Small Structure Design 101 Purdue University Road School 2011
Small Structure Design 101 Presenters: Mark Williams, PE INDOT Design, Vincennes District Richard Thomas, PE INDOT Design, Vincennes District
Site Criteria • A culvert should be used • Where a bridge is not hydraulically required. • Where debris and ice are tolerable. • Where it is more economical than a bridge.
Site Criteria (Cont.) • Length and Slope • Approximate existing topography. • Invert should be aligned with channel bottom and skew angle of stream. • Clear-zone and embankment geometry may dictate length.
Site Criteria (Cont.) • Location in Plan • Place normal to roadway if no defined channel. • Minimize channel relocation and erosion. • Locate utilities before final location is chosen. • Location in Profile • Approximate natural stream profile.
Hydraulic Design Criteria • Allowable Headwater (AHW) • Roadway Serviceability • Maximum Velocity • Minimum Velocity
Allowable Headwater (AHW) • New Alignment • 1.5 in. Maximum Backwater. • Exceptions • Backwater Dissipates to 1.5 in. or less at R/W. • Backwater is Contained in the Channel.
Allowable Headwater (Cont.) • Existing Conditions • Proposed Surcharge ≤ Existing Surcharge. • If Existing Surcharge > 1 ft., Proposed must not be > 1 ft. above the Natural Channel Flood Profile.
Allowable Headwater (Cont.) • Right of Way • The Ponding Limit Cannot Exceed the R/W for New Alignment. • Upstream Channel • The Ponding Limit Cannot Exceed the Banks of Upstream Channel for New Alignment.
Allowable Headwater (Cont.) • Other Constraints • Grades of Adjacent Drives. • Finished Floor Elevations of Buildings. • Elevation of Existing Cropland or Other Property.
Roadway Serviceability • For Q100 Design Storm • Headwater must be at least 2 ft below edge of pavement • For Design Storm less than Q100 • Headwater cannot exceed edge of pavement
Maximum Velocity Vo = Outlet Velocity
Minimum Velocity • Typical Minimum Outlet Velocity is 3 ft/s.
Culvert Sizing Process • Priority System • Interior Designation • Minimum Culvert Size • Cover
Priority System • Trial 1. Single Circular Pipe. • Trial 2. Single Deformed Pipe. • Trial 3. Single Specialty Structure. • Trial 4. Multiple Circular Pipes. • Trial 5. Multiple Deformed Pipes. • Trial 6. Multiple Specialty Structures.
Interior Designation • Run Hydraulic Design for both Smooth Interior and Corrugated Interior. • Four Possible Situations.
Interior Designation (Cont.) • Situation 1. Required Smooth and Corrugated Sizes are Identical. • Situation 2. Required Smooth and Corrugated Sizes are Different. • Situation 3. Acceptable Size for one but not the other. • Situation 4. No Acceptable Size.
Pipe Material Selection • Computer Program (INDOT Website) • Information Required • Pipe Classification • Pipe Interior Designation (Smooth/Corrugated) • Pipe Criteria (Size, Cover, Slope) • Pipe Service Life • Abrasive or Non-Abrasive Site • Structure pH
Pipe Classification • Type 1. Under Mainline or Public Road. • Type 2. Storm Drain. • Type 3. Under Drive or Field Entrance. • Type 4. Underdrain or Drain Tile. • Type 5. Broken Back or other which requires coupled pipe.
Pipe Service Life • Based on Functional Classification of Mainline Roadway • 75 Years- Freeway, Expressway, Arterial • 50 Years- Collector or Local Road
Abrasive or Non-Abrasive • Abrasive • Mainline Culvert • Public Road or Drive Culvert Installed in a Natural Channel. • Non-Abrasive • Storm-Drain, Public Road, or Drive Culvert on a Constructed Side Ditch Line.
Structure pH • Maximum pH. Cannot Exceed the Map pH (Fig. 28-6A IDM). • Lack of Sample. Use pH From Nearest Structure. If Not Available Within Project Limits Use pH Map Value. • Side Ditch Culvert. Use pH for the Most Downstream Culvert for Each Culvert in Ditch Line.
Plans • Plan & Profile Sheet. Appropriate Location for Drainage Structure Identification. • Detail Sheet. Detail Drainage Structures and Features that are not included in the Standard Drawings.
Plans (Cont.) • Structure Data Sheet. • Backfill Method, Structure Backfill (CYS), Type. • Flowable Backfill (CYS), Type. • Scour Protection Information • Sump Depth (IN), Geotextile (SYS), • Riprap (Type & Tons) • Video Inspection Length. • Pipe Material Sheet. List Acceptable Pipe Materials for each Pipe Structure.
Structure Backfill • Type 1 • Longitudinal or Transverse Structure Within 5 ft. of the back of paved Shoulder or back of Sidewalk of a New Facility. • Structure for an Existing Facility Where all Existing Pavement is to be Replaced.
Structure Backfill (Cont.) • Type 2 • Longitudinal or Transverse Structure Within 5 ft. of the back of paved Shoulder or back of Sidewalk Where Undisturbed Existing Pavement is to Remain. • Precast Concrete 3 Sided or 4 Sided Structure with Cover of 2 ft or Greater.
Structure Backfill (Cont.) • Type 3 • Behind Mechanically Stabilized Earth Retaining Walls. • Type 4 • Trench Where Utility Line is Present. • Behind Reinforced Concrete Slab Bridge End Bent.
Structure Backfill (Cont.) • Type 5 • Precast Concrete 3 Sided or 4 Sided Structure with Cover Less than 2 ft. • Filling Voids in an Underground Facility. • Filling an Abandoned Pipe or Structure. • Other Application that does not Require Excavation.
Backfill Methods • Method 1 • Under New or Replacement Mainline or Public Road. • Under Median Embankment. • New Structure Under Existing Mainline or Public Road.
Backfill Methods (Cont.) • Method 2 • Placed Under a Drive in New or Replacement Work. • Method 3 • Under New or Replacement Median Trench.
Backfill Quantities • Measured by CYD Computed from Neat Line Limits. • Hand Calculation • IDM Chapter 17 • Standard Drawings Section 715-BKFL • Computer Program • www.in.gov/dot/div/contracts/standards/07Bkfl-qt.xls
Pavement Replacement • Longitudinal Pay Limits
Culvert Modifications • Culvert Slip Lining • Culvert Extension • Culvert End Treatment • Headwalls and Anchors
Culvert Slip Lining • Two Common Types • High Density Polyethylene Pipe (HDPE) • Cured in Place (CIPP)
Culvert Slip Lining (Cont.) • Review Pipe Inspection Report • Structures with an overall rating of 2 to 5 should be considered for lining. • Overall ratings of 0 or 1 are normally structures that need to be replaced. • Field Review • Pictures • Measurements
Culvert Slip Lining (Cont.) • Factors to Consider • Structure barrel should be relatively straight and not significantly deformed. • Existing backfill free from large voids. • Should be sufficient room to work from at least one end of the existing structure. • In a location where road closure is undesirable or impractical. • Hydraulic capacity of liner. • Liner cost vs. replacement cost
Lining Design Criteria • Time of Concentration < 1 Hour • May increase backwater over existing if: • Headwater maintains road serviceability. • Headwater does not reach fixed private property structures. • Time of Concentration > 1 Hour • May not increase backwater, unless contained within channel banks or R/W. • Check Outlet Velocity
Lining Design Procedure • HDPE Liners • Manning’s n=0.012. • Use maximum pipe liner size from tables in Standard Specification Section 725. • Use largest possible liner, even if smaller liner is hydraulically adequate.
Lining Design Procedure (Cont.) • CIPP Liners • Only considered if HDPE will not work (cost). • Manning’s n=0.012. • Structures < 96” diameter or equivalent. • Will reduce existing structure size as follows: • Dia. ≤ 24”, reduce by 1” • Dia. from 27” to 48”, reduce by 2” • Dia. from 54” to 72”, reduce by 3” • Dia. from 78” to 96”, reduce by 4”
Culvert Extensions • Match Existing Pipe Size and Interior Designation. • Perform Appropriate Hydraulic Analysis.
Culvert End Treatment • Projecting • Extends beyond the roadway embankment. • Can be damaged during maintenance. • Low construction cost. • Poor hydraulic efficiency. • Anchor should be used for 42” or larger diameter. • Clear zone requirements.
Culvert End Treatment (Cont.) • Mitered • Hydraulically more efficient than projecting. • Mitered to match fill slope. • Pipe End Section • For corrugated metal or concrete pipe. • Retards embankment erosion. • May improve hydraulic efficiency.
Culvert End Treatment (Cont.) • Wingwall • Retain roadway embankment • Used where side slopes of channel are unstable. • Best hydraulic efficiency if flare angle is between 30° and 60°. • Should be used on precast concrete drainage structures.
Culvert End Treatment (Cont.) • Apron • Reduce scour • Should extend at least one pipe diameter. • Should not protrude above normal streambed elevation. • May be constructed of riprap and geotextile or concrete.
Culvert End Treatment (Cont.) • Cutoff Wall • Prevent piping along culvert barrel. • Should be used for culverts with headwalls. • Should be minimum of 20” depth.
Specialty Structures • Precast Concrete Box Culvert • Maximum span is 12 ft. • The recommended layout is to extend the box to the point where the roadway sideslope intercepts stream flowline. • End of box culvert should be protected with guardrail or located beyond the clear zone.