Newsletter Volume 10, Issue 4 December 2025

 

Professor Emeritus
Saitama University

 

1. Introduction

Geogrid reinforced soil walls (GRSWs) are recognized to have high seismic stability. Fig. 1 shows typical reinforced soil wall technologies in Japan. Table 1 summarizes the damage statistics of the investigated reinforced soil walls that were affected by the 2011 Tohoku earthquake (Miyata, 2014). Although seismic motion was much higher than the design value in a wide area, and the impact of the tsunami was not considered in the design, the ultimate limit state is less than 1% for all three types of investigated walls. More than 90% of the walls show no damage. It was also reported that more than 90% of the walls showed no damage by the investigation after the 2024 Noto earthquake. Although the walls did not collapse completely, significant wall deformations were observed in the recent large earthquakes (Kuwano et al., 2014), as seen in Fig. 2, for example. After the earthquake, rescue teams should reach the sites as soon as possible. The “Rule of threes” tells that “You cannot survive for more than 3 days without water”. Road access is a lifeline for disaster victims. However, in the case of the 2024 Noto earthquake, roads were closed at numerous locations, and rescue teams could hardly reach the most seriously damaged area of the Noto Peninsula. We should decide whether the road can be used or not without a detailed, time-consuming investigation. Stability of damaged GRSWs against the next major event is unknown. It is necessary to evaluate the extent of their damage to assess the necessity of repairing or reconstructing them. Wall inclination was proposed to be a good index to evaluate the extent of GRSW damage through a series of laboratory model tests (Izawa & Kuwano, 2011). However, determining wall inclination would be challenging in real-world scenarios due to factors like difficulty in approaching the damaged wall around the area of disaster, surveying the deformation of damaged wall which is often screened by vegetation etc., as shown in Fig. 2. On the other hand, settlement of the backfill surface is easier to be measured by UAV (unmanned aerial vehicle) than measuring the wall inclination at the site. InSAR (Interferometric Synthetic Aperture Radar) from a satellite will be used to monitor the change of configuration in the long term, as the satellite does not fly above the site frequently, but it would be useful to know the contour before the earthquake.

This article reports results of laboratory model shaking table tests carried out to study settlement profiles in the backfill surface and their correspondence with the extent of damage of GRSWs. A series of shaking table tests on GRSWs into which soil nails were incorporated was also performed to investigate the effect of soil nails on the seismic stability of GRSWs.
 

Fig. 1 Reinforced soil wall technology in Japan (modified Miyata, 2014)	Fig. 2 Example of a damaged reinforced soil wall and the surrounding condition.

Table 1. Damage statistics of reinforced soil walls affected by the 2011 Tohoku earthquake (Kuwano et al., 2014)

	Steel strip walls	Geogrid walls	Multi-anchor walls
Ultimate limit state	0.3%	0.7%	0%
Restorability limit state	1.0%	4.3%	0%
Serviceability limit state	7.0%	0.7%	3.0%
No damage	91.7%	94.3%	97.0%

Note: These statistics are based on damage investigation conducted at sites where the JMA seismic intensity scales were mostly greater than 5 upper.
 

2. Outline of the tests to estimate the damage of GRSWs

A typical layout of the model wall is shown in Fig. 3. The dimensions of the models were 750 mm in length, 300 mm in width, and 500 mm in height. The wall facing used in the model was made of acrylic panels with wedge-shaped ends that allowed rotation at the contacts but restricted any horizontal movement. Toyoura sand (Dr=80%) was used for both backfill and foundation. Optical targets were placed within the backfill soil to monitor strain condition by GoPro Hero 7 rigidly connected to the front of soil container. Intel RealSense LiDAR L515 was fixed above the model to monitor the settlement profile, as shown in Fig. 4. Two types of geogrids were used. One is with high stiffness (H-type) and the other is with low stiffness (L-type). Based on the pullout test results, the pullout resistance of H-type is around four times greater than that of the L-type. A total of six model tests were conducted using different combinations of geogrid type and geogrid length. Models H40, H50 and H60 used H-type geogrid, and models L40, L50, and L60 used L-type. The second part of the model’s name represents the geogrid length-to-wall height ratio in percentage.

The model walls were subjected to seismic loading in a shaking table using a sinusoidal wave with a frequency of 5 Hz. The amplitude of the wave was increased by 50 gal every 10 seconds until the walls collapsed completely, as shown in Fig. 5. For measuring the intensity of input motion, Arias Intensity, I_A, was used as a measure of the strength of a ground motion. It is defined as the time integral of the square of input acceleration, a(t):
 

Distributions of maximum shear strain within the backfill soil at the time of collapse, along with the settlement profile, which is the green area at the top surface, are shown in Fig. 6. The observed maximum shear strain distribution and slip lines resemble those assumed in the two-wedge theory, which is widely used in the design of reinforced soil walls. The first wedge constitutes the reinforced soil block, and the second wedge is the triangular shape sliding wedge behind the reinforced soil zone. It was observed that the primary area of settlement was distributed above the sliding wedge.

The maximum settlement, S_max, and the area of settlement, A_s, along the longitudinal center line were determined as shown in Fig. 7. They were nondimensionalized by the wall height, H, as S_max /H and A_s/H². Settlement along the center line of the top surface at various values of Arias Intensity, I_A, is shown in Fig. 8. The location of the maximum settlement and the length of the geogrid reinforcement are also illustrated. As seen in Fig. 8, the location of the maximum settlement is typically just behind the reinforced soil zone. In the cases of H40, H50, L40, and L50, the difference in settlement between the reinforced soil block and the sliding wedge was quite evident. However, in longer geogrid cases of H60 and L60, the settlement was distributed over a greater length of backfill. As a result, the settlement at a specific section appeared to be low, though the overall settlement area remained significant.

Maximum settlement and settlement area, along with the bottom facing inclination, which were used by Izawa and Kuwano (2011), were studied as parameters for indicating the extent of damage in GRSWs. As seen in Fig. 8, locations of the maximum settlement may not remain quite consistent as the shaking progressed. The shape of the settlement trough also exhibited variations during this progression. When S_max /H is plotted against the facing inclination, as shown in Fig. 9, it is found that the maximum settlement progression was not fully consistent with the facing inclination. Using only maximum settlement for assessing the extent of damage can lead to incorrect evaluation. Unlike maximum settlement, which focuses only on a specific section, settlement area accounts for settlement across the entire backfill. Fig. 10 shows the relationship between the A_s/H² and the facing inclination. The settlement area demonstrates a relatively linear increase with facing inclination for all the models. This indicates that the settlement area can serve as a better indicator of the extent of damage, though monitoring the area of settlement trough is not as easy as monitoring the maximum settlement and/or a wall top displacement.

 A_s/H² is plotted against the Arias Intensity in Fig. 11. For the same level of shaking, I_A, the wall with lower stiffness geogrid and shorter geogrid suffers more settlement. The model wall shows a point at which the wall became unstable with sudden expansion of the settlement trough for further shaking and collapsed. The walls in this test series could not bear additional shaking and collapsed when A_s/H² reached approximately 2%.
 

Fig. 11 Settlement area vs. Arias Intensity

Geogrid reinforced soil walls (GRSW) usually use only geogrids as a reinforcing material. But if it is thought that the stability of GRSW is not enough with only geogrids, or when the damaged GRSW needs to be repaired, it is expected to add soil nails. A series of shaking table tests on GRSWs into which soil nails were incorporated was performed to investigate the behavior and seismic resistance during earthquakes.
 

In this series of tests, summarized in Table 2, the 200 mm long H-type geogrid and two types of soil nails were used. The length of soil nails was 300 mm. One geogrid and two soil nails were used with each facing panel, as shown in Fig. 12. Geogrids were fixed to the wall panel through a metal jig, except in the test case Ga2Ns3, in which geogrids were not fixed to the facing panels, assuming the serious rupture damage of geogrids by the earthquake. Fig. 13 shows support in front of the facing panels and insertion of soil nails. The supports were used to keep the facing panels vertical when the GRSW model was made. The soil nails were inserted after making the GRSW model.

Fig. 12. Model geogrid and soil nails.	Fig. 13. Support in front of facing panels and insertion of soil nails.

Fig. 14 shows the observed reinforced area together with the reinforcing materials and the slip line of each test model. In the cases of G2 and G2Ns3 with fixed geogrid, the slip line was not a single straight line, i.e. an almost vertical line behind the geogrids and a slope of the active wedge. The slip line avoided the geogrid at the bottom of the reinforced area. It indicates that the wall and backfill were well integrated in the reinforced area by geogrids. In G2Ns3, confined zone of backfill seems to be thicker than that in G2. It indicates that the stability of GRSW was increased by soil nails. In contrast, in the cases of Ns3 and Nsc3 without a geogrid, the slip line could go through the reinforced area. As the φ 13 mm soil nail used in Nsc3 was a screw rod covered with cement and thicker than a φ 5 mm screw rod nail, the reinforced area was integrated more in Nsc3 than in Ns3, and the slip line needed to go through a deeper location. Ga2Ns3, in which geogrids were not fixed to the facing panel, the slip line was formed in a single straight line across the reinforced area, though the area of geogrids seems to be integrated. In the preliminary test, similar to G2, but geogrids were not connected to the facing panels, the wall collapsed as soon as the supports were removed. However, if soil nails were installed as in Ga2Ns3, backfill soil reinforced by geogrids was integrated with facing panels. This indicates that even damaged GRSW with ruptured geogrids could recover its stability with soil nails.

Wall displacements of the models caused by shaking are shown in Fig. 15. The displacements were measured by analyzing the black dots on the side of each wall panel. In the cases of walls with geogrid, G2, G2Ns3, and Ga2Ns3, the walls leaned and collapsed with the panels keeping almost straight shape. It shows that the wall and the backfill are well integrated in the reinforced area by geogrids, and the effect of reinforcement can be confirmed. The acceleration when the wall started to deform in G2Ns3 and Ga2Ns3 was larger than in G2, and it shows the improvement of seismic resistance by the soil nails. These results indicate that the addition of soil nails to GRSW is effective in improving seismic resistance, and additional reinforcement of soil nails to GRSW damaged by the earthquake is also effective.
 

Fig. 16 shows the overall wall leaning angle of each test model. This angle is [(horizontal displacement at the top of the wall) - (horizontal displacement at the bottom of the wall)] × 100 / (height difference between two points), which is the average leaning angle of the entire wall. The graph of G2Ns3 is similar to that of G2, but the acceleration to cause a similar displacement was higher for G2Ns3 than that of G2. It indicates that the seismic resistance of G2Ns3 is higher than that of G2. As seen in Fig. 16, the overall wall leaning angle of Ns3 is negative because the lower part of the wall is deformed like swelling, instead of maintaining a straight line as in G2 and G2Ns3. Therefore, it was thought that this average angle of the overall wall could not be used properly to evaluate the stability of wall. The local maximum wall leaning angle of each of the eight wall panels was calculated. The maximum local leaning angles of the walls, at the third panel from the bottom in N3, and the second from the bottom in other walls, are shown in Fig. 17. Nsc3 wall showed the high seismic resistance as thick soil nails confined wider area as seen in Fig. 14. Other walls showed almost similar shape in increase of the local leaning angle with time, therefore with shaking, but the three walls with screw rods (Ns3 walls) showed higher seismic resistance than G2 wall without soil nail. It indicates the potential of soil nails in increasing seismic stability of GRSWs.

We should decide whether the road can be used or not without a detailed, time-consuming investigation to rescue the people in a seriously damaged area. Settlement in the backfill can be used to evaluate the extent of damage in GRSWs due to an earthquake. The settlement area offers a more comprehensive representation of settlement behavior in the backfill compared to the maximum settlement and is a more suitable indicator of the extent of damage. Seismic stability of the GRSW with soil nails is higher than the GRSW without soil nails, though geogrids are not connected to the facing panels. It indicates the potential of soil nails in increasing the seismic stability of GRSWs and repairing damaged GRSWs.