2025 Volume 66 Issue 4 Pages 241-247
This paper presents a construction method for geosynthetic-reinforced soil retaining walls with rigid facing that can be used in narrow spaces. We developed settlement-following components to prevent backfill settlement from causing formwork deformation when formwork and backfill are connected. The proposed method uses these components in combination with lightweight embedded formwork to simultaneously construct the formwork and reinforced-backfill from the rear side, eliminating the need for scaffolding. A trial construction was conducted to verify the feasibility of the proposed method. Results confirmed settlement-following components functioned properly in a retaining wall with a height of 2.4 m. Considering the allowable sliding displacement of the components, the maximum height of this method is estimated to be approximately 4.0 m.
The geosynthetics-reinforced soil retaining wall with a rigid facing (RRR method), (hereinafter referred to as the “reinforced soil retaining wall”) has been widely used in railway construction. Proper construction of the rigidly integrated wall facing is critical. After constructing the reinforced-backfill, scaffolding is typically erected on the front side to build the wall facing. Construction projects such as continuous grade separation and double-track often take place in narrow areas adjacent to operating railway lines, roads, and residential areas. In such cases, there is insufficient space to erect scaffolding, which makes constructing the wall facing of the reinforced soil retaining wall difficult.
To address this issue, we developed a construction method that enables wall facing to be built from the rear side (i.e., from the backfill side), eliminating the need for scaffolding. This paper discusses the issues associated with the conventional wall construction method and outlines the proposed rear-side construction method in Chapter 2. Chapter 3 discusses the feasibility of the proposed method based on test construction. Chapter 4 describes the applicable height range for the method.
Figure 1 shows the conventional construction procedure for geosynthetics-reinforced soil retaining walls [1]. The procedure is as follows, consisting of step 1 through step 4. Step 1: Construct the foundation. Step 2: Place geogrid reinforcement wrapped with temporary holding (such as gravel bags or welded steel fabric) and then construct the reinforced-backfill using the holding effect of the temporary holding at the wall facing and the geogrid (Fig.1(a)). Step 3: After the backfill has settled, erect scaffolding in front of the retaining wall. Step 4: Construct the wall facing (Fig. 1(b)). Since the wall facing is constructed step by step, it is not affected by the deformation of the backfill. Note that scaffolding is required not only for formwork installation but also for formwork removal.
The structure of the wall facing is shown in Fig. 2. The formwork is welded to the L-steel through the separator. The L-steel is then welded to the steel for fixing external formwork on the backfill side. As a result, the formwork is supported from the backfill side via the L-steel.
The conventional procedure of installing the formwork after constructing the reinforced-backfill cannot be applied in rear-side construction without scaffolding. Constructing the formwork from the rear-side allows construction to proceed without scaffolding. However, it is necessary to build the reinforced-backfill and the formwork simultaneously. Using the current fixing method, if the reinforced-backfill and the formwork are constructed simultaneously without any countermeasures, the formwork may deform through the L-steel when the backfill settles, as shown in Fig. 3.
In addition to plywood formwork, the RRR-B method uses precast concrete panels that do not require demolding. This aims to shorten the construction period and reduce the amount of labor required for concrete work. The standard specifications for precast concrete panels for the RRR-B method are as follows: each panel weighs 584 kg and measures 2,000 mm in width, 1,008 mm in height, and 125 mm in thickness. This necessitates the use of a large crane during construction. The advantage of using precast concrete panels for the RRR-B method is that they serve as both formwork and part of the structure simultaneously.
2.2 Rear-side construction methodWe proposed a construction method for geosynthetics-reinforced soil retaining walls with a rigid facing that is suitable for narrow spaces. We developed components that can accommodate the settlement of backfill (hereafter referred to as “settlement-following components”), to prevent settlement from causing deformation of the formwork when it is connected to the backfill.
2.2.1 Rear-side construction using lightweight embedded formworkConstruction work in narrow areas adjacent to operating railway lines, roads, or residential buildings has issues related to the construction space. This makes securing the necessary working area and bringing in large construction machinery difficult. As mentioned above, the precast concrete panels used in the RRR-B method require a large crane for installation and scaffolding for placement.
Therefore, we considered using lightweight embedded formwork [2] as the formwork for the rear-side construction. The lightweight, embedded formwork is a residual formwork made of concrete that does not require demolding. The concrete mix design has a maximum coarse aggregate dimension of 15 mm and a design standard strength of 36 N/mm2. The standard specifications of the lightweight embedded formwork are as shown in Fig. 4. It is lightweight, with width of 800 mm, height of 400 mm, thickness of 30 mm, and weight of 20 kg per panel. It can be installed from the rear-side while being lifted with a small backhoe. In addition to ring connectors for connecting separators, adhesion strengthening connectors are attached to the rear-side of the lightweight embedded formwork to prevent it from falling off when in service. The attached adhesion-reinforcing connectors have been confirmed to have sufficient pull-out strength (adhesion strength).
Figure 5 shows the procedure for rear-side construction using lightweight embedded formwork. Step 1: Construct the foundation. Step 2: The lightweight embedded formwork is erected simultaneously with the construction of the reinforced-backfill. Step 3: After completion of reinforced-backfill and formwork construction, and when the reinforced-backfill has settled, concrete is placed to anchor the wall to the reinforced-backfill.
In rear-side construction, the reinforced-backfill and formwork must be constructed simultaneously. Therefore, we investigated a solution that enables this. During construction, the loads acting on the formwork are wind loads and lateral pressure from concrete placement, both of which act horizontally. When the reinforced-backfill and formwork are constructed simultaneously, in addition to the horizontal loads mentioned above, the formwork is also affected by settlement and bulging of the backfill that occur during construction, as shown in Fig. 3. It should be noted that settlement of the reinforced-backfill increases progressively with backfill placement, while the bulging tends to stabilize after the layer is compacted. For further details, refer to Reference 3.
In other words, applying horizontal restraint after constructing the first backfill layer can prevent bulging from significantly affecting the formwork and accommodate the deformation of the backfill. However, in the vertical direction, a sliding mechanism is necessary to accommodate the settlement of the backfill. Considering these factors, we developed a settlement-following component that is fixed horizontally to the L-steel but can slide vertically. This component is shown in Fig. 6. This component can easily be installed on the side of the L-steel and connected to a commonly used steel for fixing external form by welding. Installing the settlement-following component suppresses the displacement of the wall surface. It resists wind loads acting on the formwork during construction and lateral pressure during concrete placement. Furthermore, by sliding along the L-steel, the component can adapt to the settlement of the backfill. This allows the formwork and backfill to be constructed simultaneously. The settlement-following component is installed at the tip of the steel for fixing external form after the first backfill layer is constructed and is then fixed in the horizontal direction.
The settlement-following component is made of separator steel wire with a tensile strength of 540 N/mm2 or higher (equivalent to or exceeds JIS G 3532 standard for ordinary steel wire SWM-P) and has a diameter of 6.9 mm. As shown in Fig.6, the tip of the component is twisted into a hook shape and bent to follow the contour of the L-steel. The bent portion is 25 mm in length, which is half the width of the L-steel's flange. This configuration allows the component to resist horizontal loads such as lateral pressure during concrete placement, while enabling it to slide vertically along the L-steel.
We confirmed the feasibility of the back-construction method using lightweight embedded formwork and settlement-following components through test construction.
3.1 Overview of the test constructionFigure 7 shows a schematic diagram of the test specimen constructed for the test. The specimen measured 2.4 m in height, 2.0 m in width, and 2.0 m in depth. The lightweight embedded formwork was arranged in a staggered pattern.
The backfill material used was crusher run (C-40), compacted in 0.15 m thick layers to achieve a compaction degree of Dc = 90% (E-c method). The standard vertical spacing of geogrid reinforcement in reinforced soil retaining walls using the RRR method is 0.3 m. However, in this test construction, in addition to investigating the wall construction method, the influence of vertical geogrid reinforcement spacing on the stability of the retaining wall was also examined. Therefore, the vertical spacing was experimentally increased to 0.6 m―twice the standard spacing―and the geogrid reinforcement length was set to the minimum design length of 1.5 m. It has been confirmed [3] that even with the increased spacing of 0.6 m, construction stability remains approximately equivalent to that of the standard 0.3 m spacing. For further details, see Reference 3. Since one layer of geogrid reinforcement was installed for every two layers of welded steel fabric, the reinforcement in the folded-back sections wrapped around both welded steel fabrics together and embedded in the backfill. This resulted in a structure where the upper and lower welded steel fabrics act as one. The steels for fixing external formwork were installed at the same vertical intervals as the geogrid reinforcement (i.e., every 0.6 m). These fixings were equipped with settlement-tolerant members. Normally, these steels for fixing external formwork are placed at 0.3 m intervals to align with each layer of welded wire fabric. However, in this test construction, they were installed at 0.6 m intervals―half the usual frequency.
Figure 8 shows the construction procedure and the test construction. The procedure was as follows: (1) During the initial foundation work, L-steel was erected (Step 1 in Fig. 8). (2) A lightweight embedded formwork was then installed, and separators were welded and fixed to the L-steel (Step 2 in Fig. 8). (3) After constructing one layer of reinforced-backfill, settlement-following components were installed onto the L-steel, and the steels for fixing external formwork were welded together with the settlement-following components (Step3 in Fig. 8 and Fig. 6). Once the lightweight embedded formwork and the reinforced-backfill were completed, concrete was placed into the formwork (Step4 in Fig. 8).
During the construction of the reinforced-backfill, measurement items were taken the displacement in the depth direction of each level of the lightweight embedded formwork, the amount of bulging and settlement of the reinforced-backfill, and the stress of the separator member and steels for fixing external formwork. The displacement of the lightweight embedded formwork was measured using an optical range finder. The bulging and settlement of the reinforced-backfill were measured by placing targets on the welded wire fabric at vertical intervals of 0.3 m. Strain gauges were installed on the separators and steels for fixing the external formwork in order to measure stress on these members.
During concrete placement, displacement gauges were installed along the center row of the lightweight embedded formwork at each level to measure time-dependent displacement in the depth direction. Earth pressure cells were installed on the lightweight embedded formwork at each level to monitor the lateral pressure exerted on the formwork during concrete placement.
3.2 Results of the test construction 3.2.1 Deformation during construction of the wallFigure 9 shows both the measured settlement of the backfill and the displacement of the formwork. Figure 9(a) shows the amount of settlement, which was obtained by placing targets at the boundaries of each backfill layer immediately after compaction and surveying them after the completion of the backfill construction. Figure 9(b) shows the displacement of the formwork, measured at the center of each formwork level. The displacement in the depth direction was evaluated relative to the initial position at the time of formwork installation. Displacement toward the front (facing) side is defined as negative, while displacement toward the rear (backfill) side is defined as positive.
From the results shown in Fig. 9, it is verified below whether the settlement-following components functioned effectively against the settlement of the backfill induced by rolling compaction during backfill construction. If the settlement-following components do not function properly against the settlement of the backfill, the formwork is expected to undergo displacement in the depth direction as the backfill settles, as illustrated in Fig. 3. However, as shown in Fig. 9, the backfill experienced a maximum settlement of 26 mm, while the deformation of the formwork was limited to less than 2 mm. This result indicates that the settlement-following components effectively accommodated the settlement of the backfill. These findings confirm the feasibility of constructing the reinforced-backfill and lightweight embedded formwork simultaneously through the use of settlement-following components.
Figure 10 shows the depth wise installation accuracy of the lightweight embedded formwork after the reinforced-backfill is complete. The installation accuracy in Fig. 1 was evaluated as the deviation from the design slope of the wall (1:0.05). The installation accuracy of the formwork was defined as positive on the front side and negative on the back side.
Although there are no explicit standards for formwork installation in railway structures, the black dashed line in the figure indicates a reference tolerance of ±5 mm from the designed position. The variation was kept within about ±5 mm, confirming that the lightweight embedded formwork was installed with adequate precision. For reference, the specified by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) [4]'s control criteria for retaining wall construction allow for a tolerance of ±0.03H (72 mm) and within ±300 mm. This indicates that there are no issues with regard to these standards. Furthermore, the finishing tolerance for retaining walls in actual railway engineering practice is typically around ±20 mm, a criterion that the present installation meets.
3.2.2 Concrete placementThe concrete mix was designed with an ordinary Portland cement and 20 mm coarse aggregate. It has a nominal compressive strength of 21 N/mm2, a maximum water-cement ratio of 60%, and a slump of 21 cm. The concrete was placed at an approximate rate of 2.4 m/h. The ambient temperature during placement was 19°C.
Figure 11 shows the time history of the displacement of the lightweight embedded formwork during concrete placement. The displacement is plotted as the incremental change from the initial value just before placement. Outward bulging of the formwork is defined as positive. At a placement height of 1.65 m, a gap was observed between the drainage hole located at 1.8 m and the formwork. As a result, the placement was halted temporarily for approximately 3 minutes, while the gap was repaired with mortar putty.
A sudden increase in displacement was observed during placement at a height of 2.0 to 2.25 m in the fifth and sixth stages of the lightweight embedded formwork. Placement was suspended again (for about 8 minutes), and the formwork was temporarily restrained to prevent it from displacing any further outward.
The maximum displacement (bulging) of the lightweight embedded formwork was 17.3 mm. This satisfies the ±20 mm tolerance that is commonly used as a finishing standard for retaining walls. According to the current specifications, placing settlement-following components and steels for fixing external formwork at 0.3 m intervals is considered to enhance the restraining effect and reduce formwork displacement. Indeed, past construction records have confirmed that safe execution without the need for repairs can be achieved by placing the steels for fixing external formwork at 0.3 m intervals. However, in this test construction, the settlement-following components and fixing steels were installed at 0.6 m intervals, in accordance with the vertical spacing of the geogrid reinforcement. Nevertheless, to further reduce formwork displacement during concrete placement, it is necessary to adopt the 0.3 m interval as specified in the current design standards.
Figure 12 shows the time history of the time-dependent lateral pressure acting on the lightweight embedded formwork during concrete placement. The figure also includes the lateral design pressure corresponding to each earth pressure gauge installation height. which is represented by an equivalent hydrostatic pressure assuming a unit weight of 23.5 kN/m3 and a lateral pressure coefficient of 1.0 which is shown by dashed lines. Attention is focused on the earth pressure gauge installed at the lowest position (0.2 m height), where the lateral pressure is expected to be the greatest. The maximum value was recorded when the concrete placement height reached approximately 1.2 m, and the increasing trend of the lateral pressure at that point was found to be comparable to the hydrostatic pressure indicated by the dashed line. Furthermore, based on the gauges installed at 0.6 m and 1.0 m, it was observed that the lateral pressure equivalent to hydrostatic pressure only acted up to a placement height of around 1.5 m. Beyond this value, even as the placement progressed, no further increase in lateral pressure was observed. These results suggest that the design of components can be based on a pressure distribution model. In this model, hydrostatic pressure acts up to a concrete height of 1.5 m. After that, the lateral pressure remains constant. This model is based on the distribution presented in the Standard Specifications for Concrete [5]. The maximum stresses measured in the separator and the steels for fixing external formwork were 78 N/mm2 and 86 N/mm2, respectively―both sufficiently lower than the tensile strength of 540 N/mm2.
Using settlement-following components allows for the simultaneous construction of the reinforced-backfill and formwork. However, it should be noted that these components are designed to accommodate the settlement and compression of the backfill body itself. They are not intended to respond to consolidation settlement of the underlying ground.
The settlement-following components slide vertically along L-shaped steel sections to follow the settlement of the backfill. However, the separators for formwork are welded and fixed to the L-shaped steel. Figure 13 shows an example of the positional relationship between a separator and a settlement-following component. The ring connector used to attach the separator is installed 70 mm inward from the end of the formwork (Fig. 4). Depending on the positional relationship, the separator and the settlement-following component may interfere with each other. Since the separator can be welded at a slight angle if necessary, it is possible to ensure a clearance of at least 80 mm between the separator and the settlement-following component. Taking the construction tolerances and the weld leg length into account, the sliding allowance of the settlement-following component is approximately 70 mm when such clearance is ensured.
Figure 14 shows the relationship between the average interlayer compression ratio and the applied overburden load of the backfill observed in the test construction. In this test, crusher-run gravel (C-40) was used as the gravelly soil, and Inagi sand, which was compacted to a degree of compaction (Dc) of 90% using the E-c method, with a compacted layer thickness of 0.3 m , was used as the sandy soil [3]. For reference, the results from a highway project (hereinafter referred to as the “road case”) are also shown [6] and [7]. Although there was some variability between the sandy and gravelly soil, both exhibited an increasing trend in compression ratio with increasing overburden load. This is consistent with observations from the road case. Notably, the interlayer compression ratio was higher in the test construction than in the road case under increasing load. This difference is considered to be due to variations in the thickness of the compacted layers.
In the road case, the compacted layer thickness was 0.6 m. In the test construction, it was reduced to 0.3 m for sandy soil and 0.15 m for gravelly soil to achieve the target compaction degree of Dc = 90%. Although sandy soil is generally more compressible than gravelly soil, both types of soil showed similar trends in this test. This is believed to be partly due to the thinner, more compacted layers used in the test construction.
Figure 15 illustrates the relationship between the backfill height and the settlement for each layer of backfill. For the case with a backfill height of 2.4 m, the measured settlement shown in the figure is the residual settlement obtained by surveying during the construction test. The calculated settlement values were derived from the results in Fig. 14 (gravelly soil) to estimate the total backfill settlement.
Although some variability in the test at heights of 1.2 m and 1.5 m, the results generally followed the same trend as the calculated values. The greatest amount of settlement was found in the mid-height layers. This is because the layer equipped with settlement-following components experiences not only its own settlement, but also the cumulative effect of compression settlement in the underlying layers. A similar trend was also observed in the road case.
4.3 Applicable height of settlement-following componentsWhen the allowable sliding amount of the settlement-following component is set to 70 mm, the system can accommodate backfill settlement of up to 70 mm. As shown in Fig. 13, the maximum settlement of the backfill is 70 mm at a retaining wall height of approximately 4.5 m for both sandy and gravelly soils. However, considering the variation between calculated and measured values, the applicable wall height is currently set at approximately 4.0 m, which corresponds to an estimated settlement of about 53 mm.
Furthermore, according to surveys of actual reinforced soil retaining wall construction, about 45% of cases involve wall heights of less than 5 m [8]. Therefore, this component is expected to be applicable to a relatively large number of practical construction projects.
In this study, a construction method was developed for building retaining walls from the rear side to eliminate the need for scaffolding. The feasibility of the method was verified through test construction, and the applicable wall height was evaluated. The key findings are as follows:
1. A rear-side construction method for reinforced soil retaining walls suitable for narrow construction sites was proposed. By combining the newly developed settlement-following components with lightweight embedded formwork that does not require removal, simultaneous construction of the formwork and reinforced-backfill from the rear side is possible, thereby eliminating the need for scaffolding.
2. Test construction verified the feasibility of the proposed method. For a retaining wall with a height of 2.4 m, it was confirmed that the settlement-following components effectively responded to backfill settlement.
3. Considering the allowable sliding distance of the settlement-following components, the applicable height for this construction method is estimated to be approximately 4.0 m.
Furthermore, details regarding the rear‑side construction method that combines settlement‑following components with lightweight embedded formwork―including component specifications, construction procedures, and key considerations during construction―are now provided in the revised editions of the RRR Method Design and Construction Manual [9], Materials Manual [10], and Cost Estimation Manual [11], which were published in October and November 2024.
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Yuki KURAKAMI, Ph.D. Senior Researcher, Foundation & Geotechnical Engineering Laboratory, Structure Technology Division (Former) Research Areas: Geotechnical Engineering |
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Susumu NAKAJIMA, Ph.D. Senior Chief Researcher, Head of Foundation & Geotechnical Engineering Laboratory, Structure Technology Division Research Areas: Geotechnical Engineering |
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Takeharu KONAMI OKASAN LIVIC Co.,LTD. |
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Yoshio YAMASHITA Enbine Company |
