The Slope Instability During Excavation Construction Essay

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Soil is considered one of the most difficult material to deal with from engineering prospective due to its variables properties and behaviour. However Performing Geotechnical instrumentation can help to overcome many of those problems, and to have better understanding of the site as Instrumentation could give useful and invaluable data regarding geological and geotechnical condition of the ground.

In our project a road of approximately 35 m long and 10 m wide is planned to be built in the side of a hill. And due to some restriction from land use constraints approximately half of that road (5 m ) can be constructed by cutting the hillside and the other half should be placed on fill.

In this site there are many potential failure problems which engineer should take care of .The site had already undergone landslides in distant past ,which could happen again at any time, there are many indications to believe that there is high ground water table in the area which can cause many potential instability failures. The strata are weak and they are prone to landslides and slopes instability owing to many reasons such as settlement ,heave ,intense rainfall , and etc.

And to conduct successful site investigation ,all these problems be detailed and After investigating the site ,we have proposed different types of instrumentations to predict and monitor the site instability due to those various problems. We proposed monitoring pore water pressure by installing vibrating wire piezometer .We placed two Inclinometer and a single extensometer to measure lateral and vertical deformation respectively .these two instrument are helping engineer to predict lateral and vertical deformation which could cause slope failure, also to predict heave and settlement problems ,to design any future remedial process and etc.



PART 1 Introduction to Geotechnical Instrumentation

Chapter 1 Geotechnical Instrumentation: An Overview 1-1introduction 1-2 Type and Purpose of Instrumentation

PART 2 Desk Study

Chapter 2 The Site Characteristics 2-1 Purpose of This Desk Study 2-2 Introduction to the Proposed Project 2-3 History of the Site 2-4 Surface Conditions 2-5 Ground Conditions

Chapter 3 Mechanism of Failure 3-1 Introduction 3-2 Different Modes of Failure 3-2.1 Slope Instability During Excavation 3-2.2 Landslide 3-2.3 Lateral Deformation 3-2.4 Vertical Deformation 3-2.5 Soil creep 3-2.6 Ground Subsidence

Chapter 4 Instrumentation 4-1 Vibrating Wire Piezometer 4-2 Inclinometer 4-3 Extensometer


PART 1 Geotechnical Instrumentation: An Overview

Chapter 1 Geotechnical Instrumentation: An Overview

1-1 Introduction

Soil is considered one of the most difficult material to deal with from engineering prospective due to its variables properties and behaviour, and many of these variables data which engineer find may not be a genuine representative of the site since the sample area is only fractions of the total soil mass . However there is a tool which could give us an indication in defining some of these problems.

Performing Geotechnical instrumentation can help to overcome many of those problem since Instrumentation could give useful and invaluable data regarding geological and geotechnical condition of the ground, which will help engineer to have a better understanding of the parameters of the site of the project that we work on ,to minimize the hazards and potential problems which may encounter, and to check the design data or design for a remedial process which may be need.

1-2 Type and Purpose of Instrumentation The use of type of Instrumentation depend on several factors ,as we have to consider the nature of our construction ,type of investigation which is required from us ,and the nature and location of the site, Dunnicllif (1993) demonstrated the need of by Instruments, by stating :' …every instrument on a project should be selected and placed to assist in answering a specific question: if there is no question there should be no instrumentation '. Every site has its own unique characteristics and therefore we should base our selection of Instrumentation types on the site conditions and project conditions. By monitoring two basic parameters which are pore water pressure (Ground water level) and deformations through instrumentation process an engineer can tackle and solve many problems which encounter during and after construction such as slope instability, ground subsidence ,ground heave, seepage problem and many other problems which may arise.

Site instrumentation is a useful tool to engineers in all stages of the construction if it has been used properly by selecting the right choice of Instrumentation for that project. We can sum up some of the benefits of Instrumentation in the following :

In pre-construction stage, engineer can verify his data through data obtained from instrumentation process.

During construction stage ,instrumentation is used to maintain safety by determining potential failures due to construction processes such as excavation and hence providing safety to the work to avoid any unexpected failures which could cost money and life. Also it plays a role in reducing construction cost.

And in post-construction period, Instrumentation is used to monitor the project performance and ensure safety of the project by detecting any potential problem such as sliding instability , seepage ,heave and etc.

PART 2 Desk Study

Chapter 2 The Site Characteristics

2-1 Purpose of This Desk Study The purpose of this Desk study is to study the site characteristics carefully, and hence to find all instability defects, at least those we may consider from our point of view , in the site location. Then we have to come up with possible failure mechanisms that could happen in the site during different stages of the project having considered the ground and the site conditions , potential problems and ,instability issues that we encountered during our desk study. And finally to install proper instrumentations from both point of view of type , locations and numbers based on our perception to the instability features that we believe that they could exist in the site.

2-2 Introduction to The Project A road of approximately 35 m long and 10 m wide is planned to be built in the side of a hill. And due to some restriction from land use constraints approximately half of that road (5 m ) can be constructed by cutting the hillside , so we are compelled to build the other half on fill.

2-3 History of the site Based on the information that is given by Dr Chapman , the proposed area for the road construction may had some landslide failures in the distant past. And this give us indication that there is possibility of new landslide failure since there is no guarantees that this landslides activities will not emerge again.

2-4 Surface condition of the Site Also Dr Chapman states :'the slope is generally covered with grass and in some areas ,both above and below the line of the road , there are small areas of hydrophilic vegetation'. The existence of grass along the slope is giving us an indication that there is a high water level at that area. Also as mentioned above in Dr Chapman's statement that there is hydrophilic vegetation in small areas along the sloped area ,and according to US Army Corps of Engineers(1987) : 'this type of vegetation occurs in areas where the frequency and duration of inundation or soil saturation produce permanently or periodically saturated soils of sufficient duration ….'. By another means we can determine from the presence of hydrophilic vegetation that the area is saturated during most of the year, and this will cause some instability issues such as sliding ,heave and etc. Which engineers should take care of during design and construction process.

2-5 Ground Conditions The anticipated ground conditions along the line of the road as it is given in the assignment are :

0.1 to 1 m Topsoil. The top soil does not make any concern because it is probably removed during excavation.

1 to 2 m Firm to stiff grey silty clay Silty clay commonly occurs as soft ,wet unconsolidated deposit and they are termed as problematic soil because of their high compressibility and low shear strength and due to this properties they prone to cause certain instability issues. And from the colour of the second layers (silty clay ) we can determine that the area is saturated for most of the time in the year ,Since the grey colour of soil is an indication of that and this leads to the conclusion that there is a potential for slope instability due to this saturation problem Also.

2 to 10 m weak fine to medium grained pale brown sandstone. For this layer we also face some instability problem due to the fact that there is a firm to stiff layer overlying this layer and there is a weak layer underlying it ,so there is possibility of land slide problems. However, sandstone itself also has the potential of collapsing ,occurring of subsidence in overlying material and the water inflow may cause cavities.

10 to 20 m Weak grey silty mudstone. Mudstone is considered as a weak rock. It has a high swelling, slaking properties once it is exposed to water it will turn to a soft material and hence it will cause slope failure problem to the site. Also its weakness could cause settlement problem \\.

Another ground condition which is also a potential problem to the construction is the variation between the strata dip angle (10 degree towards the dace of the hillside ) and the ground dip angle which is 15 degree and this variation can cause many potential problem

Chapter 3 Mechanism of Failure

3-1 Introduction after studying and analysing the surface and the ground conditions at the previous chapter, we should now be concern about some probable failure mechanism that can occurs in the site during and after the construction project , due to slope instability ,landslide, ground water and bearing capacity.

3-2 Different Modes of Failure

3-2.1 Slope Instability During Excavation

During the cut and fill process of road construction there is a potential for slope failure and this could risk the life the working staff who are working in the site. And this problem is basically due to high ground water and the deformation that could occur.

3-2.2 Landslide

According to the assignment the land may have undergone some landslide failures in distant past and hence there is no reason why this landslide does not occurs again.

A- Translational landslide :Translational landslide is a mass that slides downward and outward on top of an inclined planar surface Abbott (2004).

Abbott, P,L., 2004, Natural Disasters, New York, McGraw-Hill Companies, Inc., 460 p.

B - Rotational landslide :This type is also another type of slope failure and the slip surface in this case is tended to be curved.

High water content, along with intense rainfall is the main cause of landslides .However , the cutting of the hillside due to work can escalate the failure. And in our site The presence of grass , hydrophilic vegetation , and grey colour of the silty clay soil are quite reasonable indications that the area is in saturation mode during most of the time in the year. Also excavation is required due to the road construction ,so there is a high possibility of this type of landslide.

3-2.3 Lateral Deformation Lateral deformation is another potential cause of failure in our site due change in ground condition caused by the road construction. And it can cause instability to the slope of the ground , the cut , and the filling part .

3-2.4 Vertical Deformation This is the second type of deformation which can cause settlement and heave problems to our project. The main cause of this deformation are change in water content. We have saturation condition in our soil also external applied load to the soil And since we have to apply compaction process in order to compact the fill then the load also exist, so the site has the two basic required parameters for vertical deformation .and settlement failure can exist which may cause slope instability to the slope.

3-2.5 Soil creep soil creep is a slow movement of the soil mass . and the primary cause of this problem is raising and drawdown of the water table level. And in this site the water level is in saturation level most of the time in the year and there is no guarantee that ground water level will not fluctuate so there is a potential creep problem in our site. This could cause slope stability failure in the long term.

3-2.6 Ground Subsidence this is another potential hazards that we may face in this project. This problem mainly occur when water is extracted from the ground for any reason and due to this the clay layer will get compact and settle .and in our project since we have a high water table , we may be compelled to lower water table in order to construct the road which it will lead us to this problem .

3-2.6 seepage and leakage The high water table and variation in the soil strata can cause Seepage and leakage problem which they will in turn will affect cause slope instability.

Chapter 4 Instrumentation 4-1 Vibrating Wire Piezometer the presence of grass and hydrophilic vegetation alongside with the grey colour of silty clay layer lead us to the conclusion that the area is in saturation status during most of the time in the year. And this will urge us to investigate water level and pore water level in the ground. The most appropriate instrument to measure the pore water pressure and ground water level is by excavating a borehole in the ground up to the end of the mudstone layer(21 m) and placing a vibrating wire piezometer since it is suitable to all type of soil and provide quick result. We use the Vibrating Wire Piezometer in Pre-construction stage, During construction stage and Post-construction stage In pre-construction it is essential to install this instrument since the ground water level and pore water pressure has major impact on many aspect of the site such as landslide , slope instability, settlement measurement ,predicting heave problem, seepage and leakage in the site ,to design safe slope for excavation in the cut side , it is needed in design of retaining structure to support the slope of the hillside ,to calculate seepage rate .and to calculate the shear strength of the soil. So due to all these it is essential to calculate the pore water pressure and water level in the ground at this stage. During the construction process it is also essential to monitor the water level and pore water pressure because any change in water level can lead to serious problems, e.g. potential landslide, slope failure , possible settlement and heave ,excavated slope failure. And at post-construction ,the piezometer can be used to monitor any change in water level and pore water pressure which can many potential hazards such as landslides ,slope failures ,heave or settlement. And to regarding numbers and locations of the vibrating wire piezometers, due to cost restriction we should try to use less instrument to monitor the field. However, Clayton and others stated that: '…fortunately only one measurement is required at any point to define the regime'. So according to them only one piezometer is needed to define the regime and if this can give us reasonable value there is no need for extra piezometers which will cost more,and the location of our piezometer is at the top of the hillside since we want to keep it out from damages during construction so we can use it afterward for monitoring the serviceability of the road in the post construction process.

4-2 Inclinometer It is the main Instrument which is used to monitor the lateral ,subsurface deformations . And it is proposed to be used in the pre-construction , during construction and post-construction stage in this project, hence we need to excavate another borehole to place the inclinometer.

in pre-construction and during construction stage it is used to evaluate design data , evaluate slope stability of the slope ,monitor any lateral movement that could affect the design , monitor sliding of rock mass ,detect shear zone of the slope and shape of the shear zone if it is circular or planner. And in post construction stage beside the previous function which already it has , it is also used to monitor the retaining wall stability(if it exist ) by measuring slope deformation behind the wall, and to check the retaining wall stability against rotation.

The number of installed Inclinometer depend on the cost , and to reduce the cost we shall place only two inclinometer to monitor the area, one is to be installed at mid span of excavation boundaries , the second at the fill side, and the last one behind the retaining wall to monitor slope failure of the soil.

4-3 Extensometer Borehole Extensometers are used to measure vertical deformation , and heave in our site. And these two parameters are quite essential in assessing slope stability. This Instrument is used during construction and after construction since will help us to predict heave problem in the site ,monitor consolidation rate ,and also to monitor ground settlement which in turn affect slope stability of the ground. And the above mentioned may happen during construction and in the life time of the structure ,so to ensure the safety of the work during and after construction and to initiate any future remedial process it is necessary to install extensometer so that the engineer be aware of any potential slope failure due to heave, settlement ,or consolidation. Also to reduce the cost, we recommend one Extensometer which is enough to measure any vertical deformation , heave and etc. And for the location of the Extensometer to avoid interference of location of the extensometer with the construction work we will place the extensometer behind the excavated slope (behind the retaining wall, if exist ) so we can use it in post-construction stage also without getting damage in the construction stage.