NCMA SRW Design & Construction Best Practices Guide. Scott Vollmer, PE SRW Market Manager Oldcastle Architectural

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1 NCMA SRW Design & Construction Best Practices Guide Scott Vollmer, PE SRW Market Manager Oldcastle Architectural

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3 First What s an SRW? Segmental Retaining Wall (SRW) - Earth retention wall comprised of drystacked concrete block units and compacted fill. Often includes geosynthetic reinforcement.

4 Why do we need Best Practices for SRWs? With success can come problems

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7 NCMA Solution What significant issues need to be addressed? What can the industry doing to address these issues?

8 NCMA Strategic Plan for SRWs Quality of Units Quality of Walls Industry Communication 8

9 Quality of Walls Program Zero failures initiative & philosophy Best Practices Guide Education for Specifiers Wall design engineers Contractors Inspectors

10 Building a Path to Zero Wall Failures What s the focus? Who assumes overall design responsibility? What are appropriate design guidelines? What are the appropriate materials? What should be included for a typical wall design? Water Management Tall Wall Considerations Detailing What are keys for construction & inspections

11 Best Practices Guide -Contents Roles & Responsibilities Design Materials Construction Inspection Special situations tiered walls, etc.

12 Roles & Responsibilities Owner or Owner s Rep Hire the Wall Design Engineer is best Establish minimum criteria Hire the Geotechnical engineer and make sure the report covers the retaining wall area Make sure global stability and foundation is assessed Hire the QA Inspector

13 Roles & Responsibilities Wall Design Internal Design Engineer Sliding & Overturning Global Stability do or coord. w/ Geotech Foundation/Bearing coord. w/ Geotech Account for all design variables within design envelope

14 SRW Design Envelope Limits Example The wall designer should account for loads at the top of the wall from a fence, but does not design the fence

15 Roles & Responsibilities Civil Engineer Retaining wall location and site grading plan Verify selected wall fits within site Address all surface water movement around retaining wall Design scour prevention measures as needed for walls in water applications Consider utility conflicts with wall zone Address needs for handrails & fences

16 Roles & Responsibilities Geotechnical Engineer Include area where retaining walls are placed in evaluations Assess the foundation soil s ability to support the wall including settlement Provide soil design strength parameters for the site soils Ground Water levels Do or assist with global stability analysis Provide seismic design parameters

17 Design -Design Method

18 Design -Factors & Criteria

19 Resisting Force Driving Force Up to 90% of a reinforced wall is made up of soil

20 Compound and Global Stability Internal Compound Stability (ICS) A special case of Compound limits search to 2H or H+L Compound Stability Global Stability

21 Internal Compound

22 Internal Compound Stability This failure mode is possible and needs to be reviewed

23 Global Stability

24 Design -Global Stability

25 Best Practices -Special Design Better Backfill Geogrid Length, 0.6H Criteria 0.7H minimum when deviating from BPG Limit Reinforcement spacing 2 x SRW unit Depth and 24 Maximum Reduce spacing when using marginal backfill Better drainage Increase depth of facing aggregate Toe embedment

26 Best Practice -Geogrid Spacing The lessor of 2 times SRW unit depth or 24 - only when all criteria met When criteria not met such as using soils that are marginal for wall H, tighten spacing to 16 maximum

27 Best Practice Gravel at Face Greater of 24 from face or 12 from back Increase when using marginal soils or tall walls, etc.

28 Best Practice - Embedment Added Stipulation 4 Bench The top of the leveling pad (bottom of wall) should not be above the ground surface elevation 4 feet in front of the wall

29 Best Practice -Materials Selection

30 SRW Components SRW Unit (Modular Block) Geogrid Reinforcement Leveling Pad Gravel Fill at face Reinforced Soil Fill Drainage Pipe 30

31 Materials SRW Facing Units There are over 20 different styles of face designs: 8 inch / 6 inch / 4 inch tall units Front Lip Units Rear Lip Units Pin Connections Core Locking Units Hollow Units Solid Unit 1 ft Deep Units 2 ft Deep Units Modular Units (multiple depth assemblies) 31

32 Best Practice -SRW units ASTM C1372 except for: Height tolerances: ±1/16 in. Durability based on Weather Zone and De-icing Salt Exposure

33 SRW Durability Freeze-Thaw

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35 Best Practice -Reinforcement Can be a complex subject, but this makes it easy Use a geosynthetic reinforcement with a National Transportation Product Evaluation Program (NTPEP) report ( Great resource is the NCDOT Geogrid Evaluation Program

36 NCDOT Geogrid Evaluation Program

37 Best Practice Foundation Soils Geotechnical evaluation for: Support of Retaining Wall i.e. bearing, settlement, repair Design parameters for Global Stability

38 Best Practice Reinforced Soil One of the goals of the document is to tighten the requirements Limit use of marginal materials the current NCMA Design Manual allows to smaller, less consequential walls (generally less than 10 tall)

39 Best Practice Reinforced Soil Normal Criteria <35% Fines & PI<20, LL<40 For walls < 10 only Fines limit waived (allows SC, ML & CL) with geotechnical acceptance For walls > 10 Normal Criteria, but tighten to PI<6 Compaction testing to include light compaction equipment zone (0 to 4 behind face)

40 Best Practice -Water Management It is necessary during and after construction both surface and subsurface Includes: Grading and swales Chimney drains Blanket drains Geocomposites

41 Surface Water above the wall

42 Surface water during construction

43 Water Management

44 Best Practice -Water Management Detail to make sure collected water drains

45 Best Practice -Water Management Filter fabric over gravel zone at top of wall Retards fines infiltration into gravel

46 Best Practices Special Applications Tall Walls Terraced Walls Seismic Water applications Top wall loadings

47 Best Practices for Tall Walls What s a tall wall? >10 Tall >20 Very Tall Stresses increase at the square of the H Compressibility of properly compacted backfill and foundation soil can be a concern

48 Best Practice Tall Walls Taller walls need better reinforced soil May need higher compaction standards 95% Modified Proctor, or 98% Standard Proctor More intense inspection & testing Global stability analysis

49 Best Practices -Tall Walls -Soils Better Backfill For walls over 10 use soil with <35% fines and PI < 6 For walls over 20 use well graded soil with <15% fines and PI < 6

50 Best Practices Tall Walls Gravel Depth Limits differential settlement between face units and backfill Increase gravel fill from 24 back from the face to 36 for the portions of the wall over 10 tall

51 Best Practice -Terraced Walls Global stability will govern Fill supports upper walls so good compaction a must

52 Best Practice -Seismic Extend top two geogrid layers Tighten reinforcement spacing 16 maximum

53 Best Practices Water Applications Free draining fill material up to high water Geotextile filter Scour toe protection

54 Best Practices Top of Wall Install sleeves for fence posts during wall construction eliminates need to auger into reinforced zone

55 Best Practices Top of Wall Consider adding reinforcement to slopes steeper than 3:1 above the wall

56 Best Practices Top of Wall Extend length of top geogrid layer(s) with level back slopes to avoid getting cracks in the soil immediately above end of geogrid

57 Construction Best Practices

58 Typical Wall Construction Considerations Have you established allowable infill soil & verified intended soil meets criteria? How are soils and base verified? Minimum grid lengths, spacing and locations Minimum compaction levels and locations Capping requirements

59 Best Practice -Sequence Start at face and work progressively back Keep reinforcement taught & level One course at a time Lifts 8 Compact and test entire backfill zone

60 Best Practice -Sequence

61 Best Practice Geogrid Placement First layer no more than two courses up from the base Top layer no more than two courses and a cap down from the top of wall Keep geogrid taught and level

62 Geogrid Levelness & Spacing

63 Best Practice -Inspection QA is not the same as QC by contractor QA should be done by testing firm working for the owner so there is no conflict of interest Includes Verification testing of soils Verification of compaction throughout entire reinforced zone Documents proper geogrid placement & length

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65 Questions Thank you for your time! 65