# Construction of a Railway

Problem 1: Horizontal Alignment (5 marks)
A 4-lane road (2 lanes in each direction with no median separation) has a circular curve with a
superelevation of 5% and a central angle of 50 degrees. The CT of the curve is at station 8+880
and the PI is at 8+806. The road has 3.6m lanes and 3.0m shoulders on both sides with high
retaining walls going up immediately next to the shoulders (both sides of the road).
Considering speeds in increments of 10km/hr (starting with 40km/hr), what is the maximum
safe speed of this curve and what is the station of TC?
(5 marks)
Problem 2: Vertical Alignment (5 marks)
A team of transport engineers have addressed safety concerns at a railway crossing on a existing
level highway (Grade = 0) by proposing the construction of a highway overpass (above the existing
railway line). The following design constraints have been imposed:
• The existing highway has a design speed of 70km/hr
• Symmetrical vertical curves are designed for the overpass, the sag curves must be
designed with a rate of vertical curvature of 23 ( = 23) while the crest curves must be
designed with a rate of vertical curvature of 17 ( = 17).
• The road surface of the overpass structure must be 7.5m above the existing highway.
• The overpass structure (bridge) must be level (Grade = 0), centred above the railway line
and the length of the level bridge must be 60 metres.
a) Draw a clear labelled diagram presenting the overpass design described above.
Clearly indicate the railway line, the vertical curve components, the bridge and
indicate the dimension constraints described above.
(2 marks)
b) Calculate the minimum length of the existing highway that must be reconstructed to
provide the appropriate vertical alignment.
(3 marks)
Problem 3: Cross Section Design (5 marks)
A section of the Pacific Highway (2-Lane, 2-way) contains a spiral curve – circular curve – spiral
curve which has been designed to connect two tangents of the horizontal alignment. The speed of
this section of the highway is 80km/h. The following design principles are applied:
• The superelevation at the circular curve is 5%,
• The normal crown has a cross-fall of 2.5%,
• The width of carriageway is 8.0m
• The length of the transition curve is 100m.
a) Calculate the cross-fall for the cross-section of the carriageway at a distance of 40m
from TS
(3 marks)
b) Present a drawing of the carriageway of the cross-section at a distance of 40m from
TS, showing the heights of the outside edge and inside edge of the carriageway
relative to the centreline.
(2 marks)
49106: Road Engineering Practice – Assignment 1 3
Problem 4: Combined Design – Grade Separated Intersection (20 marks)
In the dynamic and expanding urban environment of “Scienceberg”, a new highway (“Edison
Expressway”) is to be constructed over the existing “Westinghouse Way”. The following conditions
apply to the site:
• Edison Expressway and Westinghouse Way intersect at right angles (90 degrees).
• As the highways are grade-separated (Edison Expressway bridges over Westinghouse
Way), there is a desirable clearance height of 5m between the two highways.
• Both highways are at a level grade (constant elevation).
• Edison Expressway is oriented east-west at an elevation of 185m. The bridge structure is
such that the bridge girder thickness is 2.5m (measured from the road surface to the bottom
of the girder, assume a clearance of 7.5m from road surface to road surface).
• A single-lane ramp is to be constructed to allow eastbound traffic to go northbound. A
single horizontal curve, with a central angle of 90 degrees, is to be used (no transition curve
is designed). The design speed for the curve is set at 60km/h and a required superelevation
is 5%.
Figure 1 and Figure 2 present diagrams of the alignment of the two highways.
Figure 1 : Horizontal Alignment of Edison Expressway and Westinghouse Way
Figure 2 : Vertical Alignment of Edison Expressway and Westinghouse Way
49106: Road Engineering Practice – Assignment 1 4
question in relation to the grade separated intersection design:
a) What are the stationing of TC, PI and CT, assuming the curve begins at station
3+512.000?
(5 marks)
b) What are the stationing and elevations of all key points (PVC, PVI and PVT for each
designed vertical curve) along the vertical alignment?
(6 marks)
c) What is the distance that must be cleared from the inside of the horizontal curve so
that the line of sight is sufficient to provide adequate stopping sight distance?
(5 marks)
d) Based on your assessment and calculation procedures, was it correct to NOT
include a transition curve in this design? Clearly justify your answer (maximum ½
page response).
(4 marks)

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