Strip for use in stabilized earth structures and method of making same

A strip for use in a stabilized earth structure has a tensile portion in the form of an elongate core which supports lateral friction wings and optional ribs. The elongate core may be steel or a polymer material and the wings are fabricated from a plastic material. Alternatively, the strip may be made from a single material (e.g., plastic) with a thickened region defining the core and thinner regions providing the friction wings.

CROSS REFERENCES TO RELATED APPLICATIONS 
This application corresponds to PCT International Application No. 
PCT/GB94/02286 filed Oct. 19, 1994 and priority applications filed in the 
United Kingdom as Application No. 9321792.5 filed Oct. 22, 1993 and 
Application No. 9417134.5 filed Aug. 23, 1994 upon which a claim for 
priority is made. 
BACKGROUND OF THE INVENTION 
This invention relates to a stabilising strip for use in stabilised earth 
structures. 
A stabilised earth structure is one in which stabilising elements, such as 
elongate strips, are combined with backfill, such as earth, in order to 
form a composite material. The strips extend rearwardly from a facing into 
the backfill and are horizontally and vertically spaced from each other. 
Such structures are commonly employed to provide retaining walls and 
abutments for bridges. They are known from, for example, GB-A-1 069 361 
incorporated herewith by reference. 
In the vast majority of cases, the stabilising elements are provided in the 
form of strips having a length of between about 3 and 10 m, although 
shorter strips and occasionally longer ones of up to about 20 m may be 
used. The width of the strips is generally between 4 and 6 cm although it 
is known to use strips of up to 10 or 25 cm in width. Their thickness 
ranges from about 1 mm to a few centimeters and is generally in the range 
of 1 to 6 mm. 
The purpose of the stabilising strips is to transmit forces within the 
earth mass and to distribute stresses. In particular, it is firstly 
necessary to transmit forces between a strip and the backfill in which it 
is placed and therefore the strip must have a sufficiently large surface 
area to develop through friction the required shear resistance per unit 
length. In order to increase the shear resistance, the width of the strip 
must be increased. The surfaces of the strip may also be provided with 
laterally extending ribs to increase the frictional interaction with the 
earth, as is known from GB-A-1 563 317 incorporated herewith by reference. 
Secondly, the strips must be capable of transmitting forces along their 
length and therefore it is necessary that they have a high tensile 
strength. 
As well as these two main functions which are fundamental to the basic 
operation of the stabilised earth structure, various other characteristics 
are also highly desirable. A reinforcing strip should be able to flex in a 
vertical plane in order to accommodate soil deformation, such as 
settlement or shrinkage, without being damaged; the strip should have a 
high breaking strain, to give good elongation before it breaks; and the 
strip should also be durable, having a slow and predictable rate of 
degradation with time, even in an aggressive backfill environment. When 
steel strips are used, these requirements generally make it necessary to 
use strips of at least 4 or 5 mm in thickness in order to provide the 
necessary strength, bearing in mind the effects of degradation over time. 
When this thickness is combined with the width of the strip which is 
required in order to provide sufficient frictional interaction with the 
earth, the result is a technical over-design in terms of the tensile 
capacity of the strip, particularly for low structures and the upper part 
of higher structures. It will be appreciated therefore that strips having 
a high weight per unit length are employed, such that the strips are heavy 
to transport and install, as well as expensive. 
SUMMARY OF THE INVENTION 
The invention provides a strip for use in stabilised earth structures, 
comprising a longitudinally extending tensile portion for resisting 
tensile force, and a lateral portion which projects laterally of the 
tensile portion for frictional engagement with earth. 
Thus, the tensile portion can be designed or selected in accordance with 
the required tensile resistance of the strip, whilst the lateral portion 
can be separately designed or selected in accordance with the required 
frictional engagement forces to be mobilised. Therefore the strip may be 
designed to optimise its performance in both of these respects whilst 
providing a more economical use of the material or materials from which 
the strip is made. 
It will be appreciated that the tensile portion will generally extend 
across only part of the width of the strip, for example less than half or 
less than one third. Preferably the tensile portion extends across about 
one quarter of the width of the strip, and possibly one tenth. The lateral 
portion will then project laterally across the remaining part of the 
width. Thus, in general, the extent of lateral projection of the lateral 
portion will be substantially greater than the thickness of the strip. 
In practice, the surfaces of the strip immediately above and below the 
tensile portion will be in contact with the earth above and below and will 
thus make a greater or lesser contribution to the frictional engagement 
with the earth, depending on the size and profile of these surfaces. This 
will be in addition to the frictional capacity of the lateral portion. 
Moreover, the lateral portion may make a contribution to the tensile 
resistance of the strip. 
However, as the function of transmitting the stresses within the structure 
along the length of the strip is primarily carried out by the tensile 
portion of the strip, only this portion need be strong enough to carry the 
tensile loads. Therefore, there is no need for the lateral portion to have 
a high tensile strength and so in order to save materials it is preferred 
that the tensile portion has a higher total tensile strength than the 
lateral portion. 
In a typical example, a 6 m length strip may be subject to a maximum 
tensile load of 15 to 20 kN. Assuming a maximum tensile force of 16 kN and 
a cross-sectional area of the tensile portion of 80 mm.sup.2, there will 
be a tensile stress of 200MN/m.sup.2. The maximum shear stress resulting 
from the transmission of frictional forces from the lateral portion to the 
tensile portion may typically be about 1MN/m.sup.2. Hence, although the 
two stresses are not directly comparable since one is a tensile stress and 
the other one a shear stress, the stress which will have to be sustained 
by the tensile portion is about two orders of magnitude larger than the 
stress that will have to be sustained by the lateral portion. 
This explains why considerable savings may be obtained by separating the 
two functions of tensional and frictional capacity, for example by using 
two different materials, or two different thicknesses, or a combination of 
both, or the same materials and thicknesses and with the lateral portion 
including perforations. 
The tensile portion and the lateral portion may thus be of the same 
thickness, if the tensile portion is made from a stronger material than 
the lateral portion, or if the portions are of the same material but the 
lateral portion includes perforations. 
Preferably, however, the tensile portion is thicker than the lateral 
portion. The minimum thickness to provide the necessary tensile strength 
is only required in the tensile portion and therefore the lateral portion 
may be made much thinner because the stresses acting upon it are 
comparatively low. In fact, even if a localised area, for example a few 
square centimeters, of the lateral portion were to disappear through 
degradation, this would not significantly impair the stability of the 
structure. Thus the cross-sectional shape of the lateral portion is in 
theory ideally a wide and thin section, but for practical purposes may be 
other shapes where the ratio of its perimeter/cross-sectional area is 
large. Preferred shapes are a thin rectangle or two or more thin 
rectangles. 
A thicker tensile portion is generally preferred, in order to provide a low 
ratio of its perimeter/cross-sectional area. This decreases the contact 
with the environment for any given cross-sectional area of the tensile 
portion. Consequently the section of the tensile portion is in theory 
ideally circular, but for practical purposes may be circular, oval, 
square, rectangular or other shapes where the perimeter/cross-sectional 
area ratio is small. 
The preferred requirement that the tensile portion is thicker than the 
lateral portion will generally apply along the full length of the strip. 
However, it may be appropriate in certain circumstances to provide regions 
where the tensile portion is thinner than it is in the remainder of the 
strip, for example, at the end of the strip furthest from the facing where 
the tensile forces are lowest. 
If different materials are used for the lateral portion and the tensile 
portion, they may have substantially different properties. For example the 
tensile portion could be formed of a strong material such as steel or 
polymer yarns or drawn bulk polymer and the lateral portion may be made of 
a material which may not be mechanically very resistant but is resistant 
to degradation such as some plastics materials like polyethylene or 
polypropylene. The use of plastics to provide the lateral portion is also 
advantageous because these materials are lightweight and easy to form with 
appropriate surface texture, perforations etc. 
The tensile and lateral portions of the strip may be provided in different 
ways depending upon the profile chosen for the strip and the choice of 
materials. For example, the lateral portion may project laterally on one 
side only of the strip. Preferably however the tensile portion is a core 
from which lateral portions project laterally on both sides. Thus, the 
lateral portions may be in the form of friction wings extending on 
opposite sides of the core. 
Alternatively, the tensile portion may comprise a plurality of cores 
interconnected by the lateral portion. For example, two cores may be 
provided. Thus, the tensile loading could be divided between the two cores 
and these cores may be connected together by the lateral portion, which 
may also have wings at the sides of the strip. 
It is possible that the strip could be formed with a smooth outer surface 
and the frictional interengagement with the earth could then be provided 
simply by having a sufficiently large surface area for the lateral 
portion. However, it is preferred that the lateral portion be provided 
with a surface which has been adapted to resist longitudinal motion 
through the earth. Therefore, preferably, the lateral portion is provided 
with ribs and/or corrugations and/or perforations to improve the 
frictional interaction with the earth. Corrugations, ribs or a rough 
surface may be obtained by pressing the strip into hot or cold dies, or by 
gluing, pasting or welding to the strip a corrugated/ribbed/rough coating. 
The various means for improving frictional interaction may if desired be 
provided across the surfaces of the strip above and below the tensile 
portion, and not just on the lateral portion. 
In general, the lateral portion will extend longitudinally of the strip, 
preferably continuously but if short breaks are provided at longitudinal 
intervals this may not significantly affect its performance. Any such 
breaks will normally be shorter than the remaining lengths of the lateral 
portion. 
The tensile portion of the strip may typically have a cross-sectional area 
in the range from about 15-400 mm.sup.2, for example about 100 mm.sup.2. A 
tensile portion of circular cross-section may have a diameter of about 
5-16 mm and more usually about 8-12 mm, whilst a tensile portion of 
rectangular cross-section may have a width of about 15-30 mm and a 
thickness of about 4-15 mm but more usually about 5-8 mm. The total strip 
width may be about 20-80 mm and more usually about 40-70 mm. The thickness 
of the lateral portion may be about 1-5 mm or 1-3 mm, plus 1-3 mm ribs if 
these are provided. The ratio of the tensile portion thickness/lateral 
portion thickness may typically be in the range 2-4. The above figures are 
merely examples and other dimensions may be desired or required in certain 
circumstances, for example with very short or very long strips. The 
figures given are also particularly applicable to substantially all steel 
strips or strips with a steel tension portion and a plastics lateral 
portion, although in practice they may also apply to other forms of strip, 
such as polymer/plastics strips. 
The strips may be designed to be impermeable to water. However, in some 
circumstances it may be useful to provide a strip which permits entry of 
water thereto and longitudinal transport of water therealong. Because 
water can flow into such a strip and then along it, the strip may be used 
to drain the backfill within which it is located, reducing the pore water 
pressure. For example, water may be transported to the front of an earth 
structure and allowed to drain away. By removing water, the adherence 
between the strip and the backfill material is increased and so, for a 
given number or surface area of stabilising members, a lower quality, less 
well draining backfill material may be used. Thus, these strips may be 
employed in areas where the available backfill material is, for example, 
clay, without the need to incur the expense of transporting better quality 
backfill material from elsewhere. The drainage property of the strip also 
speeds up consolidation of the backfill. 
The transport of water from the inside of a structure to the surface is 
also advantageous when the facing includes vegetation growth and 
conditions are dry, since water can be transferred to the vegetation. 
Various methods may be used to secure the strip to the facing of a 
stabilised earth structure. The strip may have at one end an integral pad 
adapted to have formed therethrough an aperture suitable to receive 
fastening means, such as a vertical pin or bolt, to locate the strip in a 
stabilised earth structure, the thickness of the pad being greater than 
the thickness of the lateral portion. The pad may be thicker than the 
tensile portion or it may be more convenient for it to have the same 
thickness. In one example, the cross-sectional shape of the pad is 
rectangular, whereby the pad has a uniform thickness across its width. In 
another example, when viewed in cross-section, the pad has a thick central 
region and a thinner lateral region on each side thereof, both the central 
and the lateral regions being thicker than the lateral portion elsewhere 
in the strip. Typically, the pad is 40-100 mm in length. The pad may be 
formed by various means such as by hot forging, but preferably the strip 
is rolled to include thickened pads at longitudinal intervals, as is known 
from GB-A-2 177 140. The strip is then cut so that one of the pads is 
located at an end of the strip and can be formed with a vertical hole for 
receiving a pin for connection to the facing. In the case of the composite 
strips, the pad will normally be formed integrally with the tensile 
portion thereof. 
In a stabilised earth structure the facing is designed to take the local 
loads exerted by the adjacent backfill. If the strips are widely spaced 
from each other, a stronger facing is required to resist backfill 
pressure; in other words, the required strength of the facing is directly 
dependent on the strip spacing. As discussed above in relation to known 
strips, there is a lower limit to the strip thickness to allow for 
degradation and there is a lower limit to the strip width to ensure 
adequate frictional interaction with the earth, with the result of a 
practical lower limit to the strength of the strip. In existing structures 
this has led, for reasons for economy, to a relatively small number of 
strong strips at wide spacings and to the requirement for a rather strong 
facing. Another advantage of the strip of the present invention is that 
the minimum strength of the strip can be decreased leading to a less 
strong and less expensive facing. 
The present invention also extends to a stabilised earth structure 
comprising stabilising strips as set out herein. Such a structure can 
benefit both from economical strips and an economical facing. 
There are two main classes of manufacturing methods by which strips 
according to the invention may be made. First, the strip may be made from 
a single type of material. Second, a plurality of materials may be 
combined. 
In the first case, separate pieces of the same material may be glued, 
welded, or joined together by other means. Preferably, however, the strip 
is made from a single piece of material formed into an appropriate shape, 
for example by casting. A preferred method of making a stabilising strip 
comprises rolling a blank to form the strip. 
The strip may be rolled in a single stage, or, alternatively, a first 
rolling stage may provide the general outline and then a further stage may 
add ribs, corrugations or perforations as appropriate. Thus, for example, 
the method may further comprise the step of cutting apertures in the 
lateral portion. 
In the second case, the lateral portion could be attached to the tensile 
portion in a number of ways, such as by welding or by the use of clamps, 
bolts, adhesives, etc. However, it is particularly preferred that the 
tensile portion be surrounded by the material which forms the lateral 
portion. By encasing the tensile portion in this way it may be protected 
from corrosion by the material which also forms the lateral portion, as 
well as providing a strong connection between the portions. 
In order to improve adherence between the lateral portion and the tensile 
portion, the tensile portion may be provided with ribs or the like. 
The material forming the lateral portion may be moulded around the tensile 
portion or, alternatively, it may be provided in two separate parts which 
are brought together with the tensile portion sandwiched therebetween. The 
parts may be glued, hot welded (e.g. by a pair of press and weld 
cylinders), hypersonically or ultrasonically welded or attached by other 
appropriate means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning first to FIG. 1 there is illustrated a strip 1 for use in a 
stabilised earth structure which has a tensile portion in the form of a 
steel core 2. This is surrounded by a casing 3 which has a pair of 
laterally projecting portions in the form of friction wings 4. At the tip 
of each wing 4 a thickened bead 6 is provided. In order to assist in the 
engagement of the friction wings with soil, the wings are provided with 
small vertically projecting ribs 5, provided alternately on the top and 
bottom faces of the strip (the bottom ribs not being shown). 
The core 2 may be made out of a steel bar which may have a smooth surface, 
or be sanded, gritted or otherwise roughened or deformed to enhance 
adherence to the friction wings. Thus a deformed or high adherence 
reinforcing bar of the type normally used to reinforce concrete may be 
used, or a smooth reinforcing bar may be used. The core may alternatively 
be made of other materials such as stainless steel or aluminium alloys. 
The casing 3 may be made of a polymer material such as polyethylene, 
polypropylene, PVC etc. This may be treated to resist UV light by the 
addition of carbon black and its strength may be increased by the addition 
of glass fibre. Other additives such as talc may be used to enhance 
durability or resistance to impact. 
The diameter of the core 2 may be anything from a few millimeters to a few 
centimeters but is usually in the range 5-15 mm. In the example of FIG. 1, 
the diameter is 10 mm. The thickness of the friction wings 4 is 3 mm, the 
diameter of the beads 6 is 5 mm, and the height of the ribs 5 is 2 mm. 
A similar structure may be employed when a core of polymer is used. Such a 
strip 1 is illustrated in FIG. 2. In this figure the core 2 is somewhat 
flatter in profile than that of FIG. 1 and therefore the casing 3 is 
shaped accordingly. It will be noted that in this embodiment the ribs 5 
extend most of the way across the width of the strip 3 and therefore 
extend above and below the core 2 as well as on the friction wings 4. The 
core 2 may consist of a tension resistant polymer in its bulk form, in 
drawn members, or in jointed yarns. 
FIG. 3 illustrates a third type of strip 1, in which two steel cores 2 are 
provided, each formed from a reinforcing bar. These are laterally spaced 
and interconnected by the casing 3 which has two small friction wings 4 on 
opposite lateral sides of the strip and a larger central interconnecting 
part 26 between the two cores. Ribs 5 are provided on all three parts of 
the casing 3 although because of its larger size, larger ribs are formed 
on the central part 26. 
FIGS. 4 to 6 illustrate embodiments of the invention which are made from a 
single type of material. FIG. 4 illustrates a strip 1 which is the fourth 
embodiment of the invention. This has a thick core 2 which extends along 
the centre and has thinner laterally projecting portions on each side 
which form friction wings 4. Ribs 5 project at intervals from the wings. 
FIGS. 5 and 6 illustrate the fifth and sixth embodiments of the invention 
respectively, which are similar to the fourth embodiment, except for the 
method in which the frictional engagement with the earth is increased. The 
embodiment of FIG. 5, rather than having ribs projecting from its friction 
wings 4, instead has perforations 57 for assisting frictional interaction 
with the earth. The perforations 57 are formed by cutting slots at 
intervals along the friction wings and opening the slots. Thus, the 
lateral edges of the wings have projections 58 and recesses 59 caused by 
this action. These further assist in engagement with the earth. In FIG. 6 
the friction wings 4 have corrugations 60; in other words they are rippled 
up and down with a respect to the core 2. 
A seventh embodiment 610 is shown in FIG. 12. This strip has an outer 
covering 611 of perforated PVC which surrounds a plastics water 
transmitting portion 612 and two cores 615. The outer covering 611 is 
provided with small perforations (not shown) to allow the ingress of water 
as will be explained below. It is further provided with moulded ribs 612A 
to improve the engagement of the strip with backfill material 619. 
The water transmitting portion 612 is a wafer like member of plastics 
material which has channels 613 along its length in order to allow water 
to be transmitted along the strip. Communicating with the channels are 
openings 614 to enable water to flow into the channels. Thus, in use, 
water in the backfill material, which will be under pressure, will be 
forced through the perforations in the outer covering into the openings 
614 and will then be able to flow along channels 613 towards the ends of 
the strip. 
The cores 615 provide the strip of the present embodiment with the 
necessary tensile strength to enable it to transmit forces along its 
length. Each core 615 comprises a sheath 616 which holds a bundle of 
filaments 618 which may be wires or polymer fibres. The latter are 
preferably used because they will not be corroded if water penetrates the 
sheath 616. 
FIG. 13 illustrates a way of providing a connection to the stabilising 
strip 610 of FIG. 12. This is achieved by allowing parts of the cores 615 
to project from one end of the strip where they are connected together to 
form a loop 617. This loop may be connected around a suitable device 620 
attached to a facing element 622. As an alternative to connecting the two 
cores together, a continual core may be used which forms both cores 615 
and the loop 617 without the need for a joint. 
FIG. 7 illustrates an apparatus 100 which is particularly suitable for 
producing strips of the first embodiment. A coil of reinforcing bar steel 
102 is provided adjacent to a reinforcing bar straightener 103. The steel 
is fed from the coil through the straightener and then to a cutting 
machine 104 which is adjusted to cut the steel to lengths of bar 101 
corresponding to the length of strip 1 which is required. The bars are 
then passed into a hopper 105 from which they may be fed to the remaining 
part of the apparatus. 
Next, each bar 101 is fed from the bottom of the hopper 105 into a plastics 
extrusion dye 106. Into this dye is fed the raw plastics material 107 from 
a bin 108 in which it has been mixed with appropriate additives. The bin 
is heated and the molten plastic is fed by a screw pump 109 into the 
extrusion dye 106. The extrusion dye is provided with heater coils in 
order to maintain the plastics material in the molten state whilst it is 
moulded around the bar. The bar may be pushed or pulled through the dye 
and as it advances it is encased in the plastics material which forms the 
casing 3. After it leaves the dye 106 the bar, now coated with plastics, 
is hot rolled through a pair of rollers 110 which produce the ribs 5 on 
the wings 4 of the coated strip 1. If desired, the end of the strip may be 
sealed in order to protect the ends of the bar and then the plastics 
material is allowed to set thoroughly. 
FIG. 8 illustrates an apparatus 200 which is generally similar to that of 
FIG. 7, except that it is adapted for producing strips of the second 
embodiment in which the tensile portion is a core of polymer yarns. The 
illustrated apparatus provides a continuous process in which the core is 
pulled in the direction of arrow P through the apparatus. The extrusion 
components are identical to those of FIG. 7. However, the reinforcing bar, 
coil straightener, cutting machine and hopper are replaced by a number of 
coils 201 of yarn 202 and a device 203 in which the yarns are brought 
together to form a single core 204. This core is then pulled through the 
extruding die 106 and rollers 110. The strip 1 once made could be wound 
onto a drum and cut to desired lengths on site, or, alternatively, cutting 
apparatus could be added after the extrusion apparatus. Depending on the 
type of core material, on the type of soil in which the strip is to be 
placed, and on the service life required for a structure to be erected 
using the strips, their ends may or may not be sealed when the strips are 
cut. 
FIG. 9 illustrates an apparatus 300 for making strips of the first 
embodiment in which the casing 3 is formed (for example by extruding and 
rolling) in two separate parts 301,302 between which the core 2 is 
sandwiched. The core is fed from a coil of reinforcing bar material 102 
through a reinforcing bar straightener 103 and then between coils 
containing the casing parts 301 and 302. One of these coils is located 
above the bar and the other directly beneath it, and as the core 2 is fed 
between them the material unrolls from the coils thereby sandwiching the 
core. A pair of press and weld cylinders 306 is provided downstream of the 
coils to seal the plastics material in position around the core. In an 
alternative arrangement, the coils may be located on each side of the bar, 
with the cylinders 306 having their axes arranged vertically. The 
apparatus may be arranged such that the action of pulling the core 2 will 
by itself unwind the parts 301,302 of the casing material from the coils. 
Further apparatus may be provided downstream of the press and weld 
cylinders 306 in order to cut strips of the required length. It will be 
appreciated that similar apparatus could be useful producing the strips of 
the second embodiment if yarn-handling apparatus of the type illustrated 
in FIG. 8 were used in place of the reinforcing bar coil 102 and 
reinforcing bar straightener 103. 
FIGS. 10 and 11 illustrate apparatus for rolling strips of, for example, 
steel in order to provide strips of the fourth and fifth embodiments 
respectively. FIG. 10 illustrates an apparatus 400 in which a strip of 
steel 401 passes through a pair of rollers 402. The rollers have a waist 
403 which provides a raised, central portion 404 to the strip, thereby 
forming its core 2. On each side of the waist there are a series of 
indentations 405 which provide ribs 5 on the friction wings 4. The spacing 
of the rollers 402 determines the thickness of the wings. 
In the apparatus 500 of FIG. 11 two pairs of rollers are provided. The 
first pair 501 serve to form the core 2 of the strip 1 and are provided 
with a waist portion 502 of an appropriate profile to achieve this. The 
rollers 501 are also spaced from each other by a distance which determines 
the thickness of the friction wings 4. After passing through the first 
rollers, the strip passes through a second pair of rollers 503 which as 
well as having a waist portion 504 to accommodate the core 2 of the strip 
are provided with pairs of cutters 505. It will be noted that these 
cutters are arranged to cut slits in the wings of the strip and also to 
open up the slits in a lateral direction as the strip passes between the 
rollers 503. In this way a series of apertures 57 is provided in the wings 
4 and the lateral sides of the wings are provided with a series of 
projections and recesses. 
If desired, the cutting pair of rollers 503 may be provided independently 
of the rollers 501, for example in another plant. In fact, such cutting 
rollers may be used directly on a conventional steel strip, i.e. one 
without a thickened core. In this case, the tensile portion of the strip 
comprises an uninterrupted longitudinal region, whilst the lateral portion 
for frictional engagement with the earth comprises a region of the same 
thickness but formed with apertures. The provision of the apertures 
increases the overall width of the strip for the amount of material used. 
While preferred embodiments and methods have been set forth, the invention 
is to be limited only by the following claims and equivalents thereto.