Mechanically Stabilized Earth Structures By Blaise J. Fitzpatrick, P.E. Fitzpatrick Engineering Associates, P.C.

Transcription

1 Mechanically Stabilized Earth Structures By Blaise J. Fitzpatrick, P.E. Fitzpatrick Engineering Associates, P.C. I. History or MSE Structures II. Industry Design Methods and Programs III. Products and Soils Testing IV. Design Outline V. Modes of Failures A) External Stability B) Internal Stability C) Connection Stability VI. Civil Design Considerations for Mechanically Stabilized Earth Walls VII. Why Do MSE Walls Fails VIII. MSE Slopes I. History History of MSE Structures Ancient Babylonian Ziggurats (2,500 BC) - Reed mats (Iraq) Great Wall of China (2,000 BC) - Tree branches (Only portions of the wall are reinforced) Roman Levees (2,000 years) - Reed mats (Italy - along Tiber River) Ziggurat of Ur in Mesopotamia About 2500 B.C. Blaise J. Fitzpatrick, P.E. Page 1 11/1/2008

2 Ziggurat of Ur in Mesopotamia About 2500 B.C. History of MSE Structures Modern First modern documented use in 1850 s in Europe, most military structures. Technique rediscovered for retaining walls by Vidal in France in the early 1960 s. First MSE Wall in United States in 1972 along Highway 39 in the San Gabriel Mountains, California. Geosynthetics have greatly accelerated use of soil reinforcement over the past 30 years. First Highway Use of Modern MSE Earth Wall France between Nice and the Italian Border (1968) Blaise J. Fitzpatrick, P.E. Page 2 11/1/2008

3 First MSE Structure in the United States 1972 along Highway 39 San Gabriel Mountains, California. II. Industry Design Methods and Programs Design Methodologies NCMA - National Concrete Masonry Association uses the NCMA Design Manual for Segmental Retaining Walls, Second Edition, AASHTO - American Association of State Highway and Transportation Officials uses FHWA Publication No. NHI , Mechanically Stabilized Earth Walls and Reinforced Soil Slopes - Design and Construction Guidelines, March Basic Difference Between NCMA and AASHTO NCMA L/H ratio 60 % of wall height. Variable reinforcement lengths. Re-use of on-site soils (if possible). Uniform Loads Limited Design Reduced block embedment depths. Commercial projects. Minimum design life of 75-years AASHTO L/H ratio 70 % of wall height. Uniform reinforcement lengths. Select fill in the reinforced zone. Uniform & Strip Loads - Full Design Minimum embedment of 2-feet. Public & Highway projects. Minimum design life of 75-years NOTE: Both design methods work. However, watch out for in-house design methods based on years of experience of designing walls. At this point in time, the NCMA and AASHTO design methods are the only industry standards of practice in the United States. Blaise J. Fitzpatrick, P.E. Page 3 11/1/2008

4 Design Software 1. MSEW (AASHTO, NCMA & LRFD) 2. SRWall (NCMA) 3. AnchorWall 4. KeyWall 5. MiraWall 6. StrataWall 7. RisiWall 8. TensWall 9. ABWalls MESA-Pro III. Products and Soil Testing - Components of MSE Walls Soil Geosynthetic Reinforcement (geotextile or geogrid) Masonry Block Facing Units Drainage System Leveling Pad Soil - Soil makes up 95% of all reinforced structures. Soil is inexpensive and abundant. The quality of the soil used in MSE system is critical. The MSE wall design engineer should provide a proper soil specification. Reinforcement - there are several brands of soil reinforcement products available for construction. Geotextiles and geogrids are man-made products comprised of High Tenacity Polyester or High Density Polyethylene HDPE. Reinforcement Types and Manufacturers Amoco PP & PET Geotextiles GeoStar PET Geotextiles Huesker PET Geotextiles and Geogrids LINQ PP & PET Geotextiles Lückenhaus PET Geogrids & PET Geotextiles Strata Systems PET Geogrids Synteen PET Geogrids Synthetic Industries PET Geotextiles TC Mirafi PET Geogrids & PET Geotextiles Tensar HDPE and Polypropylene Geogrids Blaise J. Fitzpatrick, P.E. Page 4 11/1/2008

5 Geosynthetic Reinforcement Geotextiles Geosynthetic Reinforcement Geogrid Components of MSE Walls - Masonry Block Units Masonry block facing units - there are several brands of masonry block units. Block units are categorized as having a mechanical or frictional connection. Blaise J. Fitzpatrick, P.E. Page 5 11/1/2008

6 Segmental Block Manufacturers Mechanical Connection Newcastle, MESA HP & Cornerstone Frictional Connection Allan Block Anchor Wall Systems StoneGrid GeoStone Keystone Amastone Risi Stone Rockwood Stonewall Versa-Lok Examples of Commercially Available Segmental Units (NCMA, 1997) Blaise J. Fitzpatrick, P.E. Page 6 11/1/2008

7 Components of MSE Walls Drainage system - drainage systems must be constructed to contain and/or control surface and subsurface water. Blanket drains are required when ground-water is close to the MSE wall foundation. Blanket and chimney drains are required ground-water rises above the MSE wall foundation. Drainage Aggregate, Soils, and Gravel Leveling Pad Drainage Aggregate, Soils, and Chimney Drain Leveling Pad (NCMA, 1997) Blaise J. Fitzpatrick, P.E. Page 7 11/1/2008

8 IV. Design Information Information Needed for a MSE Wall Design Soil Data Information Civil Drawings - Site Specific Information Geosynthetic Reinforcement Data Block Information Connection Strength Testing Unit to Unit Shear Testing The Role of Soils o Remember that 98% of a segmental retaining wall system consists of soil. o There is no other structural system where you assume 98% of the system and it will still work. o Therefore, correct soil parameters are essential to an accurate design. Soil Zones What do we need to know about these soil zones? o Friction Angle () - Triaxial or Direct Shear Test o Unit Weight () - Proctor Test or Density Test What do we need to know about these soil zones? Blaise J. Fitzpatrick, P.E. Page 8 11/1/2008

9 In order to successfully design a segmental retaining wall, you need to know or be able to define the Internal Friction Angle (). This is a property of the soil type and defines the soil in terms of shear strength. The higher the angle, the stronger the soil in terms of resistance to sustained loads. You also need to know or define a moist unit weight of the soil, known as gamma (). This affects the driving and resisting forces. Another function of the soil shear strength is cohesion. This should also be determined. However, cohesion is ignored for the reinforced and retained soil zones, and is only used in the foundation soil zone. Geotechnical engineers describe soil shear strength using Mohr-Coulomb failure criteria. Direct Shear Test (granular soils only!!) Blaise J. Fitzpatrick, P.E. Page 9 11/1/2008

10 Triaxial Test (cohesive soils only!!) There are several Triaxial Tests The Correct Triaxial Test Needed for Wall Design is the Consolidated Undrained (CU) Test with Pore Pressures. This test can take up to several weeks. This test is expensive. Therefore, plan your timetable wisely! Standard or Modified Proctor Test (performed in the lab) Compaction Test (on site soils) Blaise J. Fitzpatrick, P.E. Page 10 11/1/2008

11 Discussion on Allowable Soil Backfill The success or failure of a MSE wall or slope is greatly dependent upon the soil used to construct the geosynthetic-reinforced zone and, to a lesser extent, the retained zone (soils located behind the reinforced zone). The selection of soil backfill with respect to the reinforced zone is extremely critical since about 98 percent of the structure is soil. Finegrained soils such as SC, ML, CL, MH and CH can have a negative effect on the behavior of a wall or slope and therefore should not be used with any of the Mechanically Stabilized Earth walls and/or slopes for the above-referenced Project. Fine-grained soils have a much greater potential for time-dependent movement (creep deformation) of the Mechanically Stabilized Earth wall or slope system, leaving Mechanically Stabilized Earth walls and slopes more susceptible to failure if backfilled with fine-grained soils. The use of backfill with a large amount of fines is also a problem with all types of retaining walls. The lack of drainage, which may evolve over time, can eventually cause failure of any wall unless the wall is designed to retain water. FEA notes that the liquid limit, LL, and Plasticity Index, PI, can have a significant effect on the performance of a Mechanically Stabilized Earth wall or slope. Soils used to construct the geosynthetic-reinforced zone must have a LL<35 and PI<10. This is to assure that time dependent deformation will not be excessive and that backfill drainage will not be minimized. Non-creeping soil types must be used for construction. Creep of Mechanically Stabilized Earth walls and slopes depends largely on the creep characteristics of the geosyntheticreinforced soil. Field performance data have indicated that creep deformation of Mechanically Stabilized Earth walls and slopes is minimized when a "well compacted granular fill" is used. The creep rate of geosynthetics and soil are different. Where non-creeping soil is used (e.g., granular with less than 35% passing the #200 sieve), the reinforcement will creep faster than the soil and thus the soil will serve to restrain creep of the reinforcement, causing it to relax (reducing the load in the reinforcement by increasing the load transfer into the soil along its common interfaces with the reinforcement). By contrast, research performed at the University of Colorado, Denver, has shown that clayey backfill enhances creep of geosynthetics by creeping itself more than the geosynthetics. If a fine-grained silty or clayey soil (e.g., more than 50% passing the #200 sieve) were to be used to construct the reinforced zone, it may creep faster than the reinforcement and transfer load to the geosynthetic, resulting in increased load and rate of creep in the geosynthetic, leading to possible failure. Fine-grained silty or clayey soil, including SC, ML, CL, MH and CH, should therefore not be used for construction of the MSE wall or slopes. Blaise J. Fitzpatrick, P.E. Page 11 11/1/2008

12 What s the ideal soil for a MSE structure? SAND vs. CLAY Advantages of granular soils Easier to place and compact. Higher permeability which assists drainage. Greater friction angle which reduces stresses. Generally less susceptible to creep. If you re going to use clay. o Make absolutely sure that proper drainage is installed. o Be sure that the soil has a low to moderate frost heave potential. o The internal cohesive shear strength parameter c is ignored. o Pay special attention to the creep potential of the soil. o Never greater than 20 PI Determining the geosynthetic allowable design strength Begin with the ultimate tensile strength of the reinforcement... this value is adjusted by the Creep Reduction Factor, RF cr... along with the Durability Reduction Factor, RF d... and the Installation Damage Reduction Factor, RF id... finally, apply a load reduction factor of FS UNC = 1.5 The end result is defined by the equations.. T Ultimate LTDS = RF CR x RF ID x RF D T Allowable = LTDS FS UNC Reduction Factors are available through reinforcement suppliers or are published in the annual Geosynthetics Specifier's Guide. SRW Block Information Needed For Design Block Dimensions Block Setback Weight of the Individual Blocks Infilled Weight of the Blocks Unit to Unit Shear Block/Grid Connection Strength You need to choose a specific SRW facing unit and reinforcement combination. Why do you need to know the specific block and grid combination used? because each unit has unique unit-to-unit shear-strength properties as well as unique connection properties with each individual reinforcement type. Blaise J. Fitzpatrick, P.E. Page 12 11/1/2008

13 Tests for determining these properties: SRWU-1 Determination of Connection Strength between Geosynthetics and Segmental Concrete Units SRWU-2 Determination of Shear Strength between Segmental Concrete Units Connection Test Connection Strength Diagram n h T h Blaise J. Fitzpatrick, P.E. Page 13 11/1/2008

14 Connection Capacity Curve 1500 Connection Strength, T c (lb/ft) 1000 T max 500 T c = a c + n tan Normal Load, n IF YOU DO NOT HAVE THESE TEST RESULTS, UNIQUE TO EACH COMBINATION OF SRW UNIT AND REINFORCEMENT, YOU CANNOT COMPLETE THE RETAINING WALL DESIGN! IV. Design Procedure and Failure Mechanisms Defining the Wall Geometry Establish wall profile (top and bottom of wall elevations). Determine crest and toe slopes. Identify surcharge loads (traffic & structural). Consider drainage issues. Usually, this information can be obtained from the site grading plan. Blaise J. Fitzpatrick, P.E. Page 14 11/1/2008

15 Example of a Grading Plan Wall Design Procedure The MSE wall must be analyzed for stability with respect to: External Stability Internal Stability Facing Connection Global Stability Seismic Analysis Blaise J. Fitzpatrick, P.E. Page 15 11/1/2008

16 Main Modes of Failure for Reinforced Soil SRWs (NCMA, 1997) Recommended Minimum Factors of Safety & Design Criteria for Reinforced Soil SRWs Mode Design Parameters Required FS External FS - Base Sliding 1.5 External FS - Overturning 2.0 External FS - Bearing Capacity 2.0 Internal FS - Sliding Along Reinforcement Layers 1.5 Internal FS - Reinforcement Pullout 1.5 Internal FS - Reinforcement Tensile Overstress 1.5 Internal FS - Facing Connection Break and Pullout 1.5 Internal FS - Material Uncertainty 1.5 Global FS - Rotational Failure (Bishop s Modified Method) 1.3 Global FS - 2 Part Wedge Translational Failure (Spencer s Method) 1.3 Global FS - 3 Part Wedge (Spencer s Method) 1.3 Blaise J. Fitzpatrick, P.E. Page 16 11/1/2008

17 When does Global Stability Control? Steep slopes above and/or below the wall Poor foundation soils Heavy surcharges with a toe slope and... terraced walls! Global Stability HAS to be checked by SOMEBODY! This is the point in the design process where you need to have the product specific shear and connection strength testing for the masonry block units. If you do not have this testing, you will be unable to complete the design. Plans and Specifications Must Include Wall Elevations Blaise J. Fitzpatrick, P.E. Page 17 11/1/2008

18 Sections and Details Wall Specifications The Design Must Address: Soil Compaction Specifications External & Internal Drainage Provisions Adjacent Utilities Surcharge Loads Crest & Toe Slopes Vertical & Horizontal Penetrations Who Should Design SRW s? Recommended minimum qualifications of the design engineer: o Strong background in geotechnical and geosynthetic engineering (Masters or PhD). o 5-years design experience in mechanically stabilized earth design and analysis. Who Should Construct SRW s? Recommended minimum qualifications of the reinforced soil wall contractor: o 5-years minimum construction experience. o Constructed at least 500,000-ft 2 of mechanically stabilized earth walls. Blaise J. Fitzpatrick, P.E. Page 18 11/1/2008

19 VI. Civil Design Considerations for Mechanically Stabilized Earth Walls Civil Design Considerations for Mechanically Stabilized Earth Walls Some Practical Do s & Don ts o When at all possible. Get everything but soil and reinforcement out of the reinforced zone! Avoid Creating Low Spot Behind Wall - Original Grading Plan Avoid Creating Low Spot Behind Wall - Original Profile Top of wall grades must be set to allow for positive surface water flow across the top of wall and to exit at one or both ends of the wall. Low spot elevations graded in the middle of the wall, as noted in the example profile, serves as a concentrated point to collect water and creates a situation in which washout or wall failure can occur. Blaise J. Fitzpatrick, P.E. Page 19 11/1/2008

20 Civil Design Considerations Removal of Low Spot Behind Wall - Preferred Grading Plan Removal of Low Spot Behind Wall - Preferred Wall Profile Removing the low spot in the above example keeps surface water from collecting and flowing over the wall at the 90-degree outside corner. Standing water collected at this location could cause a failure. Blaise J. Fitzpatrick, P.E. Page 20 11/1/2008

21 Walls Go On Top Of Slopes Slopes on Top of Wall is Not Preferred If a mechanically stabilized earth (MSE) wall or slope is to be constructed, it is preferred to locate the wall or slope on top of the toe slope. This type of geometry results in significantly less stress, both internally and externally, on the geosynthetic-reinforcement and the MSE facing system. As shown in the example above assuming a friction angle of 30-degrees o A level backfill produces an earth pressure coefficient of Ka=0.33 o A 2H:1V backfill produces an earth pressure coefficient of Ka=0.54 Blaise J. Fitzpatrick, P.E. Page 21 11/1/2008

22 Should any repairs to the wall or slope be required post construction during the design life, they can be made much more easily without the slope on top of the wall. If a 2H:1V, 2.5H:1V or 3H:1V toe slope is to be constructed or exists, a minimum "5-foot wide level bench" should be graded immediately in front of the MSE wall or slope. The 5- foot wide level bench provides a working platform for the contractor to begin the wall construction. Civil Design Considerations Add swales to walls with crest slopes greater than 5-feet in height. Remove low spots from walls. Provide scour protection. For Crest Slopes Longer Than 10ft Swale If a backfill or crest slope is to be graded at the top of a MSE wall or slope and the backfill or crest slope length exceeds 10-feet, then a drainage swale must be constructed behind the wall crest. To provide room for the swale, the wall height must be increased accordingly based on the swale width and depth as determined by the civil engineers hydraulic study. The grading and drainage plan must also reflect the presence of a swale. Blaise J. Fitzpatrick, P.E. Page 22 11/1/2008

23 At Parking Lots - Drain Water Away From Wall Where parking lots or roadways are constructed behind the crest of a MSE wall or slope, it is important to make sure surface water or sheet flow is directed away from the wall and collected in drop inlets located outside of the geosynthetic-reinforcement zone. Many times the grading plan allows for surface water to sheet flow towards the wall and be collected in curb inlets located within the reinforced zone. If cracks develop in the pavement structure the water could flow through the cracks and into the reinforced zone. Water pressure could then cause the wall to deform or fail. Blaise J. Fitzpatrick, P.E. Page 23 11/1/2008

24 Locate The Following Structures Outside Of The Reinforced Zone Underground utilities o Storm pipes (use neoprene 0 -ring gaskets, minimize joints). o Electric, cable, etc. (wall contractor can install conduits). o Strom Water and Sewer lines. Locate Utilities Outside of Reinforced Zone. If possible, all utilities must be located outside of the geosynthetic-reinforced zone. If a pipe must be located within the geosynthetic zone of "any MSE wall or slope" and the pipe has to be serviced or repaired after the structure is built, then to service the pipe layers of geosynthetic-reinforcement will have to be cut and the wall be dismantled to the elevation of the pipe. Also, it makes construction of the wall and pipe more difficult when two separate contractors (pipe and wall contractor) are working in the same area trying to coordinate the pipe elevation within the layers of geosynthetic-reinforcement. If liquid bearing utilities are located within the geosynthetic-reinforced zone, the following must be considered. Storm water pipes are subject to separation at joints. If this occurs, water will seep into the adjacent soil and soil can migrate into the pipe. If the pipe is located within or next to the reinforced zone of a MSE wall or slope, it can cause excessive hydrostatic loads or result in settlement at the ground surface. Storm water pipes located inside or within 10-feet of the geosynthetic-reinforced zone should consist of either continuous pipe sections, or neoprene o-rings should be properly installed at the pipe joints. A double lined pipe system or a leak management system could also be implemented into the storm water design. Design and detail of all pipe systems is the responsibility of the project civil engineer. Blaise J. Fitzpatrick, P.E. Page 24 11/1/2008

25 Storm Water Pipes Need Water Tight Joints Try to get the following structures out of the reinforced zone: Drainage structures/stand-pipe man holes Convert curb inlets to drop inlets Manhole Structures No Closer Than 6-ft To Wall If a vertical storm water riser is placed within the geosynthetic-reinforced zone, it should be located such that there remain at least 6.0-feet between the edge of the riser and the back of the MSE facing system. This amount of space allows for proper compaction of soil. However, it is strongly recommended that all utilities, i.e., storm pipes, electrical, gas, catch basins, drop inlets, and etcetera be located outside the geosynthetic-reinforced zone. Blaise J. Fitzpatrick, P.E. Page 25 11/1/2008

26 Landscaping - Control Excavation & Irrigation Landscaping above the geosynthetic-reinforced zone should be controlled. The wall designer should be consulted before excavating through layers of reinforcement for the planting of trees and shrubs. As a minimum, small shrubs should be located 5-feet from the MSE facing system and tress located 10-feet from the MSE facing system. Subsurface irrigation systems are prone to leaking and saturating adjacent soil. If this occurs behind a MSE wall, the increase in horizontal pressure due to hydrostatic loading may induce wall failure. Therefore, subsurface irrigation systems should not be installed in slopes above or below the reinforced zone or within 20-feet behind the geosyntheticreinforced zone of the MSE wall. Where parking lots or driveways are to be constructed behind a wall crest, the civil engineer must provide for adequate space behind the crest to account for wall better, fence posts and guard rails. Generally, if guardrails are located no closer than 3.0-feet behind the segmental blocks, and or fence post no closer than 2.0-ft, a structural engineer must design and detail these features. To calculate the batter of a MSE wall the following equation can be used: x=h tan() where, o x is the horizontal wall batter as measured from the toe to the crest front wall face. o H is the total wall height (feet) o is the angle of the batter (most SRW walls have batter between 3 and 7-degrees). Example: If the maximum wall height is 30-feet and the segmental retaining wall block has a 3-degree batter, the maximum batter is: x=30-feet [tan (3-degrees)] x=1.57-feet x=18.87-inch Most civil plans use a wall line thickness at 1-ft. The depth of the block is 1-ft so the total batter from the bottom of the wall at the face to the top of wall at the backside of the block is 2.57-ft. A wall line thickness of 3-ft based on the tallest wall section should be used to determine the wall line thickness. Blaise J. Fitzpatrick, P.E. Page 26 11/1/2008

27 VII. Why Do MSE Walls Fail The two main reasons walls fail is due to drainage issues and poor compaction. Results of Poor Compaction Blaise J. Fitzpatrick, P.E. Page 27 11/1/2008

28 Poor compaction in the reinforced soil zone leads to settlement of the backfill zone causing cracks to develop in the pavement structure. If not fixed surface water enters the crack and has the potential to cause failure. Too much fill at one time (over 3 feet, too much for the small walk behind compactor). The compacted lift thickness should be limited to 8-inch and monitored by the project geotechnical consultant as would be noted in the MSE wall specifications. Compaction is the process of mechanically densifying a soil or aggregate. Densification is accomplished by pressing the particles into closer contact while expelling air from the soil mass. Compaction of a soil or aggregate will increase its density and shear strength and reduce its permeability. These changes are all desirable, and compaction is the simplest and most effective way to improve a soil s or an aggregate s engineering properties. Moisture content is defined as the ratio of the weight of water to the weight of solids in a given soil mass, usually expressed as a percentage. The amount of moisture in a soil mass affects the ability to compact the soil. The moisture content of the soils used to construct the wall may vary considerably with weather conditions during construction. Drying or wetting of the soils may be necessary to achieve the recommended compaction criterion. If backfilling occurs during wet weather, these materials can likely not be dried sufficiently to obtain a satisfactory degree of compaction. As a practical consideration, these materials would generally be wasted and select materials trucked to the site. Any off-site materials should conform to the structural fill criteria discussed in the construction specifications. In-place density testing should be performed as verification that the recommended compaction criterion has been achieved. In-place density testing for retaining walls is typically performed on a will-call basis, with a testing frequency of one test for every 100 to 150 linear feet of in-place fill. Tests are performed on at least 2-foot vertical increments. Areas failing to achieve the recommended compaction criteria are reworked and retested prior to proceeding with subsequent phases of construction. Blaise J. Fitzpatrick, P.E. Page 28 11/1/2008

29 Internal settlement within the geosynthetic-reinforced zone may occur if improper soils, such as organic material or fine-grained soils with more than 50% passing the #200 sieve along with high liquid and plastic limits, are used to construct the MSE wall. Internal settlement can also occur if the reinforced soils are not compacted in strict accordance with the MSE wall Construction Specifications. The contractor must compact the reinforced soils in strict conformance with the compaction requirements outlined in project construction specification. The on-site geotechnical engineer must closely monitor contractor s fill operation to ensure that the maximum fill lift thickness does not exceed 8- inches. Not backfilling each course, contaminating drainage aggregate with fine grained soil. End of course. Blaise J. Fitzpatrick, P.E. Page 29 11/1/2008