Pipe piles

Using pipe piles in thick soft soil areas—the result was a tragedy! The foundation of the main building in a residential community in Fujian subsided, with tilt exceeding limits, causing cracks in the main structure.

Owner's report of the situation

In ** Mansion Community in Fuzhou, Area A is resettlement housing and Area B is commercial housing, with the basements of Areas A and B interconnected. Currently, buildings A12, A13, and A8 in Area A surrounding Area B are severely tilted and have been sealed off, and the community gate is severely tilted and has been demolished; the three buildings in Area B are still accelerating in settlement. Due to uneven settlement of the main structure (pile foundation), the main structure of the Area B basement has tilted, with through shear cracks in beams and through cracks such as crushing and shear cracks in columns, which has also caused misalignment and deformation of gas pipelines. At the same time, the structural appraisal reports for Buildings A12 and A13 issued in February 2022 were both Grade D.

Expert panel opinions

    1. The survey report for Building A12# recommended the use of bored (drilled) cast-in-place piles, while the construction drawing documents used high-strength prestressed concrete pipe piles.

    2. The survey report recommended that if high-strength prestressed pipe piles are used, the soft soil layer needs to be hardened, but no relevant content is seen in the construction drawing documents.

    3. According to the survey report ⑤-1, the soil layers above the sand require consideration of negative skin friction, but the influence of negative skin friction was not fully considered when arranging piles for the main building walls and columns around the basement side walls.

    4. Some buildings in this project used high-strength prestressed concrete pipe piles, failing to comply with the provisions on pile foundation selection based on soft soil layer thickness in Article 2, Paragraph (5) of the Several Provisions on Building Structure Design of Fujian Province.

    5. The in-line pile arrangement under the shear walls of Building A13# requires high construction precision and is difficult to control in actual construction.

    6. In the static load test report of the trial piles before pile foundation construction (Report No.: YZJJ1800285) selected for spot check, the settlement data of Pile A13-01# is questionable.

The design institute that selected pipe piles replied to the expert panel's opinions, and the overall idea of the reply was that there was no problem with the design. Ultimately, because the construction unit cut corners, it was fined 13.6943 million yuan. Since there was no problem with the design and the cause of the accident was also the construction unit's cutting corners, why did the expert panel unanimously believe that there were problems with the design institute's drawings?

Design institute's reply

Reply regarding the 'Expert Opinions from the Design Document Analysis Meeting on Quality Issues of Some Buildings in Yunjin Mansion (Sanjiang Lixiang City)'

Fuzhou Municipal Urban and Rural Construction Bureau:

In accordance with your bureau's 'Expert Opinions from the Design Document Analysis Meeting on Quality Issues of Some Buildings in Yunjin Mansion (Sanjiang Lixiang City)', our company replies to the expert panel's opinions as follows:

1. The survey report for Building A12# recommended the use of bored (drilled) cast-in-place piles, while the construction drawing documents used high-strength prestressed concrete pipe piles (PHC500-125AB);

Reply: The survey report for Building A12# revealed that the silt thickness at the bottom of the pile caps at the 7 borehole locations of the main building was ZK75--3.4 m thick, ZK76--5.2 m thick, ZK77--6.2 m thick, ZK78--6 m thick, ZK79--5.7 m thick, ZK80--miscellaneous fill at the pile cap bottom, and ZK81--10.7 m thick (average thickness 6.2 m); around the main building, ZK178 (Section 15, 6.4 m thick) and ZK190 (Section 28, 4.3 m thick). At ZK81, the earthwork excavation at the pile cap bottom was 1.3 m, and there was no silt at the pile cap bottom. Therefore, according to the Several Provisions on Building Structure Design of Fujian Province, the use of high-strength prestressed concrete pipe piles is permitted and also complies with the provisions of current relevant codes.

2. The survey report recommended that if high-strength prestressed concrete pipe piles are used, the soft soil layer needs to be hardened, but no relevant content is seen in the construction drawing documents;

Reply: The survey report's recommendation to 'harden the soft soil layers present at the foundation pile caps and basement when prestressed pipe piles are used' was to meet the requirements of relevant codes and documents. However, the buildings in this project using prestressed pipe piles can already meet the requirements of relevant codes and documents without hardening treatment.

3. According to the survey report ⑤-1, all soil layers above the sand require consideration of negative skin friction, but the influence of negative skin friction was not fully considered when arranging piles for the main building walls and columns around the basement side walls;

Reply: The influence of negative skin friction was already considered when arranging piles for the main building walls and columns around the basement side walls. Under the dead + live load condition, the vertical reaction of a single pile + pile negative skin friction ≤ 2500 kN; at Axis (8-1) of Building A8#, the maximum vertical reaction of a single pile under the DL condition is 1918 kN, and the negative friction of the pile group calculated according to the geological profile at this location is 250 kN; the sum of the two is 2168 kN < 2500 kN; at Axis (8-1) of Building A12#, the maximum vertical reaction of a single pile under the DL condition is 1918 kN, and the negative friction of the pile group calculated according to the geological profile at this location is 250 kN; the sum of the two is 2168 kN < 2500 kN; the calculation sheet for pile negative skin friction is detailed in Attachment 1.

4. Some buildings in this project used high-strength prestressed concrete pipe piles, failing to comply with the provisions on pile foundation selection based on soft soil layer thickness in Article 2, Paragraph (5) of the Several Provisions on Building Structure Design of Fujian Province;

Reply: Upon review, although the silt (with sand) soil layer at Borehole ZK81 of Building A12# slightly exceeds the requirements of Article 2, Paragraph (5) of the Several Provisions on Building Structure Design of Fujian Province, the average thickness of the silt layer at the pile cap bottom of Building A12# is 6.2 m, which is less than the requirement of less than 8.0 in Article 2, Paragraph (5) of the Several Provisions on Building Structure Design of Fujian Province. In addition, the survey report mentions that the water content of Soil Layer ③ at Borehole ZK77 is 43.4% (only slightly greater than the liquid limit of 37.7%), and the structural height of Building A12# is 61.15 m (only 1.15 m higher than the building height specified in Article 2, Paragraph (5) of the Several Provisions on Building Structure Design of Fujian Province); in summary of various factors, Building A12# uses high-strength prestressed concrete pipe piles (PHC500-125AB).

5. The 'in-line' pile arrangement under the shear walls of Building A13# requires high construction precision and is difficult to control in actual construction;

Reply: The 'in-line' pile arrangement under shear walls allows the vertical force of the shear walls to be transmitted more directly to the foundation, and is the most reasonable pile arrangement method, widely used in a large number of practical projects. During the pile foundation construction of Building A13# and the subsequent basement construction of pile caps, base slabs, etc., our company did not receive any feedback that the pile foundation of Building A13# had deviations exceeding the code limits.

6. In the static load test report of the trial piles before pile foundation construction (Report No.: YZJJ1800285) selected for spot check, the settlement data of Pile A13-01# is questionable.

Reply: There is no such static load test report in the materials submitted by our company, and our company has never received this document.