1.0 HYBRID SYSTEM

Any wall which uses facing units (of any type) tied to rods or strips (of any material) which have

their ends anchored into the ground is a Hybrid Wall.

These are walls which combine elements of both externally stabilized walls (e.g. gravity walls)

and internally stabilized walls (e.g. reinforced soil):

a) Tailed Gabion

b) Tieback Anchor Wall / Anchored Earth

c) Steel Sheet Pile Wall

d) Tailed Concrete Block

1.1 Tailed Gabion

1.1.1 Definition

Gabion walls are compartmented units filled with stone that are 4” to 8” in size. Each unit is a

rectangular basket made of galvanized steel, geosynthetic grid, or polyvinylchloride (PVC)

coated wire. Each gabion unit is laced together on-site and filled with select stone. Gabion walls

can be designed with wire mesh or geosynthetic reinforcement that extends back into the

retained soil from between the gabion unit. These wall systems are termed tailed gabions.

Figure: Typical section of tailed gabions

Main components of tailed gabions are:

i) gabion structure

-consists of gabion box and stone filling

ii) mesh reinforcement

iii) compacted soil

- for back-filling behind the gabion.

Mesh reinforcementGabion box

1.1.2 Functions

This tailed gabion system is similarly functioned as the normal gabion. It can be used in many

situations including the stabilization of earth movement and erosion, river control, reservoirs,

canal refurbishment, landscaping, channel linings, gabion wires and also as retaining walls.

a) Retain at the river banks

It is useful to avoid erosion in streams where the mattress goes into the river and

underneath a gabion wall to stop the river undercutting the wall at the edge where the water

speed is fastest. It is also offer free drainage providing a higher bank stability when used for

river bank protection.

b) As landscaping

Gabions are customizable and can fabricate curves for home garden use. The gabion can

be planted in to soften edges. When it is used for landscaping purpose, the versatility is

limited by imagination.

c) As retaining walls

Gabion walls are suitable for retaining walls near to roads, dams, military context and also

domestic project. Gabions are easy enough to install that no experience is necessary to get

a good result.

d) As gabion cladding

Gabion used as cladding can provide an attractive finish to the buildings, walls and many

other situations. Gabions of 0.3m to 0.5m generally used and need to be attached to the

existing building using clamp bars. At 4m to 5m heights, 4mm diameter wire basket will be

used. Meanwhile, for more than 5m heights, 5mm diameter wire basket will be used.

e) As gabion fencing

Gabion now becomes an important feature in many situations and the construction thin walls

from narrow depth gabions. As required vertical faces and narrow depth, extra support to

provide stability and also can use metal or concrete posts at regular intervals inside the

gabion basket.

1.1.3 Design considerations

i) Must be flexible and free draining system

ii) The design must follow the Design Criteria BS8002- The Code of Conduct for Earth

Retaining Structure

iii) It is normal to incline the wall from vertical by 6/10° for aesthetic and stability reason.

iv) It can be build with speed and economy in all circumstances and particularly suitable for

liable to subside.

v) The capacity of gabion to deform makes them preferable to concrete wall which would

crack and collapse.

vi) It is fabricated by wire coated with polyvinyl chloride to prevent corrosion.

vii) It is usually filled with stones which require qualities of weather resistance, non-

dissolution and high stiffness.

1.1.4 Method of construction

i) Assembly and Installation

For easy handling and shipping, gabions are supplied folded into a flat position. They are readily

assembled by unfolding and by simply wiring the edges together and the diaphragms to the

sides. Adjoining gabions are wired together at their edges. Empty gabions, stacked on filled

gabions, are connected with lacing wire to the filled gabions at front and back. After the gabion

is filled, the top is folded shut and wired to the ends, sides, and diaphragms. Where practical,

several gabions should be wired together and stretched before filling to reveal weak areas of

lacing. Gabions may be filled by almost any type of earth handling equipment such as front-end

loader, crane, or modified concrete bucket. Some hand stone adjustment during the filling

operation is required to prevent undue voids. Exposed faces should be hand placed using

selected stone. This will add to the appearance of the structure and prevent the gabions from

bulging.

ii) Mechanically Applied Fasteners

Mechanically applied fasteners (rings or clips) can be used to connect panels, diaphragms or

adjacent cells in lieu of lacing wire. Only Class 3 coated galvanized clips or rings should be

used with galvanized gabions. Only stainless steel rings or clips should be used with PVC

coated gabions. Hog rings must have a minimum overlap of 1in (25mm) when closed.

iii) Erection of the Gabion Structure

This section applies to 3ft (1.0m), 1.5ft (0.5m), and 1ft (0.3m) thick gabions used in the

construction of retaining walls, weir walls, stepped revetments, and flared zones of transition

between retaining walls and sloped revetments. When gabion units, or modules, are placed into

the structure the inspector must be concerned with the alignment, grade, stretching, and

attachment of the individual units to each other. The alignment and grade are established by the

field engineer or surveyor but should be checked. The stretching and attachment of the gabions

to each other are of primary interest to the inspector. When several units, up to 100 ft. (33.0m),

are in place and aligned, they should be stretched to tighten the wire fabric. This operation

makes it much easier to control bulging during the filling operation. An anchor for the stretching

operation can best be obtained by completely filling the end cell of the end unit. This may be

accomplished either by machine or by hand filling. The inner tie wires (or stiffeners) shall be

properly installed in both directions in this anchor cell (tie wires will be discussed later). The

pulling force is then applied to the opposite end of the string. The attachment method for

applying the pulling force should be such that the force is applied to at least 4 places on the

heavy vertical selvedge wires forming the corners. The force should NOT be applied to the

fabric of the end panel.

While the stretching force is applied, the next to the last cell of the string is filled in the same

manner as before. This will leave the last cell empty to provide for easier attachment of

additional units. Subsequent gabions or gabion modules will be placed in the same manner with

only the next to the last cell being filled during the stretching operation. There will be times,

depending upon the type of structure or the physical limitations of the project site, when it will be

impossible to stretch the units. This will not affect the structural integrity of the structure.

However, additional care will be needed during the filling operation to maintain alignment and

prevent bulging. Typical situations in which this can occur include: (a) counter fortes behind

retaining walls, (b) the main wall of weirs or drop structures, and (c) curved or serpentine walls.

Gabion counter fortes and weir walls extend into narrow notches excavated into hill sides or

stream banks which allow for very little working room. The shorter the radius

of curvature of serpentine or curved walls, the more difficult it will be to apply a stretching force

to a long string of gabions.

iv) Filling Gabions Cells

The rock used to fill gabions must be of sound rip-rap quality, preferably graded from 4in to 8in

(100mm to 200mm) in size, and roughly cubical in shape. This will allow for the maximum

amount of machine filling. There will be some minus 4in (100mm) material present, due to

breakage in transit, but this should be limited to a maximum of about 5%. No minus 4in

(100mm) material should be permitted on any exposed face of the structure since it can fall out,

be washed out, or pulled out, thus creating a void in the structure. Soil (picked up from the

bottom of the rock pile) shall not be permitted to remain in the structure as a seam or “block”.

Some dirt or fine material is inevitable, but the amount must be minimal. Soil should be brushed

or manipulated in a manner so as to provide for the required point to point contact of the large

rock fill.

Excessive amounts of sand/soil will wash out and create large voids in the gabion structure.

Rock should be placed in 1ft lifts in the gabion structure, moving from cell to adjacent cell. A row

of 5in to 8in (125 to 200mm) size rock should be hand placed against the exposed face(s)

during the filling to present a pleasing appearance and minimize the size of exposed voids. The

first fill layer is then leveled to permit the installation of the inner tie wires. When at all possible,

the maximum difference of the rock level in adjacent cells should not exceed 1ft (0.3m). When

1.5ft (0.5m) deep gabions are used in a retaining wall or weir wall configuration they shall have

the inner tie wires, placed only at mid-height. 1ft (300mm) deep gabions, in the same situation,

do not need the inner tie wires, but should be filled in half height lifts. Inner tie wires or stiffeners

are needed to help brace exposed, or temporarily unsupported, faces of the gabions.

In a long retaining wall configuration they are placed transverse to the long axis of the wall.

Only the end cells, or temporary end cells, will need inner tie wires in both directions. The inner

cells of walls which have thick layers, say 9ft (225mm) or 12ft (300mm) or more, do not need

the inner tie wires if the adjacent cells are filled in 1ft (0.3m) lifts uniformly. After the first 1ft

(0.3m) lift of rock is laced and the inner tie wires installed, the second layer and inner ties and

then the third layer are placed using the same sequence of operations. The last layer should be

filled approximately 2in (50mm) above the top of the gabion to allow for subsequent settling of

the rock fill. The top should be roughly leveled (no large humps or voids), and the lid closed, and

then attached to the tops of the diaphragms, the ends, and the fronts.

v) Backfilling behind the Gabion Structure

The inspector should be very familiar with the entire project specifications involving the type of

backfill material and degree of compaction required. Backfilling any gabion structure is

performed in the same manner as in any other type of construction. Most care is needed to

ensure the gabion mesh wires are not damaged or broken by contact with the compaction

equipment. Caution should be exercised when backfilling a single long “string” of gabions (as in

a retaining wall). To achieve the normally specified degree of compaction, the compactive effort

may be sufficient to push the gabions out of alignment. If the gabions are not embedded below

grade, or laced to gabions which are embedded, alignment control may become a problem.

One possible solution would be to place a temporary fill in front of the gabion string to help hold

alignment. Sheets of plywood can be used to keep the temporary fill from infiltrating the voids of

the front face of the gabions. When compaction is completed, the temporary fill may be

removed. This method may cost a little in time and effort, but it is far cheaper, and much more

satisfactory, then trying to push the filled gabion string back into alignment. The degree of

compaction of backfill around gabions placed in notches in stream banks (weirs), or around

gabion counter fortes for retaining walls, is critical to satisfactory performance of the structure. If

doubts arise the Resident Engineer and the Design Engineer should be contacted immediately.

1.1.5 Advantages

a) Flexibility

It is subjected to alternating forces of tension and compression, the inherent flexibility of a

gabion structure enables it to deform rather than break. This is to prevent loss of efficiency.

Deforming in response to subsidence of foundation or internal stress is a functional feature.

b) Strength

Since gabions are bound together as a monolithic unit, the wire mesh is extremely strong

under tension. The wire mesh shell is not simply a container for the stone filling, but a

reinforcement of the entire structure. Additional strength is achieved by the use of the

vertical diaphragms. These diaphragms are affixed to the base of the gabions to restrict

internal movement of the stone filling and provide further reinforcement. Gabion efficiency ia

actually increasing despite of increasing in ages. During early periods of use, silt and

vegetation will collect within the rock filling to form a naturally permanent structure,

enhancing the environment.

c) Permeability

Interstitial spaces in the stone filling within the baskets provide a great degree of

permeability throughout the structure eliminating the need for a drainage system and

preventing build up of hydrostatic pressure which will displace and crack concrete

structures. In the river works, pressure and counter pressure on the banks due to the

variations in water depth between flood and low water are therefore also eliminated.

d) Economy

Wire mesh gabions are less expensive than most construction materials. Graded stone fill is

usually available and waste materials such as crushed concrete may be specified in place of

stone. Construction costs are reduced, unskilled labors can easily learn to erect modular

gabion system, fill them and close them properly. Many gabion structures may be built

without any mechanical equipment. Pilings, underwater drainage systems and excavations

are unnecessary. There is no need to drain the site or to construct a cofferdam for

underwater installation. The first layer gabions can be laid in water or in mud. Upon

completion, a gabion structure will take its full load immediately, without the waiting periods

of up to one month normally associated with concrete structures. Gabion structure are

virtually maintenance free.

e) Extendable

Extensions are simple. Additional units are simply attached to the existing ones. The gabion

wire diameter can range from 2mm to 8mm and the gabion basket can either be welded or

woven, galvanised or plastic coated, depending on application. Baskets come in flat pack

form and are assembled on site. They can be used in a range of applications from small

domestic projects to large industrial schemes. The infill to the baskets is usually locally

sourced stone ranging from 200mm - 100mm. Hand facing ensures that the wall has good

cosmetic appearance. Gabion advantages include flexibility, durability, strength,

permeability, economy, and landscaping.

f) Very porous, durable, reliable, easily constructed and ecological.

1.1.6 Disadvantages

May be considered unsightly by some people, they may take up quite a lot of space up on the

beach as lots of gabions are needed to provide plenty of protection. This makes the beach less

desirable to people as the beach may seem more crowded.

If the rocks inside the cage become worn down the whole cage will need to be taken out and to

be replaced with new ones. If the gabions are stacked up in a wall and a lower one needs

replacing which means to take out all the above ones to replace it as well as to rebuild the wall.

Hence, this would take lots of time and cost.

1.2 Tieback Anchor Wall/ Anchored Earth

1.2.1 Definition

An anchored wall is any non-gravity cantilevered wall (i.e. sheet-pile wall, soldier pile and

lagging wall, slurry (diaphragm) wall, tangent pile/secant pile wall, or soil mixed wall (SMW))

which relies on one or more levels of ground anchors (tiebacks) or deadman anchors for

additional lateral support. The use of anchors enables these walls to be higher and deflect less

than walls without anchors, (i.e., cantilever walls). An anchor is a structural system designed to

transmit tensile loads to the retained soil behind a potential slip surface. Construction of the

vertical wall elements and lagging (if required) for an anchored wall proceeds from the top-down

as for all non-gravity cantilevered walls. When the elevation of the excavation in front of the wall

reaches approximately 3 feet below the specified elevation of an anchor, the process of

excavation is temporarily suspended and anchors are installed at the specified elevation. An

anchor is installed using drilling and grouting procedures consistent with the anchor type and

prevailing soil conditions. Each anchor is tested following its installation. Typical permanent

facing panels include CIP or precast concrete with natural, textured, or architectural finishes.

Figure: Helical tieback anchor system

Figure: Tieback anchor wall or anchored earth

Main components of tieback anchor wall are:

i) Lead section

Lead sections are composed of the steel anchor shaft with the helix plates welded to it.

Helix plates are typically spaced three helix diameters from one another along the lead

section shaft. Lead sections can have a variety of lengths, generally ranging from 10

inches to 10 feet (0.25 to 3 meters).

ii) Extension rod

It connects the helical anchor to adapter through no-load zone.

iii) Thread Bar Adapter

It connects the Helical Pier anchor and the thread bar together.

1.2.2 Functions

i) In order to minimize wall movement and ground settlement, tieback anchors are

designed to achieve the highest stiffness possible within economical considerations.

ii) Tiebacks for permanent retaining walls constructed of any materials such as cast-in-

place concrete, shotcrete, gunite, soldier beams and wood or concrete lagging.

iii) Permanent tension hold-downs for wind and seismic loads.

iv) Tiebacks for permanent or temporary shoring.

v) Anywhere where lateral loads must be resisted.

vi) All locations where grouted tiebacks are specified and the anchor zone is not in

competent rock.

1.2.3 Design Considerations

i) The wall system shall be designed to resist maximum anticipated loadings calculated for

the effects of any special loadings shown on the contract plans.

ii) The wall shall be designed to ensure stability against passive failure of the embedded

portion of the vertical wall elements (below the base of excavation). The minimum FS

shall be 1.5, unless otherwise noted on the contract plans.

iii) The axial load carrying capacity of the embedded portion of the vertical wall elements

(below the base of the excavation) shall be evaluated. The minimum FS shall be 2.5 for

elements terminating in soil and 2.0 for elements terminating on rock, unless otherwise

noted in the contract plans. The wall shall be designed to resist vertical loads including

vertical anchor forces and the weight of the lagging and the vertical wall elements.

Relying on transfer of vertical load into the soil behind the wall by friction shall not be

permitted, unless approved by the Engineer.

iv) Permanent facing shall be precast or cast-in-place reinforced concrete. Architectural

facing treatments, if required, shall be as indicated on the contract drawings. The facing

shall extend a minimum of 2.0 ft below the gutter line or, if applicable , the ground line

adjacent to the wall unless otherwise indicated on the contract drawings.

v) The external stability of the wall shall be evaluated. Failure surfaces extending beyond

the ends of the ground anchors and below the bottom of the wall shall be checked using

slope stability calculations. The minimum FS with respect to external stability shall be

1.3, or 1.5 for critical wall systems as designated in the contract plans.

vi) Wall Drainage. The wall drainage system shall operate by gravity and shall be capable

of relieving water pressures on the back face of the wall under anticipated worst case

water pressure conditions. When drainage systems are incorporated into the specific

design, hydrostatic head on the back of the wall shall not exceed 6 inches above the

elevation of the drainage collection pipe.

1.2.4 Method of Construction

Typically round corner square shaft anchors are utilized employing a wide variety of helical

leads. The shaft size and helical plate sizes are chosen for site specific soil and construction

needs. Anchors are placed typically using small hydraulic excavators to which is mounted a

rotary drive head. Pressure monitoring of the drive head during installation coupled with site

anchor tests, allows control of installation torques. Anchors placed to the defined depths and

installation torques give excellent repeatability of verifiable anchor capacity at each installation

point, a huge advantage over grouted bar anchors. Ultimate Helical anchor load capacities

using the A.B. Chance square shaft anchors is between 70 -200 kips (315- 900 kN). The

photographs shown on the next few pages demonstrate a wide variety of possible retaining wall

structures or tiebacks that can be easily done utilizing helical anchors.

A tieback is made by first drilling a hole with an auger and then placing a bar (tendon) in the

hole, concrete is then poured in the hole and the connection with wall is made (Figure 3).

Different types of augers are used to drill the tieback holes. The choice of the drilling method

depends on the soil/rock conditions on the site.

Figure 3: Tieback configuration, free and fixed lengths (Adapted from Schnabel, 1982)

Drilling should be done carefully since inadequate procedures can cause significant soil losses.

The biproduct of drilling is removed by flushing the hole with either air, water, or slurry. Air is

most efficient in dry ground, but it requires special attention because it can become entrapped

during drilling, building up zones of high pressure in the soil that can eject material for several

feet and at high speeds (potentially injuring workers). Water flushing is best used in sticky

clayey soil, and it also cleans the sides of the hole by its sweeping action, providing a stronger

bond at the grout-anchor interface. Bentonite slurry flushing works the best since it keeps

particles in suspension, while the sealing action keeps the hole from collapsing.

Figure: Steps in making a tieback: (a) hole drilled; (b) bar placed in hole; (c) concrete poured for

anchor; (d) wall connection made (Adapted from Schnabel, 1982).

Significant soil losses through the tiebacks cause significant settlements even if the retaining

walls do not move towards the excavation. In granular soils the drilled hole must be cased to

avoid collapse. Some tieback creep can be expected especially if the ties are very short and

the fixed length of the tie is within soft ground. For stability reasons, the fixed anchor should be

located beyond the active zone of movements. As a result, tieback anchors may not be an

option at sites congested where there are adjacent underground utilities or when adjacent

owners do not grant permission to drill them under their properties.

Special attention should be given to the waterproofing details at the anchor heads and at the

tieback holes. Significant leakage can be caused by inadequate waterstopping details at these

locations.

1.2.5 Advantages of tieback anchor wall

i) Tiebacks may be installed with conventional rotary drilling equipment

ii) Screw anchors are quick to install. Typical installing time per tie back is 20-30 minutes

with no spoils removal necessary.

iii) Elimination of grout allows for immediate loading or testing.

iv) In some cases the screw anchors can be withdrawn and reused

1.2.6 Disadvantages of tieback anchor wall

Like all foundation alternatives, there are some conditions where helical anchors should not be

used. Helical anchors should not be installed in subsurface conditions where the encountered

material may damage the helices or the shaft, or where installation depths are restricted. This

would include soils containing cobbles, large amounts of gravel, boulders, and construction

rubble. In extremely soft soils, typically those with standard penetration test (SPT) N-values less

than five, buckling of anchors in compression must be considered during design. Also, standard

helical anchors are not effective in supporting structures with large lateral loads and/or

overturning moments.

1.3 Steel Sheet Pile Wall

1.3.1 Definition

Sheet pile walls are constructed by driving prefabricated sections into the ground. Soil

conditions may allow for the sections to be vibrated into ground instead of it being hammer

driven. The full wall is formed by connecting the joints of adjacent sheet pile sections in

sequential installation. Sheet pile walls provide structural resistance by utilizing the full section.

Steel sheet piles are most commonly used in deep excavations, although reinforced concrete

sheet piles have also being used successfully.

Main components:

i) Steel sheet pile

1.3.2 Functions

i) Waterfront structures

Steel sheet piling is suitable for offshore construction as it is designed to cater for heavy

vertical loads and large bending moments with fast execution. Few types of retaining

structures in waterfront structures are ship berthing area, wharf, breakwater and slipway.

ii) Canalization

Steel sheet piling is used as the support, protection and beautification of river

embankments. It can also function as flood protection wall during monsoon or high-tide

season.

iii) Basement

Steel sheet piling can be used for top down construction (permanent) or bottom up

construction (temporary); both methods require minimal construction width offering

significant time and cost savings. Steel sheet pile can also support vertical loads from

the structure above.

iv) Railway and Tunnel

As support structure for railway and tunnel construction, steel sheet piling helps to

minimize excavation and disposal works, thus speed up the construction duration.

v) General Excavation

General excavation works involve temporary excavation for foundation construction and

protection to underground structure, minimizing excavation volume and area.

vi) Highway Construction

Most of the time, steel sheet piles act as retaining structure for excavation works. They

can be installed easily and extracted anytime when the construction work is completed.

The steel sheet piles can then be re-used and carried forward for later construction

stages.

vii) Bridge

Steel sheet piles can act as foundation and abutment of the bridge, requiring minimum

space and construction time.

1.3.3 Design Considerations

i) Walls of Varying Thickness

Different restraint conditions are created with abrupt changes in wall geometry and by

encasing steel sheet piles in concrete. Under thermal loads produced by heat of

hydration and ambient temperature effects, stress related racking can occur. Generally,

the top of the sheet piles should be placed 9 inches below the point at which the

concrete section thickness is increased, except at each end of the monolith. Two sheet

pile sections at each end of the monolith should be lowered an additional 9 inches,

placing these sheets a total of 18 inches below the thickness change. The sheet piles

located at the monolith joint should be notched down to 9 inches above the base of the

wall. Additional vertical and horizontal reinforcing steel should be placed at the ends of

the monoliths to provide for temperature induced loads.

ii) Corrosion

The corrosion process in sheet piling is highly dependent on the environment in which it

is placed. Generally, uncapped exposed sheet pile corrodes at varying rates averaging

from 2 to 10 mils per year depending on the surrounding atmospheric conditions, i.e.

rural versus heavy industrial. Corrosion rates usually decrease after the first few years of

exposure. Increased corrosion rates for piles in organic or fresh fills should be

anticipated due to oxygen replenishment. The most common way of protecting steel

sheet pile against corrosion is through the use of coatings. Coal tar epoxy and cathodic

protection have become widely accepted for this application.

iii) Liquefaction Potential During Driving

A total ban on driving may be warranted. Pile driving is prevented or limited based upon

the potential for liquefaction at a stage when the water level is above the landside

ground surface and pile driving is planned within 1,500 feet of the levee or flood

protection works. The extent of any limitations placed on pile driving should be evaluated

against the potential for damage to the public.

iv) Settlements

Since the wall cannot easily be modified in grade, the designer should consider the

confidence level of the settlement prediction and overbuild the wall sufficiently to prevent

settlement of the wall below grade. Concrete capping should be delayed until a major

portion of the settlement has occurred. The "after settlement" configuration is used in the

wall overturning analysis. Additionally, as the loads applied to the foundation by the wall

are essentially horizontal the designer has to be cognizant of the fact that lateral

consolidation will occur with sustained loading. This should be evaluated and the wall

system should be capable of compensating for this movement.

1.3.4 Method of Construction

Sheet pile walls are constructed by:

i) Laying out a sequence of sheet pile sections, and ensuring that sheet piles will

interlock.

ii) Driving (or vibrating) the individual sheet piles to the desired depth.

iii) Driving the second sheet pile with the interlocks between the first sheet pile and

second "locked".

iv) Repeating steps 2 & 3 until the wall perimeter is completed.

v) Use connector elements when more complex shapes are used.

1.3.5 Advantages of steel sheet pile wall

i) Provides high resistance to driving stresses.

ii) Light weight.

iii) It can be reused on several projects.

iv) Long service life above or below water with modest protection.

v) Easy to adapt the pile length by either welding or bolting.

vi) Joints are less apt to deform during driving.

1.3.6 Disadvantages of steel sheet pile wall

i) Sections can rarely be used as part of the permanent structure.

ii) Installation of sheet piles is difficult in soils with boulders or cobbles. In such cases, the

desired wall depths may not be reached.

iii) Excavation shapes are dictated by the sheet pile section and interlocking elements.

iv) Sheet pile driving may cause neighborhood disturbance.

v) Settlements in adjacent properties may take place due to installation vibrations.

vi) Noise and vibrations are generated when the sheet piles are driven into the ground by

the vibro-hammer. Silent-piler creates significantly less noise and vibrations. However, it

can be used only when the sheet piles are not required to be driven very deep into the

ground.

1.4 Tailed Concrete Block

1.4.1 Definition

A tailed concrete block also known as segmental retaining wall (SRW) is a structure, consisting

of segmental concrete units or blocks that are dry-stacked in a running bond that relies on the

self-weight of the wall to resist destabilizing forces due to retained soil and surcharge loadings

on the structure.

Main components:

i) Concrete block units

ii) Geo-grid tail/ geo-synthetic reinforcement

1.4.3 Design Considerations

Figure: Principle on wall construction

i) The designer must ensure that the ground is well-compacted and suitable to bear the

loads imposed by the wall.

ii) The levelling pad may be of concrete, mortar, compacted granular fill, or other

suitable material, depending on wall height.

iii) The Non-Woven Geotextile may be omitted if the grading of the Drainage Layer is

such that it is non-clogging and it will minimise wash-out of fines from the Wall Fill.

iv) During Construction heavy plant and compaction equipment must not be used within

2m of the wall face. In this region light compaction equipment such as a vibrating

plate must be used with thin layers of fill.

v) Reinforced fill must be compacted to the standard specified by the designer to

ensure stability of the wall and to minimise risk of settlement.

1.4.4 Method of construction

i) Prepare levelling Pad and lay base course of blocks, ensuring strict level and alignment

and place Drainage Pipes in position. To set good alignment a temporary stop, e.g

wooden battens nailed in place, can be used. This is removed when the third layer of

blocks is in place with fill compacted behind.

ii) Hold geogrid “Tail” up out of the way and lay and compact Drainage Layer, Non-woven

Geotextile and Wall Back Fill to the level of the Geogrid “Tail”.

iii) Fasten the first layer of reinforcement Geogrid to the “Tails”, stretch it out away from the

wall to tighten the joints and lay the next layer of fill on it. Note: Do not drive any

equipment directly on the geogrid. Place fill first.

iv) Lay the next course(s) of Blocks up to the next layer of reinforcement.

v) Complete laying and compacting drainage layer, non-woven geotextile and fill to the

level of the next layer of reinforcement.

vi) Repeat this sequence to the top of the wall.

1.4.4 Advantages of tailed concrete block/ masonry

i) Easy installation

Segmental retaining walls are becoming the chosen product for homeowners and

landscapers. The reasons for this is the simple fact that, they are easy to install.

Because they are dry-stacked you don’t have to wait for mortar to set and because they

are separate units, it is possible for one person to do the job.

ii) Low costs

Another very attractive benefit of segmental retaining walls is the low costs involved.

Savings are generated due to the fact that no mortar is needed, it can be a one-man job,

and no major machinery is needed. There are different costs for the type of block that is

used. One can use brick, natural stone, rocks or concrete blocks. Natural stone and

brick are usually more costly.

iii) Performance and Durability of Concrete Block Retaining Walls

The properties of the concrete used to produce segmental retaining walls consist of

characteristic low absorption and high quality materials. This enables the units to fight

the effects of weather, erosion ands insect invasion. Segmental retaining walls will not

crack, due to being ‘separate’ and not mortared together; they fit in together easily,

without affecting one another.

iv) Aesthetics

Segmental Block Retaining Walls can create an attractive feature to your garden and

landscape layout. They are available in many sizes, colors, textures and shapes.

Segmental Retaining Walls are not only aesthetically appealing, but they are structurally

sound, making them a dream for homeowners, designers and contractors.

v) Design flexibility

Segmental Retaining Walls are available in units of various sizes and weights, making it

possible to construct a Retaining Wall in the oddest of places. If you have curves or

other unique layouts, you will have no trouble installing a Segmental Retaining Wall

system. Segmental Retaining Walls are so versatile that they are ideal for both heavy-

duty walls in commercial projects and smaller sites in residential backyards.

vi) Provides support

Segmental Retaining Walls provide support to vertical slopping land. They help to retain

the soil in its place, rather than have it collapsing and eventually being washed away into

our storm water runoff systems. This helps the environment by retaining unstable soil

while preserving our waterways.