Estimating surface settlement induced by underbore or tunnel construction

It is often necessary to estimate the potential ground surface settlement caused by underground infrastructure projects involving tunnels and underbores. Such settlement assessments are used to determine if any additional protection works are necessary particularly if underbores are tunnels are to be constructed underneath roads, railways or buildings. Finite element analysis programs such as Plaxis 2D/3D, Optum G2/G3 or FLAC are typically used to model settlements in such instances. Accurate information on ground properties, tunnel parameters and loading conditions is required to provide accurate settlement assessments. 

In situations where a quick estimate with minimum data inputs is required, a common semi-empirical method developed by Peck (1969) is also commonly used. This method is based on field observations made by Peck, and the ground settlement trough profile is approximated by a Gaussian distribution curve. The volume loss in the tunnel (overbreak or annular collapse) is equated to the area under the Gaussian curve from which a settlement profile is generated. The width of the settlement trough varies between soil types and is controlled by a parameter (Kg) that is specified for different soil types and strengths. I developed a web application (https://underbore-settlement.anvil.app/), also embedded below, to estimate settlements based on the Gaussian curve method developed by Peck.

 

It should be noted that since this method does not consider any volume change in soils (consolidation or dilation), it is valid only as an initial estimate under short-term conditions. 

The figure below shows results from the above method compared to the results from a simple Plaxis 2D model. A tunnel with 1m diameter and 2m of cover subject to a volume loss of 10% bored through undrained soft clay and loose sand was modelled separately in Plaxis 2D to compare against the results using Peck's method with recommended numbers for Kg, for clay (0.5) and sand (0.3) respectively.

Comparison between Plaxis output and Gaussian curve method by Peck (1969)

It can be seen that the results from Plaxis and Gaussian curve method are similar for sand, but varies slightly for clay. Using a Kg value of 0.7 for clay leads to a curve very similar to the Plaxis 2D output. The choice of Kg for various soil types with different strengths is a subject of research, and available literature suggests that a Kg value of 0.4-0.7 is appropriate for soft clays. However, with an understanding of the limitations of the Gaussian curve method, it can be used as a rough initial estimate of settlements before embarking on detailed finite element analysis. The web application linked above will be useful for such quick assessments. 

Mining for Petroleum

Although Mining Engineering and Petroleum Engineering are two different fields of study, the two fields cross paths when it comes to the Oil Sands Industry. Oil Sands refers to the deposits of petroleum in the form of heavy bitumen being present in a mixture of sand or loosely consolidated sandstone. Because this heavy oil is too thick to flow and because it is thoroughly mixed with sand, the conventional methods of extraction of petroleum by drilling cannot be applied here. For this reason and also if the oil sand deposit lies relatively close to the surface, open pit mining is used to extract the oil sands. Once mined, the sands are processed to produce synthetic oil.

The mining for oil sands begins like in most mines by the overburden removal process. Common machinery such as excavators, bucket scrapers, and bulldozers are used to remove the overburden and the overburden is hauled by trucks and piled separately for refilling purposes. Once the overburden is removed and the oil sand layer exposed, the mining process begins. Since the oil sand deposits are unconsolidated, they can be easily be extracted by shovels or excavators. For this reason and for obvious safety reasons blasting is not performed. In addition to these machines, draglines and sometimes continuous excavators such as bucketwheel excavators are also used for mining the oil sands. The mined sand is hauled to a plant where the petroleum is extracted and refined by a variety of processes including crushing, froth flotation and distillation.

Although previously termed as an unconventional source of petroleum, the depletion of "conventional oil" and the associated rise in oil price has resulted in a shift of attention towards the oil sands industry. The largest oil sands deposits are located in Canada and Venezuela and the total volume of oil contained in the sands exceeds the conventional oil reserves of the world.  Oil sands have been commercially extracted in the Athabasca region in Canada since the 1930s and since have developed along with the advances in extraction technology. The following graph shows the value of sales  of oil sands and conventional crude oil in Canada and it can be seen that the oil sands sales have shown a significant rise during recent years.

value of sales of crude oil and oil sands (data taken from : http://www.capp.ca/library/statistics/Pages/default.aspx)

New methods have been developed to extract oil sands deposits that are too deep to extract using surface mining. These methods involve the reduction of the viscosity of the bitumen and the separation from the sand and enables the oil to be pumped to the surface. These methods are termed "In-situ methods". Cyclic Steam Stimulation (CSS), Steam Assisted Gravity Drainage (SAGD) and Toe to Heal Air Injection (THAI) are some of these methods. With advances in in-situ methods like these, the the recovery percentages of oil sands deposits have increased and now it can be considered as a viable alternative to conventional oil.

Limestone Mining in Pictures

In this post I have attempted to summarize the limestone mining process in pictorial form (and a video). Short descriptions have been included where necessary.

Overburden Removal

once the overburden is removed, the beds are cleaned and are prepared for drilling

Drilling

drilling is performed according to a pre determined pattern to accomodate explosives for blasting

Blasting

Rock Blasting is performed to fragment and loosen the consolidated limestone.

Loading and Hauling

Dispatching 

Tyre Maintenance of Mine Machinery

The performance of machinery and their availability plays a major role in the productivity and output of a mine. For this reason, every aspect of the machines should be properly maintained. The tyres of these machines are what keeps the machine in contact with the ground and bears the weight of the machine. While tyres for mining machinery are designed to withstand a beating, incorrect maintenance can reduce lifetime of the tyres, or ultimately lead to catastrophic accidents. Therefore, the proper maintenance of the tyres is as important as the maintenance of any other component of the machine.

Handling large tyres of the gigantic mine machinery is not an easy task. Even a simple task such as changing the tyre is difficult and results in a considerable amount of downtime. Therefore the best strategy in this case is preventive maintenance. This means checking tyre inflation daily, ensuring that valve caps are on, checking tread depth and removing stones and other debris lodged between the treads.

changing tyres is a difficult task

When it comes to daily maintenance, the first thing that comes to mind is the inflation pressure. Both over-inflation and under-inflation are detrimental to the tyre, and therefore, it must be ensured that the inflation pressure lies within the accepted limits. It should be noted that when running the machine the tyre pressure increases by about 10-20 percent. Therefore, this must be taken in to account when setting inflation pressure bounds. Tyres kept in good condition with the proper inflation will last longer and perform better.

External conditions of the mine also affect the tires. The road condition for example is what determines how fast the tyres will wear out. While it is not practical to maintain spotless roads, roads should be kept clear of scattered rocks and sharp pits as driving over these can cause a crack in the rubber, and subsequent running will cause it to propagate and destroy the tyre. Therefore clearing fallen debris off the roads and maintaining the road surface is also a part of tire maintenance. Wet rubber cuts better. therefore it is best to avoid wet road conditions. While most haul roads in mines are watered regularly to minimize dust, it must be ensured that the roads don't get too wet.

Bad road conditions can cause cracks to develop

The handling of the machines and their usage are two other areas that need to be considered. Driving too fast, especially in the case of dump trucks can cause several types of tire damage. In the case of heavy machinery used in mines, a combined parameter combining both speed and weight is assigned to tyres. This parameter is called the TKPH rating (Ton Kilometer per Hour). The TKPH rating should be considered when selecting and using tyres. When loading dump trucks, they should be loaded symmetrically so that the load is distributed over tyres equally. asymmetric loading can cause uneven wear and later result in more severe damages. It should also be noted that shear forces generated when steering the trucks while stationary at the loading points could cause ruptures in the tyres.

When dealing with damaged tyres, it is always best to attend to it them at the start of the problem. Neglecting small faults like cracks and running with them can cause these small faults to quickly propagate and result in serious and irreversible damage. When a crack appears it can be removed altogether by cutting around it. This however has to be done with proper equipment and knowledge of the tyre. When replacing tyres, it is always best to replace it with a tyre of similar amount of wear.

In summary, daily maintenance goes a long way in increasing the lifetime of the tyre. Inflation pressure must always be within the specified limits. The road surfaces, loading and dumping points should be free of fallen debris and excessive water, because wet rubber gets cut better. payload weight and hauling speed must be controlled. The TKPH rating assigned to the tyre must be adhered to. Proper handling of the machinery such as symmetric loading, avoiding steering while stationary and avoiding excessive tyre spinning will reduce the likelihood of damages. when repairing, the first stages of the problem is always easy to cure.

Optimization of production of a Mine

The main objective of a mine is to extract a mineral deposit in an economical manner and to maintain a continuous output. In order to do this, the operations performed in and out of the mine must complement each other. While intensive planing and monitoring of all unit activities is required to achieve this, the following steps will help to optimize production and reduce cost thereby increasing efficiency.

Calculate and plan production targets and compare with actual production.

In order to determine whether a mine is operating efficiently, first targets should be established. To do this, drilling and blasting cycles and hauling capabilities can be factored in initial calculations. To set more specific production targets, modelling software can be used. The important thing is to compare the actual data with these targets and to take corrective action when the production does not meet the target.

Maintain a buffer stock of material

While in normal conditions, production can be carried out without interruption, due to various reasons the rate of production can decrease or in worst cases, halt altogether. Interruptions due to bad whether in open pit mines is a good example for this. To cope up with these delays it is essential to have a stockpile of material ready to be dispatched. This stockpile will act as a buffer until production resumes and the mine will produce the expected output throughout.

Proactive maintenance of machinery

Several approaches can be taken in the case of maintenance of mine machinery. in order to minimize losses due to downtime and faulty operation it is best to employ a preventive or proactive maintenance strategy. When doing maintenance work, it is vital to get operator feedback as it will help to diagnose problems better. Another very important fact to be considered when acquiring a new fleet of machines to a mine is to obtain all the machines of one particular brand. This ensures inter machine compatibility. It also saves time and resources in ordering, acquiring and using of spare parts.

Streamline work and physical movement

A mine is a place of continuous movement. To increase efficiency, these movements must be streamlined and effective. This means that access paths and haulage ways must be as short as possible with sufficient room for maneuvering. The mine layout itself must be orderly and separate operations should not disturb each other(except in unavoidable circumstances such as blasting). These reduce cycle times of the mine and increase production.

Safety

This is a factor overlooked in most mines. However safety plays a major role in the overall efficiency of the mine. A mine with a lot of accidents suffers a lot of downtime and it will reduce production. Proper safety procedures will enable continuous operation and will also enhance working conditions, both of which will ultimately result in an increase in efficiency.

Asteroids as a Resource

Recently, the company planetary resources unveiled their plan to mine asteroids. Their approach to this extremely challenging task by trying to get there in 4 different stages, seems to be a practical. They are planning to mine asteroids for platinum group metals and water. With all this attention to mining from outer space it is worthwhile to consider whether asteroids actually have the potential to be a resource that is economical to exploit.

There is no denying that asteroids contain valuable metals and water in abundance. But these are even found on earth. At the moment there is no shortage of platinum here on earth although it is quite expensive due to the costly extraction process. Unless platinum occurs as pure metal on asteroids, processing will still have to be done, and doing it in outer space is going to be even more difficult.

Looking at this situation it might seem a better option to invest on the existing platinum mining and processing here on earth in order to make it more efficient. However mining from asteroids has the advantage of avoiding the environmental damage created by mining on earth. Also if the platinum demand rises sharply within the next few decades and if it becomes increasingly difficult and costly to mine for platinum on earth, mining it from asteroids will be the best solution.

Although lots of minerals and metals are found in the crust of the earth, most of the heavier metals such as iron and nickel lie inside the core of the earth due to the gradual gravitational sinking of these metals during the formation of the earth. Asteroids however are mostly of small sizes and therefore in most cases the metals are somewhat evenly distributed throughout the volume of the asteroid. This makes mining easier. Even in larger asteroids where metals have settled like on earth, accessing it will be easier.

The other important resource that is planned to mine form asteroids is water. A source of water outside earth is valuable because it would no longer be necessary to carry a heavy load of water in future space missions. It would also pave the way to colonization in space since one of the main challenges faced in such a situation is finding access to an appreciable reserve of water.

If the constituents of water, Hydrogen and Oxygen are separated, they can be used to power rockets. This could also one day lead to fuel stations in space which will not only reduce the cost of space travel but also widen our reach in space. Having both fuel and water readily available in space, our dream to colonize in space will come closer to us.

Mining asteroids seems a far fetched idea and it might not look like it is economical at a first glance. But this situation will change in the future and sometimes asteroid mining might be the only option. We will be able to advance only if we plan for the future and act accordingly. We should remember that before the internal combustion engine was invented, commercial drilling of petroleum seemed unnecessary..

image from : wikipedia

Mining Asteroids

Industries based on space exploration have received a lot of attention lately. Companies are already in the process of introducing commercial spaceflights to the general public. Among these new "space age" industries, is a new start up called Planetary Resources that focuses on mining Asteroids. Although it seems like a far fetched idea, the recently founded company Planetary Resources claim that they have a plan. They also have got some serious investors like Larry Page, co founder of Google to back them.

Asteroids are found in abundance in space and contain vast quantities of valuable minerals and metals. Asteroids also contain large amounts of water in them. Being much smaller than planets, the minerals in  an asteroid are distributed evenly throughout the entire body of the asteroid, unlike in planets where the large gravitation causes the heavier minerals to sink towards the core of the planet. This makes it easier to mine because deep excavation will not be necessary to access the resources of an asteroid.

Although the idea of mining asteroids for useful resources has been around for quite some time, this is the first time that it is actually being attempted. According to Planetary Resources, the final goal of the company which is mining asteroids commercially will take about a decade to accomplish. Despite this fact the company is set to generate revenue during the process, even before the actual mining takes takes place. In fact, one of its co-founders has claimed that Planetary resources is already a "positive cash-flow company".

Planetary resources plans to mine asteroids in 4 stages. The first stage is the "Arkyd Series100" in which a set of space telescopes known as "Leo space telescopes" will be launched. These telescopes which will be in low earth orbit and will be used to gather data about asteroids. The telescopes can also be pointed at other objects in space such as the moon  and planets as well as the earth itself. This capability gives the company the opportunity to rent out these telescopes to universities and other private and government organizations for research purposes. This is one avenue the company aims to profit in.

The second stage is the "Arkyd Series 200" also known as the interceptor in which further developed telescopes will be stationed in a higher orbit to obtain better access to asteroids and to gather data. These telescopes will also be equipped with micro propulsion mechanisms enabling them to move into different positions.

The third stage - "Arkyd Series 300" AKA Rendezvous Prospector will incorporate swarms of spacecraft capable of communicating with each other to move on to more distant asteroids and obtain specialized data regarding constituents, size shape, rotation etc while orbiting the asteroid. The technology developed during this stage is expected to generate revenue for the company by providing means of inter-planetary space flight to interested parties such as NASA and other agencies.

The fourth and final Stage is the Asteroid Mining itself. Mining will also be done using swarming robots with the ability to communicate and coordinate like in the previous stage. Several technological challenges will have to be addressed in this stage. For example whether the the processing of mined material is done on site or brought to earth before being processed. Both these methods have their advantages and disadvantages depending on the mineral to gangue ratio in the mined ore. Furthermore asteroids being situated at quite a distance from the sun, exploiting solar energy would be difficult. This creates the need for improvement of existing solar technology or the use of some other energy source. In addition to the technological issues, the legal matters pertaining to mining from outer space will have to be considered.

Planetary Resources plans mainly to extract platinum group metals and water from asteroids. Extracting water from asteroids can be very useful in future manned spaceflights and space stations because taking water from earth will not be necessary. Also the chemical constituents of water, Hydrogen and Oxygen if separated can be used as fuel for the space craft as well. This could lead to fueling stations in space and ultimately reduce the cost of space flight significantly.

The founding of Planetary Resources has redefined the limits of possibilities in the field of energy and resources. If it turns out to be successful it is bound to be the beginning of a revolutionary and extremely profitable industry while opening the doors to colonization in space. Most importantly it will be a great achievement of the human race.

images from : planetaryresources.com

Management of Mining Machinery

Mining for resources from the earth has increased steadily over the past few decades and at present is a multi-billion dollar industry. The machines used in mining operations are usually very big and expensive. Therefore in order to make a mine productive and profitable, the machines have to be managed in an effective and efficient manner.

To do this it is a mine managers duty to make sure that,
- The machines give continuous service (availability of machines)
- The machines do not fail frequently (reliability of machines )
- optimize the machines so that they operate at maximum efficiency

In order to satisfy these criteria, machines of good quality have to be obtained. When acquiring machines for a mine, several things need to be given consideration. First, the cost of the machine should be considered. Naturally machines with a lower price are likely to be of poor quality and will fail often, while machines with a higher price generally are (but not always) of high quality. Therefore in this situation the purchase price alone is not taken into account. Instead the "life cycle cost" is calculated. The life cycle cost includes the purchase price, cost of maintenance as well as the cost of disposal of the machine. This gives a good measure of the usefulness of the machine during the operation period of the mine.

Next the performance of the machines must be considered. To assess the performance of the machines, key performance parameters such as Availability, Utilization and Efficiency and Reliability are defined as given below.
- Availability = available hrs/scheduled hrs
- Utilization = utilized hrs/available hrs
- Efficiency =  productive hrs/utilized hrs
- Reliability is the probability of operating without failure.
Failure of mining machinery is a somewhat common occurrence and leads to significant losses. This is a fact that needs attention because if maintained properly, failure can be almost avoided completely. This is not impossible because aircraft and ships are maintained so well that a failure of an aircraft or ship is a rarity these days. Therefore with proper maintenance, mining machines can also reach a situation like this.

Maintenance of machines needs to be carried out with a proper maintenance strategy plan. Several approaches can be taken in this regard. They are
- Preventive maintenance  - maintenance with a schedule based on time
- Predictive maintenance  - maintenance based on current condition and future predictions drawn from it
- Proactive maintenance  - maintenance based on the root cause of possible problems
- Reactive maintenance  - continue to run until component fails and then replacing.
Maintenance costs includes the cost of spare parts as well as the losses due to any delay in aquiring the spare parts. Therefore the availability of spares also needs to be considered.

Managing mining machinery is a continuous and cyclic process and if done in the proper manner, it can improve the operating efficiency of the mine and make the mine a more profitable venture.

image from : wikipedia