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