Archive for the Category ◊ Soil Mechanics ◊

Author:
• Friday, December 17th, 2010

It is not unusual to turn on the TV and hear someone talking about sustainability. It reminds one of a time when the mobile phone was just introduced onto the market and companies and business-men started purchasing one and adding a mobile number to their business cards to promote their work.

With the environment being the hottest topic today; global warming, CO2 emission, Ozone Depletion and so on and so forth, what could work better than a term that sounds intelligent and has the flexibility of being interpreted to anything GOOD one can imagine. It fills the gaps and adds flavour to any task and has the required politic to make it easier to get an approval stamp.

This might not be the most promoting introduction to what follows but do not get it wrong!

When a body claims sustainability for its development, design or product, it inevitably brings challenge, innovation and most importantly expectation to its field. Fairly, if it was just a false claim then the minimum this field could lose is its reputation and possibly for a very long time.

But how could sustainability be applied to geotechnics, if at all? Well the answer is; sustainability simply means doing things in the best possible way and in the most responsible manner, and hence it could be implemented in anything really by anyone such as geotechnical engineers.

Sustainability in geotechnics could be categorised into four main stages; Design period, Construction time, Life time and End of service.

Design Period

The most important and effective of all in the application of sustainability in geotechnics, certainly falls into the design stages. Geotechnical team’s involvement in the decision making, in the early stages of the design of a project, is not only remarkable to fulfil a more sustainable result but is also crucial.

Geotechnical engineers could give their opinions to the architects or structural engineers, in the brainstorming stages of a project, on what type of foundations are the most efficient application in the location, on which a project is due. This will give the architects a base on which direction that their design should follow. It is perfectly understandable, especially in this period of time, that we as a human race value designs that are more innovatively sustainable, than they are awe-inspiring and appreciate the content and material of the design, as equally as its appearance, if not more.

Geotechnical engineers are sometimes given the problem and are left to deal with it. When involved in the project, their hands are more tied up on affecting the decisions, in comparison to other disciplines, while they could play a significant role in achieving a sustainable development within a project. They could even ask for relocation of a project, however extreme this may sound, if it could lead to a more sustainable approach.

Another example, when there is the possibility of using soils or relying on natural ground to carry out the imposed loads, a geotechnical engineer should act innovatively; rather than designing earth retaining structures that both: consume materials and produce waste. In highways, it is often seen that when there is an issue of land purchasing, the owner prefers to take the minimum land and hence rely on the earth-retaining structures or bridges rather than wider embankments. Embankments are a more sustainable solution, especially as they are made of natural materials, soils, and even have the possibility of being planted over (plants on the embankment add to its stability as well as its green values).

No matter how persuasive and perfect these have sounded, the real picture is different and more complicated. Ground engineering’s number one struggle is UNCERTAINTY. Unfortunately, geotechnical engineers are asked to design for many uncertain conditions. The owner, no matter how surprising this may sound, does not pay enough for the designers to carry out the adequate site investigation. This leads the designer to many predictions that often results in a conservative and sometimes over-design solution.

This is a well known issue in the field, to the extent that, it has even been recognised in the new Eurocode Design Manual (EC7), by putting a minimum limit to the number of boreholes, with a specified depth, for different types of projects, to minimise the power of the owners decision on how much site investigation they should invest on. It is not the optimum solution, but could be the start of recognising  the importance of site investigation, in achieving a safer and more sustainable design. If the designers are more certain about the parameters they have, then they will concentrate their effort on designing innovatively and more environmentally friendly, rather than being forced to consider lots of estimations and focus on how to deal with problems that might not even exist.

Construction Time

At this stage, the design has almost finished, and it is time for it to be implemented. Contractors are the main bodies to assure the constructability of the finalised design and their early involvement with the designers, plays a significant role.  A design could be absolutely green but for it to be constructed, it will possibly consume more energy and pollute more of the environment compared to the lifetime of another design, that does not possibly look as green.

Contractors are responsible to do the right thing, planned by the designers, in the right way. Increase the efficiency by acting responsibly, for example: reuse materials when possible. Although they are required to fulfill  minimum standards in order for them to be granted a contract, they should take a step forward to carry out their work more sustainably and especially when it comes to soils.

Life Time

When a project is finished, most of what a geotechnical engineer has designed, will be buried underground and unless the structure fails or is subjected to maintenance, it will possibly stay invisible for very long time. A sustainable structure under-ground, will have the same effect on the environment and the biodiversity as if it was not there, or to be optimistic – it promotes green values.

It is indeed possible to promote biodiversity even when building projects, such as infrastructures. For example; when building an embankment. Constructing a very small embankment, called beetle-bank, alongside nearby, using the excessive unwanted soil would be an excellent habitat for various insects and spiders to live in; or as mentioned earlier, the stability of the embankment could be a great excuse for the designer to include plants over the embankment.

End of service

It is clearly not sustainable if the removal of a green structure leaves contamination or adverse environmental effects behind. Neither is it, if the degrading of the foundations or disposing them, produce waste and pollution. A trustable disposal and waste management, is crucial to eliminate the impacts on the environment. By making the appropriate decisions in implementing the tasks, which could sometimes even lead to a greener solution of the underground structures not being touched or moved, mainly when removal of the base structure will lead to transition of contamination to neighbouring lands.

Conclusion

A short conclusion to this article is that geotechnical engineers can also participate in the sustainability of a design, no less than other disciplines. Design stage is the most important stage of enhancing sustainability, if all the stages have been thought through and planned accordingly. However, planning and designing the right things is not enough and it is left to the contractors to do things right decided on the right things to do. The same is correct for the waste and disposal managers that handle the structure after its lifetime service is over.

If you found this article of interest, or wish to discuss any part of it in more detail, please do not hesitate to comment.

Author:
• Monday, May 10th, 2010

Just like other construction materials soils has its own scientific analysis with regards to its abilities on dealing with forces. Being the oldest construction and probably engineering material soil is one of the most complex fields in civil engineering to the point that when it comes to the factor of safety in design whatever has direct contact with soils, e.g. foundations, or soil based constructions, e.g. embankments, requires a significantly higher safety factor compare with other construction materials, i.e. the uncertainty in soil analysis and design is higher. These is most likely resulted from the way soil originates.

Usage of soil as the main element of construction goes back to the first civilization when Sumerian built Ur, first city in the history, on south of Mesopotamia near the mouth of Euphrates River. They used bricks to build their first houses and earlier they built embankments and dams to direct the water for irrigation. The Western history of recognition the soil as a main element goes back to Romans, in the first century B.C., when their engineers used the trial and error experiences to construct foundations.

After all today soil and rock are still one of the most important materials used in construction. It is used or on its natural state or with improvements, such as compaction, reinforcement and etc., as the main component such as in dams, embankments and highways or as supporter element in every construction, i.e. foundation support.



What is soil?

All soils originate directly or indirectly from rocks and these are classified according to their mode of formation.  By a combination of physical and chemical processes rock masses are reduced to particles ranging in size down to 0.001 mm.  Soils result when collections of these particles are re-deposited, often in bodies of water, and are compressed and consolidated by further depositions above.

The nature of the subsequent soil depends not only on its parent rocks, but also on the processes and conditions of disintegration, transport and deposition – and on time.   The properties of clay minerals are important, in particular their very flaky nature.

Understanding the formation and nature of soils is an important precursor to understanding their engineering properties and their behavior under load.

Soils are, in the main, naturally occurring materials.  Engineers and builders who use soils have to take them as they find them; soils cannot be manufactured to order in the way of other materials, such as steel and concrete.  Soils are also highly variable and complex materials, possessing engineering properties that may have a wide range of possible values. Thus, at the start of any design process soils must be accurately and systematically described; classification is part of description.  The main components of soil description are:

• The nature of the soil:                shape, size and distribution of particles

• The state of the soil:                 density, relative density, water content

• The fabric of the soil:                  homogeneity or layer sequences, cementing

Physical Properties of Soils:

The basic physical properties of soils are those required to define its physical state.  The three constituent phases (solid, liquid and gas) must be quantified and relationships between them expressed in numerical terms to enable changes in physical state be measured.  A soil model is used in which the solid phases (rock or mineral particles) has one unit of volume;  the volumes and masses of the water and content are then related to this unit solid volume.  Density or unit weight and water content are important measures of physical state.

Soil Mechanics:

Soil Mechanics is the numerical science using principles of engineering mechanics such as fluid mechanics or mechanics of materials that was first used by Coulomb, 1773, a member of French Royal Engineers, to solve soil problems. It studies and defines mainly terms such as, shear strength, permeability, angle of friction, Critical state, effective stress, consolidation, slope stability, earth pressure and etc. that the website will focus on their definitions as well as their applications on today’s civil engineering.