Some features of large diameter cast-in-place concrete pipe pile technology in strengthening weak ground

Posted date 24/08/2016

3.969 view

Posted date 24/08/2016

3.969 view

Nguyen Thi Ngan - Lecturer of Faculty of Construction and Engineering

1. Problem statement :
In recent years, the process of industrialization and modernization has been taking place strongly combined with the progress of science and technology, the construction industry of our country has had rapid developments. Along with that development, the construction industry also sets out urgent requirements, in which overcoming construction incidents caused by weak ground is a matter of concern.
In our country today, the main solutions for strengthening weak soil foundations are: Pre-loading consolidation with sand or vacuum combined with wicks; Cement-soil pillars; Sand piles (sand wells); Prestressed reinforced concrete piles... However, each of these solutions has its own advantages and disadvantages.
This article will introduce a method of strengthening weak soil foundations that is quite new in Vietnam. That is the method of large diameter concrete pipe piles cast in place (abbreviated as PCC piles). This is a method researched and developed by the Ho Hai Geotechnical Research Institute, China (authors: Liu, Fei, Ma, and Gao, 2001-2002) and has been effectively and widely applied in this country. The research and successful application of PCC pile technology will provide a new solution in the construction of weak soil foundations in our country.

2. Working principle of PCC pile
Large diameter cast-in-place concrete pipe piles are piles formed after the construction of lowering the double casing pipe into the weak ground to the design depth by vibration method, the casing pipe head is arranged with a protective cap to prevent soil from entering the casing pipe during lowering. The process of pouring concrete into the hollow wall to form the pile is carried out simultaneously with the vibration and pulling up of the casing pipe. The vibration force of the pile lowering machine has the effect of compacting the pile concrete to ensure the quality of the pile and also compacting the soil around the pile.

Figure 1: Geometric dimensions of PCC piles

Figure 2: PCC pile head after construction at the site

With conventional piles, the load-bearing capacity of the pile is mobilized from the friction resistance of the lateral surfaces. But with PCC piles, the characteristic is that they are concrete pipe piles and have a soil core inside, so when bearing a load, both the internal and external friction resistance of the pile wall will be mobilized, leading to a large load-bearing capacity of the pile. Moreover, when the pile is lowered to the design depth, frictional force will form between the soil and the pile, the process of both vibrating and pulling up the casing of the concrete machine is compacted, the large concrete pressure will make the concrete distribute more widely around, so the wall thickness can be expanded more than the design, the frictional force of the wall also increases. However, to ensure safety for the design, the calculation often ignores the internal frictional resistance of the pile.

Figure 3: Description of the formation of pile friction resistance

Thus, compared to solid piles with the same cross-sectional area, tubular concrete piles have greater wall resistance; and when compared to solid piles with the same load-bearing capacity, the volume of concrete used for hollow piles is much more economical.

Figure 4: Wall resistance of solid piles and hollow piles

3. Advantages and disadvantages of PCC pile method
With existing research and applications, it is initially shown that the construction solution for foundation treatment using PCC piles has some main advantages and disadvantages as follows:
About the advantages:
- Large load capacity, high overall stability;
- Simple construction technology;
- Large diameter piles can be used;
- Pile quality as well as quality control are highly reliable;
- Competitive price;
- Fast construction time;
- The residual settlement of the foundation after treatment with PCC piles is very small.
About the disadvantages:
- With very weak geology with a lot of clay, pile quality control is relatively complicated, and the amount of concrete loss will be large;
- In case there is a thick layer of sand or obstacles, the vibration of the pile will be difficult and additional solutions must be found to vibrate the pile;
- The quality of pile construction for some complex geological types depends a lot on construction experience.

4. Applications in Vietnam:
In Vietnam, PCC pile technology has recently been applied to a number of projects, bringing certain economic and technical efficiency. However, there are still some limitations because this is a new technology, with few experienced workers and mainly experience gained during the construction process. Some projects in Vietnam have applied PCC piles to treat weak ground such as:
- Ca Mau fertilizer factory warehouse expansion project, weak ground with thickness of 17-18m, treated with PCC piles with outer diameter of 1200mm, wall thickness of 120mm and pile length of 22.5m, concrete pile grade 250.
- In Hanoi: The entrance road and the train testing track of the first station of the Cat Linh - Ha Dong urban railway construction project; PCC piles to handle weak ground here are designed with a diameter of 1000mm, pile wall thickness of 120mm, pile length depending on each area ranges from 18-22m.

Figure 5: PCC piles for the Cat Linh – Ha Dong urban railway project

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