Dissertation for Master Degree (M.Sc)
Soil and Foundation Engineering Orientation
Summer 1391
Chapter One
Introduction of micropiles
1-1 Micropiles
In general, in dealing with problematic soils such as loose soils with low load capacity, high settlement, fluidity and Manual soils, there are two ways ahead of geotechnical engineers:
A: Using load-bearing elements in the soil
B: Improving and modifying the physical-mechanical properties of the soil mass
Each of the above solutions have their own methods and specifications that have been developed over many years. Some innovative techniques are a combination of the above two categories and have the advantages of both categories to some extent. Among those categories, we can mention the use of micropiles with cement grout injection.
Micropiles refer to piles with a small diameter (less than 300 mm) which are often accompanied by light steel reinforcement and cement grout injection. In addition to acting as a load-bearing and settlement-resistant element, micropile also improves the mechanical characteristics (strength and behavior) of the surrounding soil due to the injection of cement slurry. The history of the invention of the small pile goes back to the early 1950s, when Europe was faced with a huge number of buildings that were subject to destruction due to the damage caused in the Second World War. In this period, it was very necessary to invent a bed improvement method that, in addition to efficiency and applicability among destroyed buildings, is also fast and economical. In such a situation, the invention of the micropile was done by Fondedile, a famous Italian contractor, and due to its unique features, this method was widely spread. At the beginning, the use of micropiles was only considered in the improvement of weak foundations of buildings, but gradually with the development and implementation of the method in different countries, the scope of its application was extended to other fields of geotechnical engineering, such as stabilizing slopes and dealing with liquefaction.
Figure 1-1 shows the proposed arrangement of micropiles to strengthen the existing structure. Also, in Figure 1-2, the use of a network of vertical and inclined piles to strengthen the existing structure is presented.
Figure 1-2 The use of a network of vertical and inclined piles to strengthen the existing structure
1-2 Review of studies
Until 1993, there was no comprehensive study on micro-piles. In 1993, the Federal Highway Administration (FHWA) considered a group for research and study on micropiles. The research group for this study included contractors, consulting engineers, students and employers. Evidence obtained from this study " Drilling and micro-injection of piles "
The collection includes a comprehensive international review and review, a review of the details of the research carried out and the development and expansion of the results of field and laboratory tests, design methods, identification of construction methods, construction observations and review of case studies.
By conducting the above studies, a guide reference for the design and construction of micropiles was prepared and adjusted by FHWA. The basic introduction of micropiles can be attributed to an article in the Journal of Civil Engineering published by the American Society of Civil Engineers in December 1995. This article that " An introduction to microcandles. It was named and it discussed the basic behavior of micropiles, their limitations, classification in terms of application and description of the market for the use of micropiles. 1-3 cases of using micropiles in geotechnical engineering. Use in foundation bed of structures" and "correction and improvement in place of soil" May be conducting extensive research in scientific societies– The world's expertise in micropiles and their static, dynamic and seismic behavior, as well as the definition of many researches and university theses and the presentation of various articles in international conferences about the behavior and design criteria of micropiles in different soils, It also shows the increasing acceptance of the engineering community to use this technique. Currently, the guidelines provided by (U.S.FHWA) are acceptable reference and used by designers and contractors of micropile implementation.The types of application of micropiles can be categorized as follows: A: soil modification and improvement 1- Stabilization of slopes 2- Construction of guard walls 3- Coping with liquefaction 4- Increasing the strength of the soil mass with special purposes such as tunneling 5- Chemical protection of buried parts of structures
B : Foundations of new and existing structures
B-1 – New structures
1- Reduction of settlement
2- Increasing compressive load
3- Provision of tensile load
4- Increasing lateral load
B-2- Existing structures
1- Controlling settlement of piles
2- Controlling pile load
3- Repair or replacement of pile
4- Control of foundation washing
5- Seismic strengthening of foundation
The following figures They show examples of the use of small candles in different cases.
1-4 Introduction of micro-piles
In general, micro-piles are divided into two groups in terms of construction and execution:
1- Impacted micro-piles (replacement)
2- In-situ micro-piles (replacement)
Impacted micro-piles are members that, when they are driven into the ground and move, move the surrounding soil laterally during installation, and in-situ micro-piles in holes that They are already dug, laid, or built in the place of holes, so the excavated soil is moved.
Pile pile refers to piles with a small diameter (less than 300 mm) that are often accompanied by light steel reinforcement and injection of cement slurry, which is usually reinforced. The steps of micropile implementation include drilling a well or borehole, placing reinforcing materials (reinforcement) and injecting into the borehole. Figure 1-4 shows the steps of a micropile.
For a better description of a micropile, the following can be mentioned:
* Micropiles can withstand vertical loads or lateral loads, they can also be used instead of a normal micropile, or they can be used as a member of the soil mass and micropile, according to the design idea.
* Micropiles are implemented by methods that cause the least damage to the structures and surrounding soil. During the installation of micropiles, there is minimal shaking, vibration, and noise, and they can be implemented in situations with a short distance from each other. Also, they can be used in different situations (at any angle to the horizon).
* Due to the small diameter of the pile, the bearing capacities of their tips are generally ignored. has Injection transfers the incoming load by friction from the reinforcing materials to the ground in the connection area (injection band) of the micropiles.
* Special drilling equipment is often required for the installation of micropiles in soft ground.
1-5 classification systems of micropiles
Classification of micropiles is one of the important things in their development and expansion, and this class It resolves engineering disputes about the design and behavior of components similar to micropiles and the use of common tools and materials and construction methods. In 1997, a comprehensive classification of micropiles was presented by the American Highway Administration. This classification system is based on two criteria: 1- Philosophy of design and performance 2- Construction and injection methods The first criterion describes the design method of micropiles and the second criterion describes different injection methods and the adhesion capacity between soil and materials, which is usually the most It has a structural control over the capacity of micropiles. 1-5-1 classification of micropiles based on the design philosophy and performance. Designing a micropiles or a group of micropiles is very different from designing a network of micropiles. This brings us to the definition of the micro-candle type 1 (CASE1) (Figure 5-1). These micropiles are loaded directly and are reinforced micropiles that can withstand large loads.