1.0 Project profile

1. 1 Overview of foundation pit

It is a deep foundation pit project located at downtown Wuhan. The project consists of three super high-rise buildings, six commercial podium buildings and five floors below the ground. In this project, the average excavation for the foundation of a podium building is 22.25m, and the excavation for the foundation of the tower is 22.95m ~ 23.65m. All in all, the total area for the excavation of the foundation pit is about 18,800 m2, and the perimeter of the enclosure is about 635m.

1. 2. Geological conditions of the construction

This project was situated in erosion areas, as Yangtze river Ⅲ level terrace, which is generally flat.
From top to bottom, there are five layers of soil:
(1) fill stratum;
(2) Holocene alluvial general viscous soil layer;
(3) Alluvial and diluvial old viscous soil layers;
(4) Alluvial-diluvial sandy soil and gravel pebble layer in the ancient channel of the Yangtze river;
(5) weathered silty mudstones, argillaceous siltstone and quartzite sandstone layer.

The pit excavation stratum is mainly in layer (4), and fine clay sand is the main element in this layer. For fine clay sand, medium density, the clay content is 15% ~ 20%. The sand is mainly fine, containing a specific content of medium sand, at the same time mixed with about 5% gravel pebbles.

2.0 Construction method

2. 1. The original construction plan

At first, considering that the excavation in the pit is at the construction period node, the detailed basement structure is not determined. Therefore, the targeted design of the pit in the pit cannot be carried out. Under this case, the form of well dewatering and soil nailing wall support is adopted.

Compared with other supporting structures, soil nail wall has the following advantages:
(1) small construction site required,
(2) fast construction speed,
(3) simple construction equipment and technology,
(4) limited interference to the surrounding environment,
(5) low project cost.

Before excavation in the pit, dewatering well construction is complete, and there are 3~4 pipe wells with a diameter of 300mm arranged around. Also, the capacity of pumping water for a single well is 240 tonnes per day. The slope of this unprotected open cut excavation is 60 degrees, and plain concrete C20 with the thickness of 80mm is poured in the ramp.

However, because the slope of the ramp is large and it consists of fine sticky sand with high water content, ø16mm short soil nails are applied under a quincunx arrangement of 500mm×500mm after spraying 20mm thick bottom concrete to ensure the safety and the quality of this project.

2. 2. Implementation of the construction plan

The construction process of the excavation in the pit is as follows:
(1) Engineering Surveying
(2) Replenish the precipitation well,
(3) Excavate layer by layer (2 layers),
(4) Hang net and set anchor bolt
(5) Pour the bottom.

Before excavating the slope, the concrete with a thickness of 20mm should be sprayed, the excavation slope shall be initially sprayed with concrete with a thickness of 20mm(mixed with 5% 8604-k3 accelerating agent). Mesh-bar is placed with soil nailing in the shape of quincunx section by section. Soil nailing and mesh bar are strapped, and the lap length of the net is more than 200mm. Bamboo stick and reinforcing steel bar are set for controlling the thickness of concrete, and then another 60mm concrete is poured for the second time. In the same section of the construction, concrete should be poured from the bottom to the top. The flow should be perpendicular to the surface, and the distance from the surface should be 0. 8 to 1. 5 m. Cement, sand, gravel and other materials used must meet the requirements of construction requirements. To be more specific, the water content of sand should be controlled at 5%-7%, the particle size of gravel should be smaller than 15 mm, and the mud content should be less than 3%, which must be washed clean.

2. 3 Construction problems

According to the original construction plan, the first layer of soil was excavated in pit No. 1 (about 2. 5m). However, after soil nailing, there occurred heavy sand and soil flow during the excavation of the foundation pit, and there was a considerable gap among engineering piles. Finally, this gap turns to be a hole with the depth of 1.0m to 1.5 m. Also, workers in pit No.2 have to dismantle precipitation wells to be away from excavator operation, causing water in the pit is rising gradually. Then, the soil at the top began to collapse, and the bottom layer of the poured basement floor cracked. In the end, it is impossible to carry out the next step of construction, so it has to backfill and compact the soil.

3.0 Remedial measures

3. 1. Detailed remedial measures

After the backfilling of pit No. 1, there were two remedial construction measures proposed: light well point precipitation and Lassen steel sheet pile. Both of these two methods are supplemented by the high-pressure rotary jet grouting pile in the foundation. The application of light well point precipitation is used to ensure dry at the bottom of the pit, and prevent the occurrence of sand flow effectively. Lassen steel sheet pile, with interlocks at the edge, can form steel sheet wall with large flexural rigidity and tight soil retaining. Also, it can be recycled and reused, with the characteristics of mature technology and fast construction.

Since there are many uncertain factors in the pit construction process at this time and the electricity at the bottom of the pit construction is not stable, Lassen steel sheet pile supporting structure is finally applied. The application of Lassen steel sheet pile combined with high-pressure rotary jet grouting pile in the foundation will help to ensure the safety of the project and the short work period.

3. 2. Implementation of remedial measures

Before driving steel sheet pile, two rows of ø 500mm high-pressure jet grouting piles should be set outside the excavation sideline. This measure can seal water in the soil outside the pit, and reinforce the soil in the active area outside the pit followed by the construction of steel sheet pile.

This project selects 9m Q295bz Lassen IV steel sheet pile produced by Nippon steel corporation. The process of Lassen steel sheet pile follows:
(1) Ground levelling
(2) Layout survey
(3) High-pressure rotary jet grouting pile in the foundation
(4) Equipment setting up
(5) Insertion of steel sheet piles inserting
(6) Breast beam and support welding
(7) Excavation in pit
(8) Bottom sealing by concreting (200 mm thick)
(9) Concrete enclosure wall poured
(10) Extraction of steel Sheet piles

In the end, the remedial construction of pit No.1 was relatively smooth, and the bottom coating was successfully poured.

4.0 Analysis of construction problems

4. 1. Application scope of soil nailing wall

The soil layer for excavation in the pit has lowe strength and high water content clayey, silty sand. The soil nailing wall is more applicable for viscous soil and cemented sand above the groundwater level. However, it is not suitable for the following layers:
Water-rich fine sand and relatively loose coarse sand, gravel-sand and pebble layer. Abundant groundwater may cause unstable excavation surface and the unstable concrete surface.
The dry sand layer which is lack of cohesion, and relatively small density. In these sand layers, instability of excavation face is easy to occur. Usually, the excavation in this layer is hard to control the sand flow.

4. 2. Adverse effects of groundwater

The stratum in this project presents a typical dual structure, namely the upper clay layer and the lower gravel layer. The aquifer is located in the ancient channel of the Yangtze River, which is relatively closed and difficult to supply. Because of this, the silty sand in this layer has a high clay content and a small permeability coefficient, which makes it difficult to pump water out of this sand layer, resulting in very insignificant precipitation effect. Pipe well precipitation cannot ensure the dryness of excavation stratum, and soil shear strength cannot be effectively improved. Finally, the problems of flowing sand and silt in the pit are getting serious, and the slope collapses.

5.0 Conclusion

(1) Excavation in the pit is in the final stage of pit excavation, which is relatively small size, and low deformation requirements. For most cases, there is limited consideration and attention paid to the selection of construction measures in the early design stage. In the case of poor geological conditions, it is very likely to cause construction problems.

(2) The soil nailing wall is more applicable for viscous soil and cemented sand above the groundwater level. If it is used for the fine sand with rich water content, it will lead to the failure of the concrete layer of the hanging net.

(3) This project is located in Wuhan Ⅲ level terrace. The silty sand in this layer has a high clay content and a small permeability coefficient, which makes it difficult to pump water out of this sand layer, resulting in a very insignificant precipitation effect. Finally, the problems of flowing sand and silt in the pit are getting severe, and the slope collapses.

(4) It is suggested to apply light well point precipitation and Lassen steel sheet pile supplemented by high-pressure rotary jet grouting pile in the foundation. Lassen steel sheet pile can be recycled and reused with mature technology and convenient construction, which can be given priority in emergency construction.