DRAINAGE AND SLOPE PROTECTION STRUCTURES

Drainage

- The means of collecting, transporting, and disposing of surface water originating on or near the right of way, or flowing in stream crossing, or bordering the right of way.

- about 25% of roadway funds are for drainage structures(culverts, bridges)

- bigger appropriations allotted for ditch, dikes, channels and erosion control structures.

- major investment on urban area(cities) for drainage.

Drainage System

Sanitary Sewer

(also called a foul sewer and a covered sewer) is an underground carriage system specifically for transporting sewage from houses and commercial buildings through pipes to treatment or disposal. Sanitary sewers serving industrial areas also carry industrial wastewater. The system of sewers is called sewerage.

Sewer System

Septic Tank

- a key component of the septic system, a small-scale sewage treatment system common in areas that lack connection to main sewage pipes provided by local governments or private corporations. Other components, generally controlled by local governments, may include pumps, alarms, sand filters, and clarified liquid effluent disposal methods such as a septic drain field, ponds, natural stone fiber filter plants or peat moss beds.

Types of Drainage

1. Surface Drainage- a system of drainage measures, such as open drains and land forming, to

pre- Surface Drainage vent ponding by diverting excess surface water to a

collector drain

Drainage on the adjoining land and roadway surface

Side Drainage and Cross Drainage

2. Sub-surface Drainage

Ditches

DESIGN OF SURFACE DRAINAGE SYSTEMS

Can be divided into three phases:

1. Estimation of the quantity of water that can reach any element of the system.

2. Hydraulic design of each element of the system.

3. Comparison of alternative systems and materials

Criteria-Lowest annual cost alternative

Effects of water on the pavement structure

Presence of moisture causes:

- reduction in the stability of the soil mass.

- considerable variation in volume of sub-grade in clayey soils.

- Waves and corrugations failure in flexible pavements.

- Stripping failure in flexible pavements.

- Mud pumping failure in rigid pavements.

SLOPE PROTECTION

Open Channel and Culverts

What is an Open Channel Flow?

An open channel is a waterway, canal or conduit in which a liquid flows with a free surface. An open channel flow describes the fluid motion in open channel. 

Open channels are a natural or man-made conveyance for water in which:

The water surface is exposed to the atmosphere, and The gravity force component in the direction of motion is the driving force.

Various Types of Open Channel:

Stream Channel Roadside Channel Ditch Channel Irrigation Channel Drainage Ditch

Note: The principles of open channel flow are the same regardless of the channel type.

Stream Channel Roadside Channel

Ditch Channel Irrigation Channel

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Drainage Ditch

Channel analysis is necessary for the design of transportation drainage systems in order to assess: Potential flooding caused by changes in water surface profiles, Disturbance of the river system upstream or downstream of the highway right –

of-way Changes in lateral flow distributions Changes in velocity or direction of flow Need for conveyance and disposal of excess runoff, and Need for channel lining to prevent erosion

Design of Open Channel Flow

The hydraulic analysis of a channel determines the depth and velocity at which a given discharge will flow in a channel of known geometry, roughness and slope. The depth and velocity of flow are necessary for the design or analysis of channel linings and highway drainage structures.

Channels

The design of crown ditches, gutters, stream channels and culverts flowing partially or fully are based on the principles of flow in an open canal.

Culvert

What is culvert?

A culvert is a structure sized hydraulically to convey surface water runoff under a highway, railroad, or other embankment.

Design of Culvert

The design selected should be the one that best integrates hydraulic efficiency, serviceability, structural stability, economics, environmental conditions, traffic safety and land requirements.

Culverts are used in the following condition:

Where they are more economical than a bridge Where bridges are not hydraulically required Where higher velocities can be tolerated Where greater stage increases can be tolerated, and Where debris and ice are tolerable

Different Types of Culvert:

Culvert come in many sizes and shapes including round, elliptical, flat – bottomed, pear – shaped, and box like constructions. It will depend on the design requirements and other several factors.

Round Shaped Culvert Flat – bottomed Culvert

Pear – Shaped Culvert Box Culvert

Retaining Walls

• Basic function – to retain soil at a slope which is greater than it would naturally assume, usually at a vertical or near vertical position

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Design of retaining wall

- Retaining walls have primary function of retaining soils at an angle in excess of the soil’s nature angle of repose.

- Walls within the design height range are designed to provide the necessary resistance by either their own mass or by the principles of leverage.

- Design consideration:

1. Overturning of the wall does not occur

2. Forward sliding does not occur

3. Materials used are suitable

4. The subsoil is not overloaded

Factors which designer need to take account

- Nature and characteristics of the subsoil's

- Height of water table – the presence of water can create hydrostatic pressure, affect bearing capacity of the subsoil together with its shear strength, reduce the frictional resistance between the underside of the foundation

- Type of wall

• Retaining wall failure at the Shin-Kang Dam

- Materials to be used in the construction

• Failure of retaining wall (dam) due to water pressure.

Mass retaining walls

- Sometimes called gravity walls and rely upon their own mass together with the friction on the underside of the base to overcome the tendency to slide or overturn

- Generally only economic up to 1.8 m

- Mass walls can be constructed of semi-engineering quality bricks bedded in a 1:3 cement mortar or of mass concrete

- Natural stone is suitable for small walls up to 1m high but generally it is used as a facing material for walls over 1 m

Typical examples of mass retaining wall

• Mass retaining walls

• Cantilever walls

• Counter fort retaining walls

• Precast concrete retaining walls

• Precast concrete crib-retaining walls

Brick Retaining Wall

- Cantilever wall- Usually of reinforced concrete and work on the principle of leverage where the

stem is designed as a cantilever fixed at the base and the base is designed as a cantilever fixed at the stem

- Economic height range of 1.2 m to 6 m using pre-stressing techniques- Any durable facing material can be applied to the surface to improve appearance

of the wall

• Two basic forms:

- A base with a large heel

- A cantilever with a large toe

Cantilever T Cantilever L

Counterfort retaining walls

- Can be constructed of reinforced or prestressed concrete

- Suitable for over 4.5 m

- Triangular beams placed at suitable centres behind the stem and above the base to enable the stem and base to act as slab spanning horizontally over or under the counterforts

Precast concrete retaining wall

- Manufactured from high-grade pre cast concrete on the cantilever principle.

- Can be erected on a foundation as permanent retaining wall or be free standing to act as dividing wall between heaped materials which it can increase three times the storage volume for any given area

- Other advantages- reduction in time by eliminating curing period, cost of formwork, time to erect and dismantle the temporary forms

- Lifting holes are provided which can be utilized for fixing if required

Pre cast concrete crib-retaining walls

- Designed on the principle of mass retaining walls

- A system of pre cast concrete or treated timber components comprising headers and stretchers which interlock to form a 3 dimensional framework or crib of pre cast concrete timber units within which soil is retained

- Constructed with a face batter between 1:6 and 1:8

- Subsoil drainage is not required since the open face provides adequate drainage.

SHEET PILING

- Sheet pile retaining walls are usually used in soft soils and tight spaces. Sheet pile walls are made out of steel, vinyl or wood planks which are driven into the ground. For a quick estimate the material is usually driven 1/3 above ground, 2/3 below ground, but this may be altered depending on the environment. Taller sheet pile walls will need a tie-back anchor, or "dead-man" placed in the soil a distance behind the face of the wall, that is tied to the wall, usually by a cable or a rod. Anchors are then placed behind the potential failure plane in the soil.

The Importance of a Retaining Wall

• A retaining wall might be built on property for various reasons. Some level of dirt and soil will be held away from home, garden, pool or play area by building a retaining wall.

Highway Bridges

What is a Highway Bridge?

- Highway Bridge is a major road structure that is designed to go either over or underneath an obstacle. Most highway bridges are not used by pedestrians and do not contain pedestrian walking paths. Instead, they are usually intended to allow vehicles, such as passenger cars or semi-trucks, to safely cross over or under the obstacle.

Two types of Highway Bridges:

1. Those that carry vehicular traffic and pedestrians over a large stream.2. Those that separate traffic movements as interchanges and street pedestrians over or under crossings.

Hydraulic Problems:

1. There should be available stream records which provide the usual methods for estimating water discharge under the bridge.2. Analysis of the channel relationship to peak flow, waterway opening water surface elevation at the structure and upstream from it and flow velocity.3. One major consideration is the degree of contraction of flowing water in the channel approach.4. Final structure proportions and required channel modification based from the study.5. Effect of bridge openings and approaches that might flood the adjacent properties. This is associated with hydraulic aspects of bridges design.

6. Economic, legal and social implications where cooperative planning with all affected groups agencies is necessary.7. Where bridge is to rest on erodible stream beds undermining by scour should be the primary concern. The problem of design Engineer is that if the estimated is over safe, the foundation becomes very costly, but if scour is underestimated, the foundation might be undetermined that may result to total destruction of the entire bridge.8. Recent findings show that the least scours are when piers have less resistance to flow.

Load bearing and Environmental Forces

1. The weight of the structure itself.2. The weight and dynamic effect of moving load.3. The wind load.4. The centrifugal force developed by moving vehicles on curved structure.5. Stress brought by:

a. Change in temperature e. Rib Shorteningb. Earth Pressure f. Erectionc. Shrinkage g. Current Pressured. Buoyancy h. Earthquakes

Bridge Types

1. Beam Bridges- are horizontal beams supported at each end by substructure units and can be either simply supported when the beams only connect across a single span, or continuous when the beams are connected across two or more spans.

2. Truss Bridge- is a bridge whose load-bearing superstructure is composed of a truss. The connected elements (typically straight) may be stressed from tension, compression, or sometimes both in response to dynamic loads. Truss bridges are one of the oldest types of modern bridges.

3. Cantilever Bridge- are built using cantilevers. Most cantilever bridges use a pair of continuous spans that extend from opposite sides of the supporting piers to meet at the center of the obstacle the bridge crosses

4. Arch Bridge- It has abutments at each end. The weight of the bridge is thrust into the abutments at either side.

5. Tied Arch Bridges- have an arch-shaped superstructure, but differ from conventional arch bridges. Instead of transferring the weight of the bridge and traffic loads into thrust forces into the abutments, the ends of the arches are restrained by tension in the bottom chord of the structure. They are also called bowstring arches.

6. Suspension bridge- are suspended from cables. The earliest suspension bridges were made of ropes or vines covered with pieces of bamboo. In modern bridges, the cables hang from towers that are attached to caissons or cofferdams.

7. Cable Stayed Bridge

– Bridge that uses deck cables that are directly connected to one or more vertical columns.

Length of individual spans:

1. Short span ranges up to 18 meters.a. Reinforced concrete rigid frame with slab deck.b. T-beam or Box Girders or Reinforced concrete.c. Steel or Pre-Stressed Concrete I beam with reinforced concrete deck.

2. Bridges of Large Spana. Girder type Rigid Frames or Reinforced Concrete or Steel.b. T-beam or Box Girders of Reinforced Concrete.c. Steel Plate Girders with Reinforced concrete decks.

3. Span that exceeds 90 meters long preferably use steel trusses or arches of steel or reinforced concrete.

5. Span that exceeds 150 meters are generally of steel trusses, cable stayed or suspension bridge.

Note: Highway bridges require regular upkeep in order to ensure public safety is maintained. Typically, highway bridge inspection encompasses pinpointing minor issues and ensuring they are properly repaired. Preventative bridge maintenance can help ensure costly replacements are avoided down the road. If conditions with potential risk are noted, bridge inspectors must thoroughly document this data and recommend any remedial measures. In some cases, this may mean reducing bridge traffic and vehicle weight or even temporarily closing the bridge to traffic until it can be fully repaired.