Construction of Open Caisson for Oversize Surge Shaft in Drift Gravel Stratum

Qiang Yao Ph. D. student State Key Laboratory of Hydraulics and Mountain River Engineering,

Sichuan University, Chengdu, Sichuan, 610065, China College of Water Resource and Hydropower, Sichuan University, Chengdu, Sichuan 610065, China e-mail: [email protected]

Xing-guo Yang State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University, Chengdu, Sichuan, 610065, China College of Water Resource and Hydropower, Sichuan University, Chengdu, Sichuan 610065, China e-mail: [email protected]

Hong-tao Li* State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University, Chengdu, Sichuan, 610065, China College of Water Resource and Hydropower, Sichuan University, Chengdu, Sichuan 610065, China *Corresponding author: e-mail: [email protected]

ABSTRACT Economically affordable, open caissons are a popular choice for various applications. They’re also frequently used because of their reinforcement and demolition capacities, as as for providing a large opening construction that provides safety for neighboring buildings in a complex construction field. The large cylindrical surge shaft of Xiergou Hydropower Station is located in drift gravel stratum. The upper 65 m of this surge shaft adopts an open caisson form with an internal diameter of 22 m and external diameter of 25 m. The open caisson construction can face challenges such as deviation, tilt, cracking, sink-suspension, and sudden sinking. This study analyzes the sinking stability of an open caisson by using theoretical calculations to determine sudden sinking, sink-suspension, and other risks. These calculations make use of open caisson structure optimization and auxiliary sinking methods, such as shock blasting. Moreover, using finite element numerical simulation, this study calculated the stress variations during the sinking of the open caisson. The calculations were used to optimize the construction program of an open caisson to avoid cracking and the failure of the open caisson concrete. Using a field sinking test and deformation monitoring, this study determined a set of systematic methods for the construction of a large open caisson on drift gravel stratum, which yielded a new method for open caisson construction under complicated geological conditions.

KEYWORDS: surge shaft; open caisson structure; construction; excavation; drift gravel stratum

INTRODUCTION

The principle of the open caisson suggests that the earth surface’s open caisson structure occurs gradually, in advance of the sinking to the designed elevation under the containment of the

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Vol. 19 [2014], Bund. T 5726 open caisson wall. Since the first application in the France Saloney Coal Field in 1893, the open caisson construction method has been widely applied, to more than 1,500 underground constructions in Europe. The open caisson construction method is predominantly self-sinking. However, this method is only applicable for soft strata and shallow sinking depths and lacks of related control measures. In the 1990s, construction methods based on the loaded -sinking, space system caisson (SS), and super open caisson system (SOCS) were gradually adopted. During the construction of an open caisson, the sudden sink, sink-suspension, derivation, tilt, and wall failure should be controlled. Furthermore, more dangerous construction conditions may be encountered in the sinking process compared to the completion stage of the open caisson. Therefore, it’s vital to pay close attention to the safety requirements for the open caisson structure [1-3]. Today, research on the open caisson construction period has primarily been concentrated on sinking stability [4-5], subgrade reaction [6-10], side friction resistance [11-13], strength, and deformation [14-15]. Certain researchers [16] have calculated stipulations for the construction process of small and medium-sized open caissons while ignoring the particularity of the large open caisson. Although researchers around the world have accumulated information regarding open caisson construction [17], few have investigated the construction of a large open caisson on a complicated drift gravel stratum. Xiergou Hydropower Station is a diversion hydropower station located in Zhouqu County in Gansu Province, China. Its open-top impedance surge shaft is located on the hill slope on the back of the plant, with the upper end connected to the diversion tunnel and the lower end connected to the pressure pipeline (Figure 1). The surge shaft is 102 m deep and lies in the 97m overlying stratum. The upper part of this stratum is filled with silty soil or gravelly soil and is 33.4 m thick. The low part is a 63.6 m thick pluvial drift gravel stratum with boulders that are a general diameter of 0.4~0.6 m with a maximum diameter of 1.5 m, which accounts for about 5%~8%. The stratum in the elevation range of 1537.5~1524 m is a compact conglomerate layer. The upper 65 m of the surge shaft adopts an open caisson form with an internal diameter of 22 m and external diameter of 25 m (Figure 2). The open caisson construction method can effectively prevent well wall collapse and thus reduce safety risks caused by the combination of temporary support measures with a permanent wellbore structure. However, the traditional loading-sinking, SS and SOCS based methods fail to solve the problems that arise during the construction process of drift gravel stratum. These problems include deviation, tilt, the cracking and failure of the open caisson wall, sinking-suspension, and sudden sinking. Through theoretical calculation analysis, numerical simulation, field testing, and monitoring this study analyzed the construction plans to demonstrate the sinking methods, reliability, and structure safety of the open caissons in different strata. Moreover, it investigated the sinking coefficient, construction method, sinking-blocking processing method, and sinking-slagging method of an open caisson, which provided references for the open caisson design and construction under complicated geological conditions.

Surge shaft

Hydropower house

Figure 1: Xiergou Hydropower House project Figure 2: Structure and geological section of surge shaft Vol. 19 [2014], Bund. T 5727

SINKING STABILITY ANALYSIS AND STRUCTURAL OPTIMIZATION

The sinking coefficient K refers to the ratio of the sum of the open caisson weight with the frictional resistance of the open caisson wall to soil to the counter-force of the stratum on the foot blade. It is an index for evaluating the success or failure of the sinking of the open caisson. Generally, the frictional resistance between the open caisson wall and the soil layer is calculated with the assumption that frictional resistance increases with soil depth and maximizes at 5 m, while maintaining a normal value below the 5 m, as shown in Figure 3.

Figure 3: Calculation diagram of friction resistance The sinking coefficient is calculated by: QB K  （1） TR where K is the sinking coefficient; Q is the weight and additional load of the open caisson; B is the water discharge. In the instance of the drainage sinking method, B=0; T is the frictional resistance between an open caisson wall with soil and can be obtained by Eq. (2); R is the counter-force of the foot blade and can be got by Eq. (3). The frictional resistance T between an open caisson wall with the soil layer can be obtained by: T D( H 2.5) f （2） where D is the external diameter of the open caisson ; H is the total height of the open caisson ; f is the coefficient of frictional resistance between the open caisson wall with the soil layer The counter force of the foot blade R is calculated by:

R D0 ( c n / 2) Rd （3） where D0 is the average perimeter of the open caisson; c is the tread width of the foot blade; n is the horizontal projection width of the foot blade slope with the internal contact surface between Vol. 19 [2014], Bund. T 5728 the terrain and open caisson; Rd is the limit carrying ability of subsoil. In the instance of the soil excavation in the tread and slope of foot blade, R=0. The reinforced concrete of the open caisson had a volume-weight of 25 KN/m3. The excavation section was located above the underground water line. In the calculation, we set B=0. The drift gravel stratum has a higher bearing capacity. The Rd is valued as 0.6 MPa. The open caisson in the drift gravel stratum sinks intermittently and significantly by extremely short small time intervals instead of being synchronously attributed to the fluctuations of the frictional resistance coefficient around the open caisson. Analysis on the sensitivity of the sinking coefficient of the open caisson suggested that the sinking coefficient was sensitive to the frictional resistance coefficient (Figure 4). When there was no excavating conducted on the soil in the tread and slope of the foot blade and open caisson wall thickness of 1.5 m (Figure 4 (a)), the sinking coefficient under the depth of 35 m is higher than 1.0. The high sinking coefficient of the open caisson suggests the possibility of sudden sinking. To stabilize the sinking, the friction resistance increase should be equivalent to the weight increase of open caisson. Thus the wall thickness of the open caisson should be reduced to 1.14 m under a depth of 35 m. For construction convenience, the wall thickness of the open caisson was set at 1.2 m. After the weight of the open caisson was reduced, the open caisson satisfied the stable sinking condition after the soils on the slope were excavated according to the calculation results of the subsequent excavating soil area. In the case of excavating the soils in the tread and slope of the foot blade and wall thickness of 1.5 m (Figure 4 (b)), the sinking coefficient rapidly reduces as the frictional resistance coefficient increasing from 10 kPa to 35 kPa. As the open caisson sinks below a depth of 10 m, the sinking coefficient shows little variations with the depth increase. The fluctuations of the frictional resistance coefficient in the construction caused the sinking coefficient to drop below 1.0. In these conditions, there’s a tendency of sinking. In the case of the sinking being blocked, the surrounding rocks can be lubricated by means of flushing water along the open caisson wall to reduce the frictional resistance coefficient. Also, the sinking process can be furthered by shock blasting the open caisson via a small amount of explosives (generally a bundle of 4 φ32 cartridges, 800 g) that are hung along the middle part of the open caisson with cranes. This method can induce a brief vibration of the wall of the open caisson and thus promote the successful sinking of the open caisson. During the sinking process of the open caisson, the soils around the open caisson are hard to stabilize due to in the lack of a capacity to press the backfill on the external side of the open caisson. Therefore, during the first 35 m of sinking, the open caisson thickness is set to 1.5 m to reduce the backfill. After 35 m, the open caisson wall thickness is reduced to 1.2 m to avoid the high open caisson weight. To ensure the safety of the open caisson during the sinking process, the external wall of the steel foot blade adopts a vertical structure and the tread is increased from 20 cm to 30 cm. According to the conditions of the field construction, this study increased the height of the steel plate on the inner side of the foot blade from 100 cm to 150 cm, and the installation height of the stiffening rib from 30 cm to 50 cm, all of which helped prevent the foot blade concrete failure induced by the excavation and blasting of open caisson, as well as the frictional failure of the foundation in the sinking process (Figure 5).

Vol. 19 [2014], Bund. T 5729

f=10kPa f=15kPa f=10kPa f=15kPa 2.5 f=20kPa f=25kPa 7 f=20kPa f=25kPa f=30kPa f=35kPa f=30kPa f=35kPa

2 6

5 1.5 4 1 3 2 0.5

Sinking coefficient Sinking 1 Sinking coefficient Sinking 0 0 0 20 40 60 0 20 40 60 Sinking depth/m Sinking depth/m a: Without excavating the soils in the tread and b: Excavating the soils in the tread and slope of slope of foot blade foot blade Figure 4: Sinking coefficient sensitivity curve

CONSTRUCTION METHODS OF THE OPEN CAISSON IN DIFFERENT STRATA

The excavating and sinking method in drift gravel stratum According to the sinking coefficient analysis and open caisson structure optimization results, it is required that the soil beneath the foot blade be uniformly excavated during the sinking at different depths to ensure the successful sinking of the open caisson. Moreover, to guarantee the stable sinking of the open caisson, the excavation height on each stratum should be controlled. A small backhole of 0.2 m3 was used to excavate the bottom at a thickness of 0.5~1.0 m. Along the foot blade an earth dike more than 2.0 m wide was retained. At this point, the soil layer was skived along the open caisson wall successively, comprehensively, uniformly, and symmetrically (Figure 6). The open caisson construction technology integrates the temporal support of the shaft and permanent lining into the open caisson wall. More specifically, this technology can be used to improve the conventional shaft construction process, namely, excavation→ support→ excavation →support →permanent lining, into open caisson wall production→ excavation→ open caisson sinking→ open caisson wall production →open caisson sinking in place.

Figure 5: Structure optimization of cutting Figure 6: Evenly bowl-shape excavation edge of open caisson during open caisson sinking

Numerical simulation and construction method of the open caisson in soft and hard alternated layered strata The safety monitoring results for large caissons suggests that excavation has a large influence on the stress and strain of an open caisson [10, 15]. The open caisson employed in this study had a Vol. 19 [2014], Bund. T 5730 large diameter and complex geological conditions. During the construction, a large amount of boulders emerged in the open caisson as the open caisson sunk more than 10 m. In addition, stresses were concentrated on the open caisson wall and thus resulted in the cracking and failure of the concrete. Therefore, it is important to select the optimal construction method and plan by simulating various cases, studying the stress on the open caisson wall and foot blade, and analyzing the potential locations and forms of the failure on the open caisson wall and foot blade. The finite element software ANSYS was used to calculate 6 materials, including C2 reinforced concrete, Q235 steel, soil, drift gravel, weakly weathered bedrock, and slightly weathered bedrock. The open caisson wall, bedrock, and foot blade were simulated using a linear elastic model, while overburden was simulated using ideal elastic perfectly elastic and Drucker-Prager model. Table 1 shows the physical and mechanical parameters of the two models. The compressive strength and tensile strength of concrete were set as 12.5 MPa and 1.3 Mpa, respectively. The contacts of the open caisson wall and overburdened foot blade were simulated using surface-surface contact elements. The surfaces in the contacting state were impenetrable to each other and capable of transmitting normal pressure and tangential friction force instead of normal tension. Therefore, the contact problem is freely decomposable. This effectively simulates the interactions between the open caisson wall and the surrounding soils. In the calculation, the element Targe170 was used to simulate the 3-D “target” surface; and Conta174 was used to simulate the contact surface. Figure 7 shows the finite element calculation model of the surge shaft. To determine a reasonable construction method while avoiding the disadvantageous mechanical deformation brought on by unreasonable constructions, this study selected the following cases: Case 1: symmetrical geological condition. Case 2: asymmetrical case. First, excavating the weak overburden on the tread side of the foot blade and supporting the other foot blade side through rocks. Case 3: asymmetrical case. First, excavating the rocks on the tread side of the foot blade and supporting the other foot blade side through weak overburdens.

Figure 7: The finite element calculation model of the open caisson

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Table 1: Physical and mechanical parameters of the open caisson Elastic Deformation Poisson Density φ c′ Material modulus modulus f′ ratio 3 g/cm GPa GPa 。 kPa IV class surrounding 0.34 2.78 6.5 4.00 rock III class surrounding 0.30 2.80 9.0 7.00 rock Soil 0.40 1.55 0.05 0.48 25.4 25 Drift gravel 0.40 2.24 0.06 0.53 27.6 0 Open caisson wall 0.167 2.50 28.0 Cutting edge 0.25 7.55 206 According to the results of the numerical simulation, the maximum tensile and compressive stresses on the open caisson wall in the sinking process are achievable (Figure 8). In case 1, with an increase of the sinking depth of the open caisson, the maximum tensile and compressive stresses grow gradually and reach to 1.14 MPa and 3.03 Mpa, respectively. In case 2, the maximum stress on the open caisson wall is seen on the foot blade supported by the hard base. Stresses are concentrated and increase gradually with the sinking of the open caisson. The maximum tensile and compressive stresses calculated reach to 2.48 MPa and 4.50 Mpa, respectively. Both exceed the concrete’s designated tensile strength. In case 3, the open caisson wall does not present an obvious stress concentration. The maximum tensile and compressive stresses are 1.16 MPa and 3.11 Mpa, respectively. The maximum tensile stresses on each part of the open caisson are all lower than the designed tensile strength of the concrete. During the construction process under a symmetrical geological condition, the open caisson sinks layer by layer and the stress satisfies the requirements; under the asymmetrical case, the hard rock sections should be executed first followed by the weak rock sections. In this way, the open caisson absorbs a lower stress and thus will not be damaged.

Case 1 Case 1 (unit: Pa)

Case 2 Case 2 (unit: Pa) Vol. 19 [2014], Bund. T 5732

Case 3 Case 3 (unit: Pa)

Figure 8: The calculation results of stress of open caisson wall The results of the numerical calculation show that the hard and soft alternated strata caused the open caisson cracks. Under the support of the locally hard strata, stresses are concentrated and thus induce the local tensile failure and the overall tilt of the open caisson. Therefore, this study proposes the following targeted treatment methods: (1) Before the sinking of the open caisson, the interspaces on the external side of the open caisson were filled and the fillings were compacted to increase the frictional stress of the open caisson wall and control the sinking speed. (2) Before processing the boulders on the stressing points of the foot blade, the distributions of the stressing points should be analyzed. Then, the boulders should be processed by a principle of “small explosive amount, weak blasting, low height, and repeated observation,” step by step, point by point, and layer by layer. The boulder processing height of a single time should be controlled between 0.2 m and 0.3 m. Moreover, before the borehole blasting, the drill detection holes must determine the diameter of the boulder and guide blasting borehole depth, so as to improve the quality of the blasting. The processing depth should exceed the external wall of the foot blade by 0.15 m~0.20 m, so as to eliminate the friction and damage of boulder processing on the foot blade of the open caisson (Figure 9(a)). (3) Our observations show that the internal concrete on the deformation position of the local foot blade was fractured. When the open caisson sunk steadily and the tread of the foot blade as fully stressed, the deformed steel plate of the foot blade was cut and the internal fractured concrete was removed. Subsequently, the steel plate of the foot blade is re-welded and reinforced. The C40 concrete is refilled from the preset hole (0.3 m×0.3 m) on the steel plate of the foot blade and agitated using the Φ 60 plug-in vibrator through a pre-buried grout pipe. The foot blade is processed using a steel strip 20 mm thick and 0.20 m wide (Figure 9(b)).

a: Open caisson excavation at boulder b: Deformation and cracks of cutting concentration zones edge Figure 9: The construction procedure and method of open caisson Vol. 19 [2014], Bund. T 5733

Sinking method and technological in cementing conglomerate and broken phyllite sections The cementing conglomerate can bear a high load. Moreover, its frictional resistance coefficient with the open caisson wall is significantly higher than that of the upper drift gravel layer. Therefore, the open caisson suspends and ceases sinking while the soil under the tread of the foot blade is excavated during construction. As for the dense conglomerate stratum, it is ineffective to solve the sinking-suspension of the open caisson using the frictional resistance reduction measures above; namely, by using water lubrication or shock blasts with a small amount of explosives. To reduce the contact area and fictional coefficient between the open caisson wall with a cementing conglomerate, this study extended the excavation range of the soil beneath the foot blade for 30 m to reduce the friction between the open caisson wall with soils through a field test and in reference with the principle of SS method (Figure 10). The specific measure is such that, for the sinking process in the dense conglomerate stratum, the soils around the foot blade are retained by a 2 m width. With a hydraulic hammer, buttresses are reserved along intervals according to the sections. The external excavation range of the foot blade is extended externally for 30 cm; since the bottom of the open caisson is composed of granite belt and broken phyllite sections, the open caisson-termination can be solved effectively through the decomposition and stripping by shallow borehole blasting.

Figure 10: Excavation of the compact conglomerate layer under open caisson cutting edge PROCESSING METHOD OF OPEN CAISSON DEVIATION AND CRACKS

According to the analysis of the origin of the open caisson cracks and excavation methods, the excavation of the open caisson should follow a principle of “middle part priority and then the reserved, hard base priority and then the soft base; other positions priority and then supporting points.” This will ensure the symmetry and uniformity of the excavation and the sinking of the open caisson. A single process of the boulder should be controlled in an approximate range between 0.20 m and 0.30 m. Before the boulder is processed, the soft soil layer or sand-gravel materials beneath the foot blade should not be excavated. Table 2 shows the possible problems encountered during the sinking process of the open caisson, along with corresponding countermeasures. Vol. 19 [2014], Bund. T 5734

Table 2: Problems possibly encountered in the sinking process and corresponding countermeasures Problems and Causes Countermeasures phenomena Continuously pouring concrete to increase the weight; applying a load on the open caisson roof; removing the soil below the foot blade; initiating the second bowl-shaped excavation in the open caisson; pouring thixotropic slurry 1) The friction resistance in the space between the open caisson wall between the open caisson wall and soil, to reduce the friction. The sinking difficulty of with the soil wall is too large. Removing the smaller boulders encountered the open caisson 2) The weight and sinking during soil excavation; removing the large (sinking-suspension or coefficient of the open caisson boulders or stone blocks by hole-drilling and extremely slow sinking) are not high enough. blasting using a hand drill. During the drilling, 3) There are obstacles such as the following protocol should be observed: boulders and large pebbles. The distance from the hole to the foot blade should not be larger than 50 cm; the direction of the hole should be parallel to the slope of the foot blade; the explosives should not be larger than 200 g; protection measures are necessary. Rapid sinking of the The open caisson encounters open caisson (an Supporting the designed support points using the soft soil layer during the abnormal phenomenon a brick pier or by backfilling sandy gravels; sinking process. Due to the caused by that sinking readjusting the excavation; stopping the low compression strength of speed of the open caisson excavation or partial excavation under the foot the soil, the sinking speed exceeds the excavation blade. exceeds the excavating speed. speed) As the bowl-shaped excavation goes too deep, the open caisson is temporally held in a reasonably stable Appropriately increasing the sinking condition by the external wall coefficient; pouring water along the open Sudden sinking (the frictional resistance and foot caisson wall to reduce the friction between the sudden sinking blade. By continuing the open caisson wall and soil. phenomenon caused by a excavation, the frictional lack of control over the Controlling the excavation (the bowl bottom resistance reaches to the limit sinking of the caisson) should not be too deep); do not hollow the and the open caisson wall foot blade; determine the foundation condition resistance suddenly reduces in advance. due to the thixotropy of soil. Sudden sinking is thereby induced. 1) The hardness of the soils Strengthening the observation and data beneath the open caisson is analysis in the open caisson sinking process uneven. during construction; correcting the tilt in a 2) The excavation is not timely manner. uniform and the soil surfaces Backfilling the sandy gravels for compacting Open caisson tilt (the in the open caisson show a in time. perpendicular deflection large height difference. of the open caisson from Enhancing the soil removal on the higher side the allowed limit ) 3) The foot blade is of the foot blade; stopping or reducing the over-hollowed. The open excavation on the lower side of the foot blade; caisson suddenly sinks and uniformly removing the soil layer by layer was tilted easily. after the tilt is corrected. 4) One side of the foot blade Appropriately filling sandy stones or stone is held by obstacles, and not blocks on the lower side of the foot blade to Vol. 19 [2014], Bund. T 5735

found in time. reduce the sinking speed; in case of the 5) The bias pressure on the perpendicularity of the open caisson tilting open caisson wall is induced beyond the limit, excavating the soils on the because of the removal of reverse tilt side out of the open caisson and soils or objects from the open backfilling these soils into the tilt side to caisson; the uneven increase the frictional resistance of the tilt distribution of the load on the surface. open caisson. 6) Sand boils on one side of the open caisson. 1) This phenomenon is most Preventing the open caisson from tilting commonly induced by the tilt. toward the deviation direction. When the tilt occurs and is Deliberately tilting the open caisson toward corrected, the open caisson the reverse direction of the deviation; after applies a high pressure on the several tilt corrections, the open caisson can Open caisson deviation lower part of the tilted side, recover to the correct position. (the displacement caused along with a certain by the misalignment of Deliberately tilting the open caisson toward displacement of the open the reverse direction of the deviation; then the open caisson axis caisson. The displacement with the designed axis) sinking the open caisson along the tilted size is determined by the soil direction until the centerline of the foot blade quality and the tilting times is identical to or approaches near the designed on the tilted side. center line; finally, correcting the tilt. 2) Measuring and positioning Strengthening the inspection and review of the the errors. measurement.

SLAGGING METHODS FOR THE OPEN CAISSON CONSTRUCTION The excavation of the drift gravel in the open caisson of the surge shaft is about 29475 m3. Considering the large excavation and the depth of the sinking, the upper drift gravel was dug and loaded using a 0.2 m3 small backhoe, while the slag was removed using a tower crane and 2 m3 cableway bucket (Figure 11(a)). The 33 m thick bedrocks on the lower part was dug using a 1 m3 small backhoe, while the slag was cleared using a pilot shaft 2.0 m in diameter. After the concrete construction on the fore shaft of the pilot shaft, artificial excavation was used (Figure 11(b)). The lower part of the super-deep open caisson was applied with a slagging method of pilot shaft. This method reduces the slagging difficulty and improves the slagging efficiency. Meanwhile, it determines the geological condition of the surge shaft and ensures the safety of the engineering.

a: Slagging using the tower crane at the b: Slagging method of pilot shaft at the upper part lower part Figure 11: Open caisson excavation slagging Vol. 19 [2014], Bund. T 5736

MEASUREMENT AND OBSERVATION OF THE OPEN CAISSON During the construction of the large-diameter and large-depth open caisson, the vertical and horizontal cross centerlines and benchmarks should be set on the external earth surface and wall of the open caisson. Moreover, it is required to control the perpendicularity by marking the normal axis and align the supporting wires to the scale plate below, one by one, in the open caisson to trace the perpendicular position and elevation of the open caisson. In addition, the horizontal lines used to observe the sinking depth can be set on the construction platform of the open caisson to maintain a balance with the monitoring chalk line scale on the open caisson wall. This allow for the monitoring of the open caisson pose in real time while ensuring the steady sinking of the open caisson. Table 3 shows the monitoring results. According to the monitoring results, the open caisson shows a central derivation of 46.3 cm and tilting angle of 00°25′18″ after reaching the designed elevation, which is in agreement with the design requirements.

Table 3: Monitoring results during open caisson sinking Sinking Angle of Sinking Angle of Serial off-centering Serial off-centering height inclination height inclination number (cm) number (cm) (m) (°) (m) (°) 1 2.3 3.0 0.7473 13 41.3 37.0 0.5133 2 5.3 5.5 0.5946 14 43.7 38.0 0.4982 3 8.3 9.6 0.6627 15 46.8 38.5 0.4713 4 10.3 13.5 0.7509 16 48.9 39.1 0.4581 5 12.7 17.0 0.7669 17 50.8 39.8 0.4489 6 16.5 16.2 0.5625 18 52.8 40.2 0.4362 7 20.5 18.0 0.5030 19 56.8 41.5 0.4186 8 23.0 19.5 0.4858 20 58.69 42.5 0.4149 9 27.0 24.0 0.5093 21 64.32 45.8 0.4080 10 33.0 28.5 0.4948 22 63.42 46.2 0.4174 11 36.0 31.5 0.5013 23 62.92 46.3 0.4216 12 38.0 34.7 0.5232

CONCLUSIONS

(1) The results of this study show that the open caisson construction method is applicable for drift gravel stratum and other complicated geologies. This method was conducive to environment protection since it greatly reduced the excavation amount on the overburden and saved the stacking area of the waste slag. The well wall stabilizing using open caisson increased the safety of the excavation in the well and made it possible of transferring the temporary support and permanent lining to the ground. The working environment was improved and the construction risks were reduced as a result. This creative open caisson construction method yielded significant social and economic benefits. (2) According to the sinking frictional resistance analysis and structure calculation, a reasonable open caisson structure reduces open caisson failure risk during construction. (3) Based on the numerical analysis results and sinking test, this study proposes the following Vol. 19 [2014], Bund. T 5737 suggestions: implementing a uniform bowl-shaped excavation on the drift gravel strata; for the hard and soft alternated strata, following the principle of “hard rock priority and then soft rocks”; for the gravel strata, breaking the rocks into sections using a hydraulic hammer; in regard to the strata with a sinking difficulty, assisting the sinking by shock blasts using a small amount of explosives, or, by extending the excavation to realize the stable and successful sinking of the open caisson. (4) This study (1) analyzed the causes for the sinking-termination, sudden-sinking, tilt, and derivation of the open caisson and (2) put forward corresponding countermeasures. Moreover, it investigated the tower crane transporting above the surge shaft and the pilot shaft sliding method on the lower part of the open caisson, which reduced the slagging difficulty and improved the slagging efficiency.

ACKNOWLEDGMENTS

This study was supported by the National Natural Science Foundation of China (No.51009104). REFERENCES 1 ZHU Jian-min, GONG Wei-ming, MU Bao-gang, Research on stress and settlement of super-large open caisson during first lifts[J]. Rock and Soil Mechanics, 2012, 33(7):2055-2060. (in Chinese) 2 Mu Baogang, Xiao Qiang, Zhang Licong, Effect of supporting soil stiffness on internal force of caisson during sinking[J], Journal of Southeast University ( Natural Science Edition), 2012,42(5):981-987. (in Chinese) 3 Tao Jianshan. Construction technology for draining sinkage for south caisson anchorage to Taizhou YangtzeRiver Highway Bridge with large-size sunk well [J]. Journal of Railway Engineering Society, 2009(1):63-66. ( in Chinese) 4 CHEN Xiao-ping, QIAN Pingyi, ZHANG Zhiyong. Study on penetration resistance distribution characteristic of sunk shaft foundation [J], Chinese Journal of Geotechnical Engineering, 2005,27(2):148-152. (in Chinese) 5 Vesic A S. Tests on instrumented piles, ogeechee river site[J].Journal of SMFD,1970, 96: 561– 584. 6 Tjelta T I, Guttormsen T R, Hermstad J. Large-scale penetration test at a deepwater site[C]. Proceedings of the 18th Annual Offshore Technology Conference. Houston, Texas, USA, 1986:201-212. 7 Solov'ev N B. Use of limiting-equilibrium theory to determing the bearing capacity of soil beneath the blades of caissons [J]. Soil mechanics and foundation engineering, 2008, 45(2): 39-45. 8 Yan Fuyou, Guo Yuancheng, Liu Shangqian. The Bearing Capacity Analyses of Soil beneath the Blade of Circular Cassion. Advanced Building Materials, 2011, 250-253: 1794-1797. 9 Y. W. Shin, M. Sagong. Ground Pressure Acting on Cylindrical Retaining of a Shaft in Soft Ground [J]. Chinese Journal of Rock Mechanics and Engineering, 2007, 26 (Supp. 2), Vol. 19 [2014], Bund. T 5738

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