Analysis of the Foundation Pit Collapse Accident at Guangzhou Haizhu City Plaza

I. Project Overview

1. The perimeter of the foundation pit at Haizhu City Plaza is about 340 meters. The original design had a 4-story basement, and the excavation depth of the foundation pit was 17 meters.

2. To the east of the foundation pit is Jiangnan Avenue, beneath which runs Guangzhou Metro Line 2. The distance from the edge of the Line 2 tunnel structure to the east support structure of this foundation pit is 5.7 meters. The west and north sides of the foundation pit are adjacent to a river channel, and within the river channel area to the north there is a 22-meter-wide box culvert. On the south side, the eastern part is 20 meters from the Seamen's Hotel, a 7-story building founded on Φ340 hammered cast-in-place piles; the two parts on the south side are 20 meters from Geshan Building No. 1, a 7-story building also founded on Φ340 hammered cast-in-place piles.

3. The geological conditions of the project, from top to bottom, are: fill layer, 0.7–3.6 m thick; muddy soil layer, 0.5–2.9 m thick; fine sand layer, encountered in some boreholes, 0.5–1.3 m thick; strongly weathered mudstone, with a top burial depth of 2.8–5.7 m and a thickness of 0.3 m; moderately weathered mudstone, with a burial depth of 3.6–7.2 m and a thickness of 1.5–16.7 m; slightly weathered rock, with a burial depth of 6.0–20.2 m and a thickness of 1.8–12.84 m.

II. Overview of Foundation Pit Support

1. On the east side of the foundation pit, the eastern 34 meters of the south side, and the eastern 30 meters of the north side, the upper 5.2 meters adopted a shotcrete and anchor support scheme, while the lower part adopted a scheme of excavated piles combined with steel pipe internal bracing. The bottom elevation of the excavated piles was ▽-20.0 m.

2. On the west side of the foundation pit, the upper part adopted a scheme of excavated piles combined with prestressed anchor cables, and the lower part adopted a shotcrete and anchor support scheme.

3. The remaining parts of the south and north sides of the foundation pit adopted a shotcrete and anchor support scheme. Later, due to adjustment of the ±0.00 elevation, the actual excavation depth of the foundation pit was adjusted to 15.3 meters.

4. Construction of this foundation pit began on October 31, 2002, and by July 2003 it had reached the design depth of 15.3 meters. Later, due to readjustment of the superstructure, the basement was changed from the original 4 stories to 5 stories, and the basement excavation depth increased from the original design of 15.3 meters to 19.6 meters. Because the perimeter ground beam of the basement is 0.7 meters high, the actual excavation depth of the foundation pit was 20.3 meters, which is 0.3 meters deeper than the bottom of the originally designed excavated piles.

5. After the new foundation pit design scheme was determined, construction restarted in November 2004 from the bottom of the 4-story basement pit down to the 5-story basement. By the morning of July 21, 2005, the maximum displacement of the pile-plus-steel-bracing portion on the eastern part of the south side of the foundation pit was about 4.0 cm, of which 1.8 cm occurred in the single day from July 20 to July 21. The maximum displacement of the shotcrete and anchor support portion in the middle of the south side of the foundation pit was about 15 cm.

III. Course of the Accident

The foundation pit at Zhuhai City Plaza collapsed at about 12:20 noon on July 21, 2005. According to relevant personnel of the owner, at about 9:00 a.m. on July 21, the Seamen's Hotel reported that a crack at the foot of the wall on the side facing the foundation pit had widened by about 2 cm overnight. The owner's relevant personnel immediately contacted the designer and the person in charge of the construction unit.

At about 10:30, the relevant personnel arrived at the site. Looking at the Seamen's Hotel, they indeed found a new 1.8 cm crack along more than 30 meters of the wall foot on the side facing the foundation pit. Looking inside the hotel, they found that the wall cracks had widened. After inspecting the hotel, the persons in charge from the owner, the construction unit, and the design unit went down into the foundation pit to continue checking whether there were any abnormalities in the pit. After descending into the pit, they found that at the junction between the manually excavated piles and the shotcrete and anchor surface on the south side of the foundation pit, in the third retaining pile from west to east, a vertical crack had appeared in the pile body about 1 m above the pile bottom.

By about 12:00 noon, the owner's quality and safety officer and the person in charge of the construction unit heard "ba" "ba" sounds at the bottom of the foundation pit on the south side. Initially there were about 2–3 sounds per minute; after 5 minutes, the "ba" "ba" sounds became denser and denser. The person in charge of the construction unit said: "That's the sound of anchor cable wedge plates failing; the foundation pit is going; run!" By the time the personnel at the bottom of the pit ran to the north side of the pit and before they could reach the stairs, the foundation pit collapsed.

Because the people in the temporary buildings at the southwest corner of the foundation pit could not escape in time, when the pit collapsed, 5 people were injured and 6 were buried. Of these, 3 were rescued by firefighters, and the other 3 unfortunately died. One hour before the collapse, the maximum displacement measured by the construction unit for the retaining pile plus steel pipe bracing portion was 4 cm. The monitoring unit had measured a maximum displacement of nearly 15 cm in the shotcrete and anchor support portion on the south side of the foundation pit two days before the collapse.

IV. Accident Consequences and Impact

V. Emergency Rescue Measures and Process

1. The mayor activated the municipal government's emergency plan for handling major safety accidents. A temporary emergency rescue command headquarters was formed, headed by the mayor and composed of the vice mayor, the government secretary-general, and leaders of the municipal construction commission, to organize and direct the rescue. At the same time, the Guangzhou Construction Science and Technology Commission organized and established an emergency rescue expert group to assist the rescue command headquarters.

2. All personnel of the Guangzhou Municipal Construction Commission went to the site and worked in day and night shifts to assist in the rescue. External office services at the construction commission office building were suspended for nearly a week.

3. Guangzhou Metro Corporation, Guangzhou Construction Group Co., Ltd., Guangzhou Municipal Gardening Bureau, Guangzhou Fire Bureau, Health Bureau, Epidemic Prevention Bureau, Haizhu District Government, Haizhu District Public Security Sub-bureau, street police stations, and other government departments dispatched nearly 1,000 people per day for the rescue.

4. The keys to the rescue were: first, ensuring the safety of the subway; second, ensuring the safety of nearby residential buildings.

5. Rescue methods:

① Multiple concrete pump trucks with the longest booms in Guangzhou's engineering community, up to 48 meters, were used to pump concrete for counter-pressure at locations adjacent to the subway.

② For the pile foundations of the dormitory building left hanging after the slide, ground pumps were used to pump concrete to protect the pile toes.

③ To prevent further sliding of the slip surface under rainwater erosion, ground pumps were used to pump concrete to protect the surface.

④ After resolving the above problems, the foundation pit was treated by backfilling.

⑤ To ensure the safety of subsequently rebuilding the residential roads on the slip surface, grouting treatment was carried out on the slip surface.

6. The oceanarium, one span of which had already collapsed, was demolished by blasting.

VI. Accident Losses

In terms of direct economic losses, the foundation pit collapse accident at Haizhu City Plaza exceeded 200 million yuan.

These include:

◆ Foundation pit and earthwork construction costs, and the already constructed basement base slab and one basement level;

◆ Losses from the collapsed Seamen's Hotel and related materials;

◆ Property losses of nearly 200 businesses in the annex building of the Seamen's Hotel;

◆ The large amount of materials, manpower, and equipment invested in the emergency rescue process.

During the accident, nearly 590 people were temporarily relocated; 56 households in Building No. 1 were moved out for nearly a month; compensation was paid for the three deceased; and the developer's early-stage project approval, design, and supervision fees were also involved. The land value loss from the site's change from planned commercial land to green space after the accident is also included. Direct economic losses can be calculated clearly, but the indirect losses and impacts on society, the government, and the industry are difficult to estimate.

VII. Analysis of Accident Causes

1. Over-excavation: the original design was a 4-story basement with a 17-meter pit, but it was later excavated as a 5-story basement pit (20.3 meters), turning the excavated piles into suspended-toe piles.

2. Over-time: the service life of the foundation pit support structure was one year, but in fact nearly three years had passed from excavation to the accident.

3. Overloading: dump trucks, cranes, and excavators on the slope crest were overloaded.

4. Geological causes: the rock surface was relatively shallow but inclined. The design unit still used Lizheng software to review and design the original foundation pit design scheme, while ignoring the actual situation revealed during excavation that the rock surface inclined from south to north at an angle of about 25°.

In addition, during construction, after the inclined rock surface was found and the displacement on the south side became relatively large, prestressed anchor cables were used to reinforce some areas. The reinforcement range was only 20–30 meters on the southwest side of the south part, which was too small. The owner believed that the reinforcement was caused by insufficient design and that the design unit should pay the reinforcement cost, so the design unit was under considerable pressure.

VIII. Lessons for the Geotechnical Profession

Geotechnical Investigation

1. Ensure the completeness and accuracy of geological data.

2. Ensure the accuracy and completeness of surrounding environment data.

Geotechnical Design

1. The design scheme must be targeted.

2. Design of the foundation pit monitoring scheme: the design of the monitoring scheme should comprehensively consider the allowable values and settlement of the surrounding environment, the type of support structure, and the safety grade of the foundation pit.

3. The importance of dynamic design.

Geotechnical Construction

1. Whether the surrounding environment data are consistent with the design drawings.

2. Whether the geological data referenced in the design are consistent with the geological data revealed by actual excavation.

3. Whether the construction quality of the support structure can meet the design requirements.

4. Informatized construction.