# Civil Engineering Blog

### What is Negative Skin Friction, Calculation of Negative Skin Friction?

Negative skin friction occurs when concrete piles are situated in soft soils, consolidating soil-mass, etc., resulting in a downward force that increases loading on shaft piles and reduces the bearing capacity of the piles. A new concept based on the shifting rate of piles, and the settlement rate of the surrounding soils has been suggested for the study of negative skin friction.

Negative skin friction occurs when the settlement rate of the surrounding soils is greater than that of the piles. Some relative equations have been established to define the negative friction zone of piles. Negative skin friction is dependent on the time factor and the degree of consolidation of the soil mass and can be negligible when the soil mass is nearly completely consolidated.

The use of a concrete slab combined with beams and piles was employed to treat negative skin friction on concrete piles in soft subsoil.

A relative movement between a pile and a soil produces shear stress along the interface of the pile and the soil. Such movement can be induced by a push-load on the pile pressing it down into the soil, or by a pull-load moving it upward. `

A relative movement can also be induced when the soil settles in relation to the pile, or, in swelling soils, when the soil moves upward in relation to the pile. By definition, if the movement of the pile is downward, i.e., the shear stress induced in the pile is upward, the direction of the shear is positive. If the movement of the pile is upward, the shear stress direction is negative; accordingly, the induced shear stress is called positive or negative.

In older terminology, the induced shear along a pile was called 'skin friction'. In modern terminology, the term 'shaft resistance' is used and a distinction is made between on the one hand, positive and negative shaft resistance by which is meant shear stress induced by load on the pile in the form of push-load and pull- load, respectively, and, on the other hand, negative and positive skin friction, which is shear stress induced by settling or swelling soil, respectively.

Negative skin friction produces (accumulates to) a dragload which can be very large for long piles. Johannessen and Bjerrum (1965), Bjerrum et al., (1969), and Bozozuk (1972) reported measurements of drag loads that exceed the allowable loads that ordinarily would have been applied to the piles. Bjerrum et al. (1969) also demonstrated the efficiency of coating the piles with bitumen to reduce the negative skin friction. Fellenius and Broms (1969) and Fellenius (1969) presented measurement showing that a dragload can develop alone from the reconsolidation following the disturbance caused by the pile driving. Walker and Darvall (1973) presented a comparison between bitumen coated and uncoated steel piles, and Clemente (1981) reported measurement of dragloads on coated and uncoated concrete piles. Fellenius (1975; 1979) discussed some practical aspects of bitumen coating of piles to reduce negative skin friction.

The net ultimate load carrying capacity of the pile is given by the equation
= negative skin friction
Where it is anticipated that negative skin friction would impose undesirable, large downward drag on a pile, it can be eliminated by providing a protective sleeve or a coating for the section which is surrounded by the settling soil.
To compute negative skin friction on group of piles, the minimum value from the following equations should be used.
(i) The negative skin friction as the sum of individual piles
Where n – number of piles in a group
– negative skin friction on each pile
= S x p x L (for cohesive soils)
(for granular soils)
(where c – cohesion, – reduction factor)
Where p – perimeter of the pile
L – depth of fill
= earth pressure coefficient
= unit weight of fill
f – coeffieicent of friction =
= angle of friction between pile and soil
(ii) The block skin resistance:
When the piles are placed close to each other, the negative skin friction resistance may act effectively on the block perimeter of the pile group.
S – shear resistance of soil
L – depth of fill
p – perimeter of pile group
– unit weight of soil
A – area of pile group enclosed in perimeter p.

### What is Tunnel Boring Machine (TBM), Advantages and Features

TMB is used as an alternative to drilling and blasting (DB) methods. TBMs are used to excavate tunnels with a circular cross section through a variety of subterranean matter; hard rock, sand or almost anything in between.

As the TBM moves forward, the round cutter heads cut into the tunnel face and splits off large chunks of rock.  the cutter head carves a smooth round hole through the rock -- the exact shape of a tunnel. Conveyor belts carry the rock shavings through the TBM and out the back of the machine to a dumpster.

Tunnel lining is the wall of the tunnel. It consists of precast concrete segments that form rings, cast in-situ concrete lining using formwork or shotcrete lining.

A tunnel boring machine (TBM), also known as a "mole", is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. They may also be used for microtunneling.

They can bore through anything from hard rock to sand. Tunnel diameters can range from a metre (done with micro-TBMs) to 19.25 metres to date. Tunnels of less than a metre or so in diameter are typically done using trenchless construction methods or horizontal directional drilling rather than TBMs.

TBM excavate tunnels with circular c/s through a variety of geological conditions.

Tunnel diameter can range from 1- 15 m.

One of the first TBM built was used for the construction of Hoosac tunnel- Railroad tunnel in the Northern Berkshire towns of Florida and North Adams Constructed of cast iron, it managed to drill 10 feet  before breaking down.

BMs are preferred in urban tunneling and underground mining over conventional methods.

• Reduced disturbance to the surrounding soil
• Produce a smooth tunnel wall reducing ventilation requirements
• Reduce cost of lining
• Improved personnel safety
• Working speed- 6 times greater
• Hence, wherever speed and safety becomes the main  criteria, use of TBM despite its heavy capital cost is preferred.

## MACHINE PARTS

A TBM typically consist of
Shield –metal cylinder serving as temporary support
structure while excavation
Trailing support mechanisms
• Precast concrete segment handling & installing devices
• Conveyor belt system for muck removal
• Control Cabin
• Supply tanks, exhaust fans etc.

## TYPES OF TBM

Based on no. of shields used

1.Main beam TBM
2.Single shield TBM
3.Double shield TBM

Based on the face supporting technique used

1.Earth pressure balance TBM
2.Bentonite slurry TBM
3.Compressed air TBM

4.Fluid balance TBM

### how should the guided bearings in piers of the curved region be oriented with respect to the fixed bearing in abutment?

To determine the orientation of guided bearings, one should understand the movement of curved region of a prestressed bridge. Movement of prestress and creep are tangential to the curvature of the bridge (or along longitudinal axis) while the movement due to temperature and shrinkage effects are in a direction towards the fixed pier. If the direction of guided bearings is aligned towards the fixed bearing in the abutment, the difference in direction of pretress and creep movement and the guided direction towards fixed bearing would generate a locked-in force in the bridge system.

The magnitude of the lock-in force is dependent on the stiffness of deck and supports. If the force is small, it can be designed as additional force acting on the support and deck. However, if the force is large, temporary freedom of movement at the guided bearings has to be provided during construction.

Vincent T. H. CHU

### What are the advantages of piers constructed monolithically with the bridge deck over usage of bearings?

Basically, piers constructed monolithically with the bridge deck are advantageous in the following ways:
(i) Movement of the bridge deck is achieved by the bending deformation of long and slender piers. In this way, it saves the construction cost of bearings by using monolithic construction between bridge deck and piers. Moreover, it is not necessary to spend extra effort to design for drainage details and access for bearing replacement. On the other hand, in maintenance aspect substantial cost and time savings could be obtained by using monolithic construction instead of using bearings as bridge articulation.

(ii) Monolithic construction possesses the shortest effective Euler buckling length for piers because they are fixed supports at the interface between bridge deck and piers.

Note: Monolithic construction means that piers are connected to bridge decks without any joints and bearings.

Vincent T. H. CHU

### Are diaphragms necessary in the design of concrete box girder bridges?

Diaphragms are adopted in concrete box girder bridges to transfer loads from bridge decks to bearings. Since the depth of diaphragms normally exceeds the width by two times, they are usually designed as deep beams. However, diaphragms may not be necessary in case bridge bearings are placed directly under the webs because loads in bridge decks can be directly transferred to the bearings based on Jorg Schlaich & Hartmut Scheef (1982).

This arrangement suffers from the drawback that changing of bearings during future maintenance operation is more difficult. In fact, diaphragms also contribute to the provision of torsional restraint to the bridge deck.

Vincent T. H. CHU

### What is the advantage of sliding bearings over roller bearings?

In roller bearing for a given movement the roller bearing exhibit a change in pressure centre from its original position by one-half of its movement based on David J. Lee.

However, with sliding bearing a sliding plate is attached to the upper superstructure and the moving part of bearing element is built in the substructure. It follows that there is no change in pressure center after the movement.

### What is escalator? How Escalators Work? [VIDEO]

An escalator is a type of vertical transportation in the form of a moving staircase – a conveyor transport device for carrying people between floors of a building. The device consists of a motor-driven chain of individually linked steps that move up or down on tracks, allowing the step treads to remain horizontal.

Escalators are used around the world to move pedestrian traffic in places where elevators would be impractical. Principal areas of usage include department stores, shopping malls, airports, transit systems, convention centers, hotels, arenas, stadiums, train stations (subways) and public buildings.

### Why is the span length ratio of end span/approach span to its neighboring inner spans usually about 0.75?

From aesthetic point of view, an odd number of spans with a decrease in length in the direction of abutment is desirable. Moreover, spans of equal length are found to be boring. However, the arrangement of irregular span lengths is not recommended because it gives a feeling of uneasiness.

From structural point of view, for a multi-span bridge with equal span length, the sagging moment at the mid-span of the end span/approach span is largest. In order to reduce this moment, the span length of end span/approach span is designed to be 0.75 of inner spans. However, this ratio should not be less than 0.40 because of the effect of uplifting at the end span/approach span support.

Note: End span refers to the last span in a continuous bridge while approach span refers top the first span of a
bridge.

bridge.Vincent T. H. CHU

### In the design of a simply supported skew bridge, which direction of reinforcement should be provided?

In the conventional design of steel reinforcement for a simply supported skew bridge, a set of reinforcement is usually placed parallel to free edge while the other set is designed parallel to the fixed edge. However, this kind of arrangement is not the most efficient way of placing the reinforcement. The reason is that in some parts of the bridge, the moment of resistance is provided by an obtuse angle formed by the reinforcement bars which is ineffective in resisting flexure.

In fact, the most efficient way of the arrangement of reinforcement under most loading conditions is to place one set of bars perpendicular to the fixed edge while placing the other set parallel to the fixed end as recommended by L. A. Clark (1970). In this way, considerable savings would be obtained from the orthogonal arrangement of reinforcement.

### Straddling Bus in Spotlight as Transit Elevated Bus (TEB) System [VIDEO]

Tired of traffic jams and tail gas? The design of electric "straddling buses" lets cars drive underneath them, and can help reduce air pollution.

Also known as land airbus, the new invention is less costly than subway systems.

The subject of Irrigation has assumed a worldwide importance since it holds the key to provide food to the teeming billions living in developed and developing countries. The science of irrigation engineering vary from situation to situation. The science of irrigation engineering vary from situation to situation. In the riverless middle east it depends on small discharges of wells and springs while in countries like Pakistan, Iraq, and Egypt it is based on huge river discharges. The man made canal discharge of 22000 cfs and more may be exceeding those natural rivers in other countries.
Pakistan has one of the world’s largest continuous irrigation system with three storage dams, seventeen barrages, more than 40,000 miles of canals and thousands of hydraulic structures. This was initiated in the last century and continues to expand with more areas coming under canal irrigation. Similar systems exist in other countries like India, Bangladesh, Iraq, Egypt, Jordan, Malaysia and USA.

 Irrigation and Hydraulic Structures Theory Design and Practice

## Title of the Book

Irrigation and Hydraulic Structures

Theory, Design and Practice

Dr. Iqbal Ali

## Contents of the Book

Introduction
Water Resources for Irrigation
Irrigation Canals
Silt. Control in Irrigation Canals
Structures on Canal Falls
Cross Drainage Works
Irrigation Outlets
Dams
Design of Wells
Water Management and Irrigation Systems
Environmental Impacts of Irrigation
Computer Applications in Water Resources

Note: - As per our New Policy updated on 25th August, 2016, from now onwards no Book or Software will be available for free of cost due to copyright claims, However you can enjoy the downloads by becoming our affiliate member on very reasonable and cheap rates.
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