Grade Stabilization Structure, Problem Statement - Project | BAE 4012, Study Guides, Projects, Research of Engineering

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Brian Dillard Rachel Oller Ryan Stricklin Mary Womack BAE 4012-Senior Design December 9, 2005 ii Table of Contents Mission Statement ..........................................................................................................1 Introduction .....................................................................................................................2 Problem Statement .........................................................................................................4 Current Design Specifications for Canopy and Sliced Inlets .....................................5 Research & Literature Review .......................................................................................6 Patent Search and Pipe Flow Research....................................................................6 Structural Analysis of Corrugated Metal Pipe..........................................................7 NRCS Current Guidelines for Designing Pipes and Spillways in Structures......12 Statement of Work ........................................................................................................13 Initial Investigation........................................................................................................16 Field Tour of Installation Sites.................................................................................16 Demonstration Flume ...............................................................................................17 Preliminary Calculations ..........................................................................................23 Proposed Budget ..........................................................................................................27 Conclusion.....................................................................................................................28 References.....................................................................................................................29 Appendices....................................................................................................................30 3 Figure 1. – Grade Stabilization Structure (Steichen, 1993) Blaisdale (1952) explains that the pipe may be designed for full or partially full flow and can be implemented either on a flat or steep slope. The outlet from the pipe can freely flow into the atmosphere or be submerged. In the 1950’s, various entrances for drop inlet GSSs were tested. It was found that canopy and sliced entrance structures were more effective in producing full pipe flow at lower heads than the conventional blunt entrance. (See Figures 2 and 3 for examples of sliced and canopy hood inlets in the field.) A pipe experiencing full pipe flow moves a greater volume of water in a shorter period of time than pipes flowing partially full. This is important because it reduces initial storage volume, exposure of water to soil, and is more cost effective. It was also found that vortex formation around the sliced and canopy entrances was less of a concern. (Stoner, 2000) Since the 1950’s studies, these structures have been widely implemented. 4 Figure 2. – Sliced Inlet Figure 3. – Canopy Inlet Problem Statement Since the 1980’s, Oklahoma has implemented many canopy and sliced inlet GSSs to control high runoff volumes over rural land. Though proven to be very useful over the years, an increasing number of failures of the inlets have occurred. In a report, Chris Stoner outlined the first noticed collapse on a sliced inlet. The entrance of a 42” corrugated metal pipe had failed the first time it flowed. The left side had folded inward, creating a 40% blockage of flow. Since that time, other failures have been noticed and reported. These occurrences were typical of 48” diameter or greater pipes with a 16 gauge thickness. In 1995, the NRCS recommended the use of canopy inlets instead of sliced hoods, because the canopy added extra strength to the structure. In 1997, the inlet thickness was increased to 14 gauge for pipes with diameters greater than 42”. The report by Stoner also details characteristics of the failures, which interestingly enough are all similar. Always occurring on the left side looking downstream, the pipe folded inward, consequently blocking the flow and limiting the capacity for which it was designed. Because the time of failure is difficult to determine, the magnitude of head causing the collapse is also difficult to determine. 5 NRCS is seeking an analysis of the canopy inlet to establish criteria for providing increased strength for corrugated metal canopy inlets, including: • Determining design parameters that govern the need for increased strength; • Identifying pipe sizes, corrugations, and gauges that need increased strength; • Proposing changes to the Oklahoma NRCS Conservation Practice Standards to reflect the analysis. The NRCS also requests alternative methods for strengthening and a cost comparison of options. Current Design Specifications for Canopy and Sliced Inlets The NRCS has published specifications for the dimensions of canopy inlets. For conduits with slopes less than 15%, the following equation applies: 0.75D. L 0.2D; W == (1) For conduits with slopes greater than 15%: 1.25D L 0.3D; W == (2) where: W = height of the canopy (ft) L = length of the sliced section (ft) D = diameter of the pipe (ft). The auxiliary spillway elevation must be at least 1.8D above the bottom of the pipe. The riser on the drop inlet must be at least 5D if the conduit slope is greater than the friction slope, or at least 2D if the conduit slope is less than or equal to the friction slope. The thickness of the pipe is determined based on the fill height of the grade stabilization 8 1) Dead loads- Developed by the embankment or trench backfill, plus stationary superimposed surface loads, uniform or concentrated. 2) Live Loads- Moving loads, including impacts (AISI, 1994). Spangler (1941) says that since CMP has relatively little inherent strength, a significant source in its ability to support load, is the passive pressures induced when the sidewalls begin to move outwards. Because CMP will readily deform, it consequently utilizes the passive pressures of the earth on all sides of the pipe. According to Spangler, this structural characteristic accounts for the fact that such a relatively lightweight pipe will support earth fill of considerable heights. It is assumed that loads are distributed uniformly over the top and bottom of the pipe. Loads which are caused by passive pressures of the earth are said to be greater toward the center of the pipe and can be seen in Figure 7. Figure 7. – CMP load distributions 9 The above figure does not necessarily give the characteristics of CMP under hydrostatic conditions, but it does give an idea on how CMP reacts under load stresses. Chris Stoner feels that Vortex could use the data provided from buried conduits in calculating the hydrostatic forces on CMP. With further research and modeling techniques, Vortex will determine whether or not this method will represent the forces of water acting on CMP. The following equations illustrate the load distributions that are applied to CMP under soil compression. Vertical unit load on the pipe ,v, (Spangler, 1941): r W v C 2 = (3) where: Wc = distributed load across top portion of pipe r = radius of the pipe. Vertical unit reaction on the bottom of the pipe ,v’, (Spangler, 1941): )sin( ' α v v = (4) where: α = bedding angle with respect to the vertical axis. Passive horizontal pressures on the side of the pipe, h, (Spangler,1941): 2 x eh ∆= (5) 10 where: e = modulus of passive pressure of side fill ∆x = horizontal deflection of the pipe. CMP not buried in compacted soil and subjected to external hydrostatic pressure must be designed for buckling as circular tubes under uniform external pressures (AISI, 1994). If the above method cannot be used, the two formulas below can be used to determine the critical pressure on the surface of the pipe. Critical pressure, Pcr, of a corrugated metal pipe (AISI, 1994): 32 )1( 3 R EI Pcr µ− = (6) where: E = modulus of elasticity (lb/in2) Ipw = pipe wall moment of inertia (in 4) µ = Poisson’s ratio (specific to material) R = mean pipe radius (in). The equation below calculates the estimated collapse pressure, PE, of corrugated metal pipe (AISI, 1994): 3 6 )105.49( R I PE ××= (7) Chapter 52 of the NRCS National Engineering Handbook (NEH) (2005) details basic properties for any type of flexible conduit. It includes corrugated metal pipe and also gives methods to calculate pressures and stresses on a pipe. 13 Figure 8 – Discharge vs. Head for determination of inlet structure Section 11 of the National Engineering Handbook titled Drop Spillways, details information about the design of various types of drop spillways. Technical Release 3 from NRCS documents, titled “Hood Inlets for Culvert Spillways” summarizes original research preformed on hooded inlets, including dimensions and capacities. Statement of Work The NRCS has contacted Vortex Engineers to evaluate the problems associated with sliced and canopy entrances, determine the cause of collapse, and propose design solutions to prevent future failures. The following is a brief summary of work that has been conducted and what is planned in the future. 14 First a field tour in western Oklahoma of these structures was taken to help the team visualize these structures in the field. Next, at the United States Department of Agriculture, Agriculture Research Service Hydraulics Lab in Stillwater, Oklahoma, a demonstration flume that utilizes scaled Plexiglas replicas of the inlet structures was observed. The demonstration model allowed Vortex Engineers to observe flow characteristics of the pipe with different inlet structures attached (see Figure 4). Vortex Engineers performed another demonstration using red transparency film to test if failures will occur under similar conditions, but of a different material (see Figure 5). In the model, the same failures occurred with red film as those reported from the field: after a certain head was reached, the left side of the film folded inwards. It is worth noting that though this model was helpful in demonstrating the process of how the pipe entrances failed, the model was not to scale, nor were the materials similar to that of corrugated metal pipe. Therefore, the model did not produce any measurable or useful data, but was useful in illustrating the phenomenon. 15 Figure 4. – Blunt inlet Figure 5. – Red transparency film In aid in determining where failures are occurring, Vortex has performed preliminary calculations for pressure and head analysis. These calculations will identify pressure distributions which are needed to determine forces throughout a pipe. A scale model will be built of material similar to that of corrugated pipe, possibly plastic corrugated tubing. The scale models will aid in observing what the entrances are experiencing under different flows. Vortex Engineers also hopes to gain actual pressure and velocity measurements from the scale models. In addition to scale modeling, computer modeling will be conducted. Vortex Engineers will model pressure and perform fluid flow analyses. After all modeling data is collected, strength analysis will be performed. Upon completion of these tasks, Vortex can determine whether the current designs are sufficient as specified or if they need to be modified. 18 differed through the various inlets. Figure 11 – Canopy pipe inlet replica The demonstration flume also is capable of adjusting pipe slope and water head. Following the observation of flow in the pipe models, the team marked each of the three inlet replicas with lines every 0.2 inches along the perimeter of the inlet with a permanent pen (see Figure 12). After drawing the lines on the inlet models, the team began taking pressure measurements at each increment marked around the inlet. Figure 12 – Pressure measurement layout To obtain these pressure measurements Vortex Engineers used a simple manometer constructed of a small clear plastic tube open on one end and an air pump needle attached to the opposite end. Each pressure measurement was calculated by how 19 much the water level in the manometer changed in relation to the water level at the inlet. Measurements were taken as the needle was moved each 0.2 inch increment around the circumference of the pipe. This process was preformed twice for each inlet model. In Figures 13-15 these distributions are graphically displayed. The x-axis represents the circumference of the inlet where the center is located at the lower x = 0.0 cm, and negative x represents the left side of the pipe and positive x, the right side. The most left and right boundaries of the pipe occur approximately at -0.8 cm and 0.8 cm respectively. The values at each increment show the pressure changes in terms of centimeters of water within the pipe as a function of location. All pressures are negative because the water level of each measurement dropped below the reference water level. The reference water level was taken at the surface of the impounded head on the inlet. The graphs and their discussion follow. 20 The pressure data for the canopy inlet is in Figure 13. -25 -20 -15 -10 -5 0 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Distance from vertical centerline (cm) P re ss u re ( cm w at er ) Figure 13 – Pressure distribution around circumference of canopy inlet The lowest negative pressures of -18.8 cm occurs between -0.4 and -0.6 cm and -19.5 cm on the left side at -0.6 cm. There is quite a bit of pressure distribution along the inside of the pipe, which leads Vortex Engineers to believe this could lead to unstable conditions. 23 Figure 16 – Demonstration model for blunt inlet This phenomenon could possibly be the cause of the pressure differences on each side of the inlet. The pressure measurements using this method are not considered to be hard and accurate data, and will not be treated as such by the design team. However, these data do give Vortex Engineers an idea of what the different inlets are experiencing as far as pressure is concerned. It also allows Vortex to get an idea of what forces could be resulting from these pressures. Preliminary Calculations Initial fluid flow and pressure analysis calculations were an important part of the initial investigation. The team wanted to mathematically determine the pressure gradient that existed throughout the pipe using several fluid mechanics equations. First, the flow rate through a pipe needs to be calculated by either the pipe flow or weir flow equation. At a given head, flow is calculated by both equations and the minimum of 24 the two determines the flow rate. For weir flow: 2 3 CHgQw = (12) where: g = gravity (32.2 ft/s2) C = the circumference of a circle (ft) H = head (ft), measured from the invert of the riser. The equation of pipe flow is: 2 1 2 1 )1( )'2( Lkkk gHA Q cbc P +++ = (13) where: A = area of the pipe (ft2), H’ = head (ft) measured from the top of the water surface to a point 0.6D above the outlet of the barrel L = length of the pipe (ft) ke = entrance loss coefficient kb = bend loss coefficient kc = loss coefficient due to friction of the pipe. All k values are dimensionless. To determine the total head losses throughout the pipe the Darcy-Weisbach equation was used: g V K D fL hl 2 2       += ∑ (14) where: ƒ = friction factor D = internal diameter (ft) ΣK = sum of minor losses 25 V = velocity (ft/s). After calculating the head losses, the internal pressures along the pipe were calculated by the following equation: γlhP = (15) where: P = pressure (lb/ft2) hl = head loss (ft) γ = specific weight of water (62.4 lb/ft3). Once the pressures were calculated throughout the length of pipe the hydraulic grade line was determined. CLPzhHGL l ++= (16) where: z = elevation along the pipe and CLP = centerline dimension of the pipe. Along with calculating the HGL, the Energy Grade Line (EGL) of the pipe system was calculated as well by: g V HGLEGL 2 2 += (17) 28 analysis is not possible at this point in time, as the dimensions and costs to redesign the existing flume have yet to be determined. When the team begins scale modeling, there is a possibility of using corrugated metal pipe to model the inlet structure; costs of this material and the inlet construction will be determined if needed. Conclusion This semester Vortex Engineers have focused on finding the cause of the pipe inlet failures. Investigation is ongoing into the forces the pipe experiences during various flow conditions. In the spring semester Vortex plans to perform tests using scale models to determine why these pipes are failing and what type of reinforcement might be needed. Future tasks that need to be completed can be found in Appendix A. Vortex will then use this data to recommend new design solutions for the inlet structures if applicable, as well as provide a cost analysis of the options. 29 References American Iron and Steel Institute 1994. Handbook of Steel Drainage & Highway Construction Products. Washington D.C.: AISI. Blasidell, F. Hydraulics of closed conduit spillways: Part I. Technical Paper No. 12, Series B. St. Anthony Falls Hydraulic Laborotory. United Stated Department of Agriculture. 1952. Haan, T., B. Barfield, J. Hayes, Design Hydrology and Sedimentology for Small Catchments. Academic Press, San Diego, CA. 1994. Kitoh, Osami. 1991. “Experimental study of turbulent swirling flow in a straight pipe”. Journal of Fluid Mechanics. (225):445-479. Spangler, M.G. December 24,1941. The Structural Design of Pipe Culverts. Bulletin 153. Ames, Iowa: Iowa State College of Agriculture and Mechanic Arts. Stoner, C. December 15, 2000. Memo: Research Needs –Canopy Inlet Drop Structures to Johnny Green, State Conservation Engineer, NRCS. USDA – NRCS. USDA-NRCS. 1984. National Engineering Field Handbook, Ch. 6: Structures. USDA Natural Resources Conservation Service. USDA-NRCS. 1968. National Engineering Handbook, Section 11: Drop Spillways. USDA Natural Resources Conservation Service. USDA-NRCS. 2005. National Engineering Handbook, Ch. 52: Structural Design of Flexible Conduits. USDA Natural Resources Conservation Service. USDA-NRCS. 1956. Technical Release 3: Hood Inlets for Culvert Spillways. USDA Natural Resources Conservation Service. 30 Appendices A. Task List B. Gantt Chart C. NRCS Standards and Specifications 1. Grade Stabilization Structure Conservation Practice Standard 2. Pond Conservation Practice Standard 3. Engineering Field Handbook, Chapter 6 – Structures 4. OK-Dwg-205 – Canopy Drop Inlet 5. OK-Dwg-203 – Hooded Drop Inlet D. Patent – Internally Reinforced Extruded Plastic Pipe