Failed Slopes In Engineering Soils Engineering Essay

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Failed slopes in engineering soils are caused by a variety of factors including slope geometry, material strength, hydrology and groundwater, seasonal climate variations and the effects of vegetation and wildlife. In order to improve these unstable slopes there are several site remediation techniques that can be used, these include the acquisition of additional land, reducing the slope, installing additional drainage, soil nailing and providing stability by structural methods. All these methods have their advantages and limitations and can be very costly. Another alternative approach is the use of electro kinetic geosynthetics (EKG) to improve these unstable soils. EKG have the ability to provide both passive and active roles: EKG drains actively attract water while EKG reinforcement or soil nails not only provide reinforcement, but also increase the shear strength of the soil in which they are placed improving soil/reinforcement bond as a result

Introduction

Engineering application of Geo-synthetics in civil and environmental industries has become widely utilised as reinforcements, filtration, drainage, and sediment control or used as barriers. Geo-synthetics provide resistance -reaction- to applied loads and will not affect the soil matrix structure. Newcastle University developed new technology added ability to geo-synthetics so soil matrix structure can be changed in both physical and chemical properties of using electrokinetic theory (Jones et al, 2006). This technology employs both electrophoresis and electro-osmosis functions of electro kinetics. This requires electric conductivity to be added to geo-synthetics properties so can be utilised in conventional application and the new technique. This combination of functions is often referred as Electro Kinetic Geo-synthetics (EKGs). The application of EKGs can enhance transportation of water inside fine low preamble soils that may be difficult to do by conventional methods. EKGs technique implies applying electrical field across the soil mass using EKGs so ions on charged water will be free to migrate. The cathode will attract the negative ions and the anode will attract positive ions. The migration of ions will force water to move toward the cathode for dewatering and cause soil to consolidate. EKGs are not limited to ground improvements but it can be used for dewatering of waste and mine tailing.

Electrokinetic Theory

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The design with this method requires an understanding of electrokinetic phenomena in soils. There are five major types of electrokinetic phenomena: streaming potential, migration potential, electro osmosis, ion migration and electrophoresis. Electro-osmosis effects responsible of water transport mechanism when electrical field applied to the soil mass i.e. no soil particle are transported. Electro-osmosis occurs when electrical potential is applied so charged soil particles will encourage water flow in soil . The equation of water flow in soil under electrical potential gradient is given by (Mitchell, 1993):

u = - ke/kh. γw. V

Q= ke.V/L. A

Where:

u = pore water pressure

ke = coefficient of electro-osmotic permeability

kh = coefficient of hydraulic permeability

γw = density of water

Q = electro-osmotic water flow

V = voltage applied between electrodes

L = spacing between electrodes (anode - cathode)

A = area between electrodes normal to the flow.

Figure - Electro-osmotic flow (ELECTROKINETIC LTD, 2010)

Electro-osmotic permeability ke is not related to the pore size. The most practical value is cm2/s V for clay and low hydraulic permeability kh (Jones et al, 2006).

Applications of EKGs

Beside the conventional use of Geo-synthetics in civil and environmental engineering, EKGs may be used especially for slope stabilisation, Ground consolidation and reinforcement of soils.

Slope Stabilisation

Stabilisation of clay embankment has been conducted on 22m stretch of 9m high embankment (ELECTROKINETIC LTD, 2010). EKG treatment was based on 2m spaced electrodes then DC potential is applied starting electro-osmosis process. The results of 6-week treatment yielded a reduction in plasticity and shrinkage characteristics and improved shear strength parameters. In addition, the EKG electrodes provided permanent soil nails for the embankment. The cost of this method was 26% less than the least slope stabilisation method (ELECTROKINETIC LTD, 2010).

Ground Consolidation

Application of EKGs is established to consolidate very soft clays with high water content. In conventional ground improvements, a large surcharge pressure to be applied on the soft soil layer and need long time for consolidation to take place. A full scale test was performed without using surcharge loading on very soft kaolin clay consolidation using EKGS result 14% consolidation and increased shear strength from <1kPa to over 15kPa (ELECTROKINETIC LTD, 2010). The typical application of electro-osmosis consolidation is dewatering mine waste or sewage sludge. The process of using this technique is illustrated in

Figure - Consolation process using EKGs

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Reinforcement of soils

The use of EKGs made it possible to use cohesive soils in construction of reinforced soil walls. This because of the strength bonds established between EKGs and soft soil due to cementation around the electrodes. The hardened vertical electrodes forms soil nails and increase factor of safety of the structure.

Advantages and Limitations of EKGs

The application of electro-osmosis by use of exposed metallic electrodes is very limited due to the rapid corrosion of the anode during consolidation. The addition of the geosynthetic materials to the electrodes has eliminated this problem allowing the technique to be used on a wide range of applications such as consolidation, dewatering sewage sludge and stabilising slopes. The advantages and limitations of each will be discussed below.

Traditional consolidation methods include the use of Prefabricated Vertical Drains (PVD) where the hydraulic flow of the soil will determine the amount of consolidation this is expressed by Darcy's law:

Q = kh ih A

Where;

kh is the coefficient hydraulic conductivity

Ih is the hydraulic permeability

A is the area

The process of electro-osmotic consolidation is expressed in a similar manner:

Q = ke ie A

Where;

ke is the coefficient of electro-osmotic permeability

ie is the potential gradient i.e. voltage

Figure - Comparison of electro-osmosis vs. hydraulic permeability

There is a considerable advantage in using Electrokinetic Geosynthetic Prefabricated Vertical Drains (EG-PVD). The value of ke can be up to of greater than 10,000 times the value of kh, the comparisons can be see in

Figure - Comparison of electro-osmosis vs hydraulic permeability

The value of kh can drop during the consolidation process whereas ke is stable and efficiency throughout the entire process can increase. Stage loading on weak soils is needed for conventional consolidation methods however no stage loading is needed at all with electro-osmosis. Kinks or irregularities in the EG-PVD is irrelevant, in fact efficiency increases with kinks whereas kinks in PVD hinders efficiency. The speed of consolidation through electro-osmosis can be increased by 800%.

These advantages of EKG will affect the cost efficiency of this consolidation/remediation technique. The cost of traditional consolidation can be expressed as a function of PVD, surcharge and time ie:

Ctraditional = f[PVD + surcharge + time]

Where;

PVD = cost of material and installation

Surcharge = cost of material, transportation, removal and disposal

Time = cost of lost opportunity

The cost of EKG can be expressed in the same manner as

CEKG = f [EG-PVD + operations + time]

Where;

EG-PVD = cost of material, installation, electrical connections

Operations = price of electricity (DC)

Time = cost of lost opportunity

The advantage of EG-PVD is installation equipment is the same as PVD. The cost to manufacture EG-PVD is more expensive than PVD however this may be cancelled out depending on the site and the amount and type of surcharge material. The cost of running electro-osmosis is almost equivalent to the cost of fuel required to transport the surcharge material. As mentioned before the time of consolidation using of electro-osmosis is 1/8 times that of traditional methods therefore lost opportunity will be greatly reduced.

A traditional method of sewage treatment through a belt press creates a sludge cake of solid content between 16% and 20%. The resulting water content requires the sludge cake to be mixed with straw so it can be handled and transported properly and disposal by incineration requires the sludge to be mixed with a fuel oil to aid in the increase of thermal content. The addition of EKG enables the process to create cake sludge with solid content of 30% and volume reduction of 50% the difference of the two products can be seen in

Figure - Sludge mix comparison between 20% solid content and 30% solid content

The incorporation of EKG has eliminated the required addition of straw for mechanical stability and the addition of fuels is not needed if incineration is applied.

The resulting cake sludge in can be used as an example to compare the cost between the 2 methods per machine.

Traditional Belt Press

EKG Belt Press

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Loading (kg/hr)

540

540

Operational hours

8,000

8,000

Dry solids %

20

30

Disposal cost ($AU/m3)

25

25

Disposal cost per year ($AU/yr)

550,000

336,000

Electricity cost( $AU/yr)

-

6,000

EKG saving ($AU/yr)

-

204,000

Table - Cost comparison of EKG and Traditional methods

EKG is very beneficial in the dewatering of sloped soils. The extraction of water through carefully positioned electrodes increases the shear strength of the soil and the electrodes may provide further reinforcement. The advantage of using EKG is the versatility of the application. Slope stabilisation can be tricky as surcharging may not be viable on an incline and weathering of the site can destabilise once stable soil conditions. One any potential failure planes are established electrodes can be installed and consolidation can proceed. Once consolidation has finished the electrodes can remain in place as reinforcement.

Design Techniques

The design techniques of soil improvements are often based on design codes. EKG is newly developed technique so the design philosophy usually based on test trials. However, the usefulness of soil tests undertaken to assess suitability for electrokinetic treatment are shown in

Table - The various soil tests, which EKG can treat

For successful consolidation the following criteria is to be established before the design of the technique can be created:

The acceptability of the soil to accept electro-osmotic treatment

The electro-osmotic permeability

Soil electrical resistivity

Electrode configuration

Electrode layout

The required current demand

The stabilisation of slopes incorporates a similar approach in designing the technique, with the addition of determining the expected failure planes, finding the relationship between the shear strength and the potential slip planes and the required reduction of water to achieve the necessary shear strength.

The most appropriate configuration of electrodes is found to be the use of EG-PVD which was mentioned previously. The application of reinforced walls can also be used but generally less effective than EG-PVD.

The ideal arrangement of the electrodes is found to be in the form of a hexagonal arrangement. Each corner of the hexagon contains an electrode with a cathode in the middle. This arrangement creates the most effective electrical field and inturn reducing the overall treatment time. The arrangement can be seen in

Figure - The ideal arrangement of electrodes (Red = anode, Blue = Cathode)

The required current demand can be found using the following equation:

It = ncsσV/L

Where;

It is the total required current

n is the number of anode/cathode pairs

c is the efficiency factor (0.8 < c < 0.9)

σ is the soils electrical soil conductivity

V is voltage

L is the distance between the anode and cathode

The stabilisation of slopes incorporates a similar approach in designing the technique, with the addition of determining the expected failure planes, finding the relationship between the shear strength and the potential slip planes and the required reduction of water to achieve the necessary shear strength.

The application of geosynthetics has enabled the design of electrodes in many shapes and forms. It has permitted the design to suit many different materials, settings and applications. Geosynthetics can be incorporated into the electrode itself not just acting as a protective barrier, this can make the electrodes multifunctional.

Case study: Stabilisation of a Railway embankment

This technique was successfully used to stabilise a failing clay embankment in London. From using this technique it resulted in a 26% in cost reduction and a 47% reduction in carbon footprint compared to traditional techniques.

The soil properties consisted of a mixture of weathered London Clay and other material such as brick and stone fragments onto underlying alluvium and terrace gravels. The soil conditions were identified as unstable and unsuitable for any structures to be constructed on. From the readings of an inclinometer they indicated a slip plane at a depth of 2.5m, which could result in a shallow translational slide or a deep circular failure.

In the initial stages a trial was conducted on a 22m by 9m high Victorian embankment where the slope had a Factor of safety of 0.81. Refer to for the slope profile of the embankment.

The EKG was uniquely designed to ensure that none of the two identified failure mechanisms would interfere with the technique. The EKG electrodes were arranged in a hexagon pattern as shown in .

Figure - Cross Section of the slope profile

Outcome of the Case Study

From applying this technique the treatment only took 6 weeks in order to stabilise the slope and resulted in:

An improvement in shear strength parameters,

A reduction plasticity and shrinkage characteristics

The soil was dewatered up to 25 times faster compared to control drains

There was an increase in the ground water temperature from 10°C to 20°C

A low amount of power was used in order to achieve this result (11.5kW/m3)

A slope stability analysis was undertaken before and after. The results are shown below in . These results show that after the application of the EKG the factor of safety increased to a reasonable level where it can be classified as a stable ground.

Analysis

Reinforcement

Factor of Safety

Pre treatment

No

0.81

Pre EKG treatment

No

0.96

Post EKG treatment

No

1.47

Post EKG treatment

Yes

1.71

Table - Summary of results

Compared to traditional techniques for improving unstable slopes, the EKG method had the lowest total project cost saving 26% when compared to gabion baskets and slope slackening. It also had a lower carbon footprint of up to 47% when compared to conventional methods

Summary

In summary there are many benefits that the EKG technique is able to provide an effective method for slope stabilisation, Reduced costs, the requirements for labour and materials is very low and have reduced health and safety risks.

These EKG materials open up a new range of applications for geosynthetics. Where the nature of these applications are fundamentally different to those conventionally associated with geosynthetics in that the EKGs effect can change depending on the nature and form of the material in which they are placed. Current applications of EKG materials relate to a number of main issues that can result in a brighter future for the next generation which including climate change, the reduction of carbon footprints and reduction and reuse of wastes.