casing stuck power point
Post on 08-Apr-2015
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CasingCasing--stuck troublestuck trouble
・・・・・・・・Job site examplesJob site examples・・・・・・・・Possible causes Possible causes ・・・・・・・・
CountermeasuresCountermeasuresNIPPON SHARYONIPPON SHARYO
1/27*This chapter reviews several job sites whereby the casing had been stuck in the past.
In addition, the reason why such phenomenon as casing stuck happens, and effective countermeasures for eliminating such accidents, are also instructed.
Examples of casing-stuck( Possible causes)
1.1.1.1.1.1.1.1.Unstable installation surfaceUnstable installation surface2.2.2.2.2.2.2.2.Loosed sandLoosed sand--cobble layercobble layer3.3.3.3.3.3.3.3.Underground water flowing over bed Underground water flowing over bed
rockrock4.4.4.4.4.4.4.4.Falling dry sand into hard layerFalling dry sand into hard layer5.5.5.5.5.5.5.5.Unstable inclined layerUnstable inclined layer6.6.6.6.6.6.6.6.Compaction related to Sea water levelCompaction related to Sea water level7.7.7.7.7.7.7.7.Estimate skin friction force of casing Estimate skin friction force of casing
surfacesurface
Soft installationsurface:N-value < 10
Collapses ofCollapses ofsurface surface
Steel plate
Pile specification
Bore;φ;φ;φ;φ2000
Depth; 45m
3/27
*This job site was constructing bored concrete piles having 2m in diameter and up to 45 m in depth into a reclaimed area. The installation surface of the rotator was comparatively soft. After iron reinforcement cages inserted and concrete poured in to the bored hole, the operator found that the casing could not be extracted, the rotator inclined. Because, the surface beneath the rotator had been seriously collapsed.
1.Casing stuck... ①①①①Unstable installation surface
1.Casing stuck... ①①①①Unstable installation surface
Soft installationsurface:N-value < 10
Pile specification
Bore;;;;φφφφ2000
Depth; 45m
Collapses Collapses ofofsurface surface
Steel plate
High skin friction resistance
1.Casing stuck... ②②②②Unstable installation surface
1.Casing stuck... ②②②②Unstable installation surface
4/27
The surface just beneath the rotator was involved and collapsed into the bore hole by a rotation resistance of the casing. When starting to extract the casing, the thrust cylinder generates a high extraction force, which causes the steel plates placed beneath the rotator to be bent. If the rotator sink unevenly, the casing in the bore hole is inclined which results a high degree of skin friction resistance to be generated.
Installation frameInstallation frame
1.Casing stuck... ③③③③Unstable installation surface
1.Casing stuck... ③③③③Unstable installation surface
5/27When installing the rotator on soft surface ( on reclaimed area or river bed, etc.) on which the bearing capacity is not enough to support the rotator while it is operating. We recommend that a large enough base frame shall be employed beneath the rotator for assuring stable installation of the rotator.
Stratum; Loose gravel Stratum; Loose gravel
・・・・・・・・River bedRiver bed
・・River in great River in great old time old time
Water, mud
GravelGravelcobblescobbles
2.Casing stuck... ①①①①Loosed sand/cobbles layer
2.Casing stuck... ①①①①Loosed sand/cobbles layer
6/27
Here, constructing bored piles of φ 2m into loosed gravelly stratum.While driving the casing into this kind of loose gravelly soil, the required torque to rotate the casing may increases remarkably.This stratum is pretty stable in the natural state, however, once scarified, it becomes easy to be collapsed. While the casing is rotating, the gravelly soil around the casing surface is dehydrated and the gravel articles are compacted.
Loose Loose gravel,cobblesgravel,cobbles
--River bedRiver bed
--River in old River in old timetime
DenseDense Ultra-slow
rotation speed1-2m/min.
Countermeasures
2.Casing stuck... ②②②②Loosed sand/cobbles layer
2.Casing stuck... ②②②②Loosed sand/cobbles layer
7/27
Generally said that a higher rotation speed is effective to reduce the skin friction resistance of the casing. However, this kind of gravelly stratum will be dehydrated and compacted when it is scarified by the casing, resulting the skin friction resistance to be increased remarkably.Generally, our recommended linear rotation speed of the casing is 10 m/min. in common soil, 4-5 m/min. to cut boulders, but very slow speed as 1 to 2m/min is recommendable in this case.
Pile specification
-Bore:φφφφ2000
- Depth: 12m
-Geological mapSandy gravelClay slate Rock bed ;
Sand rock
Sandygravel
Clay state
Bed rock;Sand
rock
3333.Casing stuck... ①①①①Underground water flowing over bed rock
3333.Casing stuck... ①①①①Underground water flowing over bed rock
8/27
This example was happened at the mountainous valley. The operator checked the required torque to rotate the casing just before lunch time and found that the torque was very small so that he stopped the machine with the casing end remaining in the bed rock. 50 minutes after, he started to lift up the casing gradually, but it was impossible to extract it. What change in the earth did occur between this 50 minutes ?
Countermeasures:Before stopping !・・・・Extract casing end over the bed rock.・・・・Feed water in the casing up to the bed rock surface.・・・・Rotate casing all the time with extracting casing 1~ 2 feet over the bed rock.
Impermeable layer; bed rock
10mm
Underground water current
Sand & debris
3333.Casing stuck... ②②②②Underground water flowing over bed rock
3333.Casing stuck... ②②②②Underground water flowing over bed rock
9/27
This , underground water is a cause. At the mountainous valley, even if there is no water on the river bottom, a lots of underground water is often flowing. This underground water flowing just over the bed rock surface carries lots of sand and debris which were accumulated in the gap of 10mm between the casing surface and the bore hole wall. While lifting the casing, the sand and debris are scraped up with the cutting bits and tightly compacted in the gap, resulting the casing to be stuck completely.
Dangerous stratum
・・・・Underground water flowing over bed rock
Weathered rock (Mud stone))))
Dry & fine sand
Gap
30cm
φ1500
9m
1.5m
Dry sand falls down into the gap which causes a high torque required.
Easy to drill into dry sand but torque increases remarkably when driving 1-2m into mud rock
When casing is extracted, sand in the gap is scraped up and compacted by cutting bits.
10/26
4444.Casing stuck... ①①①①Falling dry sand into hard layer
4444.Casing stuck... ①①①①Falling dry sand into hard layer
Dry, fine sand brought a high skin friction resistance. While driving the casing 1 to 2m in depth into the weathered bed rock existing 9m below G.L., the required torque increased remarkably. This is like a sand-glass, grains of sand are fallen down into the gap between bore hole wall and the casing which brings a high degree of skin friction force.
Water
6 - 9mm
500-800mm
Don’t provide any wear plate, if hard layer is overqu>400kg/cm2.
Countermeasure 1: Countermeasure 2: Feed water in the casing Provide wear plates around over the hard layer for the casing for making bore minimizing falling sand. hole wall surface smooth.
11/26
Gap
Hard layer
4444.Casing stuck... ②②②②Falling dry sand into hard layer
4444.Casing stuck... ②②②②Falling dry sand into hard layer
Effective countermeasures for preventing sand accumulation in the gap are, [To stop falling sand by keeping water level in the casing always over the hard layer, or to allow the sand being fallen down smoothly through the gap onto the bore hole bottom by making the surface of bore hole smoothly.
Countermeasures: Refill with fine materials
①①①① Drilling into rock layer ②②②② Once extract casing 1-2m over ③③③③ Clean up sand on the bore 1 - 2 m in depth. bed rock. hole bottom.
④④④④ Refilling sticky and fine ⑤⑤⑤⑤ Drive casing down up to rock layer ⑥⑥⑥⑥ Start to drill rock layer soils into bore hole. through the refilled sticky soil. (Falling sand is blocked)
12/27Before starting to drill rock sockets into the rock layer, falling sand must be blocked with sticky materials which is refilled around the casing end. Concrete is also sometimes employed for this kind of purpose instead of sticky soils.
4444.Casing stuck... ③③③③Falling dry sand into hard layer
4444.Casing stuck... ③③③③Falling dry sand into hard layer
Easy-Sliding layer
13/27
5555 Casing stuck5555 Casing stuckDrilling through out slope layer
Slope layer slides and a side force generates
2-stage casing is effective against side force
This phenomenon is so called [The mountain moves] or [Jamming-fracture layer]. When drilling through out such layers as slope, fracture or loose existing over slope rock layer, the casing end will receive a high degree of side force generated with sliding layers. Depending on stratum conditions, however, 2-stage drilling is effective to receive such side force.
6666 Casing stuck
Buoyancy
Skin Skin frictionfriction
①①①① High tide -Buoyancy occurs reducing skin friction
②②②② Low tide - compacted and a high skin friction
③③③③ Supply water for reducing skin friction
11/26
This phenomenon sometimes happens at the reclaimed area beside sea coast. At this area, there is no risk through which skin friction resistance of the casing increases remarkably during a high tide, but it will increase gradually when a low tide starts in order to reducing of buoyancy around the casing. Countermeasures;① keep high water level in the casing. ② plural-stage casing .
Confined underground water Confined underground water ①①①①①①①①
P3333
P1P1P1P1
P2P2P2P2River
Stratum:All the trumps
P2P2P2P2 <<<< P1=P3P1=P3P1=P3P1=P3
Underground
water
This phenomenon sometimes happens at beside the river embankments. Water (Rains) at the mountainside penetrated into the ground may be pressurized (P2 <P1=P2) may fountains in the casing when driving the casing into the soil.
Underground water current; 3m/min or more
Impermeable layer
Confined underground water Confined underground water ②②②②②②②②
This phenomenon sometimes happens at the stratum where a strong underground water current flows or a high pressurized underground water exists. Such strong water current results washing of concrete pored. High water pressure brings about [Boiling] just after extracting the casing.
Starting from mountainsideStarting from mountainsideConfined underground waterConfined underground water
You should think that a strong underground water current is always flowing along the valley in this kind stratums. No difficult thing is happened during constructing bored piles from ① to ⑤. But when the pile ⑥ is started to bore, a very strong boiling generated. Because, original underground water current gathered at the space of ⑥.
③ ②①④
⑤⑥
7 7 7 7 Estimate skin friction force rate per m27 7 7 7 Estimate skin friction force rate per m2
Wm
weight of rotator
Wc
F
Soils Skin friction force rate(ton/m2)
Sand/cobble N<20 0.7 <
Sand/cobble N>20 0.5-0.6
Clayey N>20 0.4-0.5& Silty
Clayey N<20 0.3-0.4& silty
Clayey N<10 0.1-0.2& silty
Loosed weathered 0.2 < rock layer
Bed rock/ 0.1 >Concrete
(((( Company experienced values))))
φφφφ2000Surface
area6.3m2/m
15/27
Required extraction force : Ef
Weight of casing
Formula of required extraction force
Ef > Wc + Wm + F The bored pile execution is not so hazardous but the casing might be stuck exceptionally with a high skin friction resistance of the casing. The table shows an estimate skin friction force per square meter classified by soil. Those skin friction force data are reference data originally experienced by NIPPON SHARYO.
7 7 7 7 Estimate skin friction force calculation7 7 7 7 Estimate skin friction force calculation
Wm
Wc
φφφφ2000
Surface area
6.3m2/mFormula of required extraction force
Ef > Wc + Wm + F
10m
10m
10m
Clay N=100.3t /m2
Sand N=250.6 t/m2
Weathered rock0.3 t/m2
GL 0-10mSurface area : 3.14 x 2m x 10m = 62.8m2
Skin friction force: 0.3 t/m2 x 62.8m2 = 18.8tGL 10-20mSurface area : 3.14 x 2m x 10m = 62.8m2
Skin friction force: 0.6 t/m2 x 62.8m2 = 37.6tGL 20-30mSurface area : 3.14 x 2m x 10m = 62.8m2
Skin friction force: 0.3 t/m2 x 62.8m2 = 18.8t
Total skin friction force FF=18.8 + 37.2 + 18.8 =74.8 tWeight of casing: Wc 2 t/m x (30m + 3m) = 66t
3m
Partial weight of rotator: WmRT-150 =17t RT-200 = 25t RT-300= 36t
Therefore, Required extraction force Efshould be larger than (74 + 66 + 36 ) 176ton.
Various stratum conditions in which a high skin friction force is generated
Soft surfaceSoft surface--Rotator inclinedRotator inclined
Sand, cobble
Boulder
Concrete
Hard rock
Sand
Casing inclined in Casing inclined in the concrete basethe concrete base
Drop of sand into Drop of sand into [Gap][Gap]
Side force from Side force from boulderboulder
Side force by obstacle
Sand-cobbles compacted
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