Soil Nail is commonly used in Malaysia especially in cut slope. The basic concept of soil nailing is to reinforce and strengthen the cut slope by installing closely-spaced steel bars, called ‘nails’, into a slope as construction proceeds from ‘top-down’. The spacing of soil nail normally used was range between 1 to 2 m at centre to centre and install using grid type or diamond type and the diameter of nail is 100mm. Several type of length was use such as 6m, 9m and 12 m length. This process creates a reinforced section that is in itself stable and able to retain the ground behind it. The reinforcements are passive and develop their reinforcing action through nail-ground interactions as the ground deforms during and following construction.
The popularity of soil nail slope is due to its technical suitability, ease of construction and is relatively maintenance free. The range of soil nail slope height up to more than 40m and then the gradient was normally range between 2:1 up to 4:1. In Malaysia, Soil nail was installed with guniting for erosion protection at the soil nail area. Even though soil nail was design in line with international practice, slope failure still occurred.
Why it happened especially in Malaysia? This matter is still questionable. Is it due to design aspect such as using wrong data /wrong assumption /mistake on theory/ mistake on correlations/ lack in assumption/ less experience/ unsustainable condition on slope and so on, but at the end of the day, an engineer always blame nature to protect his mistake, otherwise act of god?
So far, those are the soil nail failure investigation that I involved;
1. Slope Failure at CH 6100 Route 59, Ringlet to Tanah Rata, Cameron Highlands, Pahang.
On 24 February 2004, a soil nail slope at Chainage 6100 of Federal Route 59 failed. The failure debris blocked the entire two lanes of the road, covering 50 m of its length. While there was no injury, the road had to be closed for a period of 7 days, during which the failure debris was cleared, the slope inspected, the road repaired and other works undertaken to ensure safety prior to re-opening of the road.
.The slope was stabilized using 100mm diameter nailed with bar size 25mm dia. The penetration length is 12m length with spacing 1m c/c. The slope was failed during construction stage of project. An investigation of the slope failure was carried out. The investigation included a review of review of existing record, failure investigation, causes of the failure and conclusion. It was found that, factors such as geological, physical and human factor have caused to the failure but which factor contributes more to the failure is still the question. What can I say, it is very hard to find the human factor causes of failure because majority geotechnical experts have their own theory rather than expose the truth?????
Photo on soil nail failure.
2. Soil nailed wall collapsed at MRSM Bentong, Pahang.
On 6 December 2009, a soil nailed wall at MRSM Bentong, Pahang failed. The failure debris damaged all the entire sewage treatment plant. It was lucky that the failure was happened during school holidays, whilst there was no injury. Site visit was conducted a day later to overview the condition. Some technical issues were found during the site visit.
The slope was stabilized using 12m length of nail with spacing 1.5m c/c and the slope high is more than 36m high. From observation at the critical cross-section of the failure, the bond and tensile failure of reinforcement was not found. The slip failure was beyond the penetration length of soil nail. No detail investigation was carried out from our team. Nevertheless the mechanism and causes of failure still can be predicted based on evident that was found from the observation. The failure was under the external stability condition.
Based on visual inspection, the failure occurred due to the removal of the support by earthwork activity, steeping the slope and constructing the soil nail wall was unable to strengthen the global stability.
A cross-section & mechanism of the soil nailed slope failure
Photo on soil nailed wall failure at MRSM Bentong.
In conclusion, the effectiveness of soil nail to stabilize the slope still need deeper research. So far, no research was carried out to study the effectiveness of soil nail in Malaysia prospective because no research paper was found regarding Malaysian context. Most paper carried out were only focus on case study and design aspect none discuss on developing reliable theory on the effectiveness on slope soil to stabilise the slope in Malaysia.
