[American Society of Civil Engineers Seventh International Symposium on Field Measurements in Geomechanics - Boston, Massachusetts, United States (September 24-27, 2007)] 7th FMGM

A Case Study on Trench Collapse of Deep Diaphragm Wall

Jing-Wen Chen1 and Fu-Cheng Chen2

1Professor, Department of Civil Eng., National Cheng Kung University; No.1 University Road, Tainan, 701, TAIWAN; [email protected] 2PhD Candidate, Department of Civil Eng., National Cheng Kung University; No.1 University Road, Tainan, 701, TAIWAN; [email protected] ABSTRACT: This paper presents a remedy case of a large-scaled trench collapse during the construction of a deep cylindrical diaphragm wall located in the south of Taiwan. Through a well-organized field monitoring system and suitable countermeasures, the potential disaster was mitigated in time. Thirty-two panels in total were designed to form the 70 m-diametered diaphragm with 1.2 m thickness and 90 m depth. During the trench excavation, a deep and large collapse of soils, approximately 110 m3, occurred at the depth of GL.-43 m to GL.-52 m in the trench. The overlapped reinforcing bars (rebar) of previously installed panels were serious deformed and impeded the coming rebar cage insertion. A series of monitoring activities including ultrasonic sounding and piezometer measurement was performed to figure out the causes and damaged situation. Relevant remedial measures as well as the monitoring data during the post-treatment are addressed. Three main concerns are concluded : (1) a well-organized field monitoring system during construction would be helpful for judgment in case any accident occurred, (2) an improper construction activity using percussion drill inside the coming-close cylindrical diaphragm wall built up the excess pore water pressure of the confined aquifer at the collapse depth and resulted in the serious collapse, and (3) the long stand-by duration of the excavated trench would increase the possibility of collapse. INTRODUCTION Many geohazards exist in the natural geo-environment and tend to be mobilized during heavy rainfalls, typhoons or earthquakes, etc. However, some geotechnical-wise incidents sometimes result from the careless neglects of the construction management. In the past two decades in Taiwan, the diaphragm wall was widely used for the soil-retaining purpose in the civil and building construction, in which many construction problems were found and resulted in accidents or even disasters. The causes include various site conditions, such as soil characteristics, construction management, engineering judgment, and so on. Nevertheless, very few records of lessons were addressed for a reference to avoid the same mistakes. Hence, this paper

FMGM 2007: Seventh International Symposium on Field Measurements in Geomechanics 2007 ASCE

Copyright ASCE 2007 Field Measurements in Geomechanics (FMGM 2007)

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shares a practical experience on the trench collapse of a diaphragm wall for the lesson learned. The cylindrical diaphragm wall was located at a reclaimed land in the south of Taiwan. The dimensions are 70 m diameter, 1.2 m thickness and 90 m depth which is the deepest record in Taiwan by 2007. The general geology at this site is the typical alluvium strata along the western coast of Taiwan. Table 1 indicates the representative subsoil properties of the site. The reclaimed thickness of the site is 10 m~12 m in general. Underneath the reclaimed layer, SM and CL layers were interbedded until down to GL-48 m. SM4 layer (GL-43 m~GL-48 m) is the one in problem described in the paper.

CYLINDRICAL DIAPHRAGM WALL Panel Arrangement In order to form a 70m-diametered cylindrical diaphragm wall and allow a deep excavation of 50 m, 32 panels with 90 m depth were arranged which was composed of 16 A-type panels and 16 B-type panels. A-type panels (4 m arc length) were installed first and B-type panels (10 m arc length) were installed later on. The panel arrangement is shown in FIG. 1. Among the 16 B-type Panels, two particular panels (B-8 and B-16) were designed for the drainage purpose. The described serious collapse occurred in the B-8 panel. FIG. 2 shows the section details of the particular panel. Connection Joints

The joint type for panel connection is End-plate. 1300mm-lengthed horizontal extension rebars were preset at both ends of A-type panels (advanced panels) to joint the B-type panels later on. Both sides of the extension rebars (outer and inner) were protected by steel thin plate from soil scouring during joint cleaning. Please refer to the FIG. 2.

Table 1. Subsoil Properties of the Site

Depth (m) Description SPT Nave

Unit Weight(t/m3)

c (t/m2)

Permeability Coefficient (cm/sec)

0~10 Reclaimed soils 8 1.8 - - 1.5x10-3 10~12 Sea bed 2 1.9 - 36(1) - 12~15 SM1 13 2.0 - 38(1) 1.5x10-3 15~22 CL1 6 2.0 2.8 - 3.0x10-6 22~28 SM2 19 2.0 - 34 2.0x10-4 28~30 CL2 18 2.0 8.6 - 3.0x10-6 30~37 SM3 20 2.0 - 35 1.5x10-4 37~43 CL3 16 2.0 6.5 - 3.0x10-6 43~48 SM4 50 2.0 - 35 4.0x10-4 48~59 ML1 35 2.1 34 - 1.5x10-5 59~75 ML2 37 2.0 40 - 4.0x10-5

75~100 CL/ML 29 2.0 15 - 9.0x10-6 Note: (1)After improvement by SCP (Sand Compaction Piles)



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T5T5T5

Thin Plate

Drainage Pit

Tremie Pipe

End Plate 9.98m

2.40m

2.48

m

FIG.1. Panels arrangement of cylindrical diaphragm wall.

FIG.2. Section details of B-8 particular panel.



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Construction Method As a normal procedure, a precise circular RC guide wall was constructed first while

in the same time, the slurry plant was set up and bentonite slurry was ready for stabilizing the trench. In this project, the slurry trenching was performed by two types of machines. For this particular panel, a clam-shelled type MHL excavator (Masago Hydraulic Long-bucket, MHL-80120AY) was adopted for trench excavation above GL-54 m at prominent Drainage-pit and anther circumferential portion (0 m~90 m) was performed by the boring type BW excavator (Basement Wall, BWN-80120). The cutting sequence of the trench excavation for this particular panel is shown in FIG. 3. (Chung Y.T. and Chen F.C., 1994)

After finishing the trench excavation, an ultrasonic sounding device was placed across on guide wall to perform the measurement of the excavated trench. By lowering a sensor into the slurry trench, the ultrasonic wave will repeat reflecting horizontally inside the trench and processing the image on the recorder. In accordance with the images, engineer could figure out if the width and verticality are acceptable or if the sides of trench are caved. Photo 1 shows the feature of ultrasonic sounding device.

If the result of ultrasonic sounding was acceptable, then the subsequent steel cage insertion and Tremie-concrete placing were performed, meanwhile the slurry quality was carefully controlled in the slurry plant.

FIG.3. Cutting sequence of the particular B-8 panel trench.

Photo 1. The ultrasonic sounding device setting on the guide wall.

MHL Cuts (GL-0m~GL-54m) BW Cuts (GL-0m~GL-90m)

H. RebarsH. Rebars

Protection Rod

Protection Rod



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Instrumentation To monitor the behavior of the diaphragm wall for safety during deep excavation

inside, many sensors were pre-installed on the rebar cage in depths before insertion. The sensors include rebar stress meters, pore water pressure transducers, total pressure transducers, thermal couples, relative displacement transducers, accelerometer and son on. In this paper, the pore water pressure transducers installed at depth of GL-44 m in both B-5 and B-13 panels were referred. The installation of the pore water pressure transducer as well as its details is shown in Photo 2.

Photo 2. The installation of pore water pressure transducer on rebar cage.

COLLAPSE AND INVESTIGATION Collapse History

The panel in problem is one of the two particular panels, B-8 panel, which is located at the 165 clockwise from the North as shown in FIG. 1. All the panels except B-8 and B16 were finished in concrete placing before the occurrence of collapse. Two days had been suspended since the completion of the trenching of B-8 panel, while the instrument installation using percussion drill was performed for some time at the center of the diaphragm wall. The serious collapse was found in the trench using ultrasonic device before rebar cage insertion.

The inner side of the trench collapsed from GL-43 m to GL-52 m in a wedge-shape with a maximum length over 10 m and a flat top of 2.8 m in width. The collapsed soil mass was estimated to reach 110 m3. Referring to Table 1, the major collapsed soil layer was silty sand SM4 layer. Beneath SM4, silt layer (ML1) was drawn by the soil mass of SM4 layer. FIG. 4 shows the results of the ultrasonic survey at the collapsed portion. When the collapse comes to stabilization, more detailed ultrasonic surveys were performed and the estimated 3D of the huge trench collapse was schemed in FIG. 5.



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Inner

GL-52m

GL-43m

Inner Inner

Outer

Inner

Outer Outer

Section Section Section

SM4

ML1

CL3

2800

GL-43m

B-8 Panel

A-9 Panel

A-8 Panel

It is indicated that the extension rebars and thin plates of A-8 panel were pushed by the collapsed soil mass and seriously deformed as shown is FIG.6 which will obstacle the coming insertion of the rebar cage. No obvious deformation was found at A-9 panel end.

FIG.4. Ultrasonic inspection results of the trench collapse in B-8 panel. FIG.5. Inferred 3D scheme of the huge trench collapse in B-8 panel according

to the ultrasonic sounding results.

Investigation To investigate the probable causes of the collapse, the characteristic diagram was

made by collecting all the possible causes in terms of various groups. Three most possible causes were summarized: (1) Drastic increase of the pore water pressure in SM4 layer, (2) Slurry quality index approached the control value; (3) Long stand-by of the excavated trench. Among which, item (1) is judged to be the major cause of the collapse. FIG. 7 indicates the history of water pressure of SM4 monitored by B-5 and



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B-13 inside the DW and the slurry pressure in B-8 trench before and after the collapse. It is obvious that the water pressure increased drastically on the starting day of VD drilling (Vertical Displacement meter) and reached 4.78 kg/cm2 once after WP drilling (Water Pressure meter) which is greater than the slurry pressure (4.65 kg/cm2) in the trench. Subsequently, the collapse happened. The phenomenon justified the judgment made above.

FIG. 6. Deformation of Thin-plates in B-8 Panel at (A) GL-43m~GL-47m,

(B) GL-47m~GL-50m. FIG. 7. The history of water pressure of SM4 inside the DW and slurry

pressure in B-8 trench before and after occurrence of collapse.

Furthermore, one of the slurry quality index was close to the controlled value and together with the 48-hour long stand-by of the excavated trench increased the possibility of collapse. Table 2 indicates the filter volume of the slurry, 35 cc~36 cc, tended to reach the controlled values. It is believed that the higher filter volume gives less capacity of the stabilization for the trench.

Thin Plate

(B)

Thin Plate

(A)

Thin Plate

Thin Plate

V D W P D W B 5 B 1 3

4 .6 5

5 .0

4 .5

4 .0

3 .5

5 .5

6 .0

0 8 06 04 02 0

( k

g / c

m

2

)

(D A Y )

B -1 6( )

B -8

W P

B -8

D W

V D

1 2 /2 3

G L -4 4 m = 4 .6 5

1 2 /1 1

(A ) ( G L -4 4 M )

1 0 0 1 2 0

4 .7 8

1 1 /2 9

Inside of DW (GL-44m) Drilling

Elapsed Time (Day)

Wat

er P

ress

ure

( k

g/cm

2 )

Panel Panel

V. Displa.Mtr Water Pres.M Deep well

Legends :

Collapse CloseFinished

Drilling Finished Drilling

Slurry Pressure at GL-44m in the Trench= 4.65



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SOLUTION AND IMPLEMENTATION Strategy

To ensure the safety and feasibility of the solution, the following destination shall be secured: (1) Prevent the collapse from expansion; (2) Ensure the B-8 rebar cage insertion to the designated depth; (3) During the tremie-concrete placing, no impact is allowed on the other B-16 trench stability; and (4) The concrete convex formed in the collapse could be demolished safely after inner soil excavation and no harm was applied on the structure.

Solution A joint meeting with Owner / Client, Supervisor, Designer, QC, Site representatives,

Safety staffs , Operators, Instrument contractor etc., should be held shortly to approach the practical solution. Main issues included, the probable causes, countermeasures, split of work, schedule, implementation, coordination and so on. The solutions are stated in details as follows:

(1) All the construction activities were suspended immediately; (2) After the

stabilization of the collapse was observed from ultrasonic surveys, slime/slough at bottom of the trench was removed by suction pump through tremie pipes; (3) A field-made tool was introduced to rectify the deformed rebars of A-8 panel for the coming insertion of rebar cage as shown in Photo 3. Consequently, the tool successfully rectified the horizontal rebars but failed to correct the vertical ones; (4) The rebar cage was modified, in which cutting horizontal rebars, adding reinforcement, special spacer, guiding angle steel, monitoring devices (rebar-stress transducer and inclinometer), and separation plywood plates for later chip-off works, etc., were all completed as shown in FIG.8; and (5) Tremie concrete placing: In order to release the confined slurry pressure in the cave which might build up water pressure through SM4 to B-16 trench during the subsequent concrete placing, a pair of PVC pipes were installed to the cave after B-8 rebar cage insertion, as shown in FIG.9.

Table 2. Slurry test results and controlled values for B-8 Panel

In Good-Slurry Trough In Trench before Collapse Item

Measured Controlled Measured at GL-40mMeasured

at GL-90m Controlled

Specific Gravity 1.03 1.03~1.15 1.07 1.07 1.03~1.10 Viscosity(sec) 22 21~40 21.9 22.1 21~40

Sand content (%) < 0.5 < 5 < 0.5 < 0.5 < 3 Filter volume (cc) 23.5 < 40 35 36 < 40

Mud cake(mm) 0.5 < 5 1.2 1.2 < 5 pH 8.9 9~11.5 8.2 8.3 9~11.5



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Photo 3. The field-made tool for rectification.

FIG. 8. Modification of B-8 panel rebar cage.

Cutting overlapped rebars (GL-43~90m)

Make a shoe at cage bottom

Add Vertical Rebars

Reduce thickness 150mm

Special spacer

Guiding Angle Steel

Rebar stress Mtr(V. at GL-43m) (H. at GL-45m)

Inclinometer

Separation plywood plates (GL-43~50m)

A pair of PVC Pipes



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ConcreteTremie P. Flexible tube

Release Pressure

L-shape PVC PipesInner Guide Wall

Outer Guide WallRebar Cage

ML1 LayerConcrete

SM4 Layer

CL3 LayerConfined

Slurry

FIG. 9. Pressure releasing device during concrete placing.

0 2 4 10.26 8 9

5

11.5 15 13 17 19 21 23

4.9

4.8

4.5

4.6

4.7

(Hours)

k

g/cm

2

SM 4 (GL-43m Inside)Measured at GL-43m from B-5 Panel

Wat

er P

ress

ure

( k

g/cm

2 )

Elapsed Time of Concrete Placing (hour)

Implementation All the planned solutions were implemented accordingly and the results were found

satisfied as described hereinafter: (1) Tremie concrete was placed successfully while the water pressure in SM4 layer was observed from transducer installed at B-5 panel as shown in FIG. 10. It increased obviously in the duration, however no impact on B-16 panel trench was found eventually; (2) During the next stage of inner soil excavation, it was observed that the rebar stress in B-8 panel increased gradually when the excavation went deeper. Especially during the chip-off of the concrete convex, a drastic increment of the rebar stresses were found as shown in FIG.11. Nevertheless, the rebar stresses were still under the allowable stresses and the concrete convex was demolished smoothly as shown in Photo 4.

FIG.10. Water pressure history in SM4 during concrete placing for B-8.



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To ensure the safety and feasibility of the solution, the following destination shall be secured: (1) Prevent the collapse from expansion; (2) Ensure the B-8 rebar cage insertion to the designate depth; (3) During the Tremie-concrete placing, no impact is allowed on the other B-16 trench stability; and (4) The concrete convex formed in the collapse could be demolished safely after inner soil excavation and no harm was applied on the structure.

Photo 4. Appearance of huge concrete convex and chip-off in safety.

FIG. 11. Measured rebar stress at B-8 panel during inner excavation stage.

0

400

800

-400

-800

-1200

-1600

Re-

bar S

tress

(kg

/cm

)

2

93/02/04 03/01 04/01 05/01 06/01 07/01 08/01 09/01 10/01 11/01

1st Stage Excavation

(GL+0~GL-40M)

2nd Stage Excavation

(GL-40M~GL-50.27M)

Inner Side Wall Lot 1Construction

(GL-29.6M~GL-36.6M)5/113/8 8/30 10/19

(9/23~9/30)

(GL-50m Inside)

(GL-43m Inside)

(GL-45m Inside)

(GL-45m Outside)

V.RB Stress

V.RB Stress

H.RB Stress H.RB Stress

Convex Chip-off

Calendar Date

Re-

bar

Stre

ss (k

g/cm

2 )

Mar.01 Aug.01 Sep.01 Oct.01 Nov.01 Jun.01 May01 Jul.01 Apr.01

Convex Chip-off (9/23~9/30)



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CONCLUSIONS

The value of a case study can be found from the practical experience recorded as a lesson or a merit to be shared and applied for the future case. In this paper, the authors summarize the following conclusions as a reference for the similar construction:

(1). The construction of diaphragm wall gets a tight connection with the soil strata. Especially for the close-type diaphragm wall, to prevent the trench from collapse, any vibration or drilling activity is not allowed to commence before finishing all the reinforced concrete panels. Construction sequence shall be the key management for the issue;

(2). Frequent inspection on the quality control of slurry could be helpful on the stability of the excavated trench; and

(3).The period for the slurry trench to stay open shall be shortened as short as possible. Normally, the period shall not exceed 4 hours before the insertion of rebar cage.

REFERENCES Chen F.C. and Chen Y.F (1997). The Deep Excavation of 50M Depth Cylindrical

Inground Storage Tank, The Third Asian Young Geotechnical Engineers Conference (3rd YGEC), Singapore, P.141P.150.

Chung Y.T., Chen F.C., Yu M.S and Ko Y.H (1994). A Case Study of 90m Deep Cylinder Shape Slurry Wall Construction., Sino-Geotechnics, No.45, P.39P.66, Taiwan.



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Documents

[American Society of Civil Engineers Seventh International Symposium on Field Measurements in Geomechanics - Boston, Massachusetts, United States (September 24-27, 2007)] 7th FMGM