Effect of Grouting, Concrete Cover, and Combined Reinforcement on Masonry Retaining Walls

Abstract

Masonry retaining walls used in civil engineering projects, such as highway embankments and slope protections, easily crack due to complex internal pore structures and exposure to harsh environmental conditions. To address these problems, practical reinforcement methods, including grouting reinforcement, concrete cover reinforcement, and combined reinforcement, were proposed to maintain retaining walls in this study. Nine cases of different reinforcement schemes were adopted to investigate the effects of grouting volumes, grouting hole numbers, and reinforcement methods. The results showed that as the grouting volume and grouting hole numbers increased, the cracks occurred at a lower height, showing a higher moment resistance capacity. In addition, the cracking moment was enhanced with a thicker concrete cover. Furthermore, combined grouting and concrete cover reinforcement improved the structural integrity and showed the best performance, in which the failure mode shifted from brittle to ductile. However, concrete cover reinforcement is associated with a higher price and longer construction cycle. Thus, decisions should be made depending on the engineering requirement.

1. Introduction

In construction engineering, retaining walls are the most common type of support structure used to sustain embankment fills and slope soils, preventing large deformations and structure instability, such as landslides, landslips, etc. Masonry retaining walls are widely used due to the availability of local materials and their low cost. These walls are built by rubble, rough and cut stones, or masonry blocks with mortar. Due to the structural and material characteristics of masonry retaining walls, several problems may arise during construction, such as incomplete mortar filling and inadequate compaction of the internal structure, which will further lead to wall cracking, deformation, and failure [1]. Common methods widely used in retaining wall reinforcement, such as soil nailing [2] and grouting prestressed anchor cables [3], are not applicable, as disturbance during construction would further lead to the loss of wall integrity.

Since masonry retaining walls are composed of large-sized rubble, stones, and similar materials, with numerous internal voids distributed randomly, the complex internal pores structure is one of the main causes of damage to masonry retaining walls. These voids directly affect the mechanical characteristics of the wall structure. To address the above problems, various methods have been used in engineering applications, such as bacteria-induced limestone production [4], mortar with wood ash additives [5], and traditional Goan Saraswat bunds technology [6]. However, the above skills, on the one hand, depend heavily on reinforcement materials and the construction environment, which may lead to the unstable strength and poor durability. On the other hand, complex construction processes require abundant manpower and increase the cost, which limits the application of engineering scenarios.

Nevertheless, due to the lower cost and stable mechanical performance, grouting technology is widely used in tunnel [7,8,9,10,11,12,13] and mining engineering [14]. For example, Ren et al. investigated the effect of grouting diameter and thickness of grouting cover layers on tunnel stability [8], while Peng et al. researched the influence of grouting holes arrangement, grouting parameters, and the grouting scheme [9]. In addition, Qi et al. studied the impact of grouting volume, grouting distance, and grouting depth on the horizontal displacement of tunnels [10]. Above researches can provide experience for retaining wall engineering given the similarity between masonry retaining walls and both tunnel and mining engineering, where the complex internal pore structure need to be filled to meet the strength and stability requirements.

In addition, concrete structures are also used to improve the integrity of retaining walls. Akbar et al. compared the seismic responses of geogrid-reinforced retaining walls made of hollow prefabricated concrete panels, gravity-type stone masonry, and reinforced concrete walls using reduce-scaled shaking table tests [15]. It is found that the lateral earth pressure on the hollow prefabricated concrete walls and reinforced concrete walls was significantly lower than that on the gravity-type stone masonry walls, demonstrating higher resistance to lateral pressure. In addition, the hollow prefabricated concrete retaining walls exhibited the best performance in terms of top displacement and deflection, which were lower than those of the gravity-type stone masonry and reinforced concrete walls. However, in their tests, liquefaction effects were ignored, which could cause differential settlement and foundation instability of the structure [16]. Furthermore, Sano et al. investigated the seismic performance of three reinforcement structures: L-shaped reinforcement, titling reinforcement, and a combination of L-shaped and titling reinforcement [17]. It is shown that the combined reinforcement method demonstrated the best performance, resulting in the smallest bending moment, horizontal displacement at the top of the wall, and more uniform earth pressure distribution, indicating that the reinforced structure significantly affects stress state. However, the disadvantages of concrete structure reinforcement are increasing costs and construction periods.

Previous studies mainly focused on either grouting reinforcement in tunnel and mining engineering or the effect of a single reinforcement method. Due to the lacking understanding of the application of grouting technology and combined methods in masonry retaining walls, nine cases of different reinforcement schemes were adopted to assess the feasibility of different reinforcements used in the masonry retaining wall. The effect of grouting volumes, grouting hole numbers, and reinforcement methods on performance of masonry retaining walls were investigated. In addition, the cost and construction cycle were also given for a more comprehensive reference.

2. Materials and Methods

To investigate the effects of grouting volume, grouting hole numbers, and reinforcement methods on bearing capacity and failure modes of masonry retaining walls, nine groups of wall structures with a height of 270 cm and a width of 240 cm were built. The strength of the walls exceeded MU30, and the minimum thickness was more than 15 cm. The mass ratio of water, cement, and sand was 300:280:1450. The external structure of the retaining walls was dense, while the interior was loose, fitting the typical characteristics of retaining walls in actual engineering applications.

Table 1. Reinforcement program.

Wall Number	Remarks
1	Unreinforced
2	Grouted, 1 grouting hole
3	High-pressure grouting, 1 grouting hole
4	Grouted, 2 grouting holes
5	Grouted, 3 grouting holes
6	Covered with 10 cm thick C30 concrete layer
7	Covered with 20 cm thick C30 concrete layer
8	Covered with 10 cm thick C30 concrete layer, grouted, 3 grouting holes
9	Covered with 10 cm thick C30 concrete layer, grouted, 3 grouting holes with steel grouting pipes

presents the reinforcement program for the nine groups of retaining walls. Wall 1 was set as the control group without any reinforcement. Single-hole grouting was adopted in both wall 2 and wall 3, differing in the grouting volume. It is noted that the difference in grouting volume between wall 2 and 3 resulted from the different grouting pressure. Grouting for wall 2 was lower pressure, while that for wall 3 was higher pressure. In both walls, the grouting hole was located at the midline of the wall, 70 cm from the top. Multi-hole grouting was utilized in wall 4 and wall 5. Specifically, two grouting holes were installed in wall 4, while three grouting holes were set in a triangle distribution in wall 5. This is because distributed grouting holes will lead to relatively uniform grouting. In wall 4, two grouting holes were located at the midline of the wall, 70 cm and 170 cm from the top. For wall 5, the upper grouting hole was set the same as wall 4, but there were two additional lower grouting holes located 70 cm from the top and 60 cm from the sides. Unlike the grouting reinforcement used for walls 2~5, wall 6 and wall 7 were reinforced with steel mesh hanging on the wall surface and cast-in-place concrete. According to construction experience, molds used for pouring concrete are easily broken when the thickness of concrete cover exceeds 20 cm. In addition, Li et al. chose 5~20 cm thickness of concrete cover beams to investigate the effect of that on bending strength and found that the bending strength increased with the enhancement of concrete cover thickness [18]. Thus, to consider the cost and reinforcement performance comprehensively, 10 cm and 20 cm were chosen for wall 6 and wall 7, respectively. Wall 8 and wall 9 were reinforced using a combination of grouting and concrete cover. The grouting hole arrangement was the same as for wall 5, and the concrete thickness was 10 cm for both. However, there were steel grouting pipes placed inside the grouting holes of wall 9. The arrangement of grouting holes is shown in Figure 1. In addition, for walls 4~5 and 8~9, higher grouting pressures were adopted.

After the retaining walls were constructed and cured for 28 days, reinforcement measurement were carried out. For walls 6~9, a concrete cover was used for reinforcement. A mesh was hung on the surface of the retaining walls, and embedded strain gauges (Type: JMZX-215HA, sensitivity: 1 με, accuracy: 0.1% FS, range: ±1500 με) were installed at a interval of 60 cm along the midline of the retaining walls, as shown in Figure 2a. The strain gauges, labeled from top to bottom as Gauge A~E, were embedded before casting the C30 concrete cover. After the completion of the C30 concrete cover, bonded strain gauges were installed along the midline of the concrete surface at intervals of 60 cm, labeled from top to bottom as bonded strain gauge a~e. In addition, gauge a was 7.5 cm lower the top of wall. For walls 1~5, bonded strain gauges a~e were directly installed along the midline of retaining wall surface at the same interval of 60 cm, as shown in Figure 2b.

During the process of grouting reinforcement, a mass ratio of water, cement, and sand was taken as 3:3:4 to prepare grout mixture. For the lower grouting holes, 2.5% by mass of J85 accelerating agent was added to the grout for using, while 1.5% by mass of that was added to the grout for the upper grouting holes. For lower pressure grouting, when grout began to flow back from a grouting hole, it was considered fully filled. However, for higher pressure grouting, it was deemed to be finished when grout overflowed from the top of the wall. The whole grouting process conformed to Technical Specification for Grouting (YS/T 5211-2018) [19]. It is worth noting that in this process, if grout leakage occurred, the wall were sealed before resuming grouting. The grout volume for each retaining wall is shown in

Table 2. Grouting volume and accelerating agent content.

Wall Number		2	3	4	5	8	9
Grouting volume	Upper hole	0.90	1.25	0.65	0.45	0.55	0.50
	Lower hole	-	-	0.65	0.70	0.70	0.65
	Total	0.90	1.25	1.30	1.15	1.25	1.15
Content of accelerating agent	Upper hole	1.5%	1.5%	1.5%	1.5%	1.5%	1.5%
	Lower hole	-	-	2.5%	2.5%	2.5%	2.5%

, which indicates that the porosity inside the retaining walls was similar.

The loading equipment is shown in Figure 3. Two screw jacks were used to provide the load at 2/3 of the wall height by controlling the displacement of the retaining wall. During the load process, the strain, displacement, and horizontal stress were recorded. Additionally, the height of cracks on the retaining wall was observed during the test. According to Chinese standards Code for Design of Masonry Structures (GB 50003-2011) [20] and Code for Design of Concrete Structures (GB 50010-2010) [21], when penetrating cracks appeared on the surface of masonry or concrete structure, the structure was considered to be failed. This ultimate limit state commonly occurred when the displacement at the top, as recorded by cable displacement sensor 1, reached around 50~100 mm. After loading, walls were demolished, and effect of reinforced method were analyzed.

3. Results and Discussion

3.1. Unreinforced Retaining Walls

Figure 4 displays the displacement of cable displacement sensors of wall 1. It is shown that deformation was predominantly observed above 100 cm, where cracks occurred and serve as hinges, resulting in rotational deformation overall. Due to its gravity-based design and the rotational deformation, the maximum displacement under sustained horizontal loads occurred at the top of the wall.

Figure 5 illustrates the relationship between the maximum bending moment and displacement of wall 1. The curve exhibited three distinct phases. First, as the displacement increased from 0 to 0.5 mm, the maximum bending moment increased sharply from 0 to 14.4 kN·m, where no cracks were observed in the wall during this phase. Afterwards, with the further increase in displacement from 0.5 to 8.1 mm, the maximum bending moment rapidly decreased to 3.2 kN·m, accompanied by the appearance of a crack. Third, when displacement exceeded 8.1 mm, the maximum bending moment fluctuated and then remained constant in a small range. Meanwhile, cracks propagated to the front face of the wall, which resulted in small cracks appearing above the main crack and leading to separation between the upper and lower parts of the wall. The ultimate bearing capacity of retaining wall 1 obtained was 7.2 kN. Therefore, the maximum horizontal load pressure could be calculated as 2.88 kN/m considering that the height of the retaining wall above the base was 2.5 m.

Figure 6 depicts the relationship between the strain and displacement of wall 1. It is observed that gauges c, d, and e showed minimal strain variation, while gauges a and b exhibited significant changes in strain with the continuous load application. When displacement was less than 0.5 mm, the strain rapidly decreased at gauges a, while it increased sharply at gauges b, indicating tension at gauges a and compression at gauges a of the wall, respectively. Once a penetrating crack existed, strain gradually stabilized and remained relatively constant with continuous increasing displacement.

3.2. Grouted Reinforced Retaining Walls

Figure 7 shows the relationship between strain and displacement of grouted reinforced retaining walls under different grouting conditions. On the one hand, for wall 2 with 0.90 m³ grouting volume, it reveals that deformation primarily occurred above 65 cm, as shown in Figure 7a, and the cracks present at 90 cm confirmed this point. While, for wall 3 with 1.25 m³ grouting volume, it is evident that displacement increased almost linearly with the height of cable displacement sensors. The main cracks occurred at 40~55 cm above the foundation, as displayed in Figure 7b. The lower height of cracks meant the higher moment resistance capacity. Thus, wall 3 showed better strength compared with wall 2. On the other hand, for wall 4 with two grouting holes, it can be seen from Figure 7c that the displacement of cable displacement sensors showed a similar trend to wall 3. The cracks occurred 25~35 cm above the foundation. For wall 5 with three grouting holes, cracks mainly occurred 10~20 cm above the foundation. Similarly, the wall with more grouting holes displayed higher capacity.

Figure 8 shows the relationship between maximum bending moment and displacement of grouted reinforced retaining walls under different grouting conditions. Similar to the unreinforced retaining wall, all curves experienced three stages. For wall 2, the maximum bending moment reached 82.52 kN·m (Figure 8a), while that of wall 3 reached 95.00 kN·m (Figure 8b). In addition, the maximum horizontal load pressure of wall 2 and wall 3 were calculated as 16.52 kN/m and 19.00 kN/m, respectively, representing a huge enhancement compared with wall 1. This is attributed to the increased grouting volume injected, which can enhance the cementation between masonry, thus, increasing the strength of the wall. The displacements corresponding to the maximum bending moment of wall 2 and wall 3 were 0.7 mm and 0.8 mm, respectively, meaning that more grouting volume may increase the ductility of the wall. Furthermore, wall 2 has a similar damage phenomenon to wall 1, where the upper and lower parts of the wall separated. However, the upper and lower parts of wall 3 were still connected, with only a crack noted on the surface of the wall.

With regard to wall 4 and wall 5, the maximum bending moment arrived at 97.8 kN·m (Figure 8c) and 111.2 kN·m (Figure 8d), with maximum horizontal load pressures of 19.56 kN/m and 22.24 kN/m, respectively. It indicates that the strength of the reinforced retaining wall improved with the increase in grouting holes, which may be attributed to more uniform grouting due to the hole distribution. Additionally, the displacement corresponding to the maximum bending moment of wall 4 and wall 5 were 0.5 mm and 0.3 mm, respectively, showing that walls tended to exhibit brittle failure with more grouting hole numbers. Also, the upper and lower parts of wall 4 and wall 5 were split by a crack but not separated, with some masonry delaminated, which is similar to that observed for wall 3.

Figure 9 displays the relationship between strain and displacement. For both wall 2 and wall 3, it can be observed that strain gauges a, b, c, and e exhibited little change, while strain gauge d, located at crack height, showed significant compressive strain. However, the strain of wall 2 increased at the initial stage, while that of wall 3 kept constant at first and then sharply increased, which resulted from different heights of cracks. However, for both wall 4 and wall 5, strain gauges a, b, c, and d kept almost constant, while strain gauge e at the lowest height, located at crack height, showed significant compressive strain.

3.3. Reinforced Retaining Walls with Concrete Cover

The displacement at various heights of cable displacement sensors was plotted in Figure 10. It exhibits a generally linear trend, where higher displacement sensors showed more displacement.

Figure 11 shows the bending moment development curves of wall 6 and wall 7. At the beginning, when displacement at various heights was approximately approaching 0 mm, cracks were not formed. With the increase in displacement, the retaining walls began to crack. The cracking moment of wall 6 was 16.8 kN·m, slightly higher than the performance of wall 2. However, the cracking moment of wall 7 was about 40.6 kN·m, meaning that cracking moment increased with the thickness of the concrete cover. After the cracking moment remained stable for a certain displacement, the maximum bending moment continued to increase in a stepwise fashion. Wall 6 and wall 7 were only loaded to a maximum displacement of 80 mm, but the maximum bending moment was likely to continue to increase with the further displacements.

The relationship between the maximum bending moment and strain of the wall and concrete cover is shown in Figure 12. On the one hand, bonded strain gauges a, b, and c on retaining wall 6 and wall 7 show positive strains, while bonded strain gauges e and f show negative strains. This indicates that area above the crack was under tension, while the area lower than crack was compressed. Furthermore, bonded strain gauge d exhibited the largest negative strain, and bonded strain gauge c shows the largest positive strain, confirming the crack formed between their respective heights. On the other hand, all embedded strain gauges exhibited negative strain before cracking. After the walls cracked, the strain recorded by strain gauges C, D, and E rapidly increased to positive values with similar trends. This indicated that the concrete cover transitioned from a compression state to a tension state. Gauge D showed the largest strain, while gauge E displayed slightly smaller strain, which is consistent with the observations from bonded strain gauges. In addition, the strain of concrete cover of wall 7 was smaller than wall 6, indicating that the thicker concrete cover had a good strain resistance.

3.4. Composite Reinforced Retaining Wall

As explained above, similar linear trends were found in the displacement at various heights of cable displacement sensors, as shown in Figure 13.

Figure 14 shows the relationship between maximum bending moment and displacement of composite reinforced retaining walls, which were reinforced using a 10 cm thick concrete cover combined with grouting. At first, walls went through the elastic stage. After cracking, the cracking moments of wall 8 and wall 9 reached 95.0 kN·m and 128.4 kN·m, respectively. Then, the maximum bending moments of wall 8 and wall 9 decreased with increasing displacement, similar to wall 5. Meanwhile, the walls still failed in brittle mode. Subsequently, the concrete cover began to take effect. As the displacement further increased, the maximum bending moment of the walls started to increase again, which is consistent with phenomenon of wall 6. The failure mode shifted from brittle mode to ductile mode. This is because the masonry retaining wall was a discrete structure composed of block stones and mortar. The masonry retaining wall easily tended to crack or slide along weak bonding surfaces under stress, ultimately leading to brittle failure. Furthermore, stress tended to concentrate in localized areas of the masonry wall, causing rapid local failure. In contrast, the retaining wall with concrete cover exhibited increased stability. On one hand, the bonding effect of the concrete cover improved the overall integrity of the wall, effectively prohibiting crack propagation. Even when cracks occurred, the concrete cover could absorb more energy through crack extension, thereby delaying the failure process. On the other hand, the concrete cover distributed stress more evenly across the entire structure, reducing the risk of localized stress concentration. Therefore, concrete cover can help to delay the onset of cracking, thereby extending the operational life of the wall. However, the height of the crack in wall 9 was higher than that noted for wall 8. The possible explanation for this finding may be that installation of steel grouting pipes destroyed the integrity of the wall, leading to a lower moment resistance.

Figure 15 shows the relationship between maximum bending moment and strain of the wall and concrete cover for wall 8 and wall 9. Bonded strain gauges located above the crack showed minimal and almost constant values, meaning that combination of concrete cover and grouting could significantly enhance strength of the retaining wall, while those below the crack exhibited compression strain. In addition, as shown in Figure 15c,d, all embedded strain gauges exhibited small compression strain before cracking. After cracking, gauges located below the crack height rapidly increasing values, while the other gauges showed little change or remained unchanged, in line with the observations from the bonded strain gauges.

3.5. Cost and Construction Cycle

Table 3. Material cost.

Project	Unit Price
Masonry retaining wall [CNY/m3]	560
Grouting [CNY/m3]	790
Drilling grouting hole [CNY/m]	335
Steel grouting pipe [CNY/m]	100
Steel mesh [CNY/kg]	7.3
Concrete [CNY/m3]	650

lists the unit price of each project. The unit price of the masonry retaining wall was 560 CNY/m³, and each masonry retaining wall cost around 2.6 m³ masonry. For grouting reinforcement, grouting and drilling cost 790 CNY/m³ and 335 CNY/m, respectively. The grouting volume of each wall is shown in Table 2. For concrete cover reinforcement, steel mesh and concrete cost 7.3 CNY/kg and 650 CNY/m³. The steel mesh used for wall 6~9 were around 80 kg, respectively. In addition, the concrete poured for concrete cover was around 0.27 m³ and 0.54 m³ for 10 cm and 20 cm thickness, respectively. Specially, for wall 9, the unit price of steel grouting pipe placed in the grouting hole was 100 CNY/m.

Furthermore, the total cost and construction cycle are shown in

Table 4. Total cost and construction cycle.

	1	2	3	4	5	6	7	8	9
Retaining wall [CNY]	1456
Reinforcement [CNY]	-	895.25	1171.75	1666.85	2003.95	759.5	935	2942.95	2863.95
Total cost [CNY]	1456	2351.25	2627.75	3122.85	3459.95	2215.5	2391	4398.95	4319.95
Construction cycle [d]	8	8	8	8	8	37	37	37	37

. The advantage of grouting reinforcement is that it saved construction time. However, only concrete cover reinforcement seems to be more economic choice. Although combined grouting and concrete cover reinforcement are more expensive, it provided better performance. Thus, decisions should be made depending on the engineering requirement.

4. Conclusions

In this study, reinforcements of grouting, concrete cover, and combined grouting and concrete cover were carried out on masonry retaining walls to investigate the effect of grouting volume, grouting hole numbers, and reinforcement methods on performance. The following conclusions can be drawn as follows:

With the increase in grouting volume and grouting hole numbers, the cracks occurred at a lower height, and the cracking moment increased from 14.4 kN·m to 95 kN·m and from 14.4 kN·m to 111.2 kN·m, respectively, showing the higher moment resistance capacity, which is due to an increased cementation effect. The failure was brittle mode, and the strain gauge located around crack height showed a compressive strain. The advantage of grouting reinforcement is saving construction time.

Compared with grouting reinforcement, concrete cover could obviously enhance cracking moment, and this enhancement increased with its thickness, which rose from 16.8 kN·m to 40.6 kN·m. After wall cracking, the area above the crack was under tension, while the area lower than the crack was compressed. The concrete cover was under compression before cracking but under tension after cracking. In addition, the use of concrete cover reinforcement alone is a more cost-effective choice.

Combined grouting and concrete cover reinforcement showed the best capability, where the cracking moment was enhanced from 14.4 kN·m to 128.4 kN·m. The failure mode shifted from brittle mode to ductile mode, meaning that concrete cover can help to delay the onset of cracking, thereby improving the overall integrity of the wall and extending the operational life of the wall. However, it is associated with a higher price and more construction time. Therefore, this combined reinforcement shows great potential for future engineering applications.