Foundation Improvement of the Deep Soil Layer
Takashi TAKENAKA,
Water Research Co., Ltd.
1． Introduction
These days, economic development asks us to construct large-scale buildings and social infrastructures in the areas with weak foundation such as coastal areas and swamps along rivers due to the limited availability of land with solid foundation. As an example in Japan, the fourth runway of Haneda airport officially called the Tokyo International Airport is under construction as part of its extension in the Tokyo bay off the estuary of the Tamagawa River.
It is not reasonable for the foundation improvement of structures to apply the conventional method, i.e. replacement of weak soil layers by excavation, in terms of costs as well as realistic issues of construction, i.e. under-water construction in case of the extension of the Haneda airport.
Considering the above situation, new technologies called Earth Drilling Method (refer to Attachment-1) are widely applied to improve the weak foundation in Japan. This is a brief paper to introduce the Earth Drilling Method.
2. Characteristics of Earth Drilling Method
As seen in Attaachment-1, there are several ways for the Earth Drilling Method, of which we herein discuss the method to apply the drilling machine, which has a function to inject soil hardening agent, of which there are two types of powder and slurry, at the same time. That is, the drilling machine digs a hole by agitating soils and injects soil hardening agent to mix with agitated soils at the same time. After reaching to the necessary depth, the drilling machine is pulled out by rotating the blade of drilling machine reverse. This reverse rotation of drilling machine acts as a compressor to consolidate the agitated soil mixed with the soil hardening agent.
After drilling holes with an interval of 2 to 5 m to the longitudinal and horizontal directions (making holes in the mesh form) by the machine with the function to inject soil hardening agent, the soil layers between holes are received the compression of horizontal direction as well as de-watering, resulting in that the deep layer of all the designated area becomes the solid foundation. In short, the Earth Drilling Method combined with the function to inject Soil Hardening Agent can be applied to improve the deep layer of all the designated area with the weak foundation where excavation is not substantially applicable in terms of costs and technologies.
3. Earth Drilling Machines
The drilling machines are composed of a blade to dig a hole, shaft to rotate by motors and compressor to inject the soil hardening agent, and are normally mounted on the truck or bulldozer to facilitate the mobilization (Figure-1). In works under water, the drilling machines are loaded on ship.
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Figure 1 Drilling Machine with
a Function to Inject Soil Hardening Agent |
The length of blade is adjusted to the size of cobbles and stones to encounter during drilling. For example, in case where stones with a diameter of 30cm are expected to meet during drilling, the diameter of blade should be more or less 100cm. When the drilling machine is pulled out, the angle of blaze is changed. It is normal to have a double shaft in the drilling machine of Japan so that two holes can be dilled at the same time.
4. Soil Hardening Agent
Soil hardening agent is classified into two types; slurry type (a sort of liquid to resolve soil hardening agent in water) and powder type. Cement, cement-based agent and special agent belong to the former type, and quick lime, cement and cement-based agent lie in the latter. It is to be noted that there might be a chance that injection of slurry -type agent continues unlimitedly due to high permeability in the weak foundation.
It is easy to acquire cement and quick lime in the market as common materials, which however have characteristics with less hardening effects as well as easiness for weathering. Furthermore, voluminous amount is necessary to apply these materials. Considering the characteristics of cement and quick lime mentioned above as well as durability (e.g. 50 years), a soil hardening agent called Stein with the following characteristics is widely applied in Japan as one of cement-based agents:
(1) Stein can be used in the both forms of slurry (M type of Stein) and powder (R type of Stein), depending on the type of works.
(2) It is possible to gain the high bearing capacity of as high as 70kg/cm2.
(3) Permeability can be drastically improved.
(4) Nursing time period to gain the necessary bearing capacity is short.
Further discussions on Stein are referred to Attachment-2.
5. Summary
The Earth Drilling Method to apply Stein as a soil hardening agent has merits as follows:
(1) A deep soil layer can be improved without excavation.
(2) A deep soil layer has enough bearing capacity to support heavy structures even like dam including related concrete structure such as spillway.
(3) It is possible to drill a tunnel such as waterway and diversion tunnel in the improved soil layer.
(4) Construction time and cost can be saved.
It is highly recommended that the Earth Drilling Method to apply Stein be tested to improve the foundation of such infrastructures such as Ta Trach dam, river dykes and sea dykes, which have deep and weak foundation. |