Major Concerns Of Geotechnical Engineers Biology Essay

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Background

Liquefaction is one of the major concerns of geotechnical engineers affecting the stability of civil engineering structures especially those supported on loose granular soils under undrained condition. As the name suggests, liquefaction is defined as a phenomenon in which transformation of granular soil from solid state to liquid state. Liquefaction results in sudden decrease in strength and stiffness together with large deformation of the soil under the foundation which can cause major failures of geotechnical structures.

History of liquefaction studies

Generally the phenomenon happens in a short time but nevertheless long enough to result in extreme building and infrastructure damages, casualties and financial looses. It is easy to recall numerous disasters happened in past in which liquefaction was the main factor responsible for the tremendous amounts of damage. In 1999, liquefaction induced from the earthquake in Izmit Turkey caused the collapse of over 20,000 commercial and apartment buildings (figure 1.1 a) which is the main contributing reason for over 30,000 death tolls. Post earthquake liquefaction potential at the Lower San Fernando dam (California) 1917 endangered 80,000 people living downstream of the dam which required an immediate evacuation. Over $100 million was wasted on the construction project of the artificial island in the shallow water region of the Canadian Beaufort Sea which later on was abandoned due to the liquefaction slides. And last but not least, the Alaska and Niigata earthquakes of 1964 earthquakes caused enormous damage. For Niigata alone the damage in buildings and infrastructure was estimated more than $1 billion and most of this damage was related to soil liquefaction, figure 1.1 b.

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These disasters, more than anything else, proved that soil liquefaction is a considerably important aspect of soil behaviour from both a general public safety and economical point of view. Understanding of the mechanism behind the phenomenon is essential in order to develop standard procedures for evaluating the liquefaction susceptibility of soil deposits. Liquefaction studies have a strong historical background. Records on the subject can be found traced back to as far as the 17th century with chain of flow slides occured in the province of Zeeland, Netherlands (Koppejan et al., 1948). It is claimed that the start of practical soil mechanics was immediate following the terrible train disaster in the approach to a railway in Weeps in 1918, caused by the vibration form the train passing by. Hazen (1920) with his report on the failure of the Calaveras Dam, located primarily in Santa Clara County, California, as the first engineer to use the word "liquefied". In this report the phenomenon of liquefaction and the importance of excess porewater pressure were clearly recognised. The term "spontaneous liquefaction" was coined by Terzaghi and Peck (1948) to describe the phenomenon where loose sandy deposits suddenly change from solid stage into flows like viscous fluid, trigged by a mall disturbance. In 1969, Castro performed the first study looking into static liquefaction, analyzing the effect of void ratio on the undrained behaviour of sand. Since then, there have been numerous experimental state-of-art studies, journal on the subject and theoretical frameworks for liquefaction assessment were also developed over the years notably include the cyclic-strain based method (Dobry et al., 1982), nonlinear effective stress-based analyses (Finn et al., 1977) and the critical-state approach for sands (Jefferies, 1999).

(b)

(a)

Figure 1.1. Collapses of building due to liquefaction of the soil

(a) Izmit Turkey, 1999 (b) Niigata Japan, 1964 (REF?)

Types of liquefaction

From the past experience, liquefaction studies tend to be associated with earthquake. However, the term liquefaction has been used to describe number of related soil phenomena which have similar effects. In general, based on a way that porewater pressure is generated, these phenomena are classified into two main categories. These are static or flow liquefaction and cyclic liquefaction. Vast number of historical cases falls in the former group, in which static equilibrium of soil particles is destroyed by static loads in a loose soil deposits with a low residual strength. Static liquefaction can arise from two situations. Undrained failure under monotonic loading can be triggered by the decrease in the mean effective stress due to seepage, Aberfan disaster (1966) in South Wales is an example. And the second situation is post-earthquake liquefaction, where the dissipation of cyclically induced porewater pressure after the earthquake may result in further lateral movement of the ground. (EXAMPLE PIC)

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The other type of liquefaction is cyclic mobility or cyclic stress induced liquefaction in which repeated cycles of stress generates an accumulation of porewater pressure as a result of soil densification. This phenomenon can occur in any type of soils not just loose deposits as even medium to high density sands can experience densification to some extent. If the cyclic stresses have sufficient magnitude and last for long enough i.e. in terms of number of cycles, very dense saturated sand can possibly liquefy under a right density and confining stress condition. This cyclic stresses can be induced from an earthquake or from other vibrating sources from machinery, traffic and storm waves. EXAMPLE PIC

Holtz and Kovacs (1981) summarised the differences between the two categories, as shown in table 1.1.

Table 1.1. Differences between Flow liquefaction and cyclic mobility - D. Kovacs (1981)

Flow Liquefaction

Cyclic Mobility

General

Most likely in uniform, fine, clean, loose sand. Static load can cause liquefaction. Cyclic loads causing shear stresses larger than the steady-state strength also can cause liquefaction.

Any soil in any state can develop cyclic mobility in the laboratory if the cyclic stresses are large enough.

Effect of '3c at constant void ratio for '1c /'3c

Increased '3c means lar­ger deformations if lique­faction is induced. The magnitude and/or number of cyclic loads needed to cause lique­faction increases with '3c. Cyclic loads smaller than the steady-state strength cannot cause liquefaction but may cause cyclic mobility.

Increased '3c means in­creased cyclic load to cause cyclic mobility. But the cyclic mobility ratiot usually decreases with increasing '3c

Effect of '1c /'3c at constant void ratio and '3c

Smaller additional loads are needed to cause liquefaction as '1c /'3c increases. When '1c /'3c is large, a soil is more unstable and may, in the extreme, be susceptible to "spontaneous liquefaction."

In soils that have low permeability, increased '1c /'3c seems to result in somewhat smaller cyclic mobility stresses, which is a reasonable trend. In clean sands, cyclic mobility stress increases with '1c /'3c. This unusual result for clean sands is postulated to be due to the substantial test error due to redistribution of void ratio in the laboratory specimens.

Notation: '1c , '3c : in plane critical principal stresses

Liquefaction characteristics assessment

Liquefaction is dangerous and costly soil behaviour. Therefore it is a standard requirement to design the infrastructures against this phenomenon for any construction project. If there is a potential for liquefaction, it must be engineered right away. Engineering dealing with liquefaction often involves expensive processes, thus liquefaction assessment is crucial in order to successfully deliver an effective solution both in term of engineering and financial points of view.

By observation and keeping records of the site over the long periods of time, a soil can be characterised according to various criteria. For example, historical, geological compositional and state criteria can be used to determine if the soil is susceptible to liquefaction. There are various factors, namely grain size distribution, relative density, soil structure etc. contributing to the susceptibility of soil to liquefaction. Therefore, in order to come up with a suitable engineering solution, a detail report on soil characteristics of the site must be produced. As such, there have been number of different procedures developed over the years to assess the potential of liquefaction of soil deposit. Laboratory testing of soil samples subject to static or cyclic loading can be used. A number of standard tests can be mentioned here, such as cyclic triaxial test, cyclic simple shear or cyclic torsional test. The results from these tests are analysed and interpolated into cyclic resistance ratio (CRR) (DEF?) which represents the resisting capacity of the soil to cyclic loading hence giving some information on liquefaction characteristic of the soil underground. However the data collected may not very accurate due to the disturbance of the soil when sample is taken from site and transported to the laboratory. For better results, a simple and cost effective method called a cone penetration test (CPT), shown in figure 1.2, has become very popular in practice as in-situ test for determining geotechnical properties of soils. Employing a simple mechanical measurement of penetration resistance to push a tool with conical tip into the soil, the CPT can provide data with typical better than 2% accuracy. CRR value for the soil will be computed from this result. Modern CPT can also include a pressure transducer with a filter to collect porewater pressure data. (how cpt work)

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Figure 1.2. Cone Penetration Test (CPT) and results collation

Dealing with liquefaction

When dealing with liquefaction, the engineering solutions can be divided into three groups. First of all, based on the site assessment, construction on liquefaction susceptible soils can be avoided. However, as the economy expanding the land is exhausting, it not always possible to abandon a susceptible site.

Second option is to design a structure with ability to resist the effect of liquefaction. An enhanced feature can be incorporated into the foundation of the structure which enables it to accommodate the large deformation and settlement that often occurs during liquefaction. Two most common types are the mat foundation for small buildings and the pile foundation applicable to much larger structures e.g. high raise buildings, highway bridges. They both have similar purpose that is to spread out the load of the super structure evenly and transferred them onto stiffer part of the soil which is not subjected to liquefaction.

As mentioned before, the liquefaction behaviour of soil is heavily influenced by the properties of soil namely strength, density and drainage. To mitigate the liquefaction problem the obvious method would be trying a direct modification of the characteristics of the soil itself in a way that prevents the soil from liquefying. Various ground improvement techniques have been developed for this purpose. For example, vibroflotation use vibrating probe to penetrate into the ground and densifies the surrounding ground or drainage techniques which enhance the drainage ability of the soil preventing the built-up of porewater pressure.

Aims and objectives:

Many catastrophic accidents due to liquefaction over the years, It is important for engineers to get a fundamental understanding of the problem and able to deal with it. So, keeping young engineers aware of the importance of liquefaction and its consequences remains a key educational objective in geotechnical engineering. Involving the material changes from semi-solid particulate media to a nonlinear viscous material, liquefaction is a geohazard often not understood by civil engineering students, even after an undergraduate course in soil mechanics. However, for undergraduate students, understanding the behaviour of the material during liquefaction and how to mitigate its consequences is a very challenging task. A demonstration experiment tends to leave students with a lasting impression. As will be seen later, liquefaction is an intrinsically brittle process and the full scale observation in a real site when it occurs is impossible.

This leads to the main objective of this project, which is to learn fundamental concepts in soil mechanics and liquefaction, based on which to design and construct a quick sand model that will be used in teaching the static liquefaction of granular soils. This model will be a feature demonstration to inspire students who interested the phenomenon. Using this model, various aspects of liquefaction, for example, effect of soil type and its density, type of structure and foundation can be easily illustrated. Once constructed, number of tests on different types of sand will be carried out on the model. Based on the test results a liquefaction susceptibility of selected granular materials can be analysed and comparison between performance of different liquefaction resisting structures can be drawn.

Organisation of the report

The structure of this dissertation is as follow:

In this current chapter, chapter 1, a general background information on liquefaction is presented, based on which identifies and justifies the aim and objectives of the project.

Chapter 2 presents a brief literature review of liquefaction, to static liquefaction in specific as well as some information regards various types of liquefaction resistant structures. In this chapter, two of the historical failure cases due to static liquefaction will also be presented.

Chapter 3 discusses number of existing liquefaction models from other universities, outlines the methodology of the project and finally describes the final design of the model and testing procedures.

Chapter 4 analyses the testing results including numerous photos and videos of the test to confirm that the model works as predicted by the theory. In this chapter, a comparison of the various foundation types used is also performed based on their performance against static liquefaction.

Chapter 5 concludes the report with key findings and describes the limitations of the project as well as potential work that can be done in order to improve understanding on the topic in the future.

Literature review

Definition

Soil liquefaction and related geotechnical failures are commonly associated with the significant loss of strength experienced in undrained saturated granular soil or cohesionless soils under the influence of excessive increase in pore water pressures generated from either static or dynamic loading. A precise definition of term is given by Sladen et al. (1985):

"Liquefaction is a phenomenon wherein a mass of soil loses a large percentage of its shear resistance, when subjected to monotonic, cyclic, or shock loading, and flows in a manner resembling a liquid until the shear stresses acting on the mass are as low as the reduced shear resistance."

In more general terms, liquefaction is defined as the phenomenon where transformation of a granular soil from a solid to a liquefied state as a consequence of increased pore-water pressure and reduced effective stress (Marcuson 1978). When liquefied soil loses its strength and behaves like viscous liquid, hence the name "liquefaction".

Naturally, soil particles under effect of the gravitational force exerted by gravity, rest upon each other and form an interaction grid that hold the soil together and is relatively stable. As liquefaction occurs the water pressures increase high enough to counteract the gravitational force on the soil particles and eventually suspend the particles. In more technical terms, liquefaction occurs when the porewater pressure (u) and the total overburden stress () are equal. This results in a very low effective stress condition, even zero in some extreme cases (' = [ - u] = 0). This fundamental mechanism is shown in the figure 2.1. In the right hand picture, the soil is in the normal state in which the porewater pressure represented by the blue column is low. The interaction forces, represented by the arrows between the particles, are much larger in comparison to those in the left picture.

Figure 2.1. Mechanism of the liquefaction (a) normal state,(b) liquefied state

(REF)

The shear strength  of granular soil is given by:

Where: : shear strength

: apparent cohesion

: effective stress

: the effective angle of shearing resistance

No longer having the interaction force between the soil particles i.e. effective stress is zero,it can be seen from the above equation, the soil doesn't pose any shear strength. Any external disturbance even slightest can easily destroy the static equilibrium of the structure formed by the soil particles. They will then be able to move freely with respect to each other. As the structure breaks down, the strength and stiffness of the loosely-packed individual soil particles reduced significantly to a point that unable to support the structure above, results in the complete or partial failure of the structure.

Excess porewater pressure is the key factor causing the soil to liquefy, therefore based how the porewater pressure arises; liquefaction is categorized in two types:

Static liquefaction: tends to occur in undrained saturated, non-cohesion soil, excess pore pressures arise through static loading or water seepage if the soils are loose enough. Failure is triggered by static loading.

Cyclic liquefaction: used to describe the phenomenon in which excess pore pressures accumulate from repeated cycles of stress applying on the soil caused by external loading e.g. earthquakes or machenary.

As mentioned in the chapter one, static liquefaction is the type that many history cases failed in to. Also generally static liquefaction controls the stability of the soil deposit. If the soil has a sufficient residual strength, then cyclic will not be much of a thread to the structures as it only manifests itself as fatigue like strain. Therefore, this desertion will only focus on static liquefaction and literature regarding the cyclic liquefaction will not be discussed any further.

Static liquefaction

Cause of static liquefaction

Where soil liquefaction most likely occur:

One of the key condition for liquefaction to occur is soil must be saturated, it is observed most often in areas of low elevation near bodies of water e.g. near the river bank etc. Liquefaction can also be found to occur in completely submerged soil and commonly causes significant damages to bridge foundations, off-shore oil drilling facilities, and other structures that are supported by submerged soil deposits.

In general, liquefaction tends to happen in loose clean to silty sands that are saturated below the groundwater table but still within a certain distance close enough to the deposit surface. Below depths of approximately 20m, we generally do not see any evidence of liquefaction. This is due to the fact that deeper soil layers are experience a larger gravitational force from the weight of all layers above, hence better compacted than higher elevation layers.

Factor influences the liquefaction susceptibility of soil

How liquefaction susceptibility is determined

Liquefaction susceptibility is based on the soil and ground-water conditions (but doesn't include the possibility of earthquake or other external loading. This can be varied by the depth of soil as mentioned before from anywhere deeper the 20m below the ground surface, liquefaction doesn't occurs. This also varies with type of soil. Liquefaction easily happens in loosed sand and less often in coarse sand or gravel.

For detail analysis of liquefaction susceptibility of a certain type of soil, triaxial compression and extension test or simple shear test can be carried out in the lab. However the data collected may not very accurate due to the disturbance of the soil when sample is taken from site and transported to the laboratory.

Why is liquefaction important:

Severe ground shaking, lateral spreading such as earthquake, landsliding and deposit movement can cause a number of dangerous ground conditions that can lead to structural damage and failure resulting in loss of life. If these events occur over the area with susceptible soil, liquefaction may amplify the potential for those conditions and the damage they cause.

Liquefaction is responsible for extreme property damage and loss of life due to a several variations of failure potential. The stability of civil structures relies on the strength and stiffness properties of soil underneath. The failure of the soil under and around the foundation of these structures can cause the structure itself to weaken, lean, and possibly failure under its own self-weight. The damage can be even great for a structure boundaries to major bodies of water and their adjacent shores e.g. dams and retaining walls. Liquefied ground condition may result in 'subsurface landslides', during which the supporting soil at the base of the dam or wall loosens and slides out. Dam and retaining wall failure are especially problematic concerns due to the additional potential for flooding.

The damaging effects of liquefied soils are not only visible in the structural chaos left behind. Often, erosion from rivers and streams cuts into the soil along their banks, leaving behind scoured ground and gullies. The stresses produced during liquefaction can cause tension cracks to form in the soil near the embankment, or it can collapse altogether, commonly known as lateral spreading or landsliding. Soils on or near slopes, hills, or mountains can experience the same effects.

Soil improvement techniques

As mentioned in the chapter 1, there are several ways can be utilised to reduce the risk and severity of damage as a consequence of soil liquefaction. The first and most obvious is, based on testing and historical records of liquefaction events in the past to determine whether the site is susceptible to liquefaction and tries to avoid planning development on such site. However it is not always possible to do so, in the section, number of popular engineering solutions which can be applied in order to mitigate the problem of liquefaction.

Preventing liquefaction from happening or lowering the potential for it to occur is the first option. The main objective of soil improvement for reducing liquefaction hazards is to avoid the cause of the problem which is excess porewater pressure. This can be achieved by number types of ground improvement namely densification, solidification, drainage and dewatering.

From these four types, there are a variety of ground modification techniques available to reduce the potential for liquefaction to occur.

Densification

For liquefaction remediation, soil densification is the most widely used and generally highly cost effective. This includes any technique that reduces the void ratio of the soil thereby decreasing the volumetric change which could potentially lead to liquefaction. As the density of soil increases the shear deformation resistance of soil is also enhanced. However, techniques of this type can often produce objectionable levels of work vibration. Three techniques that improve the ground primarily by densification most commonly used at a new construction site are vibro-compaction, dynamic compaction, and sand compaction pile.

Vibro-compaction

Vibro-compaction is a technique for deep compaction of granular soils. It is also known as "Vibroflotation". Using specially designed vibrating probes with a rotating eccentric mass that can penetrate into the granular soil; the method can be used to densify loose natural deposits or artificially reclaimed sands to create a stable foundation.

This soil improvement technique is for purely granular soils especially works well with fully saturated and very weak soils. It relies on the basic principle that under the influence of vibration, non-cohesive soil particles can rearrange themselves into a much denser condition than it was originally in. Figure 2.X illustrates the compaction process in treating potentially liquefiable soils. Using combination of vibration and jetting of water or air, the probe penetrates into the ground up to the depth of 70m. The combined action of vibration and water saturation by jetting reduces the interaction forces between the soil particles. In this flotation state, soil particles can freely move and rearrange themselves in a more compact figuration in which the volume of the soil can be reduced up to 15%. The vibroprobe then is retracted in a 0.5m interval, which allows surrounding soil particles to collapse and flow towards the vibrobe and filling up the additional void space created. After vibro-compaction, the working platform needs to be backfilled, leveled and compacted by roller. A compacted radial zone of granular material is created.

Figure 1.X - Vibro-compaction process (Vibrotation group)

Vibro-compaction is ideally suited for clean sands where the silt content is less than 10 to 15 percent. Soils with appreciable silt content cannot be densified by vibration alone. However, the use of gravel backfill with the vibro-replacement (stone column) method extends the range of soils that can be improved to silty and clayey sands, silts and clays.

Depending on characteristics of the ground, backfill used and vibrators, the degree of compaction can be varied to meet required bearing capacity to suite the need of each project. As the result of vibro-compaction, the density of the soil increases significantly in excess of 85%, thereby improves the effective stress while reduce the permeability of the soil up to 10 fold. The risk of liquefaction in an earthquake prone area is also drastically reduced.

Dynamic Compaction

Dynamic compaction is another ground improvement technique of the densification type. Compaction of the granular soil is archived by repeatedly dropping a heavy tamper weight of steel or concrete (mass varies from 6 to 35 tons) from sufficient heights ranging from 12m to 40m on the ground. High energy shock wave created from these impacts transmits from the ground surface into the ground reaching deeper layer of soil underground. Under this applied energy, void between soil particles collapse and forcing the soil particles move into a denser configuration. The dynamic loading from the dropping can initiate local liquefaction beneath the drop point making densification of sand grains even easier. When the excess porewater pressure dissipates, additional densification occurs.

The treatment is often carried out in several passes ranging from 2 to 5 passes dependent on the soil type and condition. During each pass, the tamper weight tends to be dropped in set number of times on a predetermined grid pattern which is carefully designed so that compact zone under dropping points overlap to ensure the continuity compaction of the whole ground area. The pattern of dropping and the dropping height varies in the subsequent passes. The objective is to compact the shallower soil layers without remoulding the already compacted deeper layers.

Degree of compaction and the maximum depth can be archived by dynamic compaction are strong influenced by the total energy applied. It also depends on number of factors including dynamic response characteristics of the compacted soil as well as the underneath layers, the ground water table, the compaction procedure etc. There is empirical correlations based on soil type and total applied energy available for use in designing the appropriate amount of energy required to archive degree of densification specified. However, due to high level of uncertainty and variation in construction sites, extensive compaction trials are often carried out to optimise the design of the compaction process.

Dynamic compaction is a popular ground improvement technique, capable of improving the characteristics of a wide variety of weak soils, including silts and clays. However, this technique is most commonly used to treat old fills and low-density granular soils. Similar to vibro-compaction, results of the treatment are increase in relative density and decrease in void ratio, thereby enhancing bearing capacity and reducing settlement. It also provides an economical way of improving soil for mitigation of liquefaction hazards. However, since the heavy impact of dropping weight is employed, the process is somewhat invasive. The resulted soil surface of the process may require additional granular backfill and shallow compaction following dynamic compaction.

Sand compaction pile (SCP)

The sand compaction pile (SCP) method has been developed and widely used for many construction in japan., in which sand is fed into ground through a casting pipe and is compacted by either vibration, dynamic compaction or static excitation to construct a compacted sand pile in a soft soil ground. This method can increase the density on loose sandy ground and to increase the uniformity of sandy ground and soft clay, to improve its stability or compressibility and or to prevent liquefaction failure.

The principle concept for application is to increase the ground density by placing a certain amount of granular materials (usually sand) in the ground.

Installing compaction piles is a very effective way of improving soil. Compaction piles are usually made of prestressed concrete or timber. Installation of compaction piles both densifies and reinforces the soil. The piles are generally installed in a grid pattern and are generally driven to depth of up to 60 ft.

Solidification

Solidification is also considered highly reliable measure against liquefaction. Solidification prevents the soil particles freely movement and provides the better cohesion between particles.

Compaction Grouting

      Compaction grouting is a technique whereby a slow-flowing water/sand/cement mix is injected under pressure into a granular soil. The grout forms a bulb that displaces and hence densifies, the surrounding soil (right, HB). Compaction grouting is a good option if the foundation of an existing building requires improvement, since it is possible to inject the grout from the side or at an inclined angle to reach beneath the building.

Drainage improvement

In other hand, drainage method improves the drainage within the soil thereby accelerates the dissipation of porewater pressure and reduces potential occurrence of excess porewater pressure.

Drainage techniques

      Liquefaction hazards can be reduced by increasing the drainage ability of the soil. If the porewater within the soil can drain freely, the build-up of excess pore water pressure will be reduced. Drainage techniques include installation of drains of gravel, sand or synthetic materials. Synthetic wick drains can be installed at various angles, in contrast to gravel or sand drains that are usually installed vertically. Drainage techniques are often used in combination with other types of soil improvement techniques for more effective liquefaction hazard reduction.

Dewatering

Dewatering involves lowering the ground water level within the sand which reduces the degree of soil saturation is also very effective in dealing with liquefaction. However, it tends to be difficult and expensive process, since seepage cutoff is required at both upstream and downstream, pumps must operate all the time. Therefore dewatering is less popular in comparison to other types

Liquefaction resistant structures

In addition to soil improvement technique to lower the potential for liquefaction occurrence, consideration can be taken into account when designing the structure so that the damage resistant capacity of the structure can be enhanced.

If it necessary to construct on liquefaction susceptible soils, one can modify the design of a structure in several ways to make the structure more resistant damage potential from liquefaction.

A structure that incorporates ductility, has supports that are adjustable to accommodate differential settlement, possesses the ability to accommodate large deformations, and has a foundation design that can span 'soft' spots, can all decrease the amount of damage incurred in the case of a liquefaction event.

Shallow foundation Aspects

     It is important that all foundation elements in a shallow foundation is tied together to make the foundation move or settle uniformly, thus decreasing the amount of shear forces induced in the structural elements resting upon the foundation. The photo to the right shows a house wall under construction in Kobe, Japan. The well-reinforced perimeter and interior wall footings (KG) are tied together to enable them to bridge over areas of local settlement and provide better resistance against soil movements. A stiff foundation mat (below) is a good type of shallow foundation, which can transfer loads from locally liquefied zones to adjacent stronger ground.

     Buried utilities, such as sewage and water pipes, should have ductile connections to the structure to accommodate the large movements and settlements that can occur due to liquefaction. The pipes in the photo connected the two buildings in a straight line before the earthquake (KG).

Deep foundation Aspects

      Liquefaction can cause large lateral loads on pile foundations. Piles driven through a weak, potentially liquefiable, soil layer to a stronger layer not only have to carry vertical loads from the superstructure, but must also be able to resist horizontal loads and bending moments induced by lateral movements if the weak layer liquefies. Sufficient resistance can be achieved by piles of larger dimensions and/or more reinforcement. It is important that the piles are connected to the cap in a ductile manner that allows some rotation to occur without a failure of the connection. If the pile connections fail, the cap cannot resist overturning moments from the superstructure by developing vertical loads in the piles (see figure below).

Methodology

As mentioned in previous chapters, the objective of the project is to design and construct a "liquefaction tank" to be used during geotechnical engineering lectures and laboratory sessions to demonstrate the liquefaction phenomenon fundamental concept.

Existing liquefaction models

The liquefaction demonstration tank is not anything new. In fact, it is a classic in a geotechnical engineering program. This demonstration experiment tends to leave a lasting impression on the students mind. It is easy to find numerous figures and pictures of a basic quicksand model. For example, Holtz and Kovacs (1981) demonstrate a conceptual design diagram of a liquefaction tank, as shown in figure 3.1. The model consists of two tanks. The water tank is at the bottom and the top tank contains sand. A pump is used to pump the water from the bottom tank into the sand tank, creates the upward flow in the quicksand tank. Flowing through the porous stone layer at the bottom of the sand tank, the upward water pressure is distributed evenly over the entire base of the sand layer, keeping the porewater pressure constant throughout. Number of piezometers is installed directly onto the sand tank at different level, which enables water heads within the quicksand tank during the experiment to be observed and readings to be taken.

From the literature research, there are two existing quicksand models built at other universities, pictures of which are obtained. Essentially, the concept is similar in both tanks. There are two separate containers, one on the top contains sand specimen and a tank with water at the bottom that will be used to fill and drain the sand tank into. Figure 3.2 shows the model at the University of Illinois which is very similar to the diagram in figure 3.1. The other liquefaction tank shown in figure 3.3 is built at the Nanyang Technological University, Singapore. Instead of using pump, a standpipe is used to create and control the upwards flows in the sand tank. Also the piezometers in this model are installed on a separated board. Using flexible tubes, they are connected to valves installed at the side of the tank. A dial-gauge used to measure the vertical settlement of the object on top of the sand mass when it liquefies.

Figure 3.1 Diagram of a liquefaction tank, Holtz and Kovacs (1981)

Figure 3.2 - Liquefaction tank at University of Illinois, Luna,R. (2002)

(a) top half (sand box); (b) bottom half, (water tank);

Figure 3.3 - Liquefaction tank at Nanyang Technological University, Singapore

Design of the model

After evaluating all of the existing liquefaction models, an outline drawing including all dimensions and key features was drawn as shown in figure 3.4. The main concept of the model is kept the same as those existing models. Utilising a hydraulic bench to contain water and support the sand tank, pump and water tank is not be needed. As for the liquefaction tank, there are numerous requirements that its design has to meet. First all of the tank, measured 500x500x700mm, must be make strong enough to support the pressure created by the sand and water mass (about 200kg) in side. It also needs to be made water proof to prevent water from leaking out. One of the important requirements is that the tank must be transparent enough to enable a clear visual of the sand and the phenomenon happens in site to be observed.

Taking all consideration into account, the sand tank is made out of fabricated Perspex panels glued together using impermeable glue. Similar to the model at the Nanyang Technological University, Singapore, five piezometers are installed on a separated board and connected to the tank using flexible tubes. This enables the model to be moved around safer and easier compared with rigid piezometers installed on the tank due to the significant height required. The tank also has two valves one on side acting as the inlet and out let, which helps to control the upward flow inside the tank. An overflow tube also is incorporated at the top. At each tube and valves connections, filter is used to prevent the sand particles from leaking out.

Inside the tank, there nine plastic cylinders placed at the bottom of the tank to support the mass. The sand will sit on a layer porous stone of 40mm thick which contained by 2 layers of metal meshes with drilled holes. This allows the upward water flow to be distributed evenly over area of the sand mass base. Layers of geotextile are placed between the stone layer and the sand layer, which effectively stops the fine particles from leaking down to the porous stone layer. The sand was filled up to the height of the fifth piezometer which made up a total thickness of 430mm from the metal mesh base. The sand tank is placed on top of the hydraulic bench. There is also a steel frame support to be made in the future to secure the tank to the bench, enables it to be moved around safely.

Based on these drawings, with the assistance of our departmental senior technicians, the tank was constructed as shown in figure 3.5. There is a small modification to the design, which the valves are not connected directly onto the side panel but through a thick layer of Perspex prevents any crack to occurs at the connections. Similar to the model at the Nanyang Technological University, Singapore, a dial gauge attached to a steel bar place over the top of the tank, is used to measure the vertical settlement of the model. A metal string helps to secure the structure model to the steel bar, which enable the model to be taken out of the tank easily after completely sinking into the sand mass.

Figure 3.4 - Detail design of the liquefaction model

Figure 3.5 - Final Liquefaction model

Testing materials

Porous stone layer

As described in the final design of the model, there is a porous stone layer with a thickness of 40mm placed between the sand and the mesh support. For this project, 10mm concrete aggregate was used to make up this stone layer. A sieve analysis was carried out on a 1 kg of sample to determine the grain size redistribution of this type of aggregate.

Leighton Buzzard sand

Once constructed, the model was used for testing liquefaction resisting performance of number of different type of structures. For these tests, Leighton Buzzard Sand fraction C was used. This was supplied by the David Ball Group, Cambridge, UK, confirming to BS 1881-131:1998. After performing number of classification tests namely maximum and minimum density and sieve analysis on the 0.5kg sample, properties of the sand were determined as follows. Specific gravity of the Leighton Buzzard Sand fraction C was 2.65. Minimum and maximum dry densities were 1.40 g/cm3 and 1.68 g/cm3, respectively. These are value corresponding to the maximum and minimum void ratios which were calculated as 0.89 and 0.58, respectively. More than 80% of the coarse sand particles, which are rounded and mainly quartz, are between (around) 300 μm and 600 μm which meets the BS 1881-131:1998 standard.

Resisting structure models

For this project, three small models with similar weight were made, representing three different foundation designs as shown in figure 3.6. All three models have same shape, weight and made of the same materials. Table 3.1 shows the dimension and weight of the models. Models A represents pile foundation for high raise buildings and large infrastructures. Model B represents a typical mat foundation which is a shallow foundation for small and medium houses and apartment buildings. Model C acts as the control which is just a standard block structure without foundation.

Model A

Model B

Model C

Mass of whole model

3671.1 g

3673.5 g

3242.5 g

Dimensions

83x83x122 mm

83x83x122 mm

83x83x122 mm

Mat base

170x137x3 mm

105x105 mm

-

Piles

9 x ∅8 x 50 mm

-

-

Table 3.1 - Mass and dimensions of the models

(c)

(a)

(b)

Figure 3.6 - Resistant structure models

(a) Foundation mat, (b) Pile foundation, (c) No foundation

Testing procedures

To enable a comparison of the performance of the different types of foundations, the liquefaction tank was used to create the quicksand condition, in which the model placed on top of the sand surface, starts to sink down when the top sand layer liquefied. As mentioned in the literature review, there are various factors that can influent the liquefaction susceptibility namely soil particle grain size, upwards seepage and level of compaction. For this experiment to be accurate, all of the above parameters were kept approximately constant from one test to the other. The same sand, Leighton Buzzard (fraction C) was used in all three tests. Initial water level within the tanks as well as the flow rate controlled by the inlet valve with also was kept the same.

All nine supports were placed at the bottom of the liquefaction tank, followed by the metal mesh and the qeotextile layer. A 40mm thick layer of cleaned concrete aggregate was put on top of the geotextile and slightly compacted. Another layer of geotextile and metal mesh were put in before pouring the sand in. The sand were poured into the tank and compacted evenly in three layers. Once the model and the dial gauge were installed on top of the tank, the experiment was ready to run. While the outlet was completely closed, the inlet valve was opened to allow the water is pumped in the tank creating an upward flow, hence the change in pore water pressure between the sand particles and the increase in the water level in the piezometers. Hydraulic heads reading from the piezometers at different levels were recorded periodically and later on used to determine pore water pressure inside the tank. As predicted by the theory, as the inlet, valve 1, is opened to let the water to flow in, the head at the bottom of the sand layer will gradually increase and eventually to a sufficient value which can cause the sand to liquefy. The upward seepage forces will balance the downwards gravitational forces created by the sand mass. Hence rendering the shear strength of the sand to zero, any structure/object placed on top of the sand surface will sink in gradually sink into the sand mass. The whole procedure of the test will be recorded using a digital camera for reference when analyzing the data.

As can be noticed form the figure 3.5, a small amount of blue dye was added to all five piezometers, to enhance the visibility of the water level inside the tube, especially when taking pictures and video. This can cause the non unity density of the liquid inside the whole length of the tube and result in the inaccurate measurements of the heads in the tank. However, since the amount of dye is minimal and taking the non unity of the water inside the tank, it is assumed that the head represented by the piezometers is approximately same as the head inside the tanks. In fact, this can be shown in figure 3.7, where inlet and outlet valves are closed, the water level inside the tank is stationary. The different in heads caused by the non unity of the fluid can hardly be seen in all five piezometer.

Figure 3.7 - No significant difference between piezometer readings and water heads

Data collection

Liquefaction is a phenomenon that tends to occur very quickly. Therefore manual readings taking from all five piezometers are difficult and not very accurate. For this project, a digital camera was used to capture pictures at interval and record a video of the whole experiment.

From the reading of the piezometers, porewater pressures at different level inside the tank will be calculated during the whole experiments. From these data, graph of settlement against porewater pressure will be plotted for each experiment. Since all other parameters were kept constant from one test to the other, excepts for the type of foundation used, therefore difference between graphs will enable a comparison between the different type of foundation to be drawn.