FRST 557 Lecture 7b Cross Drainage Structures: Culverts and Bridges

Transcription

1 FRST 557 Lecture 7b Cross Drainage Structures: Culverts and Bridges Lesson Background and Overview: The last lesson introduced the topic of water management relative to forest roads and further explored the determination of water flow in channels. This and the next lesson will focus on the structures used to get the road over watercourses. Klinaklini River Lesson Objective: On completion of this lesson, you will know the basic types of structures used for cross drainage of water. You will also know some materials options available for the structures and some features of installed structures. Lesson References: BC Ministry of Forests. September Forest Road Engineering Guidebook. Forest Practices Code of British Columbia. Stream Culvert Discharge Design (This is no longer in print, but a copy is available in the T-Drive under FOPR 361 / Extra Information / FRE Guide Stream Discharge) BC Ministry of Forests. June Forest Road Engineering Guidebook. Forest Practices Code of British Columbia. Chapter 4. Road Drainage Construction BC Ministry of Forests Stream Crossing Guidebook for Fish Streams. Forest Practices Code of British Columbia. pp (Sections 3.1 and 3.2), pp (Section 5), pp (Appendix and Appendix 6) (This is no longer in print, but this information is available in the T-Drive under FOPR 361 / Extra Information / SC Guide Fish) BC Ministry of Forests. March Fish Stream Crossing Guidebook. Forest Practices Code of British Columbia. American Iron and Steel Institute Handbook of Steel Drainage & Highway Construction Products - Canadian Edition Nagy, M.M. et al Log Bridge Construction Handbook. Forest Engineering Research Institute of Canada, Vancouver

2 Types of Structures There are two basic structures used in forest operations to carry a road over a water body: Culverts are generally for smaller watercourses. Less than 10 m span Covered with the ballast (stabilizing soil) and usually a earth (gravel) surface road May be round, rectangular, or any shape between May have an open (natural) bottom or closed (culvert material) May be metal, plastic, wood, or cement Bridges are used for larger watercourses. Span is usually over 10 meters The running surface and superstructure is usually wood or cement Spans the natural watercourse with minimal disturbance. Construction materials may be logs, timbers, steel, cement, or a combination of these materials Culverts Culverts are cross drainage structures for smaller volumes of water. Frequently used to move accumulated ditch water across the road, they are also used for small creeks or streams. Culvert capacity any flow velocity calculations are often more critical than calculations for bridges because the capacity tends to be closer to actual flow than bridges. Water velocities in culverts can also present concerns for fish passage. This will be considered in Lesson 3. Some different culvert types are listed below with listed advantages and disadvantages: Culvert Construction Corrugated Metal Pipe (CMP) Corrugated Plastic Pipe Shapes Available Advantages Disadvantages Round Oval / Elliptical Arch (open bottom) Round Easily Installed Resist Deformation Fire Resistant Easily backfilled Available in custom shapes and lengths Easy to obtain Easy to transport Lighter than steel Cuts and shapes easily Resists corrosion Flexible - easier to use in flumes Easy to transport and install Concrete Any Shape Can be cast in place Low resistance to water flow Fire proof Long lasting Durable Heavy, require mechanical placement Difficult to cut or trim Can be damaged in placement Little structural resistance to deformation Not fire resistant Not available in custom shapes and lengths Vulnerable to puncture - need extra caution with backfilling Very heavy if pre-cast Labor intensive construction 2

3 Culvert Construction Shapes Available Advantages Disadvantages Wood (Log) rectangle Built with onsite materials Can increase size at minimal cost No inventory needed Less susceptible to silting - natural bed Ends don t get deformed or squashed Precision installation is not required Easily repaired Not damaged by course backfill Natural bottom Labor intensive construction Materials deteriorate Stream bed must support sills Top can collapse Not fire resistant May be difficult to grade over Culvert Installation Construction of Drainage Structures Horizontal Alignment Road C/L Dam Channel Change - Dam and Armor where needed Good Alignment Channel Maintained Poor Alignment Riprap Armor The horizontal alignment of an installed culvert needs to consider the inertia of moving water and the ability of water to erode an obstacle or restriction in its path. It is always the best practice to attempt to maintain an existing natural channel if the culvert is placed in such a location. If this is not possible, or if the culvert is diverting ditch flow, the change of direction should be minimized and erosion resistant rip-rap (e.g. course blasted rock) placed at the point of diversion. 3

4 Construction of Drainage Structures Elevation and Filling Camber in deep fills - anticipate settling Ground Line Culvert Grade is too steep - Sediment builds at the outfall end This may be designed for fish passage (embedment) Culvert is placed too deep Sediment builds through entire length When metal of plastic culverts are installed, consideration must be given to potential settling of the fill above and the bed below. To allow for this, a culvert can be installed with a camber. To allow for unrestricted water flow and to avoid erosion or sediment problems, the culvert bottom needs to be placed on the bed of the channel. Sometimes culverts are intentionally sunk into a steam bed ( embedded ) to allow for the reestablishment of a bed of natural materials that will slow water flow for fish passage. More information on culvert design and installation can be found in the Forest Road Engineering Guidebook : Bridges Although more expensive to build and maintain, bridges are required where culverts are not acceptable for any reason or where the size of a watercourse is too large for a culvert. In addition to designing for water passage, bridges require much more planning and design that do culverts. Included in the design is: Load analysis Vertical and horizontal alignment of the road approaching and exiting Although logs are still used for bridge construction, steel and concrete has become much more popular. Some bridge structures are designed to be movable. The following illustration labels components of a bridge and also identifies different possible materials for the structure. 4

5 Bridge Materials and Components Timber Risers and Guard Rail Timber ties and deck Pre-cast concrete deck Approach Guard ROAD Log Crib Sill Log or Timber Stringers Steel Beam Concrete Box Beam with integral deck Cap Steel Support Column Log or Steel Piling Pre-cast concrete backstop, crib, & spread footing Homathko River 5

6 Bridge site survey and design Site selection was discussed in the last lesson The site dictates the size and structure of the bridge Opening: Determine the height and breadth of the design flood. Make allowance for clearance of floating debris during floods (stumps, whole trees). This may be 1 to 3 meters depending on the size of timber upstream. Make greater allowance for fast moving streams. Make allowances if the bridge structure (including piers) will encroach on the flood channel. Foundation and footings: Soil or bedrock types will dictate the type of footings and the cost of site preparation Rock: abutment or sill placed directly on rock Gravel or till: requires a spread footing or crib Silt: requires piling Most soils have the potential of scouring around abutments or piers therefore appropriate control needs to be part of the design. Sub-structure: The substructure fills the space between the foundation and the superstructure (the span). Possible materials include: Log cribbing Concrete (precast or cast in place) Metal ( Bin wall or sheet piling) Superstructure or span Includes the beams (also called stringers or girders), the deck (or running surface), and guard rails In designing the bridge span, consideration must include: Length of span Life Vehicle types and load frequency Cost Stringer types Log Good for shorter spans Short useful life May be low cost if on site Limited strength Steel, Concrete, fabricated wood Longer life, stronger Must be designed by a professional engineer Deck The selection of materials between wood or concrete is a function of cost, convenience, and unique conditions of the bridge site Guard rails Wood or steel are usually chosen as materials Guard rails must be designed with repairs expected. 6

7 Bridge Approaches Both horizontal and vertical alignment is important for bridge approaches. A straight and level approach will avoid the hazards of: Unplanned forces applied to the bridge Vehicles having control problems Trailers not tracking straight onto the bridge deck Gravel and water accumulating on the deck Horizontal Alignment of Road NO YES Curve on the bridge approach does not allow for trailer to track behind tractor. Vehicles are still turning which adds centrifugal (horizontal) loads on the bridge. Tangent should be sufficient to allow trailer to resume its alignment with the tractor. No unplanned horizontal shear forces on structure Vertical Alignment of Road NO Impact loading on bridge due to grade change. Problems with water and gravel accumulating on the bridge deck and sediments directly entering the creek. YES Grade onto the bridge is the same as the bridge deck to minimize undesirable forces. Approaches climb (2%) to minimize water and gravel problems 7

8 In Conclusion: Bridges and culverts are perhaps the most significant points when designing a road. They bring together: Vertical and horizontal alignment Design for water passage Foundation design Structural design When considering the design of the structure and approaches, the environmental impact of the structure is as critical as its physical capacities. Critical Thinking: 1. A 14-meter bridge is required over a fish stream. There is good quality Douglas fir and western red cedar timber immediately adjacent to the site which could be used to construct a cedar crib / log stringer bridge. A portable sawmill is also available which could cut all necessary lumber for the ties, deck, and guardrails. In point form, what factors might dictate choosing to purchase and install a steel girder bridge with precast cement footings, retaining walls, and deck? Think beyond this lesson! 2. What factor(s) might change the decision in favor of the on-site materials? 1. Possible Factors: Need for a structure that will last longer than untreated wood Value of the logs plus the incremental cost of constructing a (labor intensive) log bridge is greater than the cost of purchasing, transporting, and constructing the fabricated bridge 2. Possible Factors: An isolated location where the incremental freight cost of the fabricated materials are excessive Isolation physically prevents expedient access 8