Shanghai Jintai SZ80-35 re-breakage problem of bite pile construction along the banks of the Minjiang River

Recently, Shanghai Tengshang Foundation Engineering Co., Ltd. was informed that in the “Wenzhou Qingtian Overseas Chinese Economic Headquarters Building (Comprehensive) Pile Foundation Project” under construction, Shanghai Jintai SZ80-35 full-enclosure drilling rig was used to construct foundation pits for bite piles. Pebble formation hole puzzle solved.

The proposed project is located on the south bank of the Lancang River, west of the Tashan Bridge, and the Minjiang River turns slowly and downwards. The west and north sides are the built-up embankments of Jiangbin Park. Five high-rise buildings were built on the site, and the pit was irregularly rectangular with an area of ​​approximately 21040m2, a perimeter of the pit of approximately 830m, and an excavation depth of 9.40-14.30m. Because the 4# and 5# buildings are excavated during the flood season and the flood season water level is higher than the excavation surface, an effective water stop curtain must be set up for this area. The plain fill layer and pebble layer in the site have a great influence on the drilling and pile formation.

Shanghai Jintai SZ80-35 re-breakage problem of bite pile construction along the banks of the Minjiang River

Shanghai Jintai SZ80-35 re-breakage problem of bite pile construction along the banks of the Minjiang River

According to the geological survey report, there are large granite blocks that have been thrown in at the bottom of the plain fill, with particle sizes ranging from 0.5 to 2.0 mm. In addition, the pebble density in the 2-2 pebble layer reaches a dense level with a content of 50 to 60%, a general particle size of 20 to 100 mm, and a maximum of 400 mm. The sub-circular shape is dominated by poor grading and texture. Hard, the mother rock composition is tuff, granite, etc., medium to micro-weathering, filled with gravel and cohesive soil, no cementation, drill hole wall easily collapsed.

The aforementioned geological conditions are obviously unsuitable for conventional cement-soil mixing piles or high pressure jet grouting piles. The design agency has taken into consideration the actual situation on the site and decided to adopt a full casing drilling process. The suitability of full casing drilling bite piles mainly manifests in the following aspects:

1. The shear strength and safety of the bite pile can meet the requirements of the envelope structure;

2. The continuous structure formed after the occlusal pile construction is completed can block the water flowing into the foundation pit from the outside of the foundation pit;

3. The use of full casing drilling rig can meet the drilling intensity, and in the construction, there is always an advanced steel casing protection wall, which will not cause hole collapse, and has little impact on the surrounding formation deformation;

The chief engineer of Shanghai Tengshang Project introduced the gold engineer: “The conventional bite pile construction equipment on the market includes pipe boring machines, full rotary rigs, and rotary excavation plus full casing. The construction process of these three types of equipment is based on hydraulic equipment. The tube is oscillated in a circle and pressed in sections, and each section of the casing is pressed into the earth and then the soil is taken out through a grab or a rotary drilling rig, and then the next section of the casing is used, and the reciprocating operation is performed until the hole reaches the design elevation. With the construction methods of casing and soil taking, the hole forming efficiency is low, and the manual and mechanical operation costs are relatively high. In order to ensure the construction period, multiple equipments need to be simultaneously constructed, which also increases the difficulty for the construction site layout.

Practice has shown that the drilling ability of the pipe boring machine is limited, and it is impossible to overcome the large-diameter pebbles and local boulders within the geological range, and the drilling depth is limited when the rotary drilling rig is equipped with a casing. Based on the above considerations, Shanghai Tengshang Basic Company decided to use the SZ80-35 double power head full casing drilling rig to carry out the bite pile construction of this project.

Shanghai Jintai SZ80-35 compared with other equipment, the significant technical characteristics are: the two power heads that respectively control the drill pipe and casing can be raised to the top of the mast, while the casing power head sinks into the casing, the drill pipe The power head drives the long auger rod to take the soil; the casing does not need to be pressed into sections, and there is no need to use other machinery to catch the earth, which can greatly improve the construction efficiency and save the mechanical and labor costs.

Even with the full casing drilling rig process, the construction of occlusal piles in compact large-size pebble strata will encounter some common problems, such as the verticality control of the bite piles, the tube boring during the hole forming process, the floatation of the reinforcement cage, and the drift during drilling. Stones, etc., these problems have already been mentioned by Shanghai Tengshan Basic Engineering Co., Ltd., which has rich experience in bite pile construction. In recent years, Shanghai Tengshang Company has successively cooperated with Shanghai Construction Engineering Group, China Railway Tunnel Group, Zhongtian Construction Group, Shanghai Tunnel Engineering Company, and Nantong Second Construction and other large construction enterprises, and completed the use of Shanghai Jintai SZ80-35 full-encased drilling rig. The representative project has provided useful exploration and practice for the construction technology of bite piles under various complex geological conditions. (This article comes from Shanghai Jintai)