Problems of spun concrete piles constructed in soft soil in HCMC and Mekong delta – Vietnam

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The use of spun concrete piles under geological layer that has the thick soft soil layer reveals many flaws, many incidents of tilting pile or broken pile have occurred during construction process in many projects, cause great damage to the cost of the project as well as the slow construction progress. In this paper, the author analyzes the causes inclined piles during construction of spun concrete piles, as well as provide the method to determine the inclination of the pile along the pile body.

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Nội dung Text: Problems of spun concrete piles constructed in soft soil in HCMC and Mekong delta – Vietnam

PROBLEMS OF SPUN CONCRETE PILES CONSTRUCTED IN SOFT SOIL IN HCMC AND MEKONG DELTA – VIETNAM Nguyen Thanh Trung Civil Engineering Faculty, Ho Chi Minh City University of Technology (HUTECH) ABSTRACT In recent years, the use of spun concrete piles for foundation of high buildings in HCMC and provinces in Mekong Delta is popular. Because of the advantage that the length of each segments of pile is longer, the number of joints is reduced, as well as load bearing capacity under material strength is greater and total cost is reduced for the Client. However, the use of spun concrete piles under geological layer that has the thick soft soil layer reveals many flaws, many incidents of tilting pile or broken pile have occurred during construction process in many projects, cause great damage to the cost of the project as well as the slow construction progress. In this paper, the author analyzes the causes inclined piles during construction of spun concrete piles, as well as provide the method to determine the inclination of the pile along the pile body. Keywords: Incident, inclinometer, soft soil, spun concrete pile, tripot. 1. INCIDENT OF PILE INCLINATION Figure 1. Photos of pile tilt at project in District 2, HCMC During the construction process, in order to shorten the construction period, contractor often uses Robot compressor, which is 600 tons and 900 tons of Robot weight and compressing load. During the pile construction process, the robot will apply the load and move above between the pile positions. With such heavy loads, Robot even moves on the thick sand layer around 1.5m to 2m, there is still able to create the sliding surface, causing the tilt of the previous pre-constructed piles. 472
Figure 2. The appearance of sliding surface during pressing Apart from the case of the piles are inclined due to the appearance of sliding surface during the pile pressing construction, there are also some reasons cause the pile inclination as follows: – Piles are inclined during deflected pile pressing construction. – Piles are inclined due to the sliding surface appearance during the pile driving machine (robot) is moving. – Previous pressed pile is inclined due to the nearby pile construction that leads to the appearance of horizontal force effects to the previous pressed pile. – Piles are inclined due to the excavation of basement 2. METHOD OF SURVEYING OF PILE INCLINATION Upon detecting potential inclined piles, aim to assess the inclination of the piles as well as the load bearing capacity of piles, here the author applies the method to measure inclination along the pile body by using inclinometer equipment. The operation of the inclinometer equipment is to measure the horizontal displacement of pile for each surveying position throughout the depth of pile under the gravitational direction. The inclinometer equipment will be sliding into the grooves of the inclinometer tube that is inserted in the body of the pile in advance. The tube is fixed by tripod system to ensure the tube position is always in the centre of the spun concrete piles. Horizontal displacement measuring equipment (Inclinometer tube) is installed inside spun RC piles after finishing construction of piles. Horizontal displacement measuring tube is composed of dedicated plastic tubes interconnect with each other, the measuring tube has a length depends on the depth of spun reinforced concrete piles. Measuring tubes are made of composite plastic reinforced fiber glass, have high elasticity and strength (ABS tubes). Measuring tubes outer diameter is 70mm. Inside the tube, there are 4 different perpendicular grooves. Each 3m tubes is connected by joints system. The tubes are placed inside the pile cage, relying on tripod 3 feet, with wheels touching the pile„s wall aim to keep the tube in the middle of the pile. Only install measuring tubes at the measured piles. 473
Figure 3. Installing of tripod system Measurement principles of inclinometer method and preparing for testing 3. COMMON TYPES OF INCLINED PILES After analyzing the results of inclination measuring, there are generally two types of inclination form of the pile as following: Figure 5. Inclination - Axis A Inclination - Axis B Figure 6. Inclination - Axis A Inclination - Axis B Inclination - Axis AD Inclination - Axis AD Type 1: Even inclined piles: Pile head are inclined Type 2: Pile is broken at joint position or in a compared to the initial pile position due to Robot‟s position adjoining the weak geology and the good weight. This is the most popular type of inclined geology, or due to the previous pressed pile piles. segments are obliqued, the following pile segments is corrected to straighten during pressing. 474
4. ANALYSIS OF CAUSES Apart from the case of the piles are inclined due to the appearance of sliding surface during the pile pressing construction. In this part, anaysis cause of problem is: – Piles are inclined due to the sliding surface appearance during the pile driving machine (robot) is moving. Using Plaxis software version 8.5 to modelling the interaction between the soil and the prestress concete piles when pressing machine (Robot) and moving of the robot with a great pressing load (42 kN/m2). Table 1. Characteractic of soil Modulus Modulus Modulus Depth  c  Layer Description Model E50ref Eoedref Eurref 3 2 (0) (m) (kN/m ) (kN/m ) (KN/m2) (KN/m2) (KN/m2) Hardening 0 Filling 1.5 10000 10000 30000 18 1 27 Soil Model Very soft, Hardening 1 Organic silt 16.3 2890 2890 8940 14.9 10 1.5 Soil Model (OH) Hardening 2 Plastic clay 24 9192 9192 27850 18.8 19.4 21.5 Soil Model Sandy, Hardening 3 20 11500 11500 34500 20 11 30.5 Silt Soil Model We will compare the tilt of pile between monitoring by Inclinometer and modelling by Plaxis result as follow: Figure 8. Monitoring and modeling Figure 7. Modelling by Plaxis software inclination by Plaxis Result 475
According modeling by plaxis result and the monitoring by Inclinometer result, the tilt of pile between 2 methods have little diferences. 5. PROBLEM SOLVING To avoid the possibility of sliding surface appearance during pile pressing leading to the pile inclination, apply the following measures: – Increase the sand layer thickness to reduce the pressure to the soft soil layer when the pressing machine. – Provide the proper moving measures for the robot to avoiding the impact on the piles were pressed earlier. – The concrete surface layer is thick enough to ensure stiffness to distribute load of robot down to sand padding underneath. Figure 9: Thick concrete surface layer to ensure the stiffness Figure 11. Monitoring and modeling Figure 10. Modelling by Plaxis software after solving inclination by Plaxis Result after solving 476
After we have 2 method to overcoming this problem, by modeling by Plaxis, we have displacement of the pile that reduced to 9.4cm. 6. CONCLUSION The use of spun concrete piles for foundation high buildings in the area with thick soft soil layer in HCMC and the Mekong Delta, may occur incidents during pile construction. There are many reasons that can cause inclined piles, but the most common cause is the sliding surface appearance during pressing piles by pressing machine (Robot) and moving of the robot with a great pressing load. The application of the inclination measurements presented above to determine the inclination of the pile when there are incidents of piles or doubt is essential to assess the remaining load capacity of the pile to make a proper measures for reinforcing for the foundation of projects are incidents. REFERENCES [1] Introduction Inclinometer Geokon. 477