|
APPROXIMATE
COSTS OF WORKS
Worked
Example 1 - Slope
Worked
Example 2 - Retaining Walls
Time
of Project
Discussion
It
is difficult to quote exactly how much slope repair works will
cost as it depends on many factors, such as the nature and
complexity of the slope problem, required level of involvement of
the consultant, unexpected conditions arising from the works etc.
Only an indication by means of worked examples is given here. Two
worked examples are given below, with one concerning remedial
works for a slope and the other for retaining walls.
Worked
Example 1 - Slope
This example concerns a
man-made slope with a plan area of about 682 m? It is 24 m
high at the western end and about 37 m at the eastern end with a
slope angle of approximately 55, as shown in Figure
1. There is a
9 m long retaining wall of about 5 m high at the toe of the
eastern corner.
Ground investigation including 3 nos. vertical boreholes (12.94 m
to 15.35 m long), 1 no. inclined borehole (5.38 m long) and 4
chunam strips, plus laboratory testing is specified. The slope
remedial works include installation of 197 nos. soil nails (38 are
8 m to 12 m long & 25 mm in diameter and 159 are 7 m to 15 m
long & 32 mm in diameter), 4 nos. 60 mm diameter horizontal
raking drains, step channels, steel staircase (at 500 mm clear
width), 300 mm U channel and sprayed concrete.
It is estimated that this project incurs a total cost of
$4,508,000 based on 1996 prices. A breakdown is given below :
|
Items
|
Cost |
%
of Cost |
| a.
Ground investigation |
$450,000 |
10.0 |
| b.
Laboratory testings |
$50,000 |
1.1 |
| c.
Topographic survey |
$50,000 |
1.1 |
| d.
Construction |
$3,200,000 |
71.0 |
|
e.
Consultant fees: Geotechnical consultancy
Authorized Person fees
Supervision fees |
$350,000
$120,000
$288,000
|
$758,000 |
16.8 |
|
Total
|
$4,508,000 |
100.0 |
For this example, it is assumed
there are 96 units in the lot with each unit contributing a sum of
about $47,000 to the work.
Worked
Example 2 - Retaining Walls
This example concerns
retaining walls of 8 m high and 33 m long, as shown in Figure
2.
The walls consist of a 2 m high masonry wall at the top and two
buttress walls with heights of 2.8 m and 3.6 m respectively at the
bottom. The buttress walls are constructed with plain concrete
with a thickness of about 200 mm and a wall area of 211 m?
Ground investigation including 1 no. vertical borehole (19 m long)
& 2 nos. trial pits (which exclude the previous ground
investigation works carried out at the site) and laboratory
testing is specified. The remedial works include installation of
73 nos. soil nails (29 are 6 m long & 40 mm in diameter and 44
are 7 m long & 40 mm in diameter) through the two buttress
walls and a 200 mm thick reinforced concrete slab at the wall
face. A total of 3 nos. 60 mm diameter horizontal drains, 52 nos.
75 mm diameter weepholes and 300 mm U channels are also
prescribed.
It is estimated that this project incurs a total cost of
$1,996,000 based on 1996 prices. A breakdown is given below :
|
Items
|
Cost
|
%
of Cost
|
|
a.
Ground investigation
|
$100,000
|
5.0
|
|
b.
Laboratory testings
|
$40,000
|
2.0
|
|
c.
Topographic survey
|
$40,000
|
2.0
|
|
d.
Construction
|
$1,300,000
|
65.1
|
|
e.
Consultant fees: Geotechnical consultancy
Authorized Person fees
Supervision fees |
$200,000
$100,000
$216,000
|
$516,000
|
25.9
|
|
Total
|
$1,996,000
|
100.0
|
For this example, it is assumed the cost is
split between two lot owners. There are a total of 116 units in
one lot with each unit contributing a sum of about $17,000 to the
work.
Time
of Project
The time taken to discharge
the Order for the two examples above is 19 and 18 months
respectively. The approximate time spent on the various phase of
the project is shown below for reference :
|
Item
|
Time
Taken (months)
|
|
Appointment of
consultant
|
1
|
|
Desk study
|
1
|
|
Award of ground
investigation contract
|
1
|
|
Completion of
ground investigation
|
2
|
|
Geotechnical study
and design of remedial works
|
2
|
|
Decision making on
design options
|
1
|
|
Approval of the
proposal by the BA
|
2
|
|
Issue of consent
to commence works by the BA
|
1
|
|
Tendering
|
1
|
|
Carrying out
remedial works
|
4 (Worked Example 1)
3 (Worked Example 2) |
|
Discharge of Order
by the BA
|
1
|
|
Contingencies
|
2
|
|
Total
|
19
(Worked Example 1)
18
(Worked Example 2) |
|
Notes
:
|
(1)
For works in certain areas (such as Mid-levels
Scheduled Area), an extra time of 3 months may be
needed for processing of plans by the BA.
(2)
The estimates above have not taken into account of the
delay due to plan rejection by the BA, which may
cause delay to the project by 3 months or more. |
|
Discussion
The figures provided above
give an indication for cost comparisons for works of similar scale
and nature. It can be seen that the costs of repair of a slope and
retaining walls of the scale and involving the works as specified
are $4.5 million and $2.0 million respectively based on 1996
price. The contribution from an unit owner in each case is about
$47,000 and $17,000 respectively, assuming that there are 96 and
116 units in each lot. A major portion of the expenditure was
spent on construction in the range of 65% to 71% of the total
cost, and depending on the scale of the project and the
involvement of the consultant, 17% to 26% of the total bill was
spent on consultancy fees.
In comparing the spending pattern of the two work examples, it can
be noted that a slightly higher commitment to consultancy fees
could result in saving in the construction cost for the project.
It is estimated that for the above two examples the time taken to
discharge the Order is 19 and 18 months respectively and the
construction takes 3 to 4 months to complete. This estimation has
not taken into account of the delay due to plan rejection by the
BA and the submission of plans in scheduled areas. |