotechnical Branch October 2016

2 Slope Failures Global & Structural Failures Erosion Piping and Undermining Mudflow 2

3 Global and Structural Often a result of a design error or oversight. Involve failure of soft ground under embankments. Failure of retaining wall and/or bridge structures. Require extensive engineering evaluation and design for repair. $$$ 3

4 Global and Structural 4

5 Global and Structural 5

6 Global and Structural 6

7 Global and Structural 7

8 Erosion Not considered a design issue. Common in sandy soils, newly placed or cut slopes. Caused by water flowing over surface. Control with vegetation, management of surface drainage. Erosion control blankets, mulch, compost. 8

9 Erosion 9

10 Erosion 10

11 Piping and Undermining Concentrated water flowing under structures. Common problem at bridge ends. Consider using Cement Stabilized Sand behind abutments. Often not obvious until structure collapses. Can be detected with inspection. 11

12 Piping and Undermining 12

13 Piping and Undermining 13

14 Piping and Undermining Early detection is key. Small problems are easy to repair. Use Flowable Fill (Item 401) to fill voids. Maintain seals and divert water when possible. Minor voids can became major problems quickly. Large, open areas can be filled with aggregate or cement-stabilized sand. 14

15 Piping and Undermining 15

16 Mud Flow Failures Slopes composed of highly plastic clay. May take considerable time to fail. Water is always a culprit. May not be preventable. (For Maintenance Forces). Designers can solve this problem for future generations!! 16

17 Mud Flow Failures 17

18 Mud Flow Failures 18

19 Mud Flow Failures 19

20 Mud Flow Failures 20

21 Mud Flow Failures 21

22 High Plasticity Clay Absorbs large amounts of water (when available). Swells and softens when wet. Shrinks and cracks when dry. Simple lab test to determine Plasticity Index (PI). PI > 30 is troublesome. 22

23 High Plasticity Clay 23

24 Why Do High PI Slopes Fail? As constructed, material is dry and well compacted. Surface immediately begins to absorb water, swell and soften. During dry periods, material s shrinks and cracks. During next wet cycle, rain penetrates more deeply, clay material swells and softens further. Finally, material can no longer hold its own weight, and flows down the slope. 24

25 Mudflow (1) Surface soils soften and begin to creep 25

26 Mudflow (2) Eventually a complete failure plane forms 26

27 Mudflow (3) Mud flow begins, forming scarp and toe bulge 27

28 Mudflow (4) Moisture penetrates more deeply, and size of failure increases 28

29 Mudflow (5) Process continues until moisture source is removed or slope is repaired 29

30 What Can Be Done To Prevent Failures? Control surface water. Avoid concentrated flows onto slopes unless in flumes. Maintain vegetation on slopes. Avoid rutting that may hold water or impede drainage. In some cases, you may not be able to avoid a failure. (For Maintenance Forces). 30

31 What Can Be Done To Repair Failures? (Permanently) Remove and replace all problem material. Flatten slope. Install retaining wall. Add drainage. Combination of above. 31

32 Remove and Replace All failed material must be removed Removal should extend beyond the failure plane. Using reinforcement layers will require even deeper excavation to anchor the reinforcement. Excavation may extend into the roadway. Shoring may be required. 32

33 Replace with What? Import New Material: Low PI fill. Reclaimed or dirty base. Modify existing high PI clay: Layers of reinforcement (geogrid, steel mesh). 33

34 Solving the Problem Item 132 Embankment Type C. Material meeting the specification requirements shown on the plans. Specify Type C and limit the PI. Do not allow material excavated on the job to be used as embankment if it does not meet the PI requirement. 34

35 Plasticity Index Recommendations SLOPE (X to 1) PLASTICITY INDEX (PI)(%) 2.5 to 1 < to 1 < to 1 < to 1 < to 1 < 85 Plasticity Index Range for Various Slopes Required for FS = 1.3 in the Long-Term (Drained) Condition 35

36 Copyright Notice Copyright 2015 Texas Department of Transportation All Rights Reserved Entities or individuals that copy and present state agency information must identify the source of the content, including the date the content was copied. Entities or individuals that copy and present state agency information on their websites must accompany that information with a statement that neither the entity or individual nor the information, as it is presented on its website, is endorsed by the State of Texas or any state agency. To protect the intellectual property of state agencies, copied information must reflect the copyright, trademark, service mark, or other intellectual property rights of the state agency whose protected information is being used by the entity or individual. Entities or individuals may not copy, reproduce, distribute, publish, or transmit, in any way this content for commercial purposes. This presentation is distributed without profit and is being made available solely for educational purposes. The use of any copyrighted material included in this presentation is intended to be a fair use of such material as provided for in Title 17 U.S.C. Section 107 of the U.S. Copyright Law. 36

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