1.0 Overview of the project

For the project of Tianjin-road canal bridge in Huaian, Jiangsu province, it has a main span of 143m, across the Beijing-Hangzhou canal. It is a national second-class inland river channel with a large number of vessels. The load level of this bridge is grade A in the city, and the main pier bearing platform of the bridge is 31m×12. 2 m×3. 5 m. The elevation at the bottom of the bridge pier is 2. 33m, and the top elevation is 5. 83 m. Also, the ordinary water level is 9. 6m. For the elevation at the bottom of the cap is 10.25 m, and plain fill is placed between 10.25m and 5.86m, and silty sand is placed between 2.86m and 0.96m. There are 21 piles with a diameter of 1.8m under each cap. The platform is the overall structure below revetments on both sides of the canal. The revetment is the mortared rubble gravitational foundation. Before the construction of pile foundation, the revetment must be cleaned up.

This project is municipal engineering to relieve the traffic pressure of the main road in the city. It is required to be completed and opened to traffic within 17 months. However, there are 18 sections of hanging basket cantilevered construction; the working period for pile-cap is not more than five months.

2.0 Comparison and selection of plans

2.1 construction plan

1) Scheme 1:
Construction of earth cofferdam
Pumping water from the cofferdam
Demolition of revetment
Construction of floating drilling platform
Construction of steel sheet pile cofferdam
Installation of internal cofferdam support
Construction of cap where it is without water

2) Scheme 2:
Construction of steel sheet pile cofferdam
installation of cofferdam internal support
Pumping water from the cofferdam
Demolition of revetment
Setting up a floating drilling platform
Pumping water
Construction of cap where it is without water

3) Scheme 3:
Construction of earth cofferdam
Pumping water
Demolition of revetment
Filling the soil to build an onshore drilling platform
Drilling into a pile on the onshore platform
Construction of steel sheet pile cofferdam
Dewatering (pumping)
Excavation
Construction of cap where it is without water

4) Scheme 4:
Construction of large earth cofferdam
converting the water construction to land construction.

For scheme 1 and 2, the construction period is more extended than expectation, and workers cannot work for different steps at the same time. Also, it is difficult to drill piles on the water, for building drilling platform requires many drilling platform equipment, which increases the cost considerably. In scheme 2, there is also a problem for solving the water leakage at the joint of steel sheet pile and concrete revetment. All in all, these two schemes are not applicable.

Scheme 4 will reduce the volume of the channel significantly, which makes it difficult for the channel and maritime administration to approve the plan. Moreover, this scheme needs to remove a large amount of earth, increasing the cost. Also, the large difference in water level between the inside and outside of cofferdam will bring safety risks. After taking all of these factors into consideration, Scheme 4 is not applicable.

At the beginning stage of scheme 3, the cofferdam is used as a water-retaining facility to dismantle the revetment successfully and work as a drilling platform. In the later stage, it can also be used as the internal support of steel sheet pile cofferdam. Earth cofferdams expand the operating space of drilling platforms, allowing for increased the number of operating drilling rig to reduced drilling time. In this plan, the concrete mixer truck directly can arrive at the orifice to pour pile foundation concrete, reducing the cost of pumping concrete. Drilling and inserting steel sheet pile cofferdam can realise to work at the same time to save the construction period. In the end, the insertion of U-shaped steel sheet pile cofferdam is taken.

The construction sequence of scheme 3 is as follows:
Filling soil
Pumping water
Dismantling revetment
Drilling into a pile
Insertion of steel sheet piles
Well-points dewatering
Digging soil
Sealing the bottom
Construction of foundation
Removing steel sheet piles
Dismantling earth cofferdam

3.0 Measures to fill the island

Because of excavator and bulldozer, the earth is pushed forward along the edge of the revetment. The vibratory roller is used to vibrate compactness during filling to compact layer by layer. To reduce water percolating capacity, earth cofferdam should be constructed by using clay instead of sand.

4.0 Dismantling the revetment

With earth cofferdam water barrier so far, the use of excavators and hammer of revetment shall be dismantled and removed to the surface when the following parts, set up in the corner of the end sump. In the process of demolition, concrete debris clean up in time to away from the position of pile caps, lest stone falls to the pile foundation liners, affect the drilling work smoothly.

5.0 Construction of steel sheet pile cofferdam

Construction of steel sheet pile cofferdam is carried out at the same time of pile foundation construction. Because of filling soil and building islands, it is relatively easy to pull the steel sheet pile. A crawler crane is used to drive and pull steel sheet pile with 90 tonnes vibrating hammer.

To reduce the consumption of steel sheet pile, U-shaped cofferdam is selected. There are two corner piles in U-shaped cofferdam. Firstly, one corner pile is inserted, and then the next one is pushed into the other corner.

6.0 Methods of dewatering in the foundation pit

The main pier cap is designed to be rectangular, and the foundation pit excavation of the cap reaches elevation +1.83m. The average excavation depth is about 8.42m, and the elevation of water level at the cap is 9.5m. The bottom of the cap is located in the non-liquefaction layer of the fourth saturated silty sand layer, which is underwater pressure.

According to the geological features of the cap, the construction of the cap dewatering selects the heavy calibre deep well scheme. There are 12 wells with a depth of 12m, and an inner diameter of 30 cm are drilled around the cap. For this cap, dry construction is realised by eliminating surface water and reducing groundwater.

6.1 Construction plan of tube well

Non-fine concrete pipe is used as a filter pipe, which double-layer filter. The inner layer is 8-16 layers nylon wire screen; the outer layer is nylon or plastic window screening (about 1.6mm hole), the filter material is the mixture of coarse sand or 3~8mm sand and gravel.

Equipment requires:
1 underwater rig.
2 mud trucks.
1 mud pump.
1 clean water pump.
15 submerged pumps.

7.0 Plan for excavation

For this foundation pit, it is 1:1, and the current water level is 9.5m. The top height of the earth cofferdam is 10.25m, and the excavation depth is 8.42m.

7.1 calculation of foundation pit slope stability after excavation

The excavation slope ratio is 1:1, and the slope angle mouth is 45. The centre position of the most slippery sliding surface was determined by drawing method, and the radius of the sliding arc was determined, which is 13.46m. The safety factor of soil slope stability is 1.39>1.25, so the slope is stable.

7.2 Excavation plan

According to the geological and hydrological conditions of the project site, the construction of foundation pit should take slope excavation based on technical specifications for highway bridge and culvert construction JTJ04—2000. Well, point dewatering shall be conducted before excavation to keep the soil within the scope of the foundation pit dry.

7.2.1 Excavation of foundation pit

Excavator is used in the foundation pit, and dump truck is used to transport the earth to the designated place for storage. The excavation slope can be adjusted according to the actual conditions of the soil on the site during the construction. Hand excavation is taken to prevent over-excavation when the excavation reaches a range of 20 cm above the base. In the construction of deep foundation pit, the method of “horizontal excavation” is adopted. In this method, the digging process is gradually from one end to the other. The excavation depth of each floor shall be no more than 3m, and a platform with a width of 1.5m shall be set up every 3m. Also, it is necessary to pay attention to the soil condition around the foundation pit during excavation. When the crack appears along the top of the foundation pit, which affects the stability of the foundation pit, there is a need to reinforce the support immediately. A 30cm high earth dam is built around the foundation pit to prevent surface water from flowing into the foundation pit. Moreover, there is a sump site at the bottom of the foundation pit. After the foundation pit is excavated, a fence is set up around it and closed with dense mesh.

At the same time, warning signs are set up around the foundation pit and construction temporary walkway. In the process of foundation construction, the dump truck must follow the established path, and no motor vehicles are allowed to pass within 10m in any directions of the foundation pit to prevent the wall collapse caused by the dynamic load. When cars are moving 10m away, pit wall monitoring should be improved.

8.0 Conclusion

It only takes four months to complete the construction of the platform, revetment demolition, steel sheet pile cofferdam, 21 root piles, sealing and caps. In this scheme, it is possible to implement different steps of work at the same time and reduce the cost of construction.