soils reinforcement retaing walls- التربة المسلحة في الجدران...

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Geotextileجیوتكستیل

تطورت أنظمة تسلیح التربة بشكل فعال حیث ظھر كثیر من المواد البولیمیرية التي

الخصائص واالستعماالت المختلفة يتم صناعتھا أساسا من األلیاف الصناعیة وذات

مثل النسیج الصناعي والشبك الصناعي وقد تم استخدام ھذه المواد في مجاالت

مختلفة أھمھا إنشاء الطرق علي األراضي الضعیفة وجسور السكك الحديدية

وأسفل المنشآت البحرية مثل حماية الشواطئ واألرصفة والمیول ودعامات الكباري

وأخیرا تم استخدامھا في تقوية التربة أسفل األساسات والحوائط الساندة

تتحسن خواص التربة الضعیفة المسلحة بزيادة عدد الطبقات مثل زيادة قدرة تحمل

ح لیس له أي تأثیر التربة وعالقة اإلجھاد االنفعال ومقاومة التربة للقص. لكن التسلی

علي ضغط المیاه البینیة داخل التربة. أثبتت التجارب المعملیة أن التربة الضعیفة

المسلحة بالشبك الصناعي تعطي تحسنا في خواص التربة أفضل نسبیاً من التربة

المسلحة بالنسیج الصناعي. تتحسن خواص التربة الضعیفة المسلحة بزيادة عدد

طويل. ولكن التحسن في خواص التربة المسلحة علي الطبقات علي المدى ال

المدى القصیر أفضل منه علي المدي الطويل. النسیج الصناعي يقلل من االزحات

الرأسیة واألفقیة ويعید توزيع اإلجھادات علي مساحة أكبر داخل التربة المسلحة.

ن تتحسن خواص التربة المسلحة كلما اتجھت طبقة التسلیح نحو األساس. ولك

طول طبقة التسلیح له تأثیر ضعیف علي خصائص التربة المسلحة. أثبتت الدراسة

النظرية أن كلما قلت المسافة بین الطبقات كلما زاد التحسن في خواص التربة

المسلحة لذلك أحسن مسافة بین الطبقات تعادل خمس عرض األساس.

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Mechanically Stabilized Earth (MSE SRW) Wall is a composite construction

material in which the strength of the engineering fill is enhanced by the addition

of strong tensile reinforcement in the form of metal strips, geotextiles, or

geogrids .

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the behaviour of the MSE wall is dependent on wall height , stiffness and

dimension of geogrid, and other parameters that is studed . also the MSE walls

costs 20 to 25 percent less than that of CIP concrete walls.

وتعني األرض , geo ان كلمة الجیوتكستیل تتألف من شطرين : (جیو)

. وتعني نسیج textile و(تكستیل)

فھي إذا عبارة عن نسیج مستخدم في علوم التربة التكنیكیة ( الجیوتكنیك

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وان كانت Geotextile), فان كانت ھذه المادة نفوذه سمیت جیوتكستیل

وان كانت شبكة بالســــــتیكیة , Geomembraneكتیمة سمیت جیوممبران

. Geogrid سمیـــت جیوغريد

1958دة االمريكیة عام ستخدم الجیوتكستیل الول مرة في الواليات المتح-

. في مرفا وكان منسوجا ووضع تحت بالطات بیتونیة

لكن البداية الفعلیة والحقیقیة كانت في فرنسا وانكلترا في الردمیات عام -

non woven geotextile" حیث استخدم الجیوتكستیل غیر المنسوج1968

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- Drainage--The geosynthetic acts as a drain to carry fluid flows through less permeable soils. For example, geotextiles are used

to dissipate pore water pressure at the base of roadway embank-ments. -

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The geogrid is sandwiched between the facing blocks, and is

hooked over the dowels that connect the blocks.

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Segmental Retaining Walls (SRW)

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Segmental retaining walls are modular block retaining walls used for

vertical grade change applications.

The walls are designed and constructed as either gravity retaining walls

(conventional) or reinforced soil retaining walls.

The system consists of dry-cast concrete units that are placed with-

out mortar (dry stacked) and rely on their unit to unit interface and mass

to resist overturning and sliding. Unit to unit interfaces include friction,

shear elements, and interlock.

The systems may also employ soil reinforcement that extends into the

backfill and allows for the construction of walls with significant height

(e.g. in excess of 50 ft (15.24 m)) that could not be accomplished with

the units alone.

Segmental retaining walls are considered flexible structures, so the foot-

ing does not need to be placed below the frost line, provided there is suf-

ficient foundation bearing capacity.

SRW units are manufactured in conformance with industry standards

and specifi cations to assure that units delivered to a project are uniform

in weight, dimensional tolerances, strength, and durability—features not

necessarily provided in site cast materials.

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Retaining Walls

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The reinforcement is placed in horizontal layers between succes-

sive layers of granular soil backfill. Each layer of backfill consists

of one or more compaced lifts.

A free draining, nonplastic backfill soil is required to ensure ade-

quate performance of the wall system.

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This mechanically stabilized earth wall uses “L” shaped wire forms stacked upon each other with an incremental setback as its facing.

Geo-grid reinforcement is strategically located between designated forms and is extended back into the granular soil backfill.

Each layer of backfill consists of one or more compacted lifts. A secondary reinforcement wrap is used at the inside face of the form to add strength to the face as well as to contain the granular backfill.

The facing of a welded wire form wall can also be con-structed with a vegetated face providing a softer, greener, aesthetically pleasing look.

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These earth retaining systems incorporate planar reinforcement, typically geotextile or geogrid, in constructed earth slopes with face inclinations of less than 70 degrees.

The reinforcement is laid down alternatively with horizontal layers of compacted soil backfill. Each layer of backfill consists of one or more compaced lifts.

If slope facing is used to prevent erosion or provide a desired ap-pearance, the facing may be constructed by: 1) extending rein-forcement layers outside the slope face and wrapping each layer around the overlying backfill and then reembedding the free end into the backfill; or 2) extending reinforcement to slope face and then either vegetating the face or placing erosion control mats or prefabricated elements against the slope face.

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Gravity retaining walls depend on the weight of their mass to resist pressures from behind and are ideal for retaining walls up to 1.25 metres high.

Suitable Secura Products include Secura Lite, Secura Major and Secura Grand.

Geogrid is the most commonly used material to reinforce retaining walls above 1.25m metres. It is a mesh-like material installed in layers between the blocks and

covered with backfill which is then compacted.

This creates a soil mass structure behind the wall, resisting the forces acting against it.

Suitable Secura Products include Secura Major and Secura Grand.

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Concrete Backfill

Concrete is poured into the space behind the blocks and the excavated space, it then acts

as a homogenous mass designed to be a mass gravity wall.

This method generally reduces the space required behind the retaining wall by around

half compared to that required for geogrids.

It is a more expensive option so is only specified where space is limited.

Suitbale Secura Products include Secura Major and Secura Grand.

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Structural Backfill Simplified

After installation of the leveling pad, base course and drainpipe,

the first lift of structural backfill can be installed behind wall blocks

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stacked at least two feet above grade.

The backfill should be placed with a skid-type loader or other

equipment, and worked into all voids and cores of the blocks.

Blocks with large cores, or large voids between adjacent blocks,

should be used to accept the larger aggregates in the backfill mix.

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Retaining walls with structural backfill zones are useful in cut wall

applications up to 10 feet high, particularly when lot lines, rock

outcroppings or other obstructions limit the ability to excavate for

geogrid reinforcement. Structural backfill cuts the required exca-

vation depth to 30-40 percent of the wall’s height, depending on

conditions; geosynthetic reinforcement typically requires an exca-

vation depth of at least 60 percent of a wall’s height.

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This construction method eliminates the need for the construction of a

mechanically stabilized earth zone behind the wall facing, reducing the

need for heavy machinery and additional manpower.

The backfill mixture, upon curing, should have at least 25 percent voids,

allowing water to drain behind the wall and serving as the drainage

zone required by most building codes.

Retaining wall systems with structural backfill, like Pavestone’s

Anchorplex wall system, can be built with smaller equipment, less labor

and better production rates than conventional grid-reinforced walls.

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The use of proprietary geosynthetics (varying polymer and construction type)

incorporated as layers into soil has the effect of increasing the shear resistance

of the soil and allows structures to be built using such “soil reinforced”

techniques in a very cost effective manner using well accepted design

methodologies.

Sometimes and by using similar design methodologies the soil reinforcement

element may be a steel strip.

These techniques will generally allow the use of “on-site” soils without the

costs associated with importing specific engineered fill

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In all such applications there will need to be a connection between the structure

face and soil reinforcement element such that a stable element is formed to resist

local face instability and assist with compaction.

Generally steepened slopes do not require facing treatments with the soil

reinforcement element generally terminated at the surface of the structure.

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Sometimes in steepened slope applications there may need to be placed an

erosion blanket on the surface to resist scour of the slope and assist in the

vegetation of the slope.

The method of construction will vary dependent upon the both the design life and

face angle of the structure. For steepened slopes the construction process is very

simple and is generally the inclusion of the soil reinforcement element at the design

elevations and length as fill proceeds with some final trim to the batter upon conclu-

sion

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Over the past two decades, geosynthetic reinforced soil (GRS) struc-tures, including retaining walls, slopes, embankments, roadways, and

load-bearing foundations, have gained increasing popularity.

In construction, GRS structures have demonstrated several distinct advantages over their conventional counterparts. Generally, GRS

structures require less over-excavation and are more ductile, more flexible (hence more tolerant to differential settlement and to seismic loading), more adaptable to low-permeability backfill, easier to con-struct, and more economical than conventional Earth structures.(2–4)

Among the various types of GRS structures, GRS walls have seen far more applications than other types of reinforced soil structures. A

GRS wall comprises two major components: a facing element and a GRS mass. Figure 1 shows a schematic diagram of a typical GRS wall

with a modular block facing.

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Figure 1. Illustration. Typical cross section of a GRS wall with modular

block facing

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Soil is weak in tension and relatively strong in compression and shear.

In a reinforced soil, the soil mass is reinforced by incorporating an in-clusion (or reinforcement) that is strong in tensile resistance.

Through soil reinforcement interface bonding, the reinforcement re-strains lateral deformation of the surrounding soil, increases its con-

finement, reduces its tendency for dilation, and, consequently in-creases the stiffness and strength of the soil mass.

Many studies have been conducted on the behavior of GRS structures; however, the interactive behavior between soil and reinforcement in a GRS

mass has not been fully elucidated.

The reinforcement strength is determined by requiring that the rein-forcement be sufficiently strong to resist Rankine, Coulomb, or at-rest

pressure that is assumed not to be affected by the configuration of the reinforcement.

Specifically, the design strength of the reinforcement, Trequired, has been determined by multiplying an assumed lateral Earth pressure at

a given depth, σh, by the value of reinforcement spacing, Sv, and a safety factor, Fs, as shown by the equation in figure 2.

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Figure 2. Equation. Design strength

Figure 2 implies that as long as the reinforcement strength is kept lin-early proportional to the reinforcement spacing, all walls with the

same σh (walls of a given height with the same backfill compacted to the same density) will behave the same.

In other words, a GRS wall with reinforcement strength T at spacing Sv will behave the same as one with twice the reinforcement strength

(2 × T) at twice the spacing (2 × Sv).

Figure 2 has important practical significance in that it has encouraged designers to use stronger reinforcement at larger spacing because

the use of larger spacing will generally reduce construction time and effort.

Some engineers, however, have learned that figure 2 cannot be true.

In actual construction, reinforcement spacing appears to play a much greater role than reinforcement strength in the performance of a GRS

wall.

Researchers at the Turner-Fairbank Highway Research Center (TFHRC) conducted a series of full-scale experiments in which a weak

reinforcement at a small spacing and a strong reinforcement (with several times the strength of the weak reinforcement) at twice the

spacing were load-tested The former was found to be much stronger than the latter.

The effects of compaction-induced stress (CIS) in unreinforced soil masses and Earth structures have been the subject of many studies.

The effect of CIS is likely to be more significant in a soil mass rein-forced with layers of geosynthetics than in an unreinforced soil mass.

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This is because the interface bonding between the soil and reinforce-ment will increase the degree of restraint to lateral movement of the

soil mass during fill compaction. With greater restraint to lateral movement, the resulting locked-in lateral stresses are likely to be-

come larger.

In addition, GRS walls with modular block facing are rather flexible.

Thus, the design of these structures should consider not only the stresses in the GRS mass but also the deformation.

The Jewell-Milligan method is recognized as the best available method for estimating lateral movement of GRS wallsHowever, it only

applies to walls with little or no facing rigidity. With the increasing popularity of GRS walls with modular block facing where facing rigid-

ity should not be ignored, an improvement over the Jewell-Milligan method for calculating lateral wall movement is needed.

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can be used in most MSE applications for soil reinforcement includ-

ing internally reinforced soil walls, segmental retaining wall reinforce-

ment, steep reinforced slopes, and reinforcement in a variety of

landfill applications including potential voids, bridging and veneer

stability. When a project specifies for long-term design strength for

structure stability use Miragrid® geogrids.

Walls

There are a large number of walls of various types being con-

structed along the West Rail Line, each holding its own significant

purpose.

Retaining walls hold earth and dirt from sliding into the rail area.

They can be concrete that is poured on site (Cast in Place-CIP),

block or Mechanically Stabilized Earth (MSE).

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MSE Walls are reinforced, soil constructed walls. Created by fit-ting together individual panels with straps that extend into the earth and then backfilling dirt behind the wall, MSE walls can be retaining or approach (examples at Kipling or Wadsworth). Soil Nail Walls are built from the top down, and can be seen un-der construction at the Jeffco Government Center.

Ballast walls hold in the ballast (rocks) that are under the ties and track.

Railing or fencing is attached to the top of these walls wherever there are no soundwalls. Soundwalls are used along the trackway to mitigate the noise from the train to the adjacent neighbors.

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MECHANICALLY REINFORCED EARTH WALLS

The general considerations for the design are: 1. Selection of backfill material: granular, freely draining material is normally specified. However, with the advent of geogrids, the use of cohesive soil is gaining ground. 2. Backfill should be compacted with care in order to avoid damage to the reinforcing material. 3. Rankine's theory for the active state is assumed to be valid. 4. The wall should be sufficiently flexible for the development of active conditions. 5. Tension stresses are considered for the reinforcement outside the assumed failure zone. 6. Wall failure will occur in one of three ways

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Figure 19.15 Principles of MSE wall design

Figure 19.16 Typical range in strip reinforcement spacing for reinforced earth walls (Bowles, 1996)

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a. tension in reinforcements b. bearing capacity failure c. sliding of the whole wall soil system. 7. Surcharges are allowed on the backfill. The surcharges may be permanent (such as a roadway) or temporary. a. Temporary surcharges within the reinforcement zone will increase the lateral pressure on the facing unit which in turn increases the tension in the reinforcements, but does not contribute to reinforcement stability. b. Permanent surcharges within the reinforcement zone will increase the lateral pressure and tension in the reinforcement and will contribute additional vertical pressure for the reinforcement friction. c. Temporary or permanent surcharges outside the reinforcement zone contribute lateral pressure which tends to overturn the wall. 8. The total length L of the reinforcement goes beyond the failure plane AC by a length Lg. Only length Lg (effective length) is considered for computing frictional resistance. The length LR lying within the failure zone will not contribute for frictional resistance (Fig. 19.15a). 9. For the propose of design the total length L remains the same for the entire height of wall H. Designers, however, may use their discretion to curtail the length at lower levels. Typical ranges in reinforcement spacing are given in Fig. 19.16.

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DESIGN METHOD

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Galvanized steel strips of widths varying from 5 to 100 mm and thickness

from 3 to 5 mm are generally used. Allowance for corrosion is normally made while

deciding the thickness at the rate of 0.001 in. per year and the life span is taken as

equal to 50 years.

The vertical spacing may range from 20 to 150 cm ( 8 to 60 in.) and can vary with depth.

The horizontal lateral spacing may be on the order of 80 to 150 cm (30 to 60 in.).

The ultimate tensile strength may be taken as equal to 240 MPa (35,000 lb/in.2).

A factor of safety in the range of 1.5 to 1.67 is normally used to determine the allowable steel strength fa. Figure 19.16 depicts a typical arrangement of metal reinforcement.

The properties of geotextiles and geogrids have been discussed in Section 19.7.However, with regards to spacing, only the vertical spacing is to be considered.

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Manufacturers provide geotextiles (or geogrids) in rolls of various lengths and widths.

The tensile force per unit width must be determined.

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The friction angle 8 between the strip and the soil may be taken as equal to 0 for a rough strip surface and for a smooth surface 8 may lie between 10 to 25°.

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Design with Geogrid Layers

A tremendous number of geogrid reinforced walls have been constructed in the past 10 years (Koerner, 1999). The types of permanent geogrid reinforced wall facings are as follows (Koerner, 1999): 1. Articulated precast panels are discrete precast concrete panels with inserts for attaching the geogrid. 2. Full height precast panels are concrete panels temporarily supported until backfill is complete.

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3. Cast-in-place concrete panels are often wrap-around walls that are allowed to settle and, after 1/2 to 2 years, are covered with a cast-in-place facing panel.

4. Masonry block facing walls are an exploding segment of the industry with many different types currently available, all of which have the geogrid embedded between the blocks and held by pins, nubs, and/or friction. 5. Gabion facings are polymer or steel-wire baskets filled with stone, having a geogrid held between the baskets and fixed with rings and/or friction. The frictional resistance offered by a geogrid against pullout may be expressed as (Koerner, 1999)

EXTERNAL STABILITY

The MSB wall system consists of three zones. Thye are 1. The reinforced earth zone. 2. The backfill zone.

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Figure 19.17 External stability considerations for reinforced earth wall

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Example

A typical section of a retaining wall with the backfill reinforced with metal

strips is shown in

Fig. Ex. 19.2. The following data are available:

Height H = 9 m; b = 100 mm; t = 5 mm\fy = 240 MPa; Fs for steel = 1.67; Fs

on soil friction

= 1.5; 0=36°; 7= 17.5 kN/m3; 5 = 25°;/? x s = 1 x 1 m.

Required:

(a) Lengths L and Le at varying depths.

(b) The largest tension Tin the strip.

(c) The allowable tension in the strip.

(d) Check for external stability.

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Check for External Stability Check of bearing capacity

It is necessary to check the base of the wall with the backfill for the bearing capacity per unit length of the wall. The width of the wall may be taken as equal to 4.5 m (Fig. Ex. 19.2). The procedure as explained in Chapter 12 may be followed. For all practical purposes, the shape, depth, and inclination factors may be taken as equal to 1.

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Example 19.3 A section of a retaining wall with a reinforced backfill is shown in Fig. Ex. 19.3. The backfill surface is subjected to a surcharge of 30 kN/m2. Required: (a) The reinforcement distribution. (b) The maximum tension in the strip. (c) Check for external stability. Given: b = 100 mm, t = 5 mm,/fl = 143.7MPa, c = 0, 0 = 36°, 8 = 25°, y = 17.5 kN/m3, s = 0.5 m, and h = 0.5 m.

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Example 19.4

Figure Ex. 19.4shows a section of a retaining wall with geotextile reinforcement. The wall is backfilled with a granular soil having 7=18 kN/m3 and 0 = 34°. A woven slit-film geotextile with warp (machine) direction ultimate wide-width strength of 50 kN/m and having 8= 24° (Table 19.3) is intended to be used in its construction. The orientation of the geotextile is perpendicular to the wall face and the edges are to be overlapped to handle the weft direction. A factor of safety of 1.4 is to be used along with site specific reduction factors (Table 19.4). Required: (a) Spacing of the individual layers of geotextile. (b) Determination of the length of the fabric layers.

(c) Check the overlap. (d) Check for external stability. The backfill surface carries a uniform surcharge dead load of 10 kN/m

The backfill surface carries a uniform surcharge dead load of 10 kN/m2

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Figure Ex. 19.4

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Example 19.5

Design a 7m high geogrid-reinforced wall when the reinforcement vertical maximum spacing must be 1.0 m. The coverage ratio is 0.80 (Refer to Fig. Ex. 19.5). Given: Tu = 156 kN/m, Cr = 0.80,

C = 0.75. The other details are given in the figure.

Figure Ex. 19.5

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D r H a m m I d a

T h a n k U