Phyllite is a metamorphosed rock baked from finely bedded Shale Rock. Quite often, the shale rock is not fully baked and its bedding layers can still be seen.
Sunday, 30 October 2016
Phyllite
Near to the Heart Centre Hospital of Kuching, about 20m from the City to University of Sarawak Highway, the underlying bedrock is mainly fractured Phyllite, very contrast to the beautiful solid Limestone cores obtained at Satok area near to City centre.
In fact, the underlying bedrock at the City Centre and near to the river front is also mainly Phyllite.
Near to the Heart Centre Hospital of Kuching, about 20m from the City to University of Sarawak Highway, the underlying bedrock is mainly fractured Phyllite, very contrast to the beautiful solid Limestone cores obtained at Satok area near to City centre.
In fact, the underlying bedrock at the City Centre and near to the river front is also mainly Phyllite.
Thursday, 27 October 2016
Wednesday, 26 October 2016
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.
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.
Tuesday, 25 October 2016
Piling On Limestone Bedrock in Kuching (1)
The Client intends to build a block of
4-Storey Shop-houses on a former swampy ground not very far from a large river.
Three boreholes were drilled into the ground. The underlying soil was found to be quite consistent comprising of 11.8m.very soft Clay/Silt, except that the last 1m contains a number of various sized limestone fragments. From 11.8m onwards, the underlying rock was mainly strong, fresh and slightly weathered Limestone.
The rock cores are as shown in the photograph.
Three boreholes were drilled into the ground. The underlying soil was found to be quite consistent comprising of 11.8m.very soft Clay/Silt, except that the last 1m contains a number of various sized limestone fragments. From 11.8m onwards, the underlying rock was mainly strong, fresh and slightly weathered Limestone.
The rock cores are as shown in the photograph.
The Engineer proposed “driving precast Reinforced concrete piles” for
the foundation of the 4-storey reinforced concrete shop-house, which was the
usual method for low rise buildings and estimated the piling length to be 11.8m.
During construction,the Contractor claimed that 42m of precast RC
pile was driven into the ground for each driving point during the first three trial piles.
Was it possible?
The Engineer and the Contractor suspected that the Soil Investigation was not accurate and called another drilling at the piled locations. The results came out to be similar to the initial Soil Investigation.
What Happened to the extra 30m length?
The Engineer and the Contractor suspected that the Soil Investigation was not accurate and called another drilling at the piled locations. The results came out to be similar to the initial Soil Investigation.
What Happened to the extra 30m length?
Monday, 17 October 2016
Failure of the Cantilever Roof Over the Window
The L-shape stretch of the 1m Cantilever Roof Cover of my house failed during one night two weeks ago. It was about twenty years old. The structure was basically triangular wooden structure nailed to an anchored wood on the brick wall, 3m above the floor. It also pulled down the lower part of the metal rain water down pipe at the corner The anchored wood is still intact on the wall.
Why the structure failed after twenty years?
The triangular truss is generally made up of Selanagn Batu timber, a good tropical hard wood, not easily worn off or attacked by insects. But the purlins for supporting concrete roof tiles and ceiling are of inferior quality timber. Some of them show signs of insect attacks and deterioration.
The rain water down pipe passes through the corner of the L-shape roof cover where water leakage had constantly wet the good quality hardwood of the anchored horizontal wood as well at the critical cantilever supports at the corner. The damp wood had encouraged the termites to thrive and together with the inferior timber purlins, had weakened the structure.
It only needed two to three trusses to fail and the rest was just progressive,pulled down by sudden large dead load from other part of the roof.
Many nails are still intact, pulled out by sudden increase of tension forces.
Lessons to learn:
(a) Cantilever structure is generally a risky structure and have to make sure every part of it is of best material and highest workmanship,
(b) galvanised steel structure is preferable in view of the high raining condition and termite problem. Timber is now as expensive as steel.
(c) Never allow rainwater down pipe to pass through critical structural supports as it tends to leak after some time.
The L-shape stretch of the 1m Cantilever Roof Cover of my house failed during one night two weeks ago. It was about twenty years old. The structure was basically triangular wooden structure nailed to an anchored wood on the brick wall, 3m above the floor. It also pulled down the lower part of the metal rain water down pipe at the corner The anchored wood is still intact on the wall.
Why the structure failed after twenty years?
The triangular truss is generally made up of Selanagn Batu timber, a good tropical hard wood, not easily worn off or attacked by insects. But the purlins for supporting concrete roof tiles and ceiling are of inferior quality timber. Some of them show signs of insect attacks and deterioration.
The rain water down pipe passes through the corner of the L-shape roof cover where water leakage had constantly wet the good quality hardwood of the anchored horizontal wood as well at the critical cantilever supports at the corner. The damp wood had encouraged the termites to thrive and together with the inferior timber purlins, had weakened the structure.
It only needed two to three trusses to fail and the rest was just progressive,pulled down by sudden large dead load from other part of the roof.
Many nails are still intact, pulled out by sudden increase of tension forces.
Lessons to learn:
(a) Cantilever structure is generally a risky structure and have to make sure every part of it is of best material and highest workmanship,
(b) galvanised steel structure is preferable in view of the high raining condition and termite problem. Timber is now as expensive as steel.
(c) Never allow rainwater down pipe to pass through critical structural supports as it tends to leak after some time.
Subscribe to:
Posts (Atom)