BRIDGE AND ROAD AND ROCKS

HOW TO SELECT SUITABLE TYPE OF PIER FOR A BRIDGE?

TYPE OF PIER

Pier is usually used as a general term for any type of substructure located between horizontal spans and foundations. However, from time to time, it is also used particularly for a solid wall in order to distinguish it from columns or bents. From a structural point of view, a column is a member that resists the lateral force mainly by flexure action whereas a pier is a member that resists the lateral force mainly by a shear mechanism. A pier that consists of multiple columns is often called a bent.

There are several ways of defining pier types. One is by its structural connectivity to the superstructure: monolithic or cantilevered. Another is by its sectional shape: solid or hollow; round, octagonal, hexagonal, or rectangular. It can also be distinguished by its framing configuration: single or multiple column bent; hammerhead or pier wall.

SELECTION CRITERIA

Selection of the type of piers for a bridge should be based on functional, structural, and geometric requirements. Aesthetics is also a very important factor of selection since modern highway bridges are part of a city’s landscape. Fig-1 shows a collection of typical cross section shapes for overcrossings and viaducts on land and Fig-2 shows some typical cross section shapes for piers of river and waterway crossings. Often, pier types are mandated by government agencies or owners.

Fig-1 Typical cross-section shapes of piers for over crossings or viaducts on land

Fig-2 Typical cross-section shapes of piers for river and waterway crossings

Many state departments of transportation in the United States have their own standard column shapes. Solid wall piers, as shown in Fig-3a and Fig-4, are often used at water crossings since they can be constructed to proportions that are both slender and streamlined. These features lend themselves well for providing minimal resistance to flood flows.

Fig-3 Typical pier types for steel bridges

Hammerhead piers, as shown in Fig-3b, are often found in urban areas where space limitation is a concern. They are used to support steel girder or precast prestressed concrete superstructures. They are aesthetically appealing. They generally occupy less space, thereby providing more room for the traffic underneath. Standards for the use of hammerhead piers are often maintained by individual transportation departments.

Fig-4 Typical pier types and configurations for river and waterway crossings

A column bent pier consists of a cap beam and supporting columns forming a frame. Column bent piers, as shown in Fig-3c and Fig-5, can either be used to support a steel girder superstructure or be used as an integral pier where the cast-in-place construction technique is used. The columns can be either circular or rectangular in cross section. They are by far the most popular forms of piers in the modern highway system.

Fig-5 Typical pier types for concrete bridges

A pile extension pier consists of a drilled shaft as the foundation and the circular column extended from the shaft to form the substructure. An obvious advantage of this type of pier is that it occupies a minimal amount of space. Widening an existing bridge in some instances may require pile extensions because limited space precludes the use of other types of foundations.

Selections of proper pier type depend upon many factors. First of all, it depends upon the type of superstructure. For example, steel girder superstructures are normally supported by cantilevered piers, whereas the cast-in-place concrete superstructures are normally supported by monolithic bents. Second, it depends upon whether the bridges are over a waterway or not. Pier walls are preferred on river crossings, where debris is a concern and hydraulics dictates it. Multiple pile extension bents are commonly used on slab bridges. Last, the height of piers also dictates the type selection of piers. The taller piers often require hollow cross sections in order to reduce the weight of the substructure. This then reduces the load demands on the costly foundations. Table-1summarizes the general type selection guidelines for different types of bridges.

TABLE-1 General Guidelines for Selecting Pier Types
Steel Superstructure
Over water	Tall piers	Pier walls or hammerheads (T-piers) (Fig-3a and b); hollow cross sections for most cases; cantilevered; could use combined hammerheads with pier wall base and step tapered shaft
	Short piers	Pier walls or hammerheads (T-piers) (Fig-3a and b); solid cross sections; cantilevered
On land	Tall piers	Hammerheads (T-piers) and possibly rigid frames (multiple column bents)(Fig-3b and c); hollow cross sections for single shaft and solid cross sections for rigid frames; cantilevered
	Short piers	Hammerheads and rigid frames (Fig-3b and c); solid cross sections; cantilevered
Precast Prestressed Concrete Superstructure
Over water	Tall piers	Pier walls or hammerheads (Fig-4); hollow cross sections for most cases; cantilevered; could use combined hammerheads with pier wall base and step-tapered shaft
	Short piers	Pier walls or hammerheads; solid cross sections; cantilevered
On land	Tall piers	Hammerheads and possibly rigid frames (multiple column bents); hollow cross sections for single shafts and solid cross sections for rigid frames; cantilevered
	Short piers	Hammerheads and rigid frames (multiple column bents) (Fig-5a); solid cross sections; cantilevered
Cast-in-Place Concrete Superstructure
Over water	Tall piers	Single shaft pier (Fig-4); superstructure will likely cast by traveled forms with balanced cantilevered construction method; hollow cross sections; monolithic; fixed at bottom
	Short piers	Pier walls (Fig-4); solid cross sections; monolithic; fixed at bottom
On land	Tall piers	Single or multiple column bents; solid cross sections for most cases, monolithic; fixed at bottom
	Short piers	Single or multiple column bents (Fig-5b); solid cross sections; monolithic; pinned at

SAFE BEARING CAPACITY VALUES OF DIFFERENT SOILS & ROCKS

VALUES OF SAFE BEARING CAPACITY

The safe bearing capacity of soil should be determined on the basis of soil test data or by performing some field test such as Standard penetration test or Plate load test etc.

However, in the absence of soil test data, the values of safe bearing capacity (S.B.C) as given in the following table may be used as a guide for preliminary analysis.

RECOMMENDED VALUES OF SAFE BEARING CAPACITY FOR PRELIMINARY ANALYSIS
Sl. No	TYPE OF ROCK OR SOIL	SAFE BEARING CAPACITY
		(kN/m2)	(kg/cm2)
ROCKS
1	Rocks (hard) without lamination and defects, for example granite, trap and diorite	3300	33
2	Laminated rocks, for example sand stone and lime stone in sound condition	1650	16.5
3	Residual deposits of shattered and broken bed rock and hard shale, cemented material	900	9
4	Soft rock	450	4.5
NON-COHESIVE SOILS
5	Gravel, sand and gravel mixture, compact and offering high resistance to penetration when excavated by tools. (Refer Note 5)	450	4.5
6	Coarse sand, compact and dry (with ground water level at a depth greater than width of foundation below the base of footing)	450	4.5
7	Medium sand, compact and dry	250	2.5
8	Fine sand, silt (dry lumps easily pulverized by fingers)	150	1.5
9	Loose gravel or sand gravel mixture; loose coarse to medium sand, dry (Refer Note 5)	250	2.5
10	Fine sand, loose and dry	100	1
COHESIVE SOILS
11	Soft shale, hard or stiff clay in deep bed, dry	450	4.5
12	Medium clay, readily indented with thumb nail	250	2.5
13	Moist clay and sand clay mixture which can be indented with strong thumb pressure	150	1.5
14	Soft clay indented with moderate thumb pressure	100	1.0
15	Very soft clay which can be penetrated several centimeters with the thumb	50	0.5
16	Black cotton soil or other shrinkable or expansive clay in dry condition (50 percent saturation) (Refer Note 2)	To be determined after site investigation
17	Peat (Refer Note 2 & 3)
18	Fills or made up ground (Refer Note 4 & 5)

NOTES

1. Values given in the above table can be adopted for design of unimportant and lightly loaded structures only.
2. No generalized values for safe bearing capacities can be given for these types of soils. In such area, adequate site investigation is required to be carried out and expert advice should be sought.
3. Peat may occur in a very soft spongy condition or may be quite firm and compact. While ultimate bearing capacity may be high in the compact cases, very large consolidation settlements occur even under small pressures and the movements continue for decades.
4. The strength of made up ground depends on the nature of the material, its depth and age and methods used for consolidating it.
5. Compactness or looseness of non-cohesive material may be determined by driving a wooden picket of dimension 5 cm x 5 cm x 70 cm with a sharp point. The picket is pushed vertically into the soil by full weight of a person and if the penetration of the picket exceeds 20 cm, then it considered as loose state.
6. The safe bearing capacity value may be increased by an amount equal to weight of the material removed from above the bearing level that is the base of foundation.

2 TYPES OF QUALITY CONTROL EMPLOYED IN ROAD CONSTRUCTION.

TYPES OF QUALITY CONTROL

One of the most important tasks of the supervision during the execution of a road contract is technical quality control, i.e. control as to whether the materials and work supplied by the Contractor meet the technical requirements in the contract specifications. There are two types of quality control, which are described below:

1.CONTROL OF METHODS

Method control is usually carried out by the Consultant`s field staff whose job it is to be on the site and supervise the Contractor during the execution of the works. At the same time the field staff will perform simple measurements, such as the recording of the thickness of fill layers, the temperature of asphalt material, and the slump of cement concrete.

Method control is carried out according to the type of work. Where the work method is of considerable importance and requires constant supervision to achieve the quality, or where in some case, the quality is difficult to improve on, there should always be a field engineer on the site. Examples are the ramming of piles, the laying of asphalt, and concreting etc. Where work methods are of less importance or quality is constantly being achieved by the contractor, there may be no need for continuous surveillance.

Examples are excavation and compaction of soil.

2.CONTROL OF END-RESULTS

End-result control includes field tests e.g. control of the evenness of completed pavement layers and laboratory tests, e.g. Marshall tests on asphalt materials. Other tests are a combination of field and laboratory tests. An example of this is the compaction control of earthworks where the achieved density is determined by means of a field test, and where the IS/ AASHTO density with which the result should be compared is found by means of a laboratory test. End results control is carried out by laboratory technicians, and most of the work consists of laboratory tests.

The frequency of end-result control depends on the quality parameters that are to be checked. Parameters which can vary considerably are continuously controlled. Examples are the composition of asphalt materials and the compaction of asphalt courses. As regards regulating laboratory tests the specification usually determines the number of tests. When the works are started and in cases where difficulties as regards compliance with quality requirements are encountered, laboratory testing will normally be intensified.

15+ DIFFERENCES BETWEEN BITUMEN AND TAR USED IN ROAD CONSTRUCTION

The following are the differences between bitumens and tars.

1. The weathering properties of bitumens are superior to those of tars. Generally greater deterioration is produced in tar than in bitumen when exposed to equal weather conditions. Bitumens have a better durability and resistance to weathering than tars.
2. Tars are more susceptible to temperature changes than bitumens. Hot weather will soften a tar surface more than a surface made with bitumen of the same viscosity, and it will become more brittle at low temperatures than bitumen. In other words tar becomes brittle in cold weather and the surface treated with tar is apt to bleed in hot weather if a little extra quantity has been used. Tar is therefore considered unsuitable for locations with wide temperature changes. Gritting or surface dressing can be delayed a little where bitumen has been used but not with tar.
3. Surface dressings with bitumen are more prone to failure by water displacement than those made with tar. Tars generally adhere better than bitumens on wet aggregate.
4. Tars can be brought to a spraying condition at lower temperatures than those needed for bitumens; stones need not be heated to high temperatures.
5. Setting time for tars is more than that of bitumens and this property is useful in the production of pre-coated aggregate which can be transported to large distances or kept for sometime before spreading.
6. Tars harden much quicker than bitumens.
7. Tars have higher specific gravity than bitumens and lower viscosity and these properties give them greater penetrating power and which are more marked during summers. Higher viscosities can generally be used with tars than with bitumens.
8. Tars produce a less slippery surface than bitumens.
9. Bitumens have a tendency to stay at or just near the surface resulting in a rich and fat surface.
10. Roads built of bitumen need constant traffic to be maintained in good order; otherwise the surface will crack and reduce the life of the road.
11. Tars make harder surfaces (but such surfaces are brittle) than bitumens and should be preferred for roads in areas where bullock carts or other hard tyred traffic predominates. Bitumens make more elastic surfaces and are better suited for pneumatic traffic. Hardening of bitumens is very gradual.
12. Tar is more suitable for dense fine grained surfaces and bitumen for more open surfaces.
13. Volume of tar required is about 10 percent less than that of bitumen for the same type of road work.
14. Tar is cheaper than bitumen.
15. A primer is not generally needed with tar.
16. Road tars do not dissolve in a petroleum distillate such as petrol, kerosene, diesel oil. As such tar carpets have proved to be good material for parking sites as it remains unaffected by spillage of oil and petrol from automobiles.

HOW TO CLASSIFY ROCKS BY ORIGIN OR GENESIS?

ROCK CLASSIFICATION

On the basis of genesis or mode of origin, rocks are classified into following three major groups.

1. Igneous Rocks
2. Sedimentary Rocks
3. Metamorphic Rocks

Igneous rocks

1. IGNEOUS ROCKS

These types of rocks are formed by the solidification of molten magma in the interior of the earth.

When a rock is formed by cooling and solidification of magma, below the surface of earth, it is called plutonic igneous rocks. Due to relatively slow rate of solidification, plutonic igneous rocks have coarse grained structure.

If a rock is formed from the solidification of lava on the surface of earth, the rate of cooling is faster and rapid solidification takes place. This type of rock is known as volcanic igneous rocks. Volcanic igneous rocks have fine grained structure.

Examples of igneous rocks are granite, dolerite, basalt, rhyolite etc.

Sedimentary rocks

2. SEDIMENTARY ROCKS

These types of rocks are formed due to weathering and decomposition of earth crust or from any rock type.

When a rock on earth crust is weathered or decomposed and transported and redeposited, and subsequently consolidated and cemented partly or fully, then the new product is known as sedimentary rocks.

Weathering agents act on the surface of a pre-existing rock which may be igneous, sedimentary or metamorphic. These weathered products later become constituents of the new sedimentary rocks. The weathering agents involved are rain, frost, wind, temperature, river, sea etc.

From an engineering point of view, the most important sedimentary rocks are sandstone, shale and limestone.

Metamorphic rocks

3. METAMORPHIC ROCK

Due to high pressure, high temperature as well as high shearing stresses on existing igneous or sedimentary rock masses, under the earth crust, re-crystallization of rocks takes place and resulting mass is known as metamorphic rocks.

The principle agents of high temperature and pressure involved in the re-crystallization can be the following

• Earth movement and pressure
• Liquid and gas, chiefly water
• Heat

Due to the effects of the above agents, new minerals are formed. Such as

• Limestone is transformed to marble
• Sandstone is transformed to quartzite
• Shale is transformed to slate