Testing deep level excavation and operation 30 Jan 2020

As underground space becomes more important for increasing urban capacity, enhancing disaster prevention, reducing traffic congestion, and for improving public infrastructure services, underground infrastructure is developing rapidly and to deeper levels. In China, the large conurbation of Shanghai is leading the way for development of deep underground space as the utilisation of the medium and shallow underground levels become saturated.

Taking excavation below the crowded near and middle levels

Underground development in Shanghai to date has been mainly within 40m deep, but this is likely to go deeper and become more complex in future, particularly for expansion of the city’s metro system, which has exhausted shallower levels, and also for deep drainage systems, as well as road and rail systems. To develop and utilise deep underground space scientifically, safely and sustainably, Shanghai has embarked on establishing an underground engineering test site and research laboratory.

Shanghai is rich in the experience of developing and utilising middle and shallow underground space and its theoretical research is also mature.

Some techniques used in middle and shallow underground space construction may reflect better adaptability in the development and utilisation of deep underground space. For example, slurry shield excavation in deep sand layers will have advantages over EPBMs in the stability control of the excavation face. The difficulty and risk of excavating ultra-deep shafts is greater than that of the traditional method of foundation pit construction. However, even though some of the existing technical methods can provide a variety of options for the development of deep underground space, their adaptability in deep underground space still needs to be studied further. Finally, with the increase in the depth of development and utilisation, the accuracy and adaptability of existing theories, equipment and materials are yet to be verified, such as the distribution law of deep water and soil pressures. These issues and difficulties in theory and technology remain to be solved in Shanghai.

Tangqiao experimental shield tunnel

Shanghai is built typically on complex soft soils in which the utilisation of deep underground space is difficult and of high risk. The proposed increase in the depth would lead to new problems for existing methods. Just a few examples of the challenges presented include:

• Shield and soil deformation issues;
• Difficult replacement of TBM tools under high soil and water pressure;
• Increased control of dewatering;
• A lack of research on methods of strengthening a horizontal foundation.

It is therefore imperative that research on new technology, new materials and new equipment for risk control is carried out and that corresponding technical standards are established.

With the construction of a deep water-storage tunnel under the Suzhou River in Shanghai and the rail transit in the central city area, the development and utilisation of deep underground space has entered its initial stage and has demonstrated that the demand for new technology, new craftsmanship, new materials and new equipment is urgent. This has prompted the proposal of building an experimental base for the development of ultra deep underground space in Shanghai. Through the field test of underground engineering, existing technology and theory can be verified and the study of new technologies can be put to use, providing a solid assurance for a sustainable use of the deep underground space.

Importance of experimental underground laboratories

In terms of constructing an underground engineering testing field, the rigorous and scientific style of test, to verify and then extend to large-scale, is certainly of merit.

In the 1960s, the late Chinese academician Lin Jianhang led a team to establish a test base in Pudong Tangqiao. They carried out research on shield tunnel lining structures, waterproofing, reinforced concrete shields, key construction technology and other topics. It was excavation of a 68m long x 4.2m o.d. shield driven test tunnel that not only verified the feasibility of shield construction beneath Shanghai, but also broke the prophecy of foreign experts that the soft soil foundation of Shanghai was not suitable for building subway tunnels, and also played a great role in the process of large-scale subway construction in Shanghai.

Planning the deep level test field

lt is proposed that the excavation depth of the new soft soil deep underground engineering test field is 100m, the range of the test field is about 2km, and the nature of the land above the test site is farmland.

The functions of the engineering test field facility include, but are not limited to:

• Tests of vertical excavation technology and equipment;
• Foundation treatment technology;
• Lateral excavation technology and equipment tests;
• Studies of the waterproofing and durability of materials and structures;
• Deep underground science testing the acquisition geotechnical parameters, deep animal and plant growth tests, study of the impact of underground engineering construction on groundwater quality, and short distance vacuum magnetic levitation test;
• Deep underground education base for experimental research by colleges and universities, and field teaching; and
• Create an underground engineering science education base with museums and exhibition halls.

Design of the proposed new deep level soft soil engineering test field

Subsequent uses of the test field include:

• As a large underground flood control facility and flood control storage reservoir facility;
• As a large underground garbage disposal centre for household garbage and construction waste;
• As a large underground storage centre for long term strategic material and cold goods;
• As a large underground recreation facility for concert halls, swimming pools, and amusement parks;
• As a deep underground scientific test platform for deep animal and plant growth tests, and short distance vacuum magnetic suspension tests
• As an underground cemetery.

With many possibilities, research on the future use of the facility should be integrated with its design and site selection.

Construction principles of the deep level test field

Proposed funding sources for the laboratory

The Shanghai engineering test field will be the first interdisciplinary comprehensive test site to be built in deep soft soil layers in the world. All current underground engineering test grounds are located in rock. With the quaternary bedrock under Shanghai at 200m to 300m deep, the test site in Shanghai will fill the gap in deep soft soil engineering in the world. lt will provide an ideal experimental environment for various disciplines and will contribute to the development of Shanghai into being a global innovation center for science and technology.

Development of the facility is based on the land being provided by central government and its operation being funded by local and central government, industry associations, enterprise investments and fees for using the facility both as a test site and under its future possible uses.

With a continuous improvement and effective operation of the deep underground engineering test field, the comprehensive base can be used as an important science education base, a new technology and official data house, and even a new tourism landmark. Plans for the facility continue to be developed towards being given approval for its start of construction.