Eng-Tips is the largest engineering community on the Internet
Intelligent Work Forums for Engineering Professionals
-
Congratulations waross on being selected by the Tek-Tips community for having the most helpful posts in the forums last week. Way to Go!
Skin Friction Pile
- Thread starter Star2009
- Start date
Normally the soil is perforated with a machine that has an helix rotating with a central pipe for axle. Water may be being added during the perforation and so the soil is partly being ejected. Then when reached "refusal" or the specified depth, concrete is injected through the pipe till the concrete overflows at the surface. Normally top 6 m of 1% or so reinforcement at least sith spiral ties is added. If in cold climate, the pile is then buried to better allow concrete hardening. Once the concrete is set enough the required length of the piles to be used in proper embedment in pilecaps at the heads of the piles are discarded (not commonly with fully hardened concrete), that is, in that length the steel is exposed.
What depth and diameter are your bored piles?
Often there will be a casing (section of pipe) which is placed in the ground for the top few metres (maybe 5m. Then the soil is excavated from within the casing and continuing down to the required level. The exvacation can be carried out by various machines including an auger drill or a bucket.
Depending on the soil conditions something will be needed to support the sides of the excavation as you go down. This could be a full depth casing, or water mixed with the soil, bentonite or polymer drilling fluid.
Once you get to full depth then assuming bentonite has been used you will have a circular hole of the correct diameter filled with bentonite. Now you have to lower a rebar cage into the bentonite. Finally you use a tremie tube to place the concrete into the bottom of the excavation and progressively displace the bentonite.
Often there will be a casing (section of pipe) which is placed in the ground for the top few metres (maybe 5m. Then the soil is excavated from within the casing and continuing down to the required level. The exvacation can be carried out by various machines including an auger drill or a bucket.
Depending on the soil conditions something will be needed to support the sides of the excavation as you go down. This could be a full depth casing, or water mixed with the soil, bentonite or polymer drilling fluid.
Once you get to full depth then assuming bentonite has been used you will have a circular hole of the correct diameter filled with bentonite. Now you have to lower a rebar cage into the bentonite. Finally you use a tremie tube to place the concrete into the bottom of the excavation and progressively displace the bentonite.
In our location, the piling contractors drill the shafts to the predetermined shaft diameter and depth, rebar cages are inserted, and concrete is poured. Fairly basic. Where sloughing and/or moderate to high rates of seepage conditions are encountered, casing is inserted to minimize accumulations of slough or water. The casing might be full length or long enough to cover the problematic zone. The casing is usually withdrawn incrementally as the concrete is placed. During the piling operation, the drilling activities are logged by the geotech to document the soil conditions, verify stratigraphy and design parameters, and to notate construction requirements such as the need for casing, etc. Pocket pens may be performed on cohesive samples retained from the auger cuttings.
what's "sloughing"?
I have always worked on bored piles using either full length casing (London), or bentonite (Malaysia, Vietnam, Thailand, Philippines) or polymer (Thailand) as excavation support. I have also seen trials of a reverse circulation technique where the existing soil (clay in this case) is mixed with water to provide support.
I've never seen bored piles without excavation support, but I suppose it depends on location, dimensions and soil types.
I have always worked on bored piles using either full length casing (London), or bentonite (Malaysia, Vietnam, Thailand, Philippines) or polymer (Thailand) as excavation support. I have also seen trials of a reverse circulation technique where the existing soil (clay in this case) is mixed with water to provide support.
I've never seen bored piles without excavation support, but I suppose it depends on location, dimensions and soil types.
usually, when the soil is above the water table and some degree of cohesion or cementation is present, an excavation support is not used, in my area.
When you come across the WT, that means you've got an aquifer with possibly loose sandy or gravelly soil, that stands for 'cave-ins', so whatever support si required is used (regular slurry being the cheapest, casing being the most espensive)
Slurry may be tricky in salty water.
Another tricky situation: a pressured aquifer.
You've got a dry hole in clayey soil, self supporting, no probs whatsoever, no slurry nor casing used, until you drill a little distance above the pressured aquifer, the clay plug resistance is overcome, water almost erupts into the hole, turbulence causes caveins of the water bearing layer and the job which you believed a shoe-in is ruined.
A geologist knowing the area is always a valuable asset in these cases.
When you come across the WT, that means you've got an aquifer with possibly loose sandy or gravelly soil, that stands for 'cave-ins', so whatever support si required is used (regular slurry being the cheapest, casing being the most espensive)
Slurry may be tricky in salty water.
Another tricky situation: a pressured aquifer.
You've got a dry hole in clayey soil, self supporting, no probs whatsoever, no slurry nor casing used, until you drill a little distance above the pressured aquifer, the clay plug resistance is overcome, water almost erupts into the hole, turbulence causes caveins of the water bearing layer and the job which you believed a shoe-in is ruined.
A geologist knowing the area is always a valuable asset in these cases.
- Thread starter
- #10
- Thread starter
- #12
No problem at all with 45m long 1500mm diameter bored piles. 60m long piles are common and up to 100m are not unusual.
You will have to get a good quality contractor and insist on a good QA regime.
The first step is the test pile. You may select a test pile with instrumentation so that you can check the perfomance of the pile at different soil levels during the load test.
You will have to get a good quality contractor and insist on a good QA regime.
The first step is the test pile. You may select a test pile with instrumentation so that you can check the perfomance of the pile at different soil levels during the load test.
- Thread starter
- #15
I've been to Sri Lanka a couple of times, last time we were pricing a precast concrete segmental alternative for the steel flyovers which are now being erected in Colombo - we didn't win the contract.
I think bored piles of the diameter and length you require have often been constructed in Sri Lanka. As a personal preference I would choose a contractor with Bauer rigs. Bauer rigs are dependable and well tested with piles of this type.
The first thing is to make sure your SI is reliable. The depth of the SI boreholes should be greater than the required pile length. I have seen cases where the boreholes are at a lesser depth than the pile and it is assumed the SPT's will remain the same or get greater - this is not always the case.
Next you need to decide about the pile test requirements. Test piles are expensive especially with instrumentation. If you only have 6 piles then probably just a load test is sufficient. But if you have many piles then instrumentation will allow for value engineering.
For QA you have to decide whther sonic logging is required or not. This is a significant expense but is getting more common.
You should discuss with proposed contractors whether they propose bentonite or polymer excavation support/drilling fluid. I think in Sri Lanka bentonite will be preferred. A suitable procedure and QA system must be provided.
I think bored piles of the diameter and length you require have often been constructed in Sri Lanka. As a personal preference I would choose a contractor with Bauer rigs. Bauer rigs are dependable and well tested with piles of this type.
The first thing is to make sure your SI is reliable. The depth of the SI boreholes should be greater than the required pile length. I have seen cases where the boreholes are at a lesser depth than the pile and it is assumed the SPT's will remain the same or get greater - this is not always the case.
Next you need to decide about the pile test requirements. Test piles are expensive especially with instrumentation. If you only have 6 piles then probably just a load test is sufficient. But if you have many piles then instrumentation will allow for value engineering.
For QA you have to decide whther sonic logging is required or not. This is a significant expense but is getting more common.
You should discuss with proposed contractors whether they propose bentonite or polymer excavation support/drilling fluid. I think in Sri Lanka bentonite will be preferred. A suitable procedure and QA system must be provided.
- Thread starter
- #17
- Status
