Case Study: Straits Regency(?) retaining wall failure

Post date: Jan 14, 2018 1:21:13 AM

There are couple of requests by friends asking about what happened in Penang. It will take some times to make some answer everything but there will be some case studies on some failures. I will try to cater all.

The failure of this retaining wall is due to global stability and not local stability of the design. Most time during submission, many councils forget about the global stability which have around five criteria to be fulfilled.

From these photos, the slippage of reinforcement is due to slip circle. In this case the initial breached would be the base or foundation for this type of retaining which is likely to be a shallow foundation. The design most likely only considers the overburden and rely the strength of anchors to withstand lateral load.

In reality, the slip circle formed behind these anchors (assuming with the length up to the end of the carriageway close to the building). The slip circle formed when water table increased (unsure if its due to subsoil or surface drainage but it is likely due to insufficient interceptor drains around the perimeter with rock bolt or soil nail behind the building).

To make it easier to shift is the drain in front of the retaining wall and tangent to the slip circle which start to saturates the soil around the foundation of the retaining wall. When breached, the retaining structure will overturn or slide forward. In this case, engineer should have prescribed the use of piles or raking piles as countermeasure against moment developed from surcharge and lateral force if there is no space to provide rock toe for the retaining wall. Nevertheless, it is only good if the local stability of the system allows rigid connection (resisting moment) between foundation to the system in event of slip or wedging occurs behind anchors.

The rock toe is the first line of defense which is vital for the global stability for this kind of retaining wall. The rock toe or bodily mass will halt the slide and restrain the slip and ensure failure will only reach its serviceability limit.

Part of the building (car porch) sheared since the column is sitting on pad footing. The rest of the structure is in tact due to the strong and shallow distance between pile cap to the rock socket. Since the pile is not slender, it is sturdy and robust to withstand movement of the soil.

Addendum:

From photos from other sources indicates that there is another retaining wall close by. What happened here is another consideration of soil state which affects the pore pressure. The at-rest, passive or active state may cause changes in load and pressure and caused instability of the retaining wall.

** This is an educational article and should be use as it is.