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Highway Engineering

Why are concrete profile barriers designed with curved surface profiles?

Safety fencings are designed to contain vehicles in the carriageway in which they are traveling and prevent them from rebounding into the road and causing hazards. For normal fencing design, when vehicles crash into safety fencings, it will give way so as to absorb as much energy as possible, thus reducing the impact forces on the vehicles. Moreover, it serves to realign the vehicles along the carriageway when vehicles hit on them. However, for concrete profile barriers they are not designed to absorb energy during vehicle crashing, but to hold the vehicles hitting on them. In this connection, concrete profile barriers are designed with curved profiles so that vehicles can mount and go up partly on them, and yet they will not cause overturning of vehicles. Reference is made to Arthur Wignall, Peter S. Kendrick and Roy Ancil.

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For shallow-angle crashing of cars, they would climb on the lower slope face of concrete profile barriers. On the other hand, when a car hits at a large angle to the barrier, the bumper collides with the upper sloping face of concrete profile barrier and the car rides upwards. This provides the uplift of the car whose wheels move up the lower sloping face of the barrier. It is not intended to lift the car too high, otherwise it may result in rolling. Since the friction between the wheels and barriers provide extra lifting forces, it is undesirable to design rough finish on these faces. In essence, the kinetic energy of the car during collision is transformed to potential energy during its lifting up on profile barrier and finally converted back to kinetic energy when the car returns to the road.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Which method of measuring road stiffness is better, Benkelman Beam Test or Falling Weight Deflectometer Test?

Pavement surface deflection measurement is the principal means of evaluating a flexible pavement structure because the magnitude and shape of pavement deflection is a function of traffic, pavement structure, temperature and moisture affecting the pavement structure. Deflection measurements can be used in back calculation method to determine the stiffness of pavement structural layers.

The Benkelman Beam measures the static deflections and it is operated on the basis of lever arm principle. Measurement is made by placing the tip of the beam between the dual tires and measuring the pavement surface rebound as the truck is moved away. The test is of low cost but it is time consuming and labour intensive in carrying out the test.

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In Falling Weight Deflectometer Test, the falling weight deflectometer is mounted in a vehicle. The sensors are lowered to the pavement surface and the weight is dropped. The test measures the impact load response of flexible pavement. It has the potential advantages that it is quick to perform and the impact load can be readily changed. Moreover, the impact action of falling weight appears to be more accurately representing the transient loading of traffic.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

In the backcalculation of moduli in Falling Weight Deflectometer, why are non-unique solutions generated?

Falling Weight Deflectometer is a non-destructive test applied to pavement for structural evaluation. The loads applied in Falling Weight Deflectometer are of impulse type (quasi-static load in Benkelman Beam) and it serves to simulate actual truck wheel load. Surface deflections induced by the impulse load are measured by some sensors located at the pavement and the pavement surface’s deflections form a deflection basin. The measured deflections can then be adopted to estimate the elastic moduli of pavement structural layers by backcalculation.

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In backcalculation process, a pavement model is firstly established to represent the exiting pavement structure. Trial values of moduli of structural layers are used to calculate the theoretical deflections of pavement model and these values are compared with the measured deflections in Falling Weight Deflectometer. The trial moduli are then adjusted iteratively until both values agree closely with one another.

However, it is observed that the backcalculated moduli based on the analysis of Falling Weight Deflectometer is dependent on software and user. Different computer programmes can generate different values of backcalculated moduli from the same deflection basin. In fact, for a given deflection basin, there are numerous combinations of trial modulus which can satisfy the deflection envelope with a certain accuracy level and hence it may result in non-unique solution developed.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What cause more damage to bituminous pavement?

Which of the following cause much damage to bituminous pavement?
(a) Low usage by heavy vehicles or frequent usage of light vehicles
(b) Low speed traffic or high speed traffic

The relationship between axle weight and the associated pavement damage is not linear but exponential. The pavement damage caused by one passage of a fully loaded tractor-semi trailer (80kN) is more than 3,000 passages of private cars (9KN). Hence, heavy trucks and buses are responsible for a majority of pavement damage.

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Slow-moving traffic imposes greater damage than fast-moving traffic. Past studies showed that when the speed is increased from 2km/hr to 24km/hr, the stress and pavement deflection is reduced by 40%.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Can subsoil drains remove all moisture to protect road formation?

Water control is essential to enhance subgrade to possess good bearing value and strength. Water movement in soils takes place by the action of gravity and capillary actions. Subsoil drains are commonly placed at least 1m below finished subgrade to maintain a lower groundwater table.

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A properly constructed subsoil drain could effectively lower groundwater table and control the capillary rise. However, they may not be able to eradicate completely the upward movement of moisture in soils. Therefore, a sand layer or granular sub-base could be placed on sub-grade to remove and intercept the moisture once it starts to accumulate.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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