Drainage and Slope Protection Drainage – is is defined as the means of collecting, transporting and disposing of surface water originating in or near the right of way, or flowing in stream crossings or boarding the right of way. Surface drainage problems follow three basic considerations: 1. Hydraulic design that deals with estimating the highest rate of run-off to be handled 2. Hydraulic design deals with the selection of the kinds and sizes of the drainage facilities that is most economical to accommodate the estimated water flow. 3. Erosion Control is to ascertain the design will not create erosion or other unacceptable conditions. Hydrology is that branch of physical geography that deals with water of the earth. The branch of hydrology that concern highway engineers are: 1. The frequency and intensity of precipitation 2. The frequencies that this precipitation brings the highest run-off which are equal or exceeded critical values 3. The distribution of precipitation throughout the seasons that influences water behavior affecting the highway surfaces 4. The prediction regarding future rainfalls or run-off from gathered statistical approaches, formula or simulated methods based on the laws of probability. Relationship and Effect of Hydraulics and Construction 1. Usually, highway construction disrupt existing natural drainage pattern. Water passing the natural right of ways is often intercepted by road cuts. 2. Construction operations may disturb the ground cover, and loosen the soil creating muddy stream as a result of erosion. 3. Erosion creates debris that are carried downstream, and deposited at points where the velocity slackens. 4. Any changes in the land use may alter the historical run-off or un-gauged rural water shed that could be disastrous on wide scale basis. Cardinal Rules on Drainage Design 1. Any existing drainage system patterns and soil cover should not be disturbed 2. Necessary Necessary changes changes in the drainage drainage patterns patterns should not not in any manner manner bring velocities velocities that may may create new erosion problems. Drainage Economic Considerations The term economy in drainage system means finding the solution to a problem that is cheapest in the long run under the following conditions: 1. Determine the estimated initial investment cost 2. Consider the maintenance cost or outlay 3. Consider anticipated loss and damage for each solution Draining the Highway 1. Water flowing across the roadway down the road slope must be thin enough in a sheet from like so that the erosion on the shoulder will be minimized. 2. Unprotected slope could be badly washed out if irregularity in the pavement or shoulder accumulates water into small streams. Another point of water accumulation that endangers the shoulder is the low portion of sagged vertical curves. 3. One way of preventing washout of the side slope is to retain the water at the outer edge of the shoulder. 4. A crown ditch or intercepting channel may be employed at the top of the cut slope. This will prevent erosion of the cut slope by surface runoff from the hillside above. 5. The surface should not be disturbed in any manner where the natural ground is already covered with grass or other ground cover. 6. The channel or canal should be formed making the original ground as its bed and a small dike of topsoil or imported materials to serve as its bank. 7. Scouring must be controlled. If possible, the water flow must be limited to a sager amount by diverting the flow at intervals. Water must be dropped down to the roadside ditch by means of specially designed channels. Draining the Urban Street 1. On city streets, rain water is guided by the road slope to the gutters and along with them the curb and inlets designed to limit the spread of water over the traffic lanes.
2. AASHTO recommended that for a grade materials water should not encroach on the outer lane by more than 1.80 meters for a 10 years flood. 3. The decision for water storm inlets and connections for gutters and depressed median should be based on the same periods. 4. For arterials, the return interval is 20 to 50 years. The water flow estimate was based on the Rational Formula using the value of runoff coefficient. Manhole, Inlets and Catch Basin 1. Manhole, inlets and catch basins are classified under Item 502 of the DPWH Standard specifications which includes construction, reconstruction or adjustment. 2. Sewer and manhole made of bricks from clay or shale must conform to AASHTO M-91 standard specifications. 3. Joint mortar mixture should be one part Portland cement and two parts fine aggregate (gravel) plus hydrated lime equal to 10% of the cement by weight. 4. Frames, gratings, covers and ladder rungs, should be assembled before shipment and may be re-assembled readily in the same position when installed.
Channel 1. The purpose in designing a channel is to determine the cross section of the canal that will accommodate water flow smoothly and cheapest to construct and maintain. 2. Side Slope with a ratio of 2:1 or even flatter is most acceptable, except on rock or other hard materials where channels are lined. 3. The design of crown ditches, gutters, stream channels, and culverts flowing partially or fully, are based on the principles of low in an open canal. 4. The condition to various channel problems rest on the following distinctions: a.) Water flowing down a mild slope in an open canal is in Sub-critical flow, which exists when the depth of water in the channel is greater than the critical depth. b.) Water flowing on steep slope is in super critical flow, which exists when the depth is less that the critical level. 5. The critical depth occurs when the velocity head is one half the average depth. The average depth or mean depth is the cross sectional area of the flow divided by its width at the liquid surface. Culvert 1. Culvert includes all closed conduits with standard designs repeatedly used, except for the storm drain. 2. Culvert is loaded vertically by the wheel load of vehicles, the earth fill covering the culvert; and the horizontal passive or active earth pressure. 3. The stress calculation for pipe culvert are based on the assumptions regarding the methods of under pipe support, soil support at its sides and loading distribution over it. 4. Corrugated metal pipe are more flexible that could tolerate greater deformations. 5. Highway engineers rarely make structural design for pipes and culverts because of the difficulties and uncertainties involved. 6. For concrete, vitrified clay and cast iron pipe, what is specified is the strength or class of pipe to be used in each condition. 7. It is now the practice of relying much on the recommendations of various manufacturers who prepared all the design data. 8. Highway agency has standard drawing of various culvert design appropriate for the more common heights and widths of openings, heights of fill, including skew angles. 9. Under Item 500 of the DPWH standard specifications, the construction or reconstruction of pipe culverts and storm drains are classified as Conduits.
Conduits 1. Conduits are laid on bedding or footing that conforms to the standard specifications. Conduit bedding is classified into Class A, B and C. 2. In laying culvert pipes, the requirement of Class C bedding is applied if no bedding class is specified. 3. Class A bedding shall consist of a continuous concrete cradle confirming with the plan detail. 4. Class B bedding is extended to a depth of not less than 30% of the vertical outside diameter of the conduit. 5. The minimum thickness of the bedding materials beneath the pipe is 10 centimeters. 6. The layer of bedding material is arranged and shaped to fit in the conduit for at least 15% of its total height. 7. When a bell and spigot type is used, the recesses in the bottom trench are shaped to accommodate the bell. 8. Class C bedding should be extended to s depth not less than 10% of its total height. 9. For flexible pipes, the bed is roughly shaped and a bedding blanket of sand is specified.
10. Installation of conduit pipes should start at the downstream end of the conduit line. 11. The lower segment of the conduit pipe should be in contrast with the shape throughout its full length. 12. The bell of the groove ends of rigid conduits and outside circumferential laps of flexible conduits are placed with longitudinal laps or seams at the side. 13. Paved or partially lined conduits pipes are laid with the longitudinal centerline of the paved segment coincide with the flow line of water. 14. Elliptical and elliptically reinforced conduits are placed with the major axis within 5 degrees of a vertical plan through the longitudinal axis of the conduit. 15. Rigid conduit pipes are the bell and spigot type, tongue and groove, or other types 16. Joining conduit pipes, the ends are fully entered into the inner surface evenly flushed. 17. Joints are connected by Portland cement mortar or grout, rubber gaskets, Oakman and mortar or joint compound, plastic sealing compound. 18. Backfilling follows after the laying installation of conduit pipes. Materials for backfill should be fine, readily compacted soil granular material selected from approved sources. Design of Underdrain 1. Underdrain is categorized under Item 501 of DPWH standard specifications. 2. The pipe is used to carry away collected water. 3. Filter materials with which the trench is backfilled or the opening in the filter, should be fine enough so that the adjacent soil will not be washed-out into o the drain. 4. The top of underdrain should be sealed with impervious soil to prevent the entrance of surface water. 5. Intercepting drains must extend into the impervious zone. 6. The pipes are laid with the flow line at least 1.20 meters below the finished grade and carefully bedded with gravel or filter materials. 7. For pipes installation, trenches are excavated to the dimensions and grades required by the plan. A minimum of 15cm bedding layer of granular backfill materials are placed and compacted at the bottom of the trench. 8. Perforated pipes are placed with the perforations downward securely joined with appropriate fittings or bands. 9. Non-perforated pipes are laid with the bell end upgrade with open joints wrapped with suitable material to permit entry of water, or unwrapped as may be specified 10. Upgrade end sections of all sub-drainage pipe installation are closed with suitable plugs to prevent entry of soil materials. 11. After the pipe installation, granular backfill materials are placed 30 centimeters above the top of the pipe and should not be displaced by the covering of open joints. 12. The remainder of granular backfill are placed and compacted at every after 15 cm. layers up to the required height. Debris Control 1. A single large span of box culvert is preferred on streams carrying large floating objects along the multi-span culverts. 2. Extend the curtain wall separating the barrels of the multi span culverts upstream, with its top slanting downward so that in time of flood, debris carried by flood water will ride up on this wall or turn to pass easily through the opening.
3. Install up stream debris racks or wire, steel rail or piling. Some design provides trap for the sand or gravel carried by the stream rather than having it pass through the structure. Problems usually encountered in the design and construction of roadways 1. The stability of fills and slope – there are materials in cut banks slips down the roadway or sometimes, apportion of high fill slides outward carrying portion of the roadway. 2. Capillarity – is the tendency of water to seek its own level as if in an open channel flows through the pores and fine channels of the soil. It is the force pulling free water through the voids of the soil in all directions. 3. Elasticity and Rutting – Elasticity is common in soils whose fines consist mainly of flat and flaky particles. This kind of soil has rubberized characteristic that rebound under heavy loads. Rutting means potholes. Slope Failure or Landslide 1. Changes in natural conditions of slope may be the result from: a.) Occurrence of earthquake b.) Subsidence of underground cavern c.) Erosion d.) Slope weakening due to cracks e.) Variations in the elevation of ground water f.) Weakening of buried soil or rock seams due to ground water flow or chemical leaching 2. Changes in natural conditions of slope induced by man a.) Increased loading on a slope or near its crest b.) Removal of earth below the toe of a slope c.) Removal of materials from slope making it steeper d.) Topographic modification like earth moving, excavations, change in elevation e.) Landslide or other conditions caused by man Classification of Slide 1. Rational slide – is associated with natural slopes and constructed embankment of homogenous materials possessing cohesion. 2. Translational slide – is associated with slope of layered materials where the mechanism of slippage occurs along a weak plane that possesses a downward dip and in cohesionless soil slopes where seepage occurs. 3. Black or Wedge failure – refers to the displacement of an intact mass of soil due to the action of an adjacent zone of earth. 4. Flows and spread failure – is the most complex type of soil mass movement. Flows involve lateral movement of soil having a characteristic of viscous fluid, although the actual consistency of having the moving mass may vary from very wet to dry. 5. Spread refers to the occurrence of multi-directional lateral movement by a fractional soil mass. Earthquake is a typical cause of lateral spreads. Improving the Stability of Slope 1. Corrective and preventive measures of reducing a mass or loading have successfully prevented further slides. 2. Improving the shear strength of the earth in the failure zone by constructing structural elements that will provide resistance to movement. 3. Consider the characteristics of the soil in the slope like: a.) Thickness and depth of the materials involved in sliding b.) The ground water conditions c.) The spaces available to undertake corrective changes d.) The topographical conditions at the vicinity of the slope 4. Flattening of the slope can be done to reduce the weight of the mass that tends to slide. 5. If base failure is anticipated, placement of beam below the toe of the slope will increase movement resistance. 6. If the zone below the toe is susceptible to severe erosion, protective rock fill blanket and riprap can be installed at the toe area. 7. To prevent infiltration, erosion and to reduce seepage force, the stone subsurface water and intercept surface water should be lowered. 8. If the soil is cohesionless, the shear strength of the slope material can e improved through densification by using explosives or vibroflotation or terra probe procedures. 9. For cohesive soils, shear strength could be improved by consolidation and water content reduction through sur-charging which may include wick drains, electro osmosis and thermal drying. 10. Where the weak condition is critical to the stability of the slope, grouting and injection methods could be utilized to add cementing and bonding agent.
11. Pile driving, sheet pilling or retaining walls are utilized to provide lateral support and increase resistance of slope tending to stocks.
Retaining Wall 1. The practical use of gravity retaining wall is controlled by height limitations. If the ground water level rises into the backfill behind a retaining wall because of either changing ground water condition or percolating water surface, the lateral pressure against the wall is also changed. 2. To avoid the rise of water building up behind the retaining wall, a weep hole or collector drainage system or both are provided as part of the design construction. Weep holes should be at least 3 inches diameter provided with granular soil filter fabric at the wall to prevent backfill erosion. The horizontal spacing of the weep holes ranges from 1.20 to 3.00 meters. 3. The base and stem of the retaining wall must be capable of resting the internal shear and bending moments as a result of soil and other loading. 4. The wall must be safe against sliding. The bearing capacity of the foundation material supporting the wall must not be exceeded. 5. Rip Rap and Grouted Rip Rap are under Item 504 of the DPWH standard specifications. This item includes furnishing and placing of riprap with or without grout, with or without filter backing. Class of stone for ripraping are: a.) Class A – ranging from 15 to 25 kg with at least 50% of the stones weighing more than 20 kg b.) Class B – ranging from 30 to 79 kg with at least 50% of the stones weighing more than 50 kg c.) Class C - ranging from 60 to 100 kg with at least 50% of the stones weighing more than 80 kg d.) Class D - ranging from 100 to 200 kg with at least 50% of the stones weighing more than 150 kg 6. The foundation of riprap is dug below the depth of scour. Stones placed below the water line are distributed properly and those above the water level are placed by hand or individually laid by machine. 7. Grouting – when grouted riprap is specified, stones are placed by hand or individually laid by machine. Spaces between stones are then filled with cement mortar sufficient enough to completely fill all voids except the face surface of the stones left exposes. 8. Beads and Joints – beads fro face stones may vary from 2cm to 5 cm in thickness. They should not extend in unbroken line through more than 5 stones. Joints may vary from 2 cm to 5 cm in thickness. They should not extend in unbroken line through more than two stones. 9. Headers – should be distributed uniformly throughout the walls of the structures to form at least 1.5 of the exposed faces. 10. Sheet piles are of three kinds. Timer sheet piles, concrete sheet piles and steel sheet piles. 11. Gabion is a wire mesh supplied in various width and length that is in multiple of 2, 3 or 4 times its width.
Highway Bridges 1. Two kinds of highway bridges: a.) Those that carry vehicular traffic and pedestrians over a large stream b.) Those that separate traffic movements as interchanges and street pedestrians over or under crossings.
2. Hydraulic Problems a.) There must be available stream record that provides the usual method of estimating water discharges under the bridge. b.) Analysis of the channel relationship as to peak flow, water way opening, water surface elevation at the structure and upstream and flow velocity. c.) The degree of contraction of the flowing water in the channel approach d.) Final structure proportions and required channel modifications based on the studies e.) Effect of bridge opening and approaches that might cause flood to adjacent properties f.) Economic, legal and social implications where cooperative planning with all affected groups and agencies is necessary. g.) Where the bridge is to rest on eroding streambed scouring is the primary concern h.) Findings showed that scours is occur when the pier has less resistance to flow. 3. Highway Bridges, Designed to Resist Loads Brought by: a.) The weight of the structure itself called dead load b.) The weight and dynamic effect of moving load called live load c.) The centrifugal forces developed by moving vehicles on curved structure d.) The wind load and the stresses brought about by temperature, earth, shrinkage, buoyancy, rib shortening, erection, current pressure and earthquake.
4. Bridges Types 1. Bridge consists of substructure of abutments and piers that supports superstructure that carry the roadway between supports. 2. Bridges type includes: slab, girder, truss, arch and cable stayed. 3. The selection of the kind of bridge to be installed depends on the length of the individual span as follows: a.) Short span up to 18 meters which either: - Reinforced concrete rigid frame with slab deck - T-Beam or box girders reinforced concrete - Steel plate girders with reinforced concrete deck b.) Bridge of large span - Girder type frames of reinforced concrete - T-Beam or box girder reinforced concrete - Steel plate girders with reinforced concrete deck c.) Span that exceeds 90 meters long steel trusses, arches of steel or reinforced concrete d.) Span that exceeds 150 meters are generally made of steel trusses, cable-stayed or suspension bridge
