CHOOSE YOUR CURRENCY

SAFETY ANALYSIS OF STRUCTURAL FOUNDATIONS BUILT ON ABANDONED SOLID WASTE SITES

Amount: ₦5,000.00 |

Format: Ms Word |

1-5 chapters |



ABSTRACT

This study adopted experimental design to investigate the suitability of abandoned solid waste site soil (ASWSS) as a foundation material for building construction. Measurements of geotechnical properties of stratified random soil samples of ASWSS and adjoining natural soil (NS) at depths 1.5, 2.0, 2.5, 3.0 and 3.5 m were obtained from six test points in Kaduna, Nigeria. The soil samples were subjected to sieve analysis, Atterberg limits (liquid limit, plastic limit, plasticity index and shrinkage limit), compaction, consolidation, triaxial, specific gravity tests as well as chemical characterization. Data treatment of ASWSS was carried out by applying 15% upper trim and 15% lower extended mean. These were done to forestall the effects of ‘reinforced earth scenario’ (unusual high strength spots caused by mix matrices of soil and fibrous materials) and unnoticed randomly distributed weak spots.  Design data were evaluated in accordance with the provision of European code (Eurocode 7). The responses of ASWSS and NS to loadings were investigated by carrying out spread foundation designs on both of them using the same loading and geometric conditions. The two sets of designs were subjected to safety measurements by first order reliability method and Monte Carlo simulation respectively. The comparative reliability of ASWSS and NS with respect to structural loading was obtained in forms of reliability index and probability of failure. Significant differences in the geotechnical properties of ASWSS and NS were observed.  The liquid and plastic limits of   ASWSS fell in the ranges of 28 – 32% and    25 – 37% respectively.  The angles of internal resistance ranged from 7 – 15º for   ASWSS and   8 º – 17º for NS. Clay and silt accounted for up to 90% of ASWSS in some cases while as low as 9 kN/m2 cohesion was recorded. The composition of organic matters in ASWSS was found to be in the range of 2.1 – 5% while that of calcium/magnesium  ranged between 106 mg/kg and 1000 mg/kg. Corrosive agents of sulphate and carbonate were found in the ranges of 235 – 903 mg/kg  and 20 –  50  mg/kg  respectively.  The  main  mineral  composition  was quartz  (silicon oxide), rutile (titanium oxide) and stolzite (lead tungsten oxide). Design  values of cohesion, angle of internal resistance and unit weight of soil were obtained in the ranges of 9.5 – 12 kN/m2, 7 – 20º and 12.9 – 14.3 kN/m3  respectively for ASWSS. The safety of foundation designs on ASWSS and NS was obtained in terms of reliability index and probability of failure. Despite the record of small probability of failure of 0.00013, corresponding to reliability index of 3.75, there were  few cases  of zero  reliability  indices  corresponding  to  probability of failure of 0.5  on ASWSS. These values placed ASWSS in the category of ‘hazardous to high’ safety index in the standard performance classification formats. Sulphate resistant cement, large reinforced concrete basement or foundations covering large areas and a minimum foundation depth of 2.0 m are recommended for all structural foundations built on abandoned solid waste sites.

CHAPTER ONE

INTRODUCTION

In most geotechnical engineering work concerning Solid Waste Sites, efforts seemed to have been directed mainly towards discovering the mechanical properties of solid waste site soil (SWSS) in order to determine the safe and reliable landfill inclination slopes and consequently landfill capacity. In other words, landfill structure is the main focus and its safe  design  the  concern.  However,  the high  cost  and  quest  for  municipal  space  for infrastructural  development  invariably  calls  for  judicious  management  and  use  of available ones by optimizing the benefits derivable from all lands including old landfills. A higher scale of this phenomenon is clearly and currently observed in most areas with the  resultant   economy  of  space   almost  over-emphasized   in   the   affected   wards. Establishing the geotechnical properties of SWSS is primary to the use of landfill for any engineering purpose though geotechnical investigation results by their very terms do not portray absolute conclusion of the elements they speak of, especially in their precise order. Spatial and geotechnical uncertainties exist significantly in different forms.

In addition to the wide ranges of approximations attending the practice, assessment of SWSS properties is categorically fraught with uncertainties in obtaining representative samples, time –dependent variations in soil characteristics and different or almost incompatible reactions of layers of SWSS to the applied stress values as a result of the heterogeneity  of  waste  composition.  These  uncertainties  are  either  compensated  for, using relevant probabilistic and statistical theories or designs and engineering judgments are made based on incomplete geotechnical information and traditional factor of safety

which by nature lacks logical competence to address the inconsistency posed by these wide ranges of uncertainties. Structural and mechanical engineering practices which deal with specified material geometries and qualities have their uncertainties arising from the prediction  of tolerances  to  which  the  structural  members  may  be  built  and  also  the stresses and environmental  conditions  to which they may be exposed (Phoon, 2008). However the practice is different in geotechnical engineering. Geological materials are investigated  in their natural state and their conditions are, of necessity, inferred  from measurements carried out on limited sample sizes (Baecher and Christian, 2003). The uncertaintities therefore, arise from the accuracy and completeness with which the geotechnical properties are discovered and the prediction of the mobilized resistance of soil and rock materials.  Reliability-based design incorporates interalia, the principles of probability,  statistics  and other mathematical  solutions  to give expressions  and  make allowances for uncertain elements in the use of evaluated soil properties for foundation design.

Due to social and economic reasons, strong preference has become an important element in the settlement pattern of most of the world cities. This has created an informal polarization of settlement and uneven distribution of population among the various wards that  make  up  the  townships.  Undoubtedly,  waste  generation  has  followed  the  same pattern.

A study of this nature therefore, is most desirable  at a time when pressures on land acquisition, coupled with lack of strict regulation, have driven individuals, public and private outfits into indiscriminate use of abandoned solid waste site soil (ASWSS) for various purposes. This study comes in to equip the public with the knowledge of the

varying risk levels involved and the required geotechnical procedures to adopt in the use of ASWSS for different developments.

1.1       Background

The  first  rational  system  of  reference  for  the  classification  of  geological  materials behaviour  and  interpretation  of observations/experience  developed  out of a scientific approach launched by Karl Terzaghi (1883-1963) in 1925 to study varying responses of soil and rock under differently specified stress characterization using the knowledge of physical science and engineering mechanics (Baecher and Christian 2003). From here geotechnical engineering took off and went through series of technological refinements to arrive at the present geotechnical reliability which is the integration and extension of the works of Freudenthal  (1947), Purgsley (1955) and Cornell (1969).

These pioneers of geotechnical engineering warned that the results of laboratory tests, their own observation/assertions or anybody’s else do not advance conclusive narrative, since applying finite efforts to discover the state of an engineering site as laid down by nature  obviously  involves  a  number  of  unpleasant  approximations  and  uncertainties which  must  be  quantified  and  compensated  for  using  reliability  based  methods.  In practical terms, reliability deals with the relationship between the loads a system has to carry and its ability to carry those loads (Baecher and Christian 2003). The interaction between the load and resistance becomes uncertain if the quantitative evaluation of the load  and  resistance  variables bears  any uncertain  elements. The  widest  and  simplest expression of reliability is in the form of reliability index and probability of failure which may be related mathematically.

There has been an intensive search for a design model that has sufficient probabilistic and statistical  robustness  to  address  soil properties  and  model  variability in  geotechnical engineering  until  1978  when  load  and  resistance  factor design  (LRFD)  method  was discovered   in  the  proposal  submitted  by  Ravindra   and  Galambos   (1978).  Their submission  which  received  approval  for  publication  in  the  first  edition  of load  and resistance  factor design  manual for steel construction, published  in 1986, formed  the basis for the development of a safety control format for steel structures in United States (US) codes. The  code clearly defines  the material  capacity as the resistance  and  the aggregate stress to be imposed on the structure as the load.

This period was actually preceded by the period of implicit consensus and understanding that the traditional methods (allowable and working stress designs) were not capable of meeting the technical challenges posed by the model and soil properties uncertainties. According to Phoon (2008), ‘LRFD is used in a loose way to encompass   methods that require all limit states to be checked using a specific multiple-factor format involving load and resistance factor’. It started as partial factor design approach (DA) in Europe; limit  state  design  (LSD)  in  Canada  and  LRFD  in  United  States,  where  practical application  of  the  model  and  development  of  its  code  have  been  recorded.  It  is noteworthy to say that the European design approach (DA) has undergone tremendous reliability-based refinement both in code calibration and design modeling that resulted in the recent design standard called Eurocode 7.

The National Research Council (2006) report acknowledged the fact that the inherent and unavoidable uncertainties resulting from soil properties measurements and model imprecision, and how they affect design decisions, need to be assessed by modern and

improved methods, simplified enough to attract world-wide acceptance. The very attempt to evaluate the reliability of a system is an acceptance of the fact that it is unrealistic to attain absolute reliability if uncertain elements have been identified in the system and thus making probabilistic analysis imperative.

Three  philosophical  issues  here  may  be  identified:  the  readiness  of  geotechnical engineering community to redirect the mind set towards a reliability based design format that has a good portion of its concept based on probabilistic analysis; the need to reduce the mathematical complexity in  reliability-based design (RBD) to a simplified model that can  be  handled  by non-specialist  in  numerical  and  statistical  analysis  and  lastly  the reliability based calibration of comprehensive  multiple factor formats that capture the variability in the sources of uncertainties (Robert et al, 2008).

RBD  was  introduced  to  civil  engineering  in  form  of  structural  reliability  theory by Freudenthal (1947) and Pugsley (1955). Like any other new concept, it was developed to improve the management of failure tendencies by carrying out design based on certain criteria that consistently reduce  the probability of failure to  its acceptable  minimum. However,  the  mathematical  rigour involved  in  the  application  of the theory even  in simple designs, made the concept unpopular. Several attempts were made towards simplifying RBD theory, but the most popular was the one by Cornell (1969) where he reformulated Gaussian equation into a model requiring just the second moment statistical descriptors (mean and covariance) of uncertain material parameters to evaluate the reliability index equation.

At this stage, however, the solution of Cornell’s equation was still found inconsistent when the performance function of factor of safety was replaced by that of margin of

safety, though their limit state equations were mechanically equivalent. Hasofer and Lind (1974) addressed this problem by proposing an equation of uncertain elements containing dimensionless  variables whose mean value and standard deviation are zero and unity respectively. They redefined reliability index whose geometric interpretation is the linear displacement between the closest point on the failure surface and the point defined by the expected values of the variables

The probability characteristics exhibited by inherent spatial variability in ASWSS properties make it exceptionally suitable for probability-based reliability treatment. It is clear that of all the sources of uncertainties, the natural random soil heterogeneity appears to have the worst effect on the failure mechanism of structural foundation soil. This fact and  the  deviation  of  the  actual  failure  surface  from  its  theoretical  domain  may  be dramatic for ASWSS.

The earlier solution of reliability equations ‘postulates an existence of average response that depends on the average values of the soil properties’ (Phoon  et al. 2003). This average response is assumed to be characteristically identical with that observed from a corresponding homogenous field having the same properties as the average properties of randomly heterogeneous soil. However recent works have revealed the possibility of the deviation of actual failure surface from its theoretically evaluated domain to a weaker part of material formation and thereby rendering the evaluated average strength of soil material higher than the actual mobilized strength. This has a serious consequence on design.

It is understandable that a comprehensive description of a random geological formation that mimics the exact spatial heterogeneity of its materials is not realistic. The progress

now, therefore, is the evolution of models and standards like Eurocode 7, first and second order reliability methods (FORM and SORM), Monte Carlo simulation (MCS), cross correlation structure (CCS), cross-spectral density matrix (CSDM) etcetera,  that reduce approximations  and  give  description  of random  fields  more  accurately (Baecher  and Christian, 2003 and Hema and Emil, 2015)..

1.2       Statement of Problem

The management of solid waste, though not totally neglected, has witnessed several but failed attempts to make it worthwhile especially in most part of the developing world.  In the absence of engineered repositories, relevant SWSS management skills and controlled disposal points, waste is indiscriminately disposed at open dumps situated at low lying areas or undeveloped and unused land masses usually not in close proximity to dwelling places (Ramaiah, et al., 2010). However, municipal expansion and proliferation of social and economic activities soon render such dump sites an environmental misnomer with subsequent abandonment of the use and re-allocation of it for infrastructural development ultimately. Not enough is seen in the treatment of SWSS by composting, and the high water content makes incineration critically seasonal.

Where   professionals   are   involved,   thorough   investigation   of   ASWSS   is   often recommended with the result revealing most of the time the anticipated weakness in soil data. Upon this weakness has been based the argument against the use of ASWSS and in favour of the search for alternative sites most of the time. Traditionally, the empirical expression of these risk levels (weakness) coherently and numerically is what has been absent. This is the solution provided by the recent development in geotechnical reliability and with the risk level of ASWSS explicitly, though not unanswerably, presented by a

tested and approved risk controlling model, ASWSS designs and decision making are made simple. This study was intended to make its impact in this direction.

The use of ASWSS for development, whether the preference of the developers or not, is a widely known practice despite the structural foundation failures recorded in the practice in  the  recent  times.  Developments  on  ASWSS  require  more  than  adherence  to  the mandatory provisions  of building codes. The design  details should be the product of tested and approved risk controlling techniques like reliability – based method.   Many catastrophic  and  fatal  failures  of landfill  structures  and  waste  dumps  were  recorded between 1997 and 2005 world-wide, resulting into the death of over 600 people and mobilization and redistribution of over 1.5 million m3 of waste (Gandolla et al., 1979; Eid et al., 2000; Blight, 2004 and Merry et al., 2005).  The environmental damage caused by these catastrophes, most of which were reported to have occurred in developing nations, was almost irreparable (Blight, 2008). One of the most recent and surprising cases was

the 2011 failure of a students workshop cited on an   ASWSS in a tertiary institution having a notable civil Engineering department in Nigeria.

In the face of the current scarcity of municipal land, old and abandoned Solid Waste Sites may not be allowed to waste without development. On the other hand, if failures are recorded despite claims of adequate site investigation and propriety of designs, it means something has to be done to acknowledge  and accept the peculiarities of SWSS that require  relevant  statistical  and  reliability  treatment  of its  soil data  to  make  it a  fair representation of both the tested sample and untested mass in the parent population. Until this is done, there will continue to be the tendency of either the design of structural

foundation members below its failure level or elusive selection of factor of safety in a defensible and uneconomical manner.

Results  obtained  from  modern  methods  (especially  Monte  Carlo  simulation)  have revealed worrisome discrepancies between the average response of spatially variable soils and the response of corresponding homogenous soil (Phoon et al 2003). For instance, Nobahar and Popescu (2000) and Griffiths et al. (2002) discovered up to 30% decrease in the  mean  value  of  bearing  capacity  of  spatially  variable  soils  having  coefficient  of variation of 50%, compared with the bearing capacity of corresponding homogenous soil with the same average soil properties.

An increase of 12% in the average settlement of spatially variable soil having coefficient of variation of 42% was equally discovered by Paice et al. (1996) over the settlement of corresponding homogenous geological formation having equivalent mean soil properties. The summary results of the work of Popescu et al. (1997) projected up to 20% increase in pore-water pressure for a non-homogenous soil deposit having coefficient of variation of

40%, over that of corresponding  homogenous  soil deposit with  equivalent  mean  soil properties.

The technical issues raised by these uncertainties constituted the target of some of the recent works. It is obvious, however, that not all the modern methods have what it takes to address the effect of these discrepancies in the use of soil data for design and analysis. While  appreciating  the  efforts  of these  researchers  and  those  mentioned  earlier,  one question  remains  unanswered  and  that  is  â€˜how  can  the  combined  efforts  of  these researchers  be harnessed  to  solve the obvious problem of variability in  geotechnical reliability?’’ This is part of the focus of this study.

1.3       Aim of the Study

The aim of the study was to explore the contrasting responses of ASWSS and adjoining NS to structural loading so as to establish the peculiar geotechnical characteristics of ASWSS.

1.4       Objectives of the Study

The specific objectives were

i.          To  obtain  the  properties  of  ASWSS  and  adjourning  Natural  Soil  (NS)  for comparison with those reported in similar and recent works of other areas.

ii.        To  obtain  design  values  of  soil  properties  using  Eurocode  7  and  reliability methods for both ASWSS and adjourning natural ground.

iii.       To  evaluate  the  reliability  indices  and  probabilities  of  failure  of  foundation designs for both ASWSS and adjourning natural soil in all cases using reliability based computer program (FORM5).

iv.         To  categorize,  from  the  results  of  ii  and  iii,  the  safety  indices  of  ASWSS foundation  designs  and  the proportion  of deviation  from  those  of the  natural adjourning  soil  and  make  recommendations  regarding  the use of ASWSS  for developments.

1.5      Significance of the Study

Despite large scale investigation of random fields for representative values, shear strength

characteristics and values so far reported in literature fall in an amazingly wide range as a result of SWSS field variation in its geotechnical character. The design engineer will therefore be in dilemma as to which value to adopt. Lack of the application of modern and  effective  probability-based   reliability  methods  in  establishing  the  engineering

behaviour of ASWS is partly responsible for the wrong selection of design data used in the prediction of failure zones that are higher than the actual field values.

The design  process  of this  study is illustrative  and  its results  the direct products  of reliability-based handling of ASWSS and thus may be reliably applied in judgments and decision making concerning the loads to be imposed on ASWSS sites. This will also save the community, developers and a relevant professional body further loses from ASWSS structural  foundation  failures  and  the  embarrassment  of  being  associated  with  such failures in the practice.

1.6       Delimitation of the Study

The application of only the geotechnical aspect of foundation design based on Eurocode

7,  Monte  Carlo  simulation  and  first  order  reliability  method  defined  the  analytical boundary of the study. The structural component of foundation design which requires second order reliability method (SORM) was not included.

A total of six abandoned  solid  waste  sites  (ASWSS),  two  in  Kaduna  North; two  in Kaduna South and two in the land between, were selected for study. This is due to the facts of literature that have shown appreciable similarity among the results of studies conducted on ASWSS.  A depth range of 1.5 to 3.5m were selected for material sampling and a scheme of geotechnical investigation was designed to include the conduct/determination  of:  unit  weight  of  soil,  water  content,  direct  shear  box  tests, atterberg limits, triaxial compression tests, grain-size distribution, consolidation and compaction  test. Direct and indirect application of some of these measurements  were made in relevant models while others were used in comparative assessment of the states of ASWSS and NS.

Bearing capacity measurement is conspicuously absent from the list, and this is due to the fact  that  there was no  intention of considering stress  imposition  on  ASWSS  soil by highway structures, though that may be a possible dimension of consideration. Furthermore, special attention is often given to heterogeneous soil outcrop and exceptionally weak formation in both design and construction of highways.

1.7       Limitations of the Study

An overall or system reliability (SYSREL) evaluation requires the analysis of both the

geotechnical and structural components of foundation structure. A higher order reliability method, however, is required for the structural analysis and this was not included in this study because of lack of requisite knowledge of the author in structural reliability theory and practice.

1.8       Research Questions

To provide a direction and focus to the execution of this study the following research

questions are generated

(i)        What are the differences between the engineering properties of ASWSS and those of the adjourning natural soil and how do they compare with those reported in literature?

(ii)       What procedures are employed to arrive at the appropriate design values of soil data?

(iii)      How do the reliability indices and probabilities of failure of ASWSS compare with those of corresponding natural soil in each case of design method?

(iv)      What are the tolerances and levels of safety indices of ASWSS on the standard expected performance classification table?

1.9       Organization of this Study

This study is divided  into  five chapters, the first of which presents  the problem and current situation of ASWSS and advances some modern techniques and approaches in design  and statistical  treatment  of data to  address  it. It highlights  the evolution  and improvement  trend  of these  approaches  and  generates  their  application  objectives  to achieve the overall purpose of the study within the defined measurement and analytical ambits. It is concluded  with a note on the potential weakness of the study, scholarly limitations of the author, benefits and beneficiaries of the results of the study. The second chapter describes mainly the published principles and practical approaches to a realistic characterization  of  ASWSS  and  corresponding  design  methodologies  that  take  into account  the  randomly  variable  distribution  of its  soil  character.  In  this  section,  the principles  of  reliability-based  design,  their  theoretical  basis  and  validity  and  the feasibility of their practical application are well discussed.

The third chapter relates to the plan for the execution of the study and contains briefs under  the  following  subheadings;  design,  subjects,  instrumentation  and procedure/methods of data analysis. The fourth chapter presents a procedural model of ASWSS data statistics and employs first order and Monte Carlo Simulation reliability to predict the expected responses of ASWSS and its corresponding NS under differently specified stress characterization. The fifth chapter is a brief that summarizes the salient features of the study including key findings from modern design and analytical efforts to discover the true state of ASWSS. It is concluded with relevant proposals on the possible geotechnical  and  statistical  solutions  to  the  problems  associated  with  the  use  of ASWSS for developments.


This material content is developed to serve as a GUIDE for students to conduct academic research



SAFETY ANALYSIS OF STRUCTURAL FOUNDATIONS BUILT ON ABANDONED SOLID WASTE SITES

NOT THE TOPIC YOU ARE LOOKING FOR?



PROJECTOPICS.com Support Team Are Always (24/7) Online To Help You With Your Project

Chat Us on WhatsApp » 07035244445

DO YOU NEED CLARIFICATION? CALL OUR HELP DESK:

  07035244445 (Country Code: +234)
 
YOU CAN REACH OUR SUPPORT TEAM VIA MAIL: [email protected]


Related Project Topics :

DEPARTMENT CATEGORY

MOST READ TOPICS