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Case Study: Straits Regency(?) retaining wall failure

posted Jan 13, 2018, 5:21 PM by jeffery jim   [ updated Oct 9, 2018, 6:10 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.

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.

This case is confirmed by Kam Suan Pheng (Soil Scientist) on on the 8th October 2018 (link: ) as follows;

Quoted Verbatim:

GEORGE TOWN: An expert told a forum yesterday how “hidden” underground rivers and waterfalls on the hills of Penang island might mean more disastrous landslides if development is allowed to continue in such areas. Soil scientist Kam Suan Pheng urged authorities to learn from last year’s rainfall and flood impact in their attempt to build a bypass road which would cross sensitive hill areas.

The road in question is the 10.53km North Coast Paired Road (NCPR), 2km of which will pass through the hill areas of Tanjung Bungah, Batu Ferringhi and Teluk Bahang. Kam reminded the authorities of the landslide on a hill slope in Tanjung Bungah during last year’s historic rainfall where a road fronting unoccupied new bungalows caved in.

She said the NCPR’s projected route might have a similar topography.

In the Tanjung Bungah incident, she said, the entire road in front of the luxury bungalows caved in, with only a retaining wall holding back the earth from apartments and other buildings below the hill. She said the authorities had failed to see that the site was sitting on what used to be a river, with a rock face above the bungalows forming a waterfall some 40 years ago.

As time passed, she said, the river was converted into concrete drains as developments cropped up in the area. Although the river might be expected to flow into the new drains, she added, the forces of nature had pushed it back to its old path. Hence, the river flowed underground despite being “buried”, she said.

According to Kam, the massive rainfall had triggered the landslide at the bungalow area. She said the old waterfall was “recreated” as rapid waters came down the rock face above the site. Coupled with the downpour, she said, the spread of the water flowing down the rock face forced it to seep below ground, destabilising the site and causing the road in front of it to cave.

“When rivers cannot take a natural course of flow, they will still try to go through the soil underground. Rivers are never dead. My take is, even if we divert (rivers), the drains will be unable to take the rapid runoff and it will definitely go underground. As water collects, the soil becomes very saturated and weakens the entire area. The area becomes waterlogged. Saturated soil is heavier than dry soil. That is how the road collapsed. This is the likely nature of the route along the NCPR. Isn’t it enough of a lesson for you to think very carefully about whether you want a road like this or not?” she said at a forum titled “NCPR, how is it going to affect you?” by Penang Forum in Tanjung Bungah last night.

The bungalows and road in the Tanjung Bungah incident were located along Persiaran Tanjung Bungah 3, where the units were going for RM1.6 million to RM2.3 million each. The city council has since assured that the bungalows are safe for occupation while the caved-in road has been repaired. In 2013, a similar landslide happened some 200m from that location. The NCPR is being built by the contractors of the undersea tunnel as part of a three-road bypass side project. It will connect Teluk Bahang, Batu Ferringhi and Tanjung Bungah. This is separate from another highway project pursued by SRS Consortium for the Penang government.

The NCPR will mostly be on the road level with 2.275km as an elevated stretch which requires cutting the untouched hills of Tanjung Bungah and Teluk Bahang. Residents have objected to the project, citing a variety of reasons including traffic and noise pollution. However, they are mostly concerned over the hill cutting and the permanent damage this will bring to the forests there.

According to the NCPR’s environmental impact assessment (EIA), 46% of the bypass will be on terrain with a higher than 25-degree slope. NGOs have argued that slopes above 25 degrees are usually “sensitive hill lands”. Despite heavy opposition from residents’ associations, NGOs and other groups, the project was given approval by the Department of Environment (DoE) last November. The contractors have vowed to follow 59 strict conditions imposed by the DoE and the project owner, the Penang government, has promised to stop the project if these conditions are not met. The project is scheduled for completion in the next seven years, pending completion of other bypass roads in the RM6.3 billion Penang undersea tunnel and three main roads project. Kam said while there was no doubt that engineering methods, mitigation and other ways might be proposed to ensure that all goes well in the road project, Malaysia’s track record in doing so had not been stellar.

“What is the track record in terms of developers and authorities being able to follow these conditions and guidelines?”