PROJECT REORGANISATION OF EXISTING MATANG –MAMBONG 275KV
TRANSMISSION LINE PROJECT
250MM MICROPILE DESIGN
FOR SPEICIAL POLE
(REOGANISATION OF EXISTING
MATANG- MAMBONG
275kV TRANSMSSION LINE
PREPARED BY
:
BORNEO GEOTECHNICAL ENGINEERING SDN BHD
MAIN CONTRACTOR:
ELITE TRANSMISSION ENGINEERING SDN. BHD
CLIENT
SARAWAK ENERGY
:
PROJECT REORGANISATION OF EXISTING MATANG –MAMBONG 275KV
TRANSMISSION LINE PROJECT
INTRODUCTION
Micropiles are common used as the remedial option or underpinning structures with foundation
problems. This is mainly due to the local geological factor of the krastic features in limestone
formation after the shallow depth. Besides its high carrying capacity, less site constraint
problems and self sustained operation due to it light and compact drilling rigs and other ancillary
equipments. The sized of pile is vary from 100mm to 350mm with the capacity of 150kN to
2000kN respectively.
Procedures of micropiles construction involving are below;
i. Install of permanent steel casing,
ii. Drill to hard layer, determine the rock founding level and drill to required socket
length
iii. Base cleaning with air compressor flushing.
iv. Install of reinforcement bar according drawing
v. Grouting by tremie method with HDPE pipe
PROJECT REORGANISATION OF EXISTING MATANG –MAMBONG 275KV
TRANSMISSION LINE PROJECT
SITE INVESTIGATION REPORT
The independent S.I contractor was engaged by the Main Contractor. BH SP-1 was carried out by
GEOSPEC Sdn. Bhd in order to confirmed the ground condition and determine the type of
foundation.
The S.I result shows the upper ground or very soft Peak and Clay from the Ground level to 9m with
the SPT value of 0-2. After the soft soil, a dense sand layer from 9.0 to 10.5m due to sediment. The
rock founding level is relatively 10.5m from the working platform which the MUDSTONE was were
and terminated at 18m.
The weak soil and shallow rock layer had narrowed the selective of type of pile. Shallow follow (pad
Footing) will need to deep excavation to 3m depth. Non displacement pile such as spun pile and R.C
pile will not gain friction along the pile due to short pile with soft layer of peak and silty Clay with
SPT N value varies from 0 to 4.
Mudstone was found after the soil layer, weather limestone with high Total Core Recovery, TCR%
which 100% for BH SP-1. The Rock Quality Designation, QRD% values are LOW which consistent
13% to final depth. Which the mudstone is HIGHLY WEATHERED rock with lower strength.
ROCK QUALITY DESCRIPTION BASED ON RQD.
RQD Value
Description of Rock
Quality
0-25 %
Very poor
25-50 %
Poor
50-75 %
Fair
75-90 %
Good
90-100 %
Excellent
The table presents the correlation of RQD to rock quality.
PROJECT REORGANISATION OF EXISTING MATANG –MAMBONG 275KV
TRANSMISSION LINE PROJECT
GEOTECHNICAL CAPACITY DESIGN
Ultimate bond stress at the grout / rock is dependent on strength of grout , rock and soil strength,
quality of contact interface resulting from drilling, cleaning and grouting operation.
When rock is encountered, following values are adopted:Allowable rock bearing is conservatively taken as 0.3 x quc, rock un confine compressive
strength (Poulos and Davis 1980), value of 0.2 quc is adopted for our design
Ultimate skin resistance is taken as 0.1 quc, using safety factor of 2, allowable skin resistance is
taken as 0.05quc however, value not exceeding 0.02 quc is adopted in our design
Following values has been adopted depending on unconfined compressive strength of rock
DESIGN CRITERIA / REQUIRMENT
Special Pole
Summary of Joint Support Reactions for
All Load Cases:
Load
Case
Long.
Force
Trans.
Force
Vert.
Force
Shear
Force
(kN)
(kN)
(kN)
(kN)
Trans.
Momen
t
(kN-m)
Long.
Moment
Vert.
Moment
Bending
Moment
(kN-m)
(kN-m)
(kN-m)
Maximum
Working
Condition
2.0
-205.80
-42.48
167.99
210.14
1066.14
-4674.50
0.02
4794.54
Broken
Earthwire
Condition
1.5
-148.94
-30.62
152.44
152.05
754.74
-3310.07
0.00
3395.02
Propose Cap
Size: 5m
PROJECT REORGANISATION OF EXISTING MATANG –MAMBONG 275KV
TRANSMISSION LINE PROJECT
MICROPILE DESIGN USING HIGH TENSILE STEEL BAR
A.
B.
B.1
PROPOSED PILE WORKING LOAD =
Diameter of micropile=
250
Area of pile = Ap=
49087
PROPERTIES OF MATERIALS
PROPOSED HIGH TENSILE STEEL BAR :Diameter of main reinforcement :
25
No. of main reinforcement :
8
Cross sectional area per bar, A :
490.87
Perimeter for each bar :
78.54
Yield strength, fs :
460.0
Total cross sectional area, As :
Total perimeter :
628.3185
Area of pile, Ac =
B.2
3926.99
mm
Micropile
Diam
500 KN
mm
mm2
250
500
mm
mm
mm
N/mm2
mm2
45160
CEMENT GROUT / CONCRETE :Characteristic cube strength at 28 days :
35
N/mm2
B.3
TYPE OF ROCK :
mudstone
C.
C.1
MICROPILE DESIGN PARAMETERS
Design ultimate anchorage bond stress between grout and steel pipe*
BS 8110 - Part 1 : 1985 Page 3/48
N/mm
C.2
(considered as plain bars anchorage in compression)
Diameter of micropile
=
=
C.3
C.4
Ac, area of concrete =
Minimum factor of safety for geotechnical calculation
Minimum factor of safety for structural calculation
=
=
C.5
mudston
Ultimate bond strength between grout and
e
(BS 8081 : 1989 Table 25)
and adopt safty factor of =
2.5
250
4516
1
2.5
2
=
0.6
2
Hence allowable bond stress =
(recommended values - 1.5 - 2.5 N.mm2)
=
0.30
Bond strength between grout and hard formation(N>50)
=
0.5
2
mm
N/mm
2
N/mm
C.6
2
KN
PROJECT REORGANISATION OF EXISTING MATANG –MAMBONG 275KV
TRANSMISSION LINE PROJECT
D.
D.1
a)
b)
PILE STRUCTURAL CALCULATION
AXIAL LOAD CAPACITY
Design as short column, N ult = 0.4 fcu Ac + 0.75 fy As
N ult =
1987 KN
Safety factor=
3.97 >
2
- therefore, OK.
Assuming the proposed working load is taken by the main reinforcement only.
Structural capacity of H.T. steel bar = As x fs =
=
3927.0
1806.42
x
KN
460
1806.4
500
Factor of Safety =
=
D.2
BS 8110 eqn 38
3.61
>
2
- therefore, OK.
STABILITY CHECK - PILE BUCKLING LENGTH ESTIMATION
Assume no support from soft soil or void inside cavities region
1/2
(pi)2 Es Itotal
K2 P
0.65
for fixed in position for both ends
Buckling length, Lc =
where K =
P=
Es =
2
x
500
x
elastic modulus of steel for H.T.bar, i.e.,
1000
=
21000
0
100000
0
N
N/mm2
(pi)
D4 + Ay2
=
2.21E+07
N/mm4
64
where d is inner diameter and D is diameter of H.T.bar, y is distance of each rebar centre
from neutral axis and A is cross sectional area of each rebar
(pi)= 3.1416
Therefore, Lc
=
10414.2
mm
Itotal =
Hence, critical unsupported length in very soft formation(N<1) is approximately
From soil report, cavity depth is less than the critical length
therefore, OK
D.3
10.4
m.
BOND BETWEEN GROUT AND REINFORCEMENT
working load(N) x factor of
KN
safety
Required bond length for
500
capacity =
total rebar perimeter(mm) x bond strength(N/mm2)
=
500
8
x
x
=
795.8
mm
=
0.796
m
1000
78.54
<
x
x
2.5
2.5
6
m
therefore,
OK
PROJECT REORGANISATION OF EXISTING MATANG –MAMBONG 275KV
TRANSMISSION LINE PROJECT
E.
E.1
GEOTECHNICAL CALCULATION
MICROPILE BOND LENGTH ESTIMATION
E.1.
1
IN
mudstone
FORMATION
Required bond length for
500
KN capacity =
working load(N) x factor of safety
pile perimeter(mm) x allowable bond strength(N/mm2)
500
=
=
(pi)
5305.2
5.3
x
1000
x
x
250
x
2.5
0.3
0
mm
m
mudstone
is weathered, working bond is further reduced by a factor of
1.1
Hence , required minimum socket length of
=
5.836 m
Proposed minimum rock socket length of
6 m
therefore, OK
*Checking with 6m rock socket length,(ηD*L*Ult. Strength)
= 3.142 x 0.25m x 6.0m x 250 kN/m²
=1178.25 kN
F.1
SETTLEMENT CHECK
The settlement of micropile is largely due in elastic compression. It can be estimated as follow:s=
PXL
where L = average length of pile =
Es = modulus of elasticity of steel
=
Ec = modulus of elasticity of
conc=
As X Es + Ac x Ec/Es
At working load , s =
At twice working load =
4.25
8.50
17
21000
0
m
26000
N/mm2
N/mm2
PROJECT REORGANISATION OF EXISTING MATANG –MAMBONG 275KV
TRANSMISSION LINE PROJECT
DESIGN OF REINFORCEMENT CONCRETE
INFILLED HOLLOW SECTION
JOB NAME:
SUBJECT:
MATANG MAMBONG SPECIAL POLE
250MM MICROPILE
BY:
A.C
Design Moment
Design Axial Load (+ve for
Compression)
150
kNm
500
kN
Casing OD
Casing Thickness
Casing Area
Casing Fy
262
6
4825
250
mm
mm
mm²
mPa
Reinforcing Size
Number of Bars
Area of Reinforcing
Reinforcing Fy
25
8
3927
460
mm
50
mm
Cover to Reinforcing Steel
(inside of casing to outside of Bar
Area of Outter Conc Ring
Area of Inner Conc Ring
Concrete Strength
(f'c)
Cube Strength of Concrete (f'c/0.875)
Area
Steel Casing Compression
Steel Casing Tension
Reinforcing in Compression
Reinforcing in Tension
As
Ast
Arc
Arc
Conc. Outter Ring in Comp.
Conc Core in Compression
Ac1c
Ac2c
3141
6
1374
9
35
35
mm²
2112
2713
1513
2414
1319
0
3658
Job No:
Page No:
172
3475
2669
2/3/202
1
91.33
-71.09
54.64
-34.24
Force.y
43882
-43882
33063
-33063
75.00
41.94
15829
2455
mm²
mPa
mm²
mm²
mPa
mPa
gm
1.1
1.1
1.15
1.15
Force (kN)
480
617
605
966
1
1
211
59
Sum of Tension Forces (kN)
Sum of Compression Forces (kN)
Moment Capacity
Squash Load
Tension Capacity
Date:
kNm
kN
kN
y
1583
1355
PROJECT REORGANISATION OF EXISTING MATANG –MAMBONG 275KV
TRANSMISSION LINE PROJECT