Piling On Limestone Bedrock in Kuching (3)

Limestone bed rock is well known to have cavities and caverns. Just go to the Mulu Cave , Niah Cave or the Bau Caves and walk through the caves, you will feel the wonder of nature. But those limestone bedrocks are all high above sea level, severe chemical erosion were expected as the accumulated acidic rainwater reacted with the limestone, gradually formed solution channels/tunnels, and flowed to the lowest water level at the nearest rivers.

The limestone bed rock at this Kuching site is 10.8m below ground level which is 3-4m above mean sea level. Not much water flow in the limestone would be expected and thus less erosion. Actually, things can be complicated if you look into the geological history of the bedrock as well as the change of sea levels of the past. But for this engineering purpose, lesser erosion is expected due to bedrock below current sea level is sufficed, i.e. only the top 1m bedrock was quite badly eroded and some limestone rock were still hanging there but cavities were filled up by the deposited alluvial soils.

These rock rock cores, although fractured badly, are very hard. When piles were driven onto this layer uncontrollably, i.e. hard driving, usually automatic mechanised hard driving, usually 1.5 ton hammer with 0.9m drop, the first contact blow would deviate the pile, the second hard blow would cause joint failure or contact failure. If there were slippery cavity channel, the pile just followed that channel until it hit obstacle and filled up by crushing debris of the heavy driving. 

In fact, this phenomenon was conveyed to me by a Senior Engineer who saw a pile driven in position A at Bau, ended up the pile coming out from position B, 10m away. Bau is a famous limestone town.

Therefore, the installed piles probably glided along the badly eroded top 1m, and if there were solution channels into the deeper layer, the piles just followed that.

The only solution is to stop hard driving, as banging the pile against the hard limestone bedrock only causes damage to the pile. It is preferable to use manual driving, lower the drop heights, such as 300mm and have to stop the moment the hammer starts to rebound, it is unlikely pile will penetrate further. Do not try a few times, as this probably will damage or deviate the piles.

It is a grave risk if  the pile is resting on a hanging piece of limestone or on the slope of bedrock surface. Engineer has to read every piling record of the piling to check that safety. Additional piles are recommended if doubts arise. In general, each pile shall be designed to half  or less the normal designed load carrying capacities.

Driving RC piles in limestone area is not the best solution, but other more reliable methods such as bored piles, micro-piles may be too expensive to use.