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CHAPTER V

FINDINGS AND DISCUSSION

5.1 Introduction

In the first part of this chapter, the findings on advantages of reinforced soil

slope using Geogrid has been explained in detail and the summary on advantage of

reinforced soil slope using Geogrid as comparison to other types of reinforced soil

structure has been tabulated. The findings on the cost comparison for Geogrid

reinforced slope with reinforced soil wall and crib wall are also been discussed in this

chapter. The last part of this chapter explains on the case study that has been carried out

on the use of geogrid for the construction of a 45(1V:1H) slope and the analysis on

the slope stability.


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5.2 Advantage of Reinforced Soil Slope Using Geogrid

The advantages of reinforced soil slope using Geogrid are :

i) A new dimension in versatility and new possibilities in

earthwork construction.

The attractiveness of reinforced structures lies not just in cost

effectiveness. First, the reinforced soil technique introduces a new dimension in

versatility and new possibilities in earthwork construction, which were

previously not feasible, e.g. in terms of slope angle, height and load carrying

capacity. This is because by reinforcing the soil with a strong structural element,

a new composite material with greatly enhanced properties is now available for

providing solutions to a wide range of problems. Whereby, this versatility could

not be achieved in the conventional earthworks solution methods.

ii) Reduce land area and volume of earthworks

Reinforced soil is a very versatile solution for slope rehabilitation (re-

construction) works. This is because by reinforcing the soil, it is feasible to alter

the slope profile by providing a steep section at any location along the slope.

This steep section can be of any angle up to 90, i.e. vertical. A steep section

requires less land area and can reduce the volume of earthworks. Wherelse, the

conventional earthworks solution only allows for gentle slopes, whereby, gentle

section requires more land area and increase the volume of earthworks.


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iii) Allowing usage of lower quality fills

Since the geogrids are not subject to corrosions, use of lower quality fills

such as cohesive are allowed. In this connection, residual soils derived from

granite, sandstone/shale and other rocks have been used successfully in many

projects. This is also an advantage in hilly terrain where access may be difficult

or in areas where good quality fills are not available. Other types of reinforced

soil structure such as reinforced soil wall, which uses metal strips as the

reinforcements, are not allowed to use lower quality fill materials. This is

because metal strips are subject to corrosions.

iv) Enhance speed of construction

Simple construction method are used in geogrid reinforced earth

structures. No heavy construction vehicles or machineries, and no construction

formworks are required. All these factors contributes to the speed of

construction. The conventional method such as reinforced concrete wall requires

heavy machineries and construction formwork, whereby slows the speed of

construction.

v) Facilitates the reduction of pore water pressure

This method provides sufficient drainage capacity by providing sand

drainage column, enabling it to drastically reduce the flow paths in the

reinforced soil. These, facilitates the reduction of pore water pressure, improves

shear characteristics and thereby increases stability. In the other types of

reinforced soil structures, woven fabrics type of geosynthetic soil reinforcement,


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viii) Cost effectiveness

Geogrids facilitates reduction in land use and earthworks volume,

allowable for lower quality fill materials, simple construction and shorter period,

no heavy machineries and construction formworks. All these factors contributes

to cost savings. Wherelse, in the conventional reinforced concrete structures or

other types of reinforced soil structures, greater earthworks volume, import of

suitable fill materials, complicated construction method, usage of heavy

machineries and construction formworks, increases the overall construction cost.


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5.3 Comparison on Advantage of Reinforced Soil Slope Using Geogrid With

Other Types of Reinforced Soil Structure

The advantage of reinforced soil slope using Geogrid as compare to other types

of reinforced soil structure are summarized in Table 5.1.

Table 5.1 : Summary on advantages of reinforced soil slope using geogrid as

comparison to other types of reinforced soil structure

Items Reinforced Soil Slope

Using Geogrid

Other Types of Reinforced

Soil Structures

Versatility By reinforcing the soil with a strong

structural element, a new composite

material with greatly enhanced

properties is providing versatility

and new possibilities in earthwork

construction, e.g. in terms of slope

angle, height and load carrying

capacity.

Not versatile in conventional

earthworks solution.

Land area and

earthworks

volume

Feasible to alter or to construct

steep slopes, up to 90angle. A

steep section requires less land area

and can reduce the volume of

earthworks.

Conventional earthworks

solution only allows for gentle

slopes, where gentle section

requires more land area and

increase the volume of

earthworks.


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comparison to other types of reinforced soil structure (Contd)


Using Geogrid


Soil Structures

Fill materials Geogrids are not subject to

corrosions, therefore, lower quality

of fill materials such as cohesive

soil are allowed.

Lower quality fill materials are

not suitable in reinforced soil

structures using metal strip

reinforcements.

Construction

durations

Simple construction method are

used and no heavy construction

vehicles or machineries and no

construction formworks are

required, therefore enhance speed

of construction.

The conventional reinforced

concrete structures requires

heavy construction vehicles or

machineries and construction

formworks, whereby slow the

speed of construction.

Pore water

pressure

This method provides sufficient

drainage capacity by providing sand

drainage column, therefore, it

facilitates the reduction of porewater pressure.

Woven fabrics type of

geosynthetic soil

reinforcement, offers lower

drainage capacity, whereby, itmay increase the pore water

pressure.

High tensile Geogrids are very high strength

polymer reinforcements, whereby

demonstrates a very low creep

tendency and provides excellent

reinforcement characteristics and

minimum deformation.

The use of reinforcing strips,

e.g. galvanized steel strips,

demonstrates a very high creep

tendency and provides high

deformation do not provides

good reinforcement

characteristics.


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comparison to other types of reinforced soil structure (Contd)


Using Geogrid


Soil Structures

Aesthetical

view

Finished slope surface can be turfed

or planted with creepers, which

creates environment friendly and

attractive green slopes.

The conventional reinforced

concrete structures or

reinforced earth walls does not

provide surface for greeneries.

Cost Reduction in land use and

earthworks volume, allowable for

lower quality fill materials, simple

construction and shorter period, no

heavy machineries and construction

formworks, are major factors

contributes to cost savings.

Greater earthworks volume,

usage of suitable fill materials,

complicated construction

methods, usage of heavy

machineries and construction

formworks increases the

overall construction cost.

5.4 Cost Comparison for Reinforced Soil Slope Using Geogrid With Other

Types of Reinforced Soil Structure

Costing for reinforced soil slope using Geogrid is shown in Table 5.2 and for the

cost comparison purpose, costing for two more types of reinforced soil structure namely

reinforced soil wall and crib wall are calculated, as shown in Table 5.3 and Table 5.4

accordingly. The summary on cost comparisons are shown in Table 5.5.

From the summary on cost comparisons as shown in Table 5.5, a relationship on

cost have been derived between the three types of reinforced soil structures. It shows

that the cost for reinforced soil slope is RM 206,913.00 (21.89%) cheaper than the cost


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for reinforced soil wall. And, the cost for reinforced soil slope is RM 313,088.00

(29.78%) cheaper than the cost for crib wall. From the above, it may established that the

cost of construction for reinforced soil slope are in the range of 20% to 30% cheaper, as

compare to the cost of construction for reinforced soil wall and crib wall.

The construction of reinforced soil wall requires reinforced concrete panels,

galvanized reinforcing strips, good quality fill materials, heavy construction

machineries and skill workers, which may contributes to the higher overall construction

cost. As for the crib wall, the crib units together with the infill are designed to act

together as a gravity structure, retaining the upper slope. Since the bearing pressure

from the reinforced block was excessive for the foundation soil, piles were provided for

external stability. The piling works and reinforced concrete crib units are the main

contribution factors for the high overall construction cost.


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Table 5.2 : Costing for reinforced soil slope using geogrid

Item

No.

Description Unit Qty Rate

(RM)

Amount

(RM)

1

2

3

3.1

3.2

3.3

3.4

Excavation to formation level for

Tensar geogrid reinforced

embankment.

Supply and install Tensar geogrids

as reinforcing element inclusive of

backfilling and compaction

cohesive frictional materials for the

construction of the reinforced

embankment from excavated

platform.

Surface and Sub-soil Drainage.

Supply and install 1000mm x

500mm thick sand column spaced

5.0m centre to centre.

Supply and install 100mm dia.

UPVC perforated sub-soil pipe

wrapped around with geotextile

space at 5.0m centre to centre.

Provide 1000mm thick crusher run

base.

Supply and install 600mm thick

sand base layer.

Cu.m

Sq.m

Cu.m

M

Cu.m

Cu.m

14,600

3,100

196

264

1,120

364

6.50

155.00

48.00

24.00

48.00

36.00

94,900.00

480,500.00

9,408.00

6,336.00

53,760.00

13,104.00

Amount Carried Forward 658,008.00


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Table 5.2 : Costing for reinforced soil slope using geogrid (Contd)

3.5

3.6

3.7

3.8

3.9

3.10

4

4.1

Amount Brought Forward

Provide rock toe inclusive of

geotextile separator to the

reinforced embankment.

Provide aggregate layer behind the

rock toe.

Provide 300mm depth drain at toe

of reinforced embankment.

Provide 300mm depth drain at berm

of reinforced embankment.

Provide cascaded drain at

reinforced embankment.

Sump with size 1000 x 1000 x

750mm with 150mm thick both

side brickwall.

Turfing.

Supply and install close turfing/

Hydroseeding to Tensar Geogrid

Reinforced slope.

M

M

M

M

M

No.

Sq.m

205

205

194

284

34

4

3,950

75.00

24.00

65.00

70.00

225.00

776.00

4.20

658,008.00

15,375.00

4,920.00

12,610.00

19,880.00

7,650.00

3,104.00

16,590.00

Total Amount 738,137.00


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Table 5.3 : Costing for reinforced soil wall

Item

No.


(RM)

Amount

(RM)

1

2

3

4

5

6

Excavate trench for levelling pad to

reduce level including backfilling,

compaction and disposal of surplus

excavated material from site.

Imported sand backfill behind

precast concrete facing wall panel.

Mass concrete Grade 20 in levelling

pad (350mm x 150mm high).

Supply of precast concrete wall

facing panel not exceeding 8.0m

high.

Erection and installation of precast

concrete facing wall panel not

exceeding 8.0m high including

providing vertical joint comprising

of 150mm concrete cube,

galvanized ribbed steel

reinforcement strips.

Supply and place geotextile

membrane.

Cu.m

Cu.m

Lin.m

Sq.m

Sq.m

Sq.m

50

9,100

200

1,500

1,500

2,000

15.00

25.00

10.50

450.00

5.00

4.00

750.00

227,500.00

2,100.00

675,000.00

7,500.00

8,000.00



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Table 5.3 : Costing for reinforced soil wall (contd)

7

8

9

9.1


Supply and install close turfing to

slope as specified.

Carry out pull-out test to

specified requirement.

Berm Drain.

Supply and construct 230mm half

round precast concrete drain

complete inclusive of 1:3:5 mass

concrete base, lean concrete,

formwork, jointing, haunching

etc. all as shown in the drawings

and all necessary excavation,

backfills and disposal of surplus

excavated material.

Sq.m

No.

M

1,000

2

200

4.20

2,000.00

80.00

920,850.00

4,200.00

4,000.00

16,000.00

Total Amount 945,050.00


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Table 5.4 : Costing for crib wall

Item

No.


(RM)

Amount

(RM)

1

2

2.1

2.2

2.3

2.4

Excavate to formation level for

crib wall embankment.

Grade 25 concrete for various

header sizes of cribwall

foundation including steel bars,

all necessary excavation, backfill,

compact and returf existing slope.

Single 1320mm header cribwall

base.

Single 1620mm header cribwall

base.

Double 1320mm header cribwall

base.

Tripple 1320mm header cribwall

base.

Cu.m

M

M

M

M

2,700

100

100

6

20

6.50

350.00

400.00

500.00

650.00

17550.00

35000.00

40000.00

3000.00

13000.00



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Table 5.4 : Calculation for crib wall (contd)

Item

No.


(RM)

Amount

(RM)

3

3.1

3.2

3.3

3.4

4

4.1

4.2

4.3

4.4

4.5

4.6

Amount Brought ForwardSupply and install cribwall of

various headers sizes, complete

with stretcher and crusher run

infill within the wall.

Height up to 3.0m

Height between 3.0m to 5.0m.

Height between 5.0m to 6.0m.

Height above 6.0m.

305mm x 305mm x 97kg/m

Sheet H-Pile.

Supply, transport, handle and

pitch 12m length sheet H-pile.

Drive vertical pile.

Cut of surplus length.

Pile connection.

Pile head.

Working load.

Sq.m

Sq.m

Sq.m

Sq.m

M

M

No.

No.

No.

No.

200

300

35

140

2,400

2,400

200

200

200

2

250.00

300.00

385.00

500.00

250.00

14.00

46.00

92.00

200.00

1,000.00

108,550.00

50,000.00

90,000.00

13,475.00

70,000.00

600,000.00

33,600.00

9,200.00

18,400.00

40,000.00

2,000.00

Amount Carried Forward 1,035,225.00


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Table 5.4 : Calculation for crib wall (contd)

Item

No.


(RM)

Amount

(RM)

5

5.1


Berm Drain.

Supply and construct 230mm

half round precast concrete drain

complete all as shown in the

drawings and all necessary

excavation, backfills and

disposal of surplus excavated

material.

M 200

80.00

1,035,225.00

16,000.00

Total Amount 1,051,225.00


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Table 5.5 : Summary on cost comparison for geogrid reinforced soil slope with

reinforced soil wall and crib wall

No. Types of Soil StructureTotal Cost

1 Geogrid Reinforced Slope RM 738,137.00

2 Reinforced Soil Wall RM 945,050.00

3 Crib Wall RM 1,051,225.00

5.5 Findings of the Case Study on the Use of Geogrid for the Constructionof a 45 (1V : 1H) slope

5.5.1 Design Calculation

Winslope Tensar Reinforced Slope design program are used to analyse the

circular failure surfaces. Circular slip surfaces are analysed using a modified form of

Bishops Simplified Method. The theory used in the Winslope design program are

elaborated in APPENDIX A1 - A4. The geogrid long term design strength are

elaborated in APPENDIX B1 - B3.


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iv) Sub grade conditions not meeting the required strength are been

removed and replaced with accepted materials.

v) Over excavated areas are replaced with structural backfill material to

the lines and grade shown on the construction drawings.

A) Geogrid Installation

i) Geogrid are oriented with the highest strength axis perpendicular to

the slope alignment.

ii) Geogrid reinforcement are placed at the elevations and extents shown

on the construction drawing.

iii) The geogrid are laid horizontally on compacted backfill.

iv) The geogrid are pulled taut and anchored prior to backfill placement

on the geogrid.

v) Geogrid are to be continuous throughout their embedment lengths.

Spliced connections between shorter pieces of geogrid are not

allowed.

vi) No joints or overlap are required between the adjacent lengths of

geogrid reinforcements longitudinal wise of slope, thus the geogrid

are just placed side by side.

D) Reinforced Backfill Placement

i) reinforced backfill are placed, spread and compacted in such a

manner that minimizes the development of slack in the geogrid.

ii) Reinforced backfill are placed and compacted in lifts not to exceed

150mm where hand compaction is used, or 250mm where heavier

compaction equipment is used.

iii) Reinforced backfill are been compacted to 90% of the maximum

density.

iv ) Tracked construction equipment are not allowed to operate directly

upon the geogrid reinforcement. A minimum fill thickness of 150mm

is required prior to operation of tracked vehicles over the geogrid.


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v) Tracked vehicle turning are kept to a minimum, to prevent tracks

from displacing the fill and damaging the geogrid.

vi) Rubber tired equipment may pass over geogrid at slow speed, less

than 16KPH. Sudden breaking and sharp turning are not allowed.

Daily site visit has been done to observe and monitor the progress of the

construction works. Frequent discussion and brain storming with the client, consultant

and contractors have been done on site to identify any site problems arise during the

construction, and to decide the best solutions for improvement and to overcome the

problems. Figure 5.3 to Figure 5.10 shows some of the construction activities during the

construction of the geogrid reinforced soil slope.

Sub-soil drainage system is provided in the reinforced soil slope to drain out

excess ground water, if any. The top of the slope is further stabilized with the provision

of berm, and surface drainage is also provided to take care of the surface runoff during

raining seasons. Figure 5.11 to Figure 5.18 shows some of the completed geogrid

reinforced soil slope features.


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Figure 5.1 : Typical section of geogrid reinforced soil slope at section 2-2

Figure 5.2 : Typical section of geogrid reinforced soil slope at section 4-4


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Figure 5.3 : Excavator used for excavation works

Figure 5.4 : Compactor used for compaction works


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Figure 5.9 : Installation of sub soil drainage

Figure 5.10 : Close view of the installation of sub soil drainage


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Figure 5.11 : Close view of the completed slope

Figure 5.12 : Close view of the completed slope and toe drain


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Figure 5.13 : Close view of the completed perimeter drain

Figure 5.14 : Close view of the completed berm drain


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Figure 5.15 : Close view of the completed interceptor drain

Figure 5.16 : Close view of the completed rock toe


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Figure 5.17 : Overall view of the completed slope

Figure 5.18 : Another view of the completed slope

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