Lessons Learned From Failure of Retaining Wall on Slope

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A high-rise waterfront residential development with ancillary facilities was proposed near a riverbank (Fig. 1). The tower block was located at the top of a gentle 10-meter-high slope with the toe of the slope meeting a flowing river. Site investigation boreholes were carried out at the tower block location and results showed good ground conditions with no presence of soft soils. Nonetheless, as the tower loadings were high, pile foundation was proposed for the tower. To facilitate the construction of the ancillary facilities consisting of light-weight structures, a retaining wall (Fig. 2) was designed to be placed at the mid-height of the slope with the top of the retaining wall meeting the platform level of the tower block. This created flat, usable space on the slope. The retaining wall base was designed to rest on grade without any piles.

Halfway into the construction of the retaining wall, a landslide occurred causing soil to be displaced into the river. The upward heaving of the riverbed even disrupted the fluency of the river flow. The retaining wall collapsed and a footpath along the river side was damaged. It was fortunate that the tower block remained intact and was unaffected by the incident. One passer-by sustained minor injuries. Post-failure investigations, which included drilling 42 additional boreholes extending to the riverbank, revealed a failure surface cutting below the slope and running through a layer of soft soil. How did a seemingly safe and “gentle” slope fail suddenly? What are the lessons learned from this episode?

The importance of site investigation in any developmental project involving the ground cannot be overemphasized. Without site investigation, it is difficult to know the stratigraphy and ascertain the appropriate ground parameters for design. Equally important, the presence of adverse ground conditions such as soft soils may be overlooked. Undetected soft soils have shown up in many cases to be problematic such as causing settlement and displacement issues. Many engineers are careful to insist on site investigation for high-rise buildings but sometimes become complacent when designing retaining walls, especially when the retaining height is not high.
Codes usually give a generic guide on the number and depth of boreholes. For example, FHWA (Federal Highway Administration) (2002) recommended “a minimum of one boring should be performed for each retaining wall." For retaining walls more than 30 m in length, the spacing between borings should be no greater than 60 meters. Similarly, in Eurocode 7 (EC7), for linear structure (roads, railways, channels, pipelines, dikes, tunnels and retaining wall), a spacing of 20–200 meters is required. It is important to note a single point information below the retaining wall base would not provide sufficient information to establish a section profile where ground conditions may differ in front and behind the wall. This is why in Eurocode 7-2, “for structures on or near slopes and steps in terrain (including excavation), investigation points should also be arranged outside the project area, these being located so that the stability of the slope or cut can be assessed.”

Before going into detailed site investigation, a preliminary desk study on the geological, hydrogeological, topographical, and historical information can be very useful. For example, differences in ground levels and presence of water bodies may suggest more complicated ground conditions such as the presence of soft deposits in buried valleys and old landslides.
A study of the location of the project site on a geological map (see Figure 3) revealed that at the project site boundary, presence of soft soils is highly probable. Coupled with the presence of an existing low-lying area and river, a cautious engineer would not assume competent ground conditions without site investigations, especially at the site boundary locality. The engineer for the project site used nearby boreholes conducted for the tower block and assumed competent ground when designing the retaining wall. He “did not know” that the presence of soft soils could be encountered so close by.

The agency having jurisdiction required site investigations to be carried out in accordance with codes and standards, including guidance issued by the local engineering fraternity. In addition, the checking of a high slope (more than 6 meters high) subjected to new loadings would require a peer review by an independent geotechnical specialist. However, it appears that these requirements were missed and not complied with during the plan submission and review process.

Many conventional reinforced concrete retaining walls adopt an L-shaped design such that the backfilled earth’s weight on the retained side bearing onto the retaining wall base helps to generate the resistance against sliding and overturning. The bearing pressure at the base of the wall should also be checked to be within allowable limits. If foundation soils are compressible, settlement or tilt could be an issue. Settlement should then be estimated and ensure any movements are still within acceptable limits. Stronger foundations or ground improvement works could be proposed to address the issue.

Another very important aspect is a check on the overall or global stability, i.e combined failure in the ground and in the structural element. This has been mentioned repetitively in codes and guidebooks. For example, the Canadian Foundation Manual stated that “where retaining walls are founded on deep layers or strata of cohesive soils, there is a possibility of failure occurring along a surface passing at some depth below the wall and well behind the backfill. The stability of the soil mass containing the retaining wall should be checked with respect to the most critical potential failure surface. A minimum factor of safety of 1.5 should be used when designing for overall stability.” Similarly, Eurocode 7 mentions overall stability checks on slopes, near rivers, retaining walls in several separate clauses. In particular, EC7 also reminded readers that “stability problems or creep movements occur primarily in cohesive soil with a sloping ground surface.” The engineer for the project site did not carry out a global stability check, as he felt the “slope was gentle” and the “ground was good” and deemed such a check unnecessary. He was overly focused on designing the retaining wall only for sliding, bearing, and overturning.

Building up a retaining wall from ground surface is likened to adding surcharge on the ground. In this case, a 4-meter-high retaining wall can hardly be considered “light-weight,” as the additional load can be easily 80 kPa.

The presence of soft soils underlying an existing gentle slope would make the slope inherently weaker than it appears to be. This means that, even before any works were started on the slope, the factor of safety could be below code requirements or even close to one, indicating impending failure, as the failure surface would pass through the weakest zone. In this incident, it can be seen that the soft soil had very little resistance against inclined loading (due to the retaining wall surcharge) because at the bottom of the slope, the boundary condition is “unconfined,” i.e. free to move. Compare this to a layer of soft soil deeper underground and loaded vertically where the lateral sides are confined, giving it added resistance. Therefore, an appreciation of factor of safety or capacity of existing slopes can be helpful.

Rain and water have a detrimental effect on slope stability in several ways. Starting from the slope surface, the most obvious is the erosion of soil (washing away at the slope face) and softening of clays when wet causing a decrease in strength and stiffness. The seepage forces inside a soil mass can also cause instability, especially at the slope toe. Saturated soil is heavier than dry soil and this increases the driving force when located near the top of slope. More fundamentally, from an effective stress perspective, an increase in pore pressures causes a decrease in strength leading to lower load-resisting capacity.

Rainfall induced landslides are not uncommon in regions receiving heavy rainfalls. In recent years, awareness of climate change has increased dramatically as more and more areas have experienced an increased frequency of returning heavy rainstorms. Prolonged periods of heavy rainfall can cause the development of excess pore water pressure, reducing effective stress and strength. Therefore, it is crucial that engineers adopt the appropriate groundwater levels in slope design, taking into account ground permeability, rainfall, and climate change.

Consider a very simplified representation of the failed slope in a finite element model (Fig. 4) with a slope height of 10 meters and gradient of about 20 degrees. The default model has sand material in dry condition (i.e. no groundwater). The model is then updated with a thin layer of soft clay. Factor of safety are computed for both cases with and without full groundwater (i.e. phreatic level following the slope surface). Results are presented in Figure 5. To capture the initial stresses correctly, a gravity (self-weight) load case was first imposed (Fig. 4c). The artificial deformations generated can be reset to zero at the next stage of loading or analysis, without affecting the stresses generated. A K0-procedure (coefficient of at-rest lateral earth pressure) computation would not be suitable for non-horizontal surfaces.

Even though the default case has a relatively high factor of safety of 3.84, Figure 5 shows that the factor of safety is reduced by half with the inclusion of full groundwater for both soil with and without the slice of soft clay. With the presence of soft clay, the failure surface is somewhat lowered slightly and forced to pass through the clay layer. When both clay and water are present, the factor of safety dropped to 1.35. Notice how a seemingly safe and “gentle” slope can have a low safety factor.

Many slope failures can be attributed to adverse geological features and groundwater conditions. Figure 6 illustrates two such examples. In the first example, the presence of a shallow impermeable layer (e.g. clay or peat) in relatively permeable (sand) ground creates a perched water table. This shallow perching can cause a rapid and large increase in pore pressures and erosion of material. The piping condition worsens over time as the permeability difference widens. In the second example, a classic soil-rock interface issue shows up in a slope stability problem. The high permeability of the loose sand at the interface again creates sub-surface erosion when water finds its way into the weakness. The channels or pipes form when a discharge outlet is created at the slope toe with the materials broken loose and carried away.

Post-failure rectification works usually involves strengthening the weak ground. Soft soils are typically treated with ground improvement such as jet-grouting or soil mixing. The insertion of structural elements (such as sheet pile and soil nails) are also effective in enhancing overall slope strength, as the failure surfaces are being cut off or intercepted by much stronger structural elements.

To strengthen an existing retaining wall, additional piles can be installed and connected to the retaining wall via tie beams. The wall base can be extended with additional reinforced concrete casting and additional counterforts or buttresses can be introduced. Changing the slope gradient or profile and adding subsoil drains can also be beneficial.

This landslide incident on a gentle slope serves as a reminder of the importance of site investigation, desk study, and checking of overall global stability involving the combined failure of the structural element and the ground. In addition, the understanding and adoption of appropriate groundwater conditions in a slope design is critical, taking into account extreme and prolonged rainstorms due to climate change. Finally, slope engineers need to be aware of the various types of adverse geological features and adverse groundwater conditions and consider the relevance and occurrence of such conditions in their specific projects. ■

About the Author

Hee Yang Ng is a Principal Engineer with a building control agency in the Asia-Pacific region.