CHAPTER 12

CONSTRUCTION METHODS

12.1

Preparation Works

12.2

Excavation and Ground Leveling

12.3

Foundation Work

12.4

Excavation of Slope of Existing Embankment

12.5

Geosynthetic Installation

12.6

Face Support

12.7

Spreading of Backfill Soil

12.8

Soil Compaction

12.9

Facing

12.10

Drainage

 

12.1 PREPARATION WORKS

Before construction, some preparations are required at the site:

1. Site surveying

2. Confirmation of buried and existing structures

3. Ground clearing, etc.

These preparation works allow subsequent construction to proceed smoothly.

 

The preparation works lead to a better understanding of the site conditions. The obstacles that may affect safety and construction of GRS-RW are removed, thus facilitate a better method, rate and quality of construction. The most efficient method is adopted with reference to the details of project.

 

1. Site surveying

If the site differs significantly from design conditions, the construction may be hampered and the design has to be conducted over again. A rapid confirmation should first be made by conducting a survey.

If the construction is to be conducted along the boundary of land, it should be confirmed at the site. During construction, the deformation or movement of form and support may result in the structure intruding the boundary. The construction and surveying methods should be carefully considered. The clarification of boundary and construction method should be discussed with all parties involved.

2. Confirmation of buried and existing structures

If the buried structures are found in the area where geosynthetic layers are to be installed, the construction will be affected, as well as the stability of GRS-RW upon completion. These buried structures and existing structures should be confirmed and removed as necessary. If the existing structure is located along the slope of excavation, its stability may be affected and thus should be evaluated.

 

3. Ground clearing.

The plants and remains of trees, etc., if located at the foundation or along the existing slope of embankment (in the case of embankment widening), should be removed prior to construction. The remains, upon decomposition, may lead to instability or may affect drainage of the GRS-RW. The ground is cleared by digging about 30 cm into the surface soil.

 

12.2 EXCAVATION AND GROUND LEVELING

The following works are done prior to installation of first geosynthetic layer at the bottom:

1. Excavating the toe of slope

2. Ground leveling

3. Drainage installation at base foundation, if necessary

 

1. Excavating the toe of slope

For GRS-RW constructed to widen an existing embankment, portion of the toe of slope may need to be excavated to accommodate the bottom geosynthetic layer (see Figure 12.1). The RRR method requires relatively short geosynthetic length, thus does not lead to large excavation. If excavation is required, the stability of existing embankment has to be evaluated. The temporary soil retaining system may need to be installed.

2. Ground leveling

The ground foundation is leveled prior to construction to ease installation of geosynthetic as well as to facilitate drainage. The leveling activities should not disturb the foundation and existing embankment.

For foundation soil of low strength and high compressibility, such as peat, it is to be replaced by the backfill soil up to a required depth so that subsequent unnecessary settlement and lateral flow will not occur.

If the foundation is composed of undulating rocks or gravels, the backfill soil should be used to level off the foundation. The undulating ground surface may damage the geosynthetic layer and/or lead to unwanted sagging during installation.

 

 

 

3. Drainage installation at base foundation

The mechanism of soil reinforcement is based on frictional resistance developed between the soil and geosynthetics. An increase in positive pore water pressure in the soil affects the stability of GRS-RW. If the water table is close to the ground surface or if the flow out is detected during investigation, the drainage channels or blankets should be installed (Figure 12.2, Photo 12.1).

 

Photo 12.1 Installation of Drainage Blanket at the Ground Surface

 

12.3 FOUNDATION WORK

The foundation is constructed with consideration to the ground topography.

 

The foundation of the facing can be very simple (see Chapter 10). The facing itself is installed only after completion of backfilling. The embedded portion of facing is constructed before the start of backfilling (see Figure 12.3).

During foundation construction, the ground should not be disturbed by excavation. The foundation material is then introduced, compacted until the desired thickness. Fresh concrete is then placed.

For rock foundation, concrete is used to level the surface. For GRS-RW located longitudinally along the slope, the construction is done in stages.

During stage excavation, the toe of the upper part of the slope may be disturbed and lead to differential settlement. Good quality soil is to be introduced and compacted at the disturbed portion (see Figure 12.4).

 

 

 

12.4 EXCAVATION OF SLOPE OF EXISTING EMBANKMENT

In the embankment widening project, the slope of existing embankment is excavated in stages. After construction, the existing embankment and GRS-RW should perform as an integrated structure to ensure adequate stability.

 

In the embankment widening project, the slope of the existing embankment may become a plane of instability. To overcome this potential problem, the slope is excavated in stages so that the GRS-RW will be attached to it as an integrated structure.

The cutting of existing embankment has to be conducted by first investigating its stability.

The height of cutting in each stage is recommended to be 60 cm, i.e., twice the thickness of each compacted soil layer (Figure 12.5).

For slope giving evidence of water flow or seepage, the drains should be installed.

 

12.5 GEOSYNTHETIC INSTALLATION

The following items are considered in installing geosynthetic layers:

1. The geosynthetic of required specification, configuration, dimension, should be installed at specified location.

2. The geosynthetic layers should be installed flatly without exhibiting undulating surface.

3. The principal axis (machine direction) of geosynthetic layer should be aligned perpendicular to the wall facing.

4. The joint of geosynthetic layer should not be made available in a direction normal to the wall facing.

 

1. The properties, configuration, and dimension are among the most important items. They have to meet the details as specified in the design and to be installed at correct location.

2. The geosynthetic functions as tensile reinforcement. Therefore, it should not be installed such that the plane remains in an undulating manner where the tensile resistant will not be fully mobilized. It has to be installed flatly in the reinforced soil zone. The undulating ground surface has to be leveled prior to installation. Any stones extruding to the ground surface should be removed since they may damage the geosynthetic.

The geosynthetic layers should be fixed using sand bags or pins so that they will not be blown or torn by strong wind until the backfill soil is placed.

3. During installation, the folding-back portion of geosynthetic layer should be laid to the outside of the wall. The principal axis (machine direction) of geosynthetic is aligned perpendicular to the wall face. After placing the gabions, this portion of geosynthetic is folded back/wrapped to give a fold-back length of 30 cm (Figure 12.6, Photo 12.2).

The overlapped portion of geosynthetic layer should be tied using wires or strings.

4. If possible, geosynthetic should be used without having joints. However, for GRS-RW construction, joints usually exist and the following measures are required.

There should not be any geosynthetic joints running parallel to the wall face. The joints do exist in the direction perpendicular to the wall face where forces are not transmitted. A 10-cm overlapping is recommended (Figure 12.7).

If the joint will exist parallel to the wall face, A 50-cm overlapping is required. The overlapped region should be tied using wires or strings.

Geosynthetic layers are cut into appropriate length at the site. A proper cutting tool should be used. The polymeric grids are normally used. They may be cut with an electrical cutter. For stiff geosynthetics, the manufacturer should be consulted regarding the cutting tool.

 

Photo 12.2 Installation of Geosynthetics

 

12.6 FACE SUPPORT

The slope face of the GRS-RW, using RRR method, is supported using gabions during construction.

In the RRR method of GRS-RW construction, the gabions are used as supports to ensure stability of the slope face during construction (Photo 12.3). The gabions function as drainage layer after completion of construction and also as a buffer to the highly rigid concrete facing and backfill soil. Thus, the buffer "absorbs" possible relative settlement between the facing and backfill soil.

 

 

A series of activities related to gabion installation, such as pouring crush stones into the woven bags, transporting them to the wall and installing them on the slope face. Human labor is involved, but for large-scale construction, the series of activities may be conducted or automated using machinery.

The gabions are not simply placed on the slope face, a vibrator/compactor should be used so that the void in the gabions will be minimized. The details of gabion are discussed in Chapter 10. For temporary support using means other than gabions, such as metals (Photo 12.4), the drainage capability and stability should be equivalent to the gabions, especially during compaction.

During installation of gabions, priorities should be given to compact them. Alignment is checked, but strict enforcement is not required since the facing will be installed at the completion of construction.

 

12.7 SPREADING BACKFILL SOIL

The following items are checked during downloading and spreading of backfill soil:

1. Before downloading and spreading the soil, confirm the type of soil.

2. The correct amount of soil is spread so that it will give the required thickness after compaction.

3. Cares should be taken so that soil spreading does not damage the geosynthetics.

4. Care should be taken on the safety aspects so that workers and machineries will not fall off the crest of slope.

 

1. The confirmation of soil type before downloading and spreading is extremely important for later quality control of GRS-RW. The backfill soil used for GRS-RW is discussed in Section 4.3. It should render stability to the structure by allowing well compaction, giving little residual settlement, and exhibiting elastic response (so that the plastic deformation will be minimized).

2. In the RRR method of GRS-RW construction, the geosynthetic layer is installed after compacting each soil layer. Therefore, the degree of compaction and thickness of soil layer should be checked accordingly.

3. The machinery and construction procedures should be considered so that the geosynthetic layers will not sag during soil spreading (Figure 12.8). The main reasons that may lead to sagging of geosynthetic layers during soil spreading are the high water content and/or the use of inappropriate machinery, among other factors.

4. For construction site with very limited space, the interaction between machinery and human, and among machineries, should be considered. Operations close to the wall face may cause the machinery or human to fall down. Safety precautions are taken.

For construction using heavy machinery, the following extra precautions are taken:

1. Direction of soil spreading

The soil spreading towards the facing normally leads to sagging of geosynthetic layers and bulging of wall face (Figure 12.9a). The spreading by heavy machinery has thus to be done parallel to the wall face.

2. Procedure of soil spreading

The soil spreading started from inside of the wall (at a greater distance from the wall face) will lead to sagging of geosynthetic layer and bulging of wall face (Figure 12.9b). It should be started from the wall face and then proceeded inward.

3. Gabion installation

The soil spreading close to the gabions or other face support is difficult since heavy machinery may damage the gabions and also presents safety hazards. Therefore, it is recommended that the labor be used for spreading within a distance of 1 m from the wall face. A well compaction is desirable, and thus machinery is used as much as possible if safety hazards do not exist.

4. Sudden stopping or turning of heavy machinery

A sudden stopping or turning of heavy machinery will disturb the soil and must be avoided. If it is done in the reinforced soil zone, the geosynthetic layer may be disturbed and damaged (Figure 12.9c).

 

Photo 12.3 Gabions Acting as Temporary Face Support

 

 

Photo 12.4 Metals Acting as Temporary Face Support

 

12.8 SOIL COMPACTION

Soil compaction affects greatly the stability and durability of GRS-RW. The following precautions should be taken during compaction:

1. The specified degree of compaction should be obtained in the field and should be uniform throughout the GRS-RW.

2. The machinery used in compaction should not exhibit potential of disturbing or damaging geosynthetics. It should be selected considering also the soil properties, scale of project and site working conditions.

3. The compaction should be completed at the end of each working day. If precipitation is expected, appropriate measures should be taken.

4. Soil compaction should not be conducted during rainy days.

 

Since the degree of compaction affects subsequent stability and durability of GRS-RW, "model construction" is normally conducted prior to actual construction. For small-scale construction involving less than 30,000 m3 of soil, the model trial construction may be conducted as part of actual construction project.

1. The degree of compaction is discussed in Chapter 13. The uniform distribution of load, possible strength degradation after absorbing water, and deformation by surcharge load, are some additional factors to be considered in constructing an uniform soil structure.

2. For safety reasons, compaction within 1 m from the slope face is normally conducted using light compaction plant. A good compaction is desirable, and thus machinery is used as much as possible if safety hazards do not exist (Photos 12.5, 12.6).

Table 12.1 shows the machineries used for compacting different types of soil. The tamping roller and other types of compaction plant that may disturb or damage geosynthetic layer during turning should be avoided.

3. The compaction should be completed within each day. If the work is terminated after soil spreading, precipitation overnight may soften the soil and thus affect compaction of the following day.

If precipitation is anticipated, the surface of compacted soil layer should be covered with water-proof sheets. The drain channels should be provided so that the precipitation will not percolate into backfill soil.

4. The construction is not proceeded during rainy days. In the presence of precipitation, the soil will be under-compacted. The high water content will also act as a source of future settlement. Following rainfall, the construction should be started by first confirming the water content of the soil and trafficability.

 

Photo 12.5 Compaction Close to the Gabions

 

Photo 12.6 Compaction of Backfill Soil Using a Compaction Plant

 

 

12.9 FACING

The facing is a rigid concrete member installed in front of the wall after completion of backfilling.

 

The structural facing is constructed to resist the earth pressure acting in the wall, thus increasing its stability. The rigidity of the wall results in smaller deformation of GRS-RW. This characteristic is highly evaluated in the RRR construction method to effectively reinforce the soil.

The quality of concrete and reinforcing bars follows that in the Highway Earthwork Specifications.

The construction method follows the JSCE Concrete Specification.

 

The general procedures are

1. to erect temporary scaffold

2. to assemble reinforcement casings

3. to put on the form

4. to place concrete

 

 

Table 12.1 Types of Soil and Compaction Plants

Types of compactor

a.

Vibrating roller, 20 ton

g.

Vibrating compactor

b.

Vibrating roller, 13 ton

h.

Rammer, tamper

c.

Vibrating roller, 5 ton

i.

Tire roller (self-driving type)

d.

Soil compactor

j.

Tire roller (pulled type)

e.

Tamping roller

k.

Bulldozer (common type)

f.

Load roller

l.

Bulldozer (wet-land type)

Notations:

t: soil thickness (cm)

1: effective

2: may be used

3: used only if other machineries cannot be used due to trafficability problem

4: used due to space limitation such that other machineries cannot be used

 

(a) Embankment and road structure

Types of soil

t

a

b

c

d

e

f

g

h

i

j

k

l

 

60-100

1

 

 

 

 

 

 

 

 

 

 

 

stiff rock mass, not easily crushed

30-60

1

2

 

 

 

 

 

 

 

 

 

 

 

30

1

2

2

 

 

 

4

4

 

 

 

 

fractured rock, soft rock

30

1

2

2

1

1

 

4

4

2

2

 

 

uniform sand or gravel, little fines

30

2

2

2

 

 

 

4

4

2

2

 

 

well-graded sand and gravel

30

2

2

2

 

 

 

4

4

2

2

 

 

silty soil (high water content)

30

 

 

 

 

 

 

 

 

 

 

3

3

 

(b) Subgrade and Road Foundation

Types of soil

t

a

b

c

d

e

f

g

h

i

j

k

l

well-graded soil

20

1

2

2

 

 

2

4

4

2

 

 

 

poorly graded sand and gravel

20

1

2

2

 

 

2

4

4

2

 

3

3

 

(c) Backfill

Types of soil

t

a

b

c

d

e

f

g

h

i

j

k

l

backfill soil*

20-30

1

2

2

 

 

 

4

4

2

 

 

 

* drop-hammer may be used

1. Temporary Scaffold

The scaffold is erected for subsequent assembly of the form and placement of the concrete. The single-pipe or assembled-form scaffold is usually adopted (Figure 12.10, Photo 12.7).

 

 

 

Photo 12.7 Scaffold

 

 

2. Reinforcement Casing

The facing used in the RRR method is normally unreinforced concrete. Sometimes, the reinforcements are used.

 

3. Form

The form used should be rigid and stable enough so that it would not deform during or after placement of concrete, or allowing mortar to flow out. Since the anchorage of geosynthetic to the facing is required to secure an increased stability in the RRR method, the form is not placed at the back of the facing.

 

The followings are several precautions to be taken in assembling the form:

a) The site supervisor should be consulted on the structure of the form and the time to disassemble it.

Since no form is employed at the back, special effort is needed in fixing them. An example of fixing is shown in Figure 12.11. The anchors installed in the wall are welded to the separator attached to the form. That is, during form installation or concrete placement, the anchor should not be pulled out from the wall or tilted. The L-shape steel bar is used to connect the anchors at the same elevation. The angle of the bar should not damage the geosynthetic, so a geotextile layer is used as protection.

b) The form is usually assembled from veneers, and sometimes metals (Photo 12.8) and other large-scale form may be considered. The crane may be used throughout construction at the site, but safety precautions should be taken. This should be taken into consideration during the stage of construction planning.

c) If aesthetic appearance is of concern, the "decorated form" may be used (Photo 12.9). The type of material of the decorated form should be consulted. Meanwhile, the instructions on usage of the material should also be confirmed.

 

Photo 12.8 Form for Facing Construction

 

Photo 12.9 Decorated Facing

 

 

4. Concrete Placement

The placement is similar to that of the common concrete construction, with the following precautions to be taken:

a) Care should be taken not to disturb or damage the drainage pipes, fasteners, and other buried items during concrete placement.

b) At the construction joint, special treatment should be conducted. This is normally done by using high-pressure jet, but other methods may be selected if appropriate.

c) If the facing is constructed of unreinforced concrete, then the steel bars should be inserted at the construction joint. (see Chapter 10, Structural Details)

d) The joint to account for expansion and contraction of concrete wall should be included. (see Chapter 10, Structural Details)

To avoid adverse effect on the wall due to ground settlements, such as by consolidation, the interval of expansion joint should be kept below 20 m, preferably between 10 and 15 m. If the longitudinal direction of GRS-RW runs along a slope, the joint interval should be considered accordingly.

For massive concrete wall, the cracks may develop due to temperature variation. This may have to be considered for construction done during summer.

The cracks may be avoided by using reinforcing bars or taking precautions for the gaps that may lead to cracks.

e) The time of concrete placement should be consulted with the supervisor. His agreement is needed before placement.

To avoid the adverse effects of foundation and wall settlements, the placement of concrete facing is done after most anticipated settlement is completed.

If the settlement of foundation and wall backfill may affect the concrete facing, the time of its placement should be changed.

The concrete is placed longitudinally for each wall block, proceeded from the bottom to the top.

 

12.10 DRAINAGE

The drains should be installed during construction and after completion of GRS-RW.

1. Surface drainage during construction

2. Drainage inside GRS-RW

3. Drainage surrounding GRS-RW

Drainage is required in GRS-RW construction and to secure stability after completion of structure.

The friction between soil and geosynthetic is required. An increase in the pore water pressure in the soil, such as by seepage or rainfall, will adversely affect the friction and thus the stability of GRS-RW. The seepage into reinforced soil should be avoided.

Geotextiles having drainage capability are sometimes used with geogrids. Since nonwoven geotextiles absorb water, they should be protected from precipitation by covering with water-proof sheets.

1. The precipitation from rainfall or snow reduces the strength of the surface soil and affects the construction. The rain also erodes the soil.

Although GRS-RW exhibits less problems due to precipitation when compared to conventional embankment, surface drainage is still required. The water-proof sheets are used to keep the water off backfill soil.

Some precautions are required during construction:

a) The surface is compacted to be flat so that pounding does not occur.

b) The drainage channels are made on the surface with a few percent gradient to allow for good drainage.

c) The debris accumulated in the drain should be removed as quickly as possible.

d) The drainage facility should be constructed as soon as possible, fully or partially.

e) If the construction has to be conducted during rainy period or when the soil of high water content has to be used, the soil should be "dried". The use of filter material or nonwoven geotextile at regular spacing is recommended.

 

 

 

 

2. The water percolating into the backfill soil will increase the pore water pressure. The drainage should be considered by using

a) Drainage pipes - Nonwoven geotextile layers are used at the inside end of drainage pipe to prevent loss of soil. During construction and compaction, the gradient of pipes may be disturbed. Therefore, it is normally done by installing a socket in between the gabions during construction. The socket is slightly larger than the pipe diameter so that the drainage pipe will later be inserted into it (Figure 12.12).

b) The channel should be constructed along the boundaries of GRS-RW and between GRS-RW and existing embankment (Figure 12.13).

3. A drainage channel is constructed along the bottom front of the facing. It drains rain water as well as possible seepage from the back of the slope and prevents erosion at the toe. The U-shape reinforced concrete channel (JIS A 5305) is typically used (Figure 12.14).

 

 

 

  

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