Deep Excavations in Soil

John Endicott

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000077094
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Transportation & Navigation
• Index: Technology & Engineering

The book describes the theory and current practices for design of earth lateral support for deep excavations in soil. It addresses basic principles of soil mechanics and explains how these principles are embodied in design methods including hand calculations. It then introduces the use of numerical methods including the fundamental "beam on springs" models, and then more sophisticated computer programmes which can model soil as a continuum in two or three dimensions. Constitutive relationships are introduced that are in use for representing the behaviour of soil including a strain hardening model, and a Cam Clay model including groundwater flow and coupled consolidation.

These methods are illustrated by reference to practical applications and case histories from the author's direct experience, and some of the pitfalls that can occur are discussed. Theory and design are strongly tied to construction practice, with emphasis on monitoring the retaining structures and movement of surrounding ground and structures, in the context of safety and the Observational Method. Examples are presented for conventional "Bottom-up" and "Top-down" sequences, along with hybrid sequences giving tips on how to optimise the design and effect economies of cost and time for construction. It is written for practising geotechnical, civil and structural engineers, and especially for senior and MSc students.

Chapter 1

What are deep excavations?

To an average person, deep excavation might conjure up thoughts of massive deep open cast gold mines. These can take dozens of years to excavate with more than a million tonnes of earth removed in one day. The largest of these goes down 700m [1], deep enough to accommodate a 230-floor tower without appearing above the original ground level, nearly as big as Burj Khalifa (829.8m) and more than Tokyo Skytree (634m) and Shanghai Tower (632m). Such deep excavations for mining rock ore are generally in undeveloped terrain. Engineering for deep open cast mines requires substantial input from rock mechanics engineers. Deep excavations in soil are generally not as deep as open cast mines and necessitate much more gentle slopes or retaining walls to hold up the sides.

Deep excavations in soil are generally carried out to enable the building of underground structures. One definition of deep excavations is based on the premise that if an excavation were to collapse, the consequences would be serious, such as causing a fatality or fatalities if people were within or adjacent to the excavation. Building Construction Regulations [2] in Hong Kong require a qualified engineer to design lateral ground support for excavations deeper than 1.2m. Soil is heavy. One cubic metre of soil that could collapse from the side of a pit 1.2m deep weighs 1.5 to 2.0 tonnes; it would seriously damage the legs of anybody standing there. Deeper excavations are potentially of even more serious concern. Excavations for six levels of a basement can exceed 18m in depth; the collapse of such an excavation would drastically undermine the surrounding area. For this reason, deep excavations in soil generally require lateral support to the surrounding ground to prevent caving-in and to limit ground movement to acceptably small amounts.

Deep excavations in soil to enable the construction of underground structures are complicated due to the variable types of ground and groundwater that they may encounter. In urban areas the surrounding land must be fully supported, and nearby structures have to be protected, sometimes by limiting ground movement and adjacent building movement to within a few millimetres. Occasionally, ancient monuments or trees with preservation orders must be protected. The sides of a deep excavation need robust Earth Lateral Support, often comprising reinforced concrete retaining walls braced by steel or concrete struts that sometimes, depending on the loads, can be quite heavy. A range of professional input is required at all stages of a deep excavation project. Professional input is required for developing concepts, for planning, for geotechnical studies, for structural design, for programming, for management, for site supervision, for certification of payment and for the resolution of any claims arising from the excavation and construction.

Successful completion of a deep excavation requires several different skills, such as planning, design, permitting, contracts, supervision, completion, payment and resolution of disputes and, if it is to be done well, a capacity for lateral thinking. Probably the man in the street does not realise just how much work is involved in excavating tens of thousands or hundreds of thousands of cubic metres of soil. This book aims to cover the whole process for people to flip through to get a taste for the subject and then to re-read to appreciate some of the world’s engineering marvels below the ground. For professional ground engineers, there is a trend towards more and more specialisation. Professional training schemes require chartered engineers to have knowledge about the whole process of civil engineering projects and not to focus on only one part, such as the cutting and pasting of soil parameters into a computer program. Whatever the specialisation, engineers who work on deep excavations and engineers for other associated civil engineering works need to have an appreciation of all of the other tasks that are required for the project and be able to exhibit the independent judgement necessary to know when and who to ask for assistance when required.

Often, construction sites are in urban areas. For reasons of safety, the sites are generally surrounded by a protective hoarding. Projecting above hoarding, intriguing 30m tall construction equipment can often be seen. Gateways in hoarding provide access to sites, and traffic controllers are stationed to admit heavy trucks with equipment or materials and trucks laden with excavated soil to leave the site dripping water after being hosed down to clean the wheels. A glimpse through a site entrance, or through a window in hoarding, can reveal excavators at work, massive shoring being erected between deep walls, various stages of partial construction of the permanent structure and in some cases substantial shoring to protect adjacent ground and buildings from movement.

Deep excavations in urban areas are adopted for multi-level basements for buildings and for underground road, underground rail and drainage infrastructure. Extensive works are particularly required for underground railways. A common layout for an underground railway station is to provide several pedestrian subway entrances down to a public area concourse level with ticketing and access through to paid areas and via escalators and staircases to platforms and rail tracks below. Excavation for a new two-floor underground station is about 15m to 18m deep to allow for a 1m- to 2m-thick base slab beneath the tracks, headroom of say 5m to 7m above the tracks for the railway system, 1.5m overhead ducting, a normal public area headroom of about 3m at the concourse level, a substantial thickness, say 1m, of roof structure and finally 2m or more space for buried pipelines and cable utilities before reaching finished ground level. Stations with cross-platform interchanges require two levels for platforms and are deeper by 5m to 7m. Where several railway lines cross each other, excavations go even deeper. For example, at Admiralty Station, Hong Kong, when constructing the South Island Line and extension works to the existing station, the excavations reached 45m below ground and included excavation beneath existing railway tunnels operating at shallower depths [3].

Railway tunnels between stations are sometimes deeper than the stations. Energy-saving tracks have downhill slopes assisting trains to accelerate on departing from stations and with uphill slopes to assist approaching trains to slow down and stop at the stations. These slopes located to either end of stations were called energy humps. They are no longer so common because the electrical power system for trains can incorporate regenerative breaking, as used on hybrid or electric cars whereby electrical braking puts energy back into the grid system. Between underground stations, shafts are sometimes required such as for ventilation, for drainage or for means of emergency escape. Whereas stations are typically 15m to 25m deep, some tunnels between stations can be much deeper.

With urban growth, more and more deep excavations are being built in urban areas. Many cities, especially in China, India and South East Asia, are getting much bigger, land above ground is getting more and more congested and land for construction is getting very rare. More utilities are needed for the burgeoning populations. In the past, utilities such as cables were slung between poles above ground in an unsightly manner and drainage was often in open channels. Excavating deeply below ground for new utilities is often necessary because of the congestion at the surface and going below ground with unsightly infrastructure is therefore sensible. With high costs for land in urban areas, basements are becoming more and more cost-effective and are also being constructed to greater depth to build more levels of floors. For example, in Jurong New Town, Singapore, new commercial basements extend to six floors below ground level. These include car parking and supermarkets where one can take a trolley loaded with purchases from a shop to the car at 15m or more below ground in air-conditioned comfort. Subways connect from one building to another obviating the need to cross busy roads in the hot open air. Direct connections from basements to underground railway stations have become popular in Hong Kong since 1978, when it was realised that entrances to underground railway stations would have large pedestrian traffic; basements that were connected to underground railway stations became prime sites for retail shops.

In many cities, underground development offers protection from extreme weather with cooling in tropical locations and warmth in cold places such as Helsinki, Montreal and other cities that are frozen in winter. Expansion of cities and buoyant economies means that the citizens expect an improved way of life and an improved living environment. With land at the surface already intensively developed and therefore in short supply, going below ground is a means of developing infrastructure that improves overall city life. Construction below ground can provide infrastructure that otherwise would take up space at the surface, in some cases releasing space that was previously occupied. In congested cities, open space is a much-valued commodity. Underground railways, which were started more than 150 years ago, reduce pedestrian traffic. If Hong Kong, Singapore and other cities had not developed underground railways since the 1970s, the streets would have become more than stagnant with twice as many vehicles. Bangkok has opened some underground lines, and where there used to be unpleasant areas with almost stationary traffic and smoke from two-stroke “Tuk-tuk” engines, there are now attractive city districts, and it is pleasant to walk by day or by night.

Mobility of pedestrians is key to life in a city. Many cities have developed, or are developing, extensive underground pedestrian subways, concourses and malls. These provide needed space for people to do activities and provide protection from the weather and, in the case of Singapore, civil defence. In busy streets, cycling can be a nightmare. Tunnels for cyclists are planned beneath Cambridge U.K., and cyclists in Paris have found routes through basements and subways. In addition to car parking and shopping, there are leisure activities below ground such as cinemas, ice rinks, swimming pools and bowling alleys. In fact, a whole host of activities can go below ground if the demand is there.

References

[1]    Twin Creeks gold mine in Nevada, United States. https://en.wikipedia.org/wiki/Open-pit_mining#/media/File:Twincreeksblast.jpg

[2]    Building Construction Regulations Cap 135, HKSAR Government. www.elegislation.gov.hk/hk/cap123B?xpid=ID_1438402645257_002

[3]    Bezzano, M., Smith, S., Yiu, J. & Wiltshire, M. Case Study: Design and Construction Challenges for Admiralty Station Expansion. Proceedings of the HKIE Geotechnical Division Annual Seminar, 2017. www.hkieged.org/download/as/as2017.pdf

Chapter 2

How deep excavations are created

Deep excavations are expensive works of civil engineering. They usually come about under a civil engineering contract let to a construction company after planning and design by engineers. The parties that are involved are a developer, usually referred to in contracts as “The Employer,” a designer who is usually referred to as “The Engineer,” who might also be entrusted to manage the contract, and a construction company who is referred to as “The Contractor.” For clarity, I will refer to “an Engineer,” using the upper case, when describing the role of an engineer under a contract, and I will refer to “engineers,” using the lower case, generally. The contractual role of an engineer and his team for a deep excavation is comprehensive from the start, when planning the excavation, until the satisfactory completion of the works and completion of the contract. In summary, the role of an engineer is to arrange for the site investigation, design and construction to be carried out, to write contract documents for the work and to arrange tendering for a construction contract to be awarded. During construction, an engineer ensures that the construction is carried out properly and that the completed excavation is handed over to the developer of another engineer or a contractor for subsequent work on the site. An engineer is usually responsible to ensure that any claims arising from the works are resolved and to certify payments.

Aspiring young engineers are often attracted to one specific the stage of the process. One engineer might want to be a designer; another might want to be a builder or a manager. Nowadays, deep excavations are generally large enough and of such complexity that there is room and a need for a team of engineers with the necessary range of qualifications and experience. However, to be the Engineer, the person and team leader who is ultimately responsible for a deep excavation, one should know about every task that is required. A leader does not have to be a specialist in all aspects himself. He (to be read as singular person throughout) needs to make sure that all the aspects are dealt with to the required standards and they need to be able to recognise when specialist advice is needed. As a member of a team, in order to fulfil one’s role, one needs to understand the respective roles of the other members of the team and how the whole process fits together.

2.1 Types of earth lateral support

Fundamentally, for deep excavations, the surrounding ground should remain stable. Movements of the sides of the excavation must be carefully controlled otherwise nearby property could move or be damaged. Generally, the adjoining ground needs to be fully supported with an Earth Lateral Support (ELS) system during the excavation process. An ELS system generally comprises a robust wall around the perimeter of the excavation and shoring to prevent the wall from moving more than a small amount. Where space is available, and the ground is strong enough, sometimes side slopes can be formed and an ELS system might not be required. For deep excavations in soil that is not self-supporting and where space is limited, an ELS system is definitely required. Especially below the ground water level and in weak soils, the ground has to be supported. Below the water table, the soils may wash into the excavation and weak soils could slump as a mass. Buildings Construction Regulations in Hong Kong[1] require that excavations greater than 2.5m deep shall be provided with adequate ELS. Failure of a 2.5m deep cutting would be potentially fatal or of serious consequence for people working in the excavation. Excavations that are considerably deeper than 2.5m require ELS all the way down. As soon as excavation reaches 2.5m depth, or even before, ELS shall be provided. ELS can be provided sequentially as the ground is excavated, by installing timbering or sprayed concrete to prevent the exposed face of the soil from ravelling, and overall support to the ground can be provided by installing props, in the case of narrow trenched excavations or by soil nails or tie-back anchors in the surrounding ground. However, these methods are not suitable for very weak soils or below-the-ground water tables where the base of the excavation could be unstable. Therefore, generally an ELS system is provided by installing robust walls from the surface before any significant excavating has taken place, and then bracing is installed as the excavation advances stage by stage.

Several techniques have been developed to install earth retaining walls from the surface before excavating the bulk of the soil from the site. The five common methods are driven steel sheet piling, king posts and lagging, contiguous bored pile walls, secant piled walls and diaphragm walls. In some countries where labour is inexpensive, piles can be excavated by hand and filled with concrete, with or without steel reinforcement, and are called hand-du...