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Transcript
1 Modules 34-35 / Topic 25 REINFORCED EARTH AND SOIL NAILING Reinforced Earth is a modern soil retaining technique, whose basic function is to contain lateral soil pressures. Even though the technique as such is relatively novel, the concept behind it, viz. interface friction, is as old as classical mechanics. The principle of reinforced earth is best illustrated by comparing it with other conventional forms of retaining structures. Thus while a retaining wall, whether of masonry or reinforced concrete, resists the lateral earth pressure by gravity forces (Fig.25.1a,b), and a sheet pile wall, by depth of penetration and tension in the tie-rod (when it is anchored – Fig.25.1c), the mechanism of retention in the case of reinforced earth is essentially one of surface friction between the backfill soil and a large number of thin elements, called reinforcing strips incorporated in the backfill (Fig.25.1d). 25.1 The mechanics of the reinforced earth technique The main components of reinforced earth are the thin, long reinforcing strips, running horizontally through the backfill soil at regular intervals – both horizontally and vertically – and terminating at the facing skin, Fig.25.2. The lateral pressure exerted by the soil on the facing element is resisted by the skin friction developing at the interface between the soil and the reinforcing strip. The uniform frictional shear on the outer surfaces of the strip induces a uniformly increasing tension in the section of the reinforcing strip, which is zero at the free end and equal to l x F at the wall end (Fig.25.3a), where l is the length of the strip, and F, the frictional resistance per unit length of the strip. This is a case of friction anchorage and in this respect the mechanism bears close resemblance to the anchorage of reinforcement in concrete by the mechanism of bond (Fig.25.3b), and hence the appropriateness of the name “reinforced earth”. One may notice that the mechanism is also similar to the case of friction pile subjected to tension (see Fig.11.6a). It is also to be noted at this stage that the mechanism of continuous friction anchorage of this type is basically different from that of ‘point anchorage’ obtaining in the case of the anchoring of the cables or rods of sheet pile bulkheads to terminal anchors (Sec.13.2) or prestressed ground anchors (Topic 22). In all these, the element under tension, viz., the anchor cable, is subjected to uniform tension. The soil not in direct contact with the reinforcing strips is held by ‘arching’ (Kurian, 2005: Sec. 13.3) between the strips. As far as the facing skin is concerned, it can be structurally very thin, if only one wants to take advantage of the arching that develops in the soil lying in contact with it. 25.2 Design 2 The geotechnical part of the design merely consists in determining the lateral pressure on the ‘wall’, and its variation with depth, by an appropriate theory, such as the Rankine’s theory of earth pressure. The major part of the structural design is concerned with the reinforcing strip, the total surface area of which must be sufficient to resist the lateral soil pressure by skin friction, while at the same time it must have enough cross-sectional area to withstand the resulting tension. (Note the similarity with tensile reinforcement in concrete whose perimeter and length must cater to bond, and section, to tension.) Thus, if the vertical and horizontal spacings of the reinforcing strip, located at a depth h from the surface are a and b respectively (Fig.25.4), the total lateral force to be resisted by the strip = Ka x 𝛾 x h x a x b = T, say (25.1) Where γ is the unit weight of the soil. To make available large surface area for resisting this tension T by friction, thin wide metallic strips are preferred whose thickness t is neglected in computing friction. Thus if w is the width of the strip, we can find the length of the strip by setting T = 2 x w x l x ( γ x h x µ) (25.2) where µ is the coefficient of interface friction between the soil and the strip (µ = tan δ, where δ is the angle of friction.) (Note that excluding the thickness of the strip from the calculation of the surface area, puts one on the safer side.) We can now write T = l x F, where F, the frictional resistance of the strip per unit length = 2w x (γ x h x µ). Equating Eqs. (25.1) and (25.2), we get: l= 𝐾𝑎 x 𝑎 x 𝑏 2𝜔 x 𝜇 which gives l for a given value of w (or vice versa). The expression shows that l is independent of the depth h. Once l and w are determined, the thickness of the strip t is obtained by setting w x t x σt = T = 2w x l x (γ x h x µ) (25.3) where σt is the permissible tensile stress in the material. (Note that T is the maximum tension in the reinforcing strip and occurs at the section where it joins the facing skin, Fig.25.3a.) t= 2𝛾 x 𝜇 x ℎ x 𝑙 𝜎𝑡 (25.4) 3 It is seen that t is proportional to h for a given value of l, and is independent of w. In the design, therefore, the maximum thickness is required at the base, which may be provided over the entire height, or decreased, uniformly or stepwise, towards the top, depending upon the availability of the strip. Since the reinforcing strip or tie is designed for both friction and tension, it may be noted that failure of the tie can occur either by pull-out, or by tie-breaking, depending upon in which respect the design is deficient. For walls above 6 m in height, practical experience has shown that a length of 0.8H, where H is the total height of the soil retained, is sufficient for the reinforcing strip, from the point of view of frictional bond. Conservative designers would prefer to place this l beyond the active wedge - assuming no contribution from the latter – such that the total length at the top (Fig.25.4) =( 𝐻 ∅ 2 tan(45+ ) )+𝑙 (25.5) which is the maximum. As stated, the thickness needed of the curved skin elements (Fig.25.5) can be surprisingly small; a 14 m high wall with steel skin theoretically requires a thickness of only about 0.2 mm. (This, however, involves an assumption that friction between soil and reinforcing strips holds back the active wedge from exerting its full thrust on the facing skin.) The thickness should, however, be considerably increased from the point of view of corrosion alone: a typical conservative design would use galvanised steel plates of thickness up to 5 mm. This type of construction, however, is obsolete; modern day construction uses concrete blocks (see Fig.25.6) in place of metallic skins. 25.3 History and development The origin of the reinforced earth technique, as it is known today, is attributed to the French engineer Henry Vidal, who developed it in the 1960s and subsequently patented it in over 40 countries. His company Terre Armee (meaning Reinforced Earth in French – Note that in French adjective follows the noun, and not precede it as in English. The same is the case with SI in SI Units – Topic 52.) based in Paris, has undertaken reinforced earth construction work in different countries. The application of the technique has since become worldwide. In USA and UK, the work is undertaken by its licensee, The Reinforced Earth Company, which offers a complete package to the client comprising all the facets of design and construction. The number of examples of reinforced earth construction reported from various parts of the world today can be counted in millions. Even though the technique by itself is new in the field of retaining structures, the concept behind it is by no means new, as we have noted before. Henry Vidal himself is reported to have discovered the principle accidentally while playing with his little 4 daughter on a beach in the Mediterranean, when he noticed that the sand which normally gave way beneath his feet, tended not to do so when it had pine needles embedded in it. In fact, the method of strengthening soil with inclusions such as rods or fibres is itself quite old. Some animals and birds use the method in building their habitation. Access roads through swampy areas are often constructed on foundations of small tree trunks and branches. Low dykes have been built of mud and sticks, to reclaim tidal flats. Fibres and branches are used to stabilise soil along river banks. The concept lies behind even the stabilising action of soils by plant roots. 25.4 Materials It needs hardly any emphasis that, in any situation, the materials used for construction must be compatible with the uses to which they are put. Thus, in the case of reinforced earth construction, the material chosen for reinforcement must be flexible, strong in tension and corrosion-resistant. The most commonly used material is strips of galvanised steel or stainless steel, a few centimetres wide but only a few millimetres thick, produced with a rough surface texture to enhance friction. Aluminium alloy is a potential alternative. Aluminium-magnesium alloy reinforcements have been used in highly corrosive brackish marine waters and industrial effluents. Fibrereinforced plastics (FRP) which are synthetic resins reinforced with materials such as glass fibre, hold considerable promise as a material for reinforcement, notwithstanding their cost and the problem of reduction in strength with time, which gets accelerated in the presence of water. Geotextiles (Topic 26) have made a significant foray as a material for reinforcement in modern times. As far as the backfill is concerned, high friction granular soils are certainly the best. At any rate the soil must have good drainage properties (Reinforced earth structures are not designed for water pressure.) and must not contain ingredients which attack the reinforcement chemically. Since the facing skin is subject to negligible soil pressure, its major design considerations would be flexibility, corrosion resistance and protection of the structure from external shocks and abrasions. The units forming the facing skin must be properly jointed to prevent the flow of backfill soil. Since it constitutes the exposed face of the structure, it is desirable that it also has an aesthetic appeal, especially when put to such use as in landscaping. To start with long steel sheets with semi-elliptical section laid horizontally and jointed flexibly (Fig.25.5) were used to form the skin (Concertina method). However, what are commonly used today are thin precast blocks cast in concrete to attractive designs and geometrical patterns, and with facilities for easy jointing with one another (Telescope method). A typical design of the latter is shown in Fig.25.6. Kurian (2013: 5 Sec.3.1.4) illustrates examples of FRP facing elements hung from steel rods to which are also fixed the reinforcing elements. 25.5 Construction The construction of reinforced earth, for example by the Telescope method, is carried out by erecting the front skin in stages of short heights while simultaneously placing the backfill behind. After compacting each layer of the backfill soil, the reinforcing strips are laid on top of the compacted soil and connected to the facing blocks, by bolting on to the projecting lugs (see Fig.25.6) which are embedded in concrete at the time of casting. The work proceeds in stages, each stage (unit operation) consisting of the operations of 1) placing the front skin, 2) laying and compacting the backfill, and 3) laying and connecting the reinforcing strips (see Fig.25.7). And when the full height is thus reached, one has an integral wall, with the soil behind reinforced and held intact in the manner stated. 25.6 Reinforced earth with geotextiles A technique in which geotextile (Topic 26) serves integrally both as the facing skin and reinforcement – popularly described as the ‘wrap-round’ construction’ – is illustrated in Fig.25.8. This is an attractive proposition in temporary constructions. 25.7 ‘Sandwich’ construction Since soil retention in the case of reinforced earth is the result of interface friction between the reinforcing strip and the soil in the backfill, the scope of this technique appears to be limited to high friction soils such as sand. It is, however, possible to extend this technique to low friction soils such as clay, by placing sand around the reinforcing strip and enveloping it (Fig.25.9), in an effort to create a high friction interface with the reinforcement. This is effected by ‘sandwiching’ layers of sand at the reinforcement locations while preparing the backfill with site soil, at the time of construction. This, however, also brings in a question of friction at the interface between sand and clay. So long as the latter is higher than that at the interface between the reinforcement and sand, the system can be expected to function integrally without pull-out of either the reinforcing strip or the sandwiching sand at their respective interfaces. 25.8 Use of anchor blocks Laymen, in whose mind ‘friction’ is an ephemeral concept, might question the long term stability of the reinforced earth technique for retaining soil permanently. In an effort which would instil greater confidence in the minds of clients in general, attempts have been made to fix the free end of the reinforcing strip to small ‘anchor blocks’ embedded in the backfill soil (Fig.25.10). This technique is widely used by Nehemiah (Sec.25.13), based in Malaysia, in their projects. The pull-out strength of the 6 reinforcement in this case is the limiting interface friction plus the passive resistance offered by the soil in front of the anchor block (but reduced by the active earth pressure of the soil at the back of the block see Fig.25.10). Theoretically, what fraction of each will get mobilised in a given design depends upon the interaction between these two effects, which a ‘finite element analysis’ of the system would reveal. 25.9 Mutually connected facing panels for embankments It is worthwhile examining at this stage whether the system illustrated in Fig.25.11 constitutes a case of reinforced earth. Here the thin non-gravity walls, retaining the embankment soil on either side, are held in place by the common strips or rods running across them, which would be subjected to tension on account of the earth pressure acting on the individual wall. This tension, however, is uniform, unlike the tension in the reinforcing strip in reinforced earth, which increases uniformly from zero at the free end. Since the system does not depend upon friction for retention, this obviously is not a case of ‘reinforced earth’ within the meaning of the term we have discussed so far. 25.10 Reinforcing foundation soil In a bid to harness the potential of the reinforcing technique from the backfills of retaining walls to foundation soil, i.e. soil underlying , say a footing, by horizontal line reinforcements (Fig.25.12), detailed investigations have been carried out by Beena (1993), under the direction of this author (Kurian et al., 1997), concerning its influence on settlement. Her studies were confined to sand, where between ‘bearing capacity’ and ‘settlement’, the latter is known to govern the geotechnical design of foundations in the majority of cases (Sec. ). Beena’s (1993) basic 2-D analysis for a continuous (strip) footing, with one-way reinforcement in the direction perpendicular to the footing, and 3-D analysis for an individual square footing, with two-way reinforcement in the length and width directions, by the finite element method, have been complemented by experimental studies covering both the cases, and supplemented by detailed parametric studies. The line reinforcements used were small diameter coir ropes and cut bamboo splinters, taking into account the need for a possible net gain from a cost-benefit angle. The technique of reinforcing soil in this manner is best attempted concomitant with ‘soil replacement’ where a pit from which in-situ soil is removed is backfilled with sand up to the top, in layers, compacting each layer and placing the reinforcement on top of it. The studies have established the influence of the reinforcement in bringing down settlements significantly. Reference may be made to Kurian (2013: Sec.3.1.10) for more details pertaining to the premises of and results from the above studies. 25.11 Uses of reinforced earth 7 The technique of reinforced earth can be gainfully employed in a variety of situations. These include retaining walls, bridge abutments and wing walls, and side walls supporting embankments carrying the approach roads, marine applications such as in port, harbour and inland waterway structures, retention dykes, foundation slabs, slide buttresses, embankments for roads and railways, earthen dams, cut and fill tunnels, and other commercial and industrial uses, and last but not the least, landscaping. These examples point to the increasing scope of application of reinforced earth in several facets of civil engineering. 25.12 Advantages of reinforced earth The advantages of reinforced earth can be summed up as: 1) Saving in material and labour, and vast overall economy, 2) Large number of potential uses, 3) Speed of execution – no special skill is needed for erection, 4) High resilience to withstand dynamic effects, such as from blasts, and high seismic stability, 5) Advantages of using precast elements, the in-situ work involved being only laying and jointing them, 6) Ease of adaptation to curved alignments, 7) Ability to withstand considerable settlements without damage, thanks to its flexibility, and 8) High aesthetics in landscaping. 25.13 The Nehemiah Anchored Earth (AE) Wall Nehemiah AE wall is a proprietary form of Reinforced Earth construction undertaken by Nehemiah Reinforced Soil Sdn. Bhd., of Malaysia. It is variant of the Telescope method (see Fig.25.6) in which hexagonal concrete panels take the place of the cruciform panels of the latter, which are fixed to small anchor blocks at the free end. Fig.25.13 shows the hexagonal R.C. unit, which forms the facing panel, with projecting lugs to fix the tendons. One can also see dowel bars used for interlocking with the adjoining panels. Fig.25.14 gives a cross section showing the components of the system and the fixing details. The reinforcing tendons are carbon steel rods, which are hot-dipped galvanised to prevent corrosion in the soil environment. Round bars are used because they are more durable than thin strips against corrosion. The anchor blocks are individual concrete blocks which act as deadmen. The tendon is passed through a preformed hole provided at the centre of the block and fixed to it using washer and nut 8 at the outer end. The anchor block enhances the pull-out resistance of the tendon (Sec.25.8). It also points to the possibility of using low-friction cohesive soils in the backfill. Analysis A full analysis of Reinforced earth involves ‘external stability’ and ‘internal stability’. As regards external stability, it is analysed as a rigid block as done in the case of gravity retaining walls, checking for factor of safety against overturning, tension at the base, and bearing and sliding at the base. Analysis for internal stability must ensure adequate factor of safety against tensile and pull-out failure of the reinforcing tendons. 25.14 Soil Nailing Soil nailing is a process of retaining soil by the incorporation of a large number of reinforcements, in the form of ‘nails’, in the soil which are free at the farther end, the other end being anchored to a thin grouted concrete wall (Fig.25.15,also see Fig.25.16). It is a variant of the reinforced earth construction with the main difference that, whereas in reinforced earth the reinforcing strips are laid and the backfill soil compacted above in layers (Sec.25.5), in soil nailing, short lengths of stiff iron rods are driven into the soil, securing them at the facing wall. In either case the mechanics of retention is based on the friction developing at the interface between the reinforcement and the soil. In order to protect the nail, and to enhance interface friction, the nail surfaces can be pressure-grouted with cement grout. Since no bulb is formed at the end, it is not a system based on point anchorage as in ground anchors, but on continuous friction anchorage as in reinforced earth. For this a large number of nails are driven at closer spacing as in reinforced earth rather than a few anchor rods at wider spacing as in ground anchors. Soil nailing can be temporary or permanent. Figs. 25.17 and 25.18 show two examples of permanent nailing. The former illustrates a scheme for retaining the soil face vertically adjacent to a multilevel car parking structure, and the latter, for cutting a steep and deep slope keeping the buildings above stable and intact. Soil nailing is an effective alternative where existing structures are so close that ground anchors cannot be laid under them due to objection from the property owners. Further, nailing can be carried out with simpler and lighter equipments which can be manoeuvred without much difficulty within limited spaces where space restrictions exist. Also, its flexibility enables its adoption to varying ground conditions. Soil nailing was originally introduced in France in the 1970s. It can be described as an in-situ reinforcing of soil using an array of nails installed as passive inclusions in a grid. The construction begins with the excavation of a shallow cut (Fig.25.19) on the face of which wire mesh is laid followed by applying shotcrete to the face. When the latter is set, soil nails are drilled through the shotcrete and grouted, followed by 9 anchoring them to the wall. The sequence is repeated until the final depth is reached. The nail being rigid, unlike the reinforcing strip in reinforced earth, can resist some bending and shear in addition to axial tension. An innovative step is the use of screw nails which are installed by rotation (like screw piles), giving rise to enhanced friction at the soil-nail interface. (This is akin to increased bond in the case of deformed reinforcement bars.) 25.14.1 Reinforced earth and soil nailing Reinforced earth and soil nailing depend on the shear stress developing at the interface between soil and the inclusion (reinforcing strip/nail). However, they differ from each other in the following respects: i) as already stated, the reinforcing strip, being flexible, can only resist tensile forces whereas the nail can resist both tensile forces and bending effects, ii) because of the difference in construction procedure, the stress pattern in the soil can differ, especially during the construction phase, iii) since we deal with natural soil in soil nailing, unlike in reinforced earth where we have the choice of the backfill material, the design must necessarily take into account these differences as they relate to soil properties. 25.14.2 Soil nailing – Advantages Soil nailing cannot replace all other methods of soil retention technically or economically. Notwithstanding the same, it has the following advantages. 1) It is not dependent on heavy equipment, 2) It is economical where the geometry of the wall is complex and where space space restrictions exist, 3) Since nails are of low strength steel, the need for corrosion protection stands reduced, 4) Construction can be carried out with little disturbance to the environment in terms of noise and vibration. In respect of facing, it must, however, be stated that mere shotcreting is not aesthetically pleasing; the same must either be supplemented or give way to more appealing facing methods. We shall close this section with an example of temporary soil nailing than can be used in construction sites to secure the faces of cuts. 25.14.3 A method of temporary soil nailing To hold back soil temporarily on the sides of excavations, plywood sheets can be used as the facing skin and deformed steel rods (rebars) as reinforcement (Fig.25.20). The work involves holding perforated plywood planks against the sides of the cut and driving rebars bent at the nearer end until the bend comes in tight contact with the 10 plank, creating thereby an integral system capable of retaining the sides. The work proceeds downwards with excavation, and repeating the above steps, until reaching the bottom of the pit. Reference Kurian, N.P., Beena, K.S. and Krishna Kumar, R. (1997), “Settlement of reinforced sand in foundations,” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Sept.1997, Vol. 123, No. 9, pp. 818-827. Video clips Vid. Cl. 25.1 Description: Soil nailing with anchorage on the inner face of the wall (European patent), Duration: 2m 23s (Source: BAU-SANIERUNGSTECHNIK Germany) Audio: German. Vid. Cl. 25.2 Description: Another variant of the above technique, Duration: 1m 38s (Source: BAU-SANIERUNGSTECHNIK - Germany) Audio: German.