Paper Geoshanghai08

8/12/2019 Paper Geoshanghai08

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Design and installation of rock fall barrier for the Pos Slim project,

Malaysia

Tiru Kulkarni & Kenneth ChooRegional Manager Managing Director

Maccaferri Asia Regional Headquarters Maraputra Sdn Bhd.Kuala Lumpur, Malaysia Ipoh, Malaysia

ABSTRACT

Rock fall protection systems are a key element in the design and maintenance of

infrastructure networks. These systems are generally categorized as either activesystems ( ie ) where the system acts before the initiation of rock mass detachment , or

passive systems ( ie ) where the system acts after the detachment of the rock mass.

Rock fall barriers comprise of a complex system of energy absorption devices thatfunction as a passive rock fall mitigation mechanism. The design of such systems is

based on a combination of site simulations and field crash tests. Due to recentdevelopments in standardizing design and testing criteria for rock fall barriers, thesesystems are rapidly gaining acceptance due to reasons of site adaptability and economy.

This paper deals with the various design and construction issues encountered on the Pos

Slim rock project in Malaysia. The project is located in a geo-morphologically unstableregion and involved several measures undertaken for the protection of a highway from

unstable rock and soil slopes.

Key words: active and passive rock fall mitigation systems, rock fall barriers.

INTRODUCTION

The concept of utilizing rock fall barriers is relatively new in Asia. Even in Europe,where these systems and applications are more common, the field of rock fall engineering

is widely considered to be an evolving code of practice ( ie ) the design and installation of

these systems depends largely on the experience and expertise of the engineer and thevisualization of the problem.

The main parameters affecting the design of a rock fall barrier system are:

Falling energy Falling velocity Height of impact

Falling energy and Falling velocity are related by the equation :

E = 0.5*m*v + 0.5*I*


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where:

m is the mass of the block

v is the translational velocity of fall

I is the moment of inertia of the block

is the angular rotational velocity

Generally, for large block sizes the component of rotational energy ( given by the factor

0.5*I*) is minimal, and can be neglected in practice.

Therefore,

E = 0.5*m*v

Falling velocity ( ie ) velocity of free fall can be estimated from the following graph

The height of impact depends on:

Morphology of the slope and Trajectory of the block


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The trajectory of rocks which bounce while falling is more difficult to forecast as

compared to unstable rock wedges which follow primarily a sliding pattern, as theirtrajectories are defined by the morphology of the slope.

A rock fall barrier is a complex system consisting of:

Posts Connection structures which act to transfer the energy to the energy dissipation

devices Energy Dissipation Devices which help in the dissipation of the generated energy

on impact. Intervention structure which acts to catch the falling rock mass Foundations and anchors

A typical rock fall barrier is shown in the figure below

The mechanism of energy dissipation in a rock fall barrier is a complex process and very

difficult to simulate and compute analytically. A live crash test is hence the accepted

norm for certifying a stipulated capacity for the rock fall barrier.


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THE POS SLIM PROJECT

The Pos Slim project involved slope stabilization works in Cameron Highlands, in the

state of Pahang in Malaysia. The geology of Cameron Highlands comprises mainly

granite rocks followed by small portions of metamorphic rocks and alluvium.

The geomorphology of this area is mainly dominated by denudational process as it is

situated in a mountainous area and has constituent of deep highly weathered martial.

Most parts, especially the exposed unprotected areas like slope-cut abandoned agriculturesites and slope-from agriculture sites, are highly affected by this process. Therefore,

erosion feature such as rill gullies can be clearly seen, and as a consequence, in certain

areas the phenomenon of mass movement is predominant as these features worsen. Thiscan be clearly seen from the photograph below

The site was located between

Northing 507975.593 and 508139.493 and

Easting 372754.721 and 372656.941 for the lower berm

And

Northing 507975.593 and 508139.493 and

Easting 372754.721 and 372656.941 for the lower berm


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Northing 508161.910 and 508337.210 and

Easting 372724.410 and 372638.040 for the upper berm

Total length of the site was 400 m.

Boreholes taken at five locations on the site showed a mix of stiff gravel and silt up to a depth

ranging from 10m 20m followed by highly fragmented slate. The table below gives a summaryof the borehole findings at the site:

Borehole number 0-10 m depth 10-20 m depth 20m + depth

BH 1 Stiff sandy gravel Weathered slate. End

of BH @ 13.0 m due

to bed rock

BH 2 Stiff sandy gravel Stiff sandy gravel Stiff sandy gravel till

22.0m. Weathered

slate from 22.0m. End

of borehole @ 37.0mdue to bed rock.

BH 3 Sandy gravel Stiff sandy gravel Weathered slate. End

of BH @ 22.1m due

to bed rock

BH 6 Stiff sandy gravel.

End of BH @ 6m due

to bed rock.

BH 7 Weathered slate. End

of BH @ 4.5m due to

bed rock

The photograph below depicts the type of weathered slate formation prevalent at the site


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DESIGN OF THE ROCK FALL BARRIER

One of the first steps in the design of the rock fall barrier involved deciding the capacityand the height of the barrier. This involved a rock fall trajectory analysis carried out on a

standard cross section at the site, whose results are shown below

Assumed weight of the rock mass considered 1800 kg 350 kgs

Coefficient of tangential restitution considered 0.4 with a standard deviation of 0.04

Coefficient of the normal restitution considered 0.85 with a standard deviation of 0.04


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Total Kinetic Energy on LOWER BERM

0

50

100

150

200

250

300

1.7

2E+

03

1.6

0E+

04

3.0

3E+

04

4.4

6E+

04

5.8

9E+

04

7.3

2E+

04

8.7

5E+

04

1.0

2E+

05

1.1

6E+

05

1.3

0E+

05

1.4

5E+

05

1.5

9E+

05

1.7

3E+

05

1.8

8E+

05

2.0

2E+

05

2.1

6E+

05

2.3

0E+

05

2.4

5E+

05

2.5

9E+

05

2.7

3E+

05

Total Kinetic Energy [J]

NumberofRocks


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Using a Kolmogorov Smirnov distribution with a cutoff percentage of 95, the design

kinetic energy was established at 146.99 KJ along with a design height of 1.287m for thecentre of gravity of the boulder.

Hence, keeping a minimum accepted Factor of Safety of 1.5, the capacity of the rock fall

barrier proposed was 250 KJ with a design height of 2.0m

FOUNDATION AND ANCHOR DESIGN

The foundation design was carried out using a force vector analysis. Rakered micropiles

were designed to resist a force of 213 KN.

There is no specific standard for micropile design. Relevant standards for individual

design components were followed for the purpose of this design. These standards are BS

8081 and BS 449. Also referred was Micro pile and anchor design by MichelBustamante.

The steps involved in the design were as follows:

The force on each micropile was calculated using a Force Vector diagram, basedon the measured upslope anchor force of 300 KN in the live crash test.

The micropile was designed to resist this force considering the strata encounteredfor each borehole.

The maximum micro pile length of 13m ( encountered for BH 2 & 3 ) wasselected as the standard length for all micropiles.

Given under are is the micropile analysis for Borehole 2 & 3


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FORCE VECTOR DIAGRAM FOR MICROPILES ( NTS )

Considering the anchorage angle as 45 degrees ( w.r.t ) horizontal plane , we get

Horizontal component of Force = 300 Cos 45

= 212.13 KN

Say 213 KN

From the force vector diagram, the components of the force on the micropiles are 213 KN

each ( equilateral triangle ).

For Borehole 2 & 3

Consider pile diameter = 0.1 m

Tl = [ * Ds1 * Ls1 * qs1 * 1000 ( KN )] + [ * Ds2 * Ls2

* qs2 * 1000 ]

Where Ds1 = * Dq1

= 1.1 * 0.1= 0.11

Ls1 = 5m ( consider top 4m as free length )

qs1 = 0.05 ( refer Bustamante : Fig 16 )

Ds2 = * Dq2

= 1.2* 0.1


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= 0.12

Ls2 = 4 m

qs2 = 0.20 ( refer Bustamante : Fig 16 )

Therefore,

Tl = 387.98

Say 388KN

Therefore FS = 388 / 213

= 1.82 > 1.5

Therefore OK

Therefore total micro pile length at BH 2 = 4m + 5m + 4m = 13m

Tendon design

Consider 40mm rebar ( min yield stress lim = 600 N / sq mm )

Assume sacrificial thickness 8mm

Net diameter of rebar = 32 mm

Yield force of rebar = * D / 4 * lim

= 482 KN

FS = 482 / 213

= 2.26 >> 1.5

Hence OK

Provide 40 mm dia rebar for micropile.

Micro pile details

Diameter of micropile 100 mm

Diameter of bore 120 mmLength 13 m


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Anchors were designed on a similar methodology for a force of 300 KN.

Summary for anchors

Diameter of micropile 100mmDiameter of bore 120 mm

Length 14m

A typical cross section is shown in the figure below

CONSTRUCTION ISSUES

Construction started in July 2007. Some of the issues encountered during construction

were:

Difficulty in drilling at 45 angle Drilling proved to be very difficult due to thefragmented strata present at the site. The fragments would choke up the drill hole

and slow down the progress. This was circumvented by bringing in high powereddrilling rigs and increasing the number of drilling machines. In total, two

mechanized drilling rigs were used for the project.

Difficult topography and terrain Simple tasks like provision of water forconstruction became difficult due to the location of the site. As there was nohabitation nearby, temporary accommodation was constructed for the labourers

on site. Frequent labour turnover caused delays, as the labour force was not used

to working in such harsh conditions.

Some photos showing construction work in progress


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Picture shows installation of the posts in progress


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Picture shows material sorting at site


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Picture shows a monkey wrench being used for tensioning the diagonal wire ropes.


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Picture shows installed barrier on the upper berm

The construction and installation work for the entire stretch of 400 m was completed inOctober 2007, four months after commencement.

Conclusion

The efficiency of a rock fall barrier is decided by its performance during a rock fall

event. Since the construction of the barrier is quite recent, and there have been no rock

fall events after it was installed it is difficult to comment on this aspect.However, the design and installation of the rock fall barrier provided a number of

engineering and project management challenges which were met successfully, thus

completing a successful installation.

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