Damage to a plant caused by construction-induced settlement

Transcription

1 Sustain Environ Res, 22(5), 7-4 (2012) 7 Damage to a plant caused by construction-induced settlement 1, 1 2 1,3 Gordon Tung-Chin Kung, * Chih-Ta Tsai, Yu-Geng Tang and Juu-En Chang 1 Sustainable Environment Research Center National Cheng Kung University Tainan 70955, Taiwan 2 Department of Architecture Hwa-Hsia Institute of Technology New Taipei City 23568, Taiwan 3 Department of Environmental Engineering National Cheng Kung University Tainan 70101, Taiwan Key Words: Geotechnical engineering, settlement, landfill, hazards ABSTRACT This study presents a case study regarding the damage to the Laboratory Waste Treatment Plant caused by construction-induced ground settlement This plant is located in the Annan Campus of National Cheng Kung University in Tainan, Taiwan The purpose of the plant is to treat the wastes produced by laboratories of universities and high schools in Taiwan After the construction of this plant, the differential settlement of ground and slab around this plant was observed in 2006 A number of cracks can be found on the surface of ground slab In addition, the tanks for storage of medicament and wastes tilt due to the differential settlement of slab The facilities of this plant may be damaged and the leakage of hazardous waste may be triggered if no measures are given to deal with the settlement problem This study investigates the settlement problem encountered in this plant and presents a procedure to effectively treat settlement-induced damage for preventing the probable hazards INTRODUCTION Construction-induced ground settlement has received great attention due to damaged structures and buildings adjacent to the construction site caused by induced settlement Although technologies and analytical methods for construction have been significantly improved or developed over the past decades, building damage incidents caused by construction-induced ground settlement are still reported occasionally The factors causing the building damage are complicated It would be desirable to learn the lessons through studying case histories, in which the detailed construction information and damage condition are available This paper investigates a forensic case regarding the damage to a laboratory waste treatment plant caused by construction-induced ground settlement In this case, the laboratory waste treatment plant was constructed by the National Cheng Kung University based on a 10-yr project funded by the Ministry of Education, Taiwan After completing the construction of this plant, the considerable ground settlement was gradually observed and would potentially cause the damage to the treatment facilities The owner of this plant (National Cheng Kung University) and the contractor have frequently negotiated to resolve this problem according to the contract of construction, but a conflict between two parties still exists In this regard, it is necessary to conduct the complete site investigation and evaluate the damage potential of treatment facilities for discussing the subsequent means and process of settlement treatment A series of site and damage investigations have been conducted and measures for treating the settlement-induced problem proposed This study first conducted the site investigation, including stratigraphy, damage evaluation, and structure safety analysis The original design was reviewed as well as a series of consolidation tests were conducted for settlement analysis Then, factors causing settlement-induced damage to the plant were comprehensively discussed Finally, the measures for mitigating the settlementinduced damage to the plant were proposed *Corresponding author tckung@mailnckuedutw

2 8 Kung et al, Sustain Environ Res, 22(5), 7-4 (2012) LABORATORY WASTE TREATMENT PLANT (LWTP) Waste produced by laboratories at schools and research institutions as well as that produced by industrial testing facilities differs from waste produced by normal factories Such waste is small in quantity, complex in terms of types, generally toxic, flammable, corrosive, and often includes newly added materials; most is dealt with through contract processing However, waste management methods have been continually updated and have become more strongly regulated Legal processing methods of contracted processing enterprises have been limited, leading to large increases in waste management fees Due to problems of an inability to find legal processors under financial constraints, some schools have been forced by excessively high waste management fees to store toxic waste within campuses, causing severe dangers to teachers and students The Ministry of Education has responded to the problem in the 400th Department of the Treasury review Combined Planning Report on Improving Pollution Prevention Public Facilities for School Campuses at Various Levels in the expectation of satisfactorily resolving processing problems and potential threats to campus safety arising from laboratory waste (including waste liquids) for schools at various levels and associated laboratories supervised by education administrative institutions National Cheng Kung University was selected to establish an Environmental Resource Research and Management Center and to establish a laboratory waste recycling stations and a laboratory building to properly process laboratory waste from across the nations and to provide processing, educa tional, research, and demonstration services The LWTP is located in the Annan campus of the National Cheng Kung University in Tainan, Taiwan (Fig 1) The National Cheng Kung University was granted a 10-yr project, in which the budget of approximately USD 11 million is used to construct the LWTP and an annual budget of about USD 135 million is given to operate this treatment plant for the period of 10 yr (a) bird's eye view Annan Campus of National Cheng Kung University Fig 1 Location of the Annan campus of National Cheng Kung University To establish a professional LWTP is a very important policy for the Ministry of Education, Taiwan in 2000s due to: (i) the chemical wastes produced by laboratories of universities and colleges are very complicated and difficult to treat, (ii) the produced laboratory wastes without proper treatment may pollute the environment and violate the law, (iii) research and teaching related to treatment of laboratory wastes are the crucial objective in the Ministry of Education, and (iv) the treatment fee for laboratory wastes is high due to the few qualified treatment plants in Taiwan Based on these considerations, the Ministry of Education has proposed a 10-yr project for establishing a professional LWTP with a annual treatment capacity of 500 t The LWTP was constructed in the period during 2002 to 2005, and the operation was initiated in late 2005 Figure 2 shows the bird's eye view and side views of the LWTP The plane view of the treatment plant, including the equipment section and the receiving and storage section, is shown in Fig 3 In general, laboratory waste processing is performed according to the principles of stability, resourcification, and harmlessness, and involves three processing system operations of physical-chemical processing, incineration, and melting (see equipment section of Fig 3) in order to detoxify and stabilize toxic waste (b) side view Fig 2 Appearance of the LWTP (c) side view

3 Kung et al, Sustain Environ Res, 22(5), 7-4 (2012) 9 Fig 3 Plane view of the LWTP and observed differential settlement, cracks and damage and further perform recycling In the course of processing, the overall operations, which do not emit harmful gases, liquids, and solids, can aid schools at various levels within the nation in appropriately processing laboratory waste and restoring safe and clean learning environments to schools INVESTIGATION OF DAMAGE TO THE PLANT In the middle of 2005, the settlement of ground slabs and surrounding ground surface could be observed At that stage, the contractor had adopted a ground improvement measure to reduce the settlement However, such settlement of ground and slab around the LWTP obviously increased in 2006, which implied that the effectiveness of the ground improvement in this case was low Figure 3 also shows various visible differential settlement and settlement-induced damage to the plant As shown in this figure, a number of cracks can be found on the surface of ground slab both in the equipment section and the receiving and storage section, and the significant differential settlement can be observed around the LWTP Many cracks can be observed on the walls of restrooms and offices of the LWTP as well In addition, the tanks for storage of medicament and wastes tilt due to the differential settlement of ground slab With the increase of settlement induced in the LWTP, the equipment may be damaged, piping may be broken, and/or the leakage of hazardous waste may be triggered An examination to evaluate the safety of structure has been conducted In the design phase of the LWTP, a 3-m landfill was planned and it was expected that significant ground settlement would be induced accordingly Therefore, the engineer designed piles to bear each of columns of the structure for preventing the effects of ground settlement The results of structure safety examination demonstrated that installation of piles indeed effectively reduce the risk of ground settlement-induced damage However, the treatment equipment and piping still encountered potential risk of failure As a result, the settlement problem in the LWTP must be effectively treated as soon as possible for preventing the possible hazards STRATIGRAPHY To study the mechanics of settlement induced in the LWTP, this study first investigated the stratigraphy of the LWTP The Annan campus of NCKU, in which the LWTP is located, belongs to an extensive saline, called the Tai-Chiang Inner Sea, before the eighteenth century Then, the Tai-Chiang Inner Sea had gradually vanished due to the deposits accumulated Most of the area of the Tai-Chiang Inner Sea became wetlands and was often used as the salt pan over the past hundreds of years According to the report of construction of the LWTP, a fill of 3 m was performed mainly using the sandy soils prior to the construction of the LWTP due to the low-lying wetland site According to the report of site investigation and analysis for construction of LWTP [1], the stratigraphy of the LWTP mostly consists of soft clay within the depth from 6 to 24 m (Fig 4) In this site, the uniform fill at depths from 0 to 3 m and uniform sandy layer at depths from 3 to 6 m are presented Essentially, the settlement observed around the LWTP might be attributed to the consolidation settlement of clayey layers caused by the loading of landfill and the structure and facilities of the plant Certainly, the consolidation characteristics of clay may also play an important role in the settlement-induced damage in the LWTP This paper conducted an extensive site investigation and took undisturbed sample to conduct a series consolidation tests The results of consolidation tests reveal that the compression index of the clay measured by the consolidation test is in the range of FACTORS CAUSING SETTLEMENT- INDUCED DAMAGE TO THE PLANT It is desirable to further investigate the factors causing the settlement-induced damage in the LWTP

4 3 Kung et al, Sustain Environ Res, 22(5), 7-4 (2012) Fig 4 Layout and results of stratigraphy investigation Fig 5 Plan view of foundation of the LWTP although the effect of clay consolidation is considered to be the crucial factor The foundation pattern of the LWTP may be another crucial factor causing the settlement problem Figure 5 shows the designed foundation of the LWTP In the design phase, the piles were employed to bear the columns and provide the additional capability against the settlement expected after the construction of the LWTP In addition, the foundation girders, constructed to connect the various columns and piles, can strength the stiffness of foundation of the LWTP In addition, the ground slab was designed not to be connected with the structure of the LWTP This means ground slab would settle freely to avoid the occurrence of differential settlement According to the damage investigation, no significant cracks can be observed in upper area of the equipment section (Figs 3 and 5) This may be due to the contribution of construction of additional girders Such design reflected that the settlement problem had been analyzed and corresponding measures adopted

5 Kung et al, Sustain Environ Res, 22(5), 7-4 (2012) 3 Figure 6 schematically displays the settlement profile of ground slab observed in-situ Note that the direction of settlement profile shown in Fig 6 is the y-direction denoted in Fig 5 The in-situ observations reveal that the settlement of the pile cap is significantly less than that of ground slab in the central part The curviform settlement profile of ground slab can be clearly observed, which reflects that the effect of the absence of the foundation girder in y-direction on the settlement characteristics is significant In practice, it is common to construct the foundation girder to upgrade the capability against settlement However, the reason that the contractor did not adopt such design may be attributed to the fact that the span of 175 m is larger Hence, the ground slab without the girders was designed not to be connected with the structure of the LWTP solidation analysis are determined from the results of laboratory tests The estimation of increment of effective stress sustained in the soils is performed using the method proposed by US Navy [3] Figure 7 exhibits the consolidation settlement contours determined based on the estimation of consolidation settlement of 1 data points and the assumption of the absence of pile foundations Also shown in Fig 7 is the contour of settlement within the office building for managing the LWTP Essentially, the maximum settlement is expected to occur in the central part of the LWTP The estimated settlement within the site of the LWTP is in the range of -42 cm, which is considerable and probably causes damage to the facilities of the LWTP As mentioned previously, for preventing the possible damage, the pile foundations were designed to bear the structure of the LWTP and thus reduce the CONSOLIDATION SETTLEMENT ANALYSIS The consolidation analysis was conducted based on the results of consolidation tests using the undisturbed sample taken from the site of the LWTP Note that the effect of the pile foundation was not considered in the consolidation analysis, which means such analysis is intended to estimate the maximum consolidation settlement The loading of the fill and LWTP structure is simultaneously considered in the consolidation analysis For simplicity, the one-dimensional consolidation theory [2] and the averaged soil layers are employed The parameters required in the con- Fig 6 Schematic settlement profile of ground slab 100 Distance (m) Office 28 LWTP Settlement (cm) Distance (m) Fig 7 Estimated maximum consolidation settlement of ground surface

6 3 Kung et al, Sustain Environ Res, 22(5), 7-4 (2012) expected consolidation settlement According to the in-situ observations and estimations of settlement herein, the presence of pile foundations can reduce more than 10 cm differential settlement According to the one-dimensional consolidation theory, the percent consolidation of clay computed from 2005 to 2008 is about 40% In other words, the theoretically accumulated settlement up to present is in the range of 13 to 17 cm, which is practically identical to the in-situ observed settlement approximately 15 cm This study continuously observed the settlement of ground and slab from 2008 to 2011 The results reveal that approximateky 10 cm settlement was induced in this period These results indciate that the one-dimensional consolidation theory is practically capable of reasonably estimating the settlement in this case Such results indicate that the settlement of about 9 to 15 cm will be expected in the future STRUCTURE DEFORMATION ANALYSIS Essentially, the differential settlement sustained by a structure may play a more crucial role in the evaluation of damage potential than the maximum settlement For comparison, the ground slope (GS) defined below [4] as shown in Fig 8 is employed in this study to preliminarily evaluate the damage potential: GS = (1) L where is the differential settlement and L is the distance between two footings When assuming the absence of pile foundation, the estimated GS in right part of the LWTP (Fig 7) is equal to 1/350, which is considered very slight to slight damage potential estimated by others [5-7] Note that the pile foundation was designed in the LWTP to bear the columns of the LWTP for reducing the excessive consolidation settlement In this regard, the settlement of the pile caps and foundation girders would be much less than the ground surface settlement (Fig 7; about Fig 8 Definition of ground slope cm) It should be noted that although the purpose of constructing piles is to reduce the settlement, the presence of piles and absence of foundation girders in y-direction (Fig 7) cause the significant differential settlement profile, which may cause the damage to facilities of the LWTP Based on the the analysis result of settlement and in-situ observations, the estimated GS would become approximate 1/50, which means the structure may be severely damaged In this case, the tilt of tanks located in receiving and storage section can reflect this point MEASURES FOR MITIGATING SETTLEMENT-INDUCED DAMAGE Figure 9 shows the principle of treatment to mitigate the settlement-induced damage in the LWTP The potential damage to structure and facilities is primarily induced by the differential settlement, which is affected by the interaction between the ground settlement and the foundation Conceptually, there are two principles to treat the settlement problem; (1) reducing the ground settlement, and (2) increasing capability of structure against differential settlement For the former, serveral measures may be taken such as improving the ground properties and reducing the loading (eg, removal of the fill) For the latter, the reduction of Fig 9 Principle of treatment in the LWTP case

7 Kung et al, Sustain Environ Res, 22(5), 7-4 (2012) 3 ground settlement is not the key point, whereas the measures capable of increasing the capability of structure and facilities against differential settlement are more desirable The construction of additional foundation girders or piles may be the options to achieve this goal The feasibility of applying such principle (eg, reducing the ground settlement) is first evaluated A set of waste treatment equipments have been installed in the LWTP The cost of equipments is as high as about USD 6 million and such heavy equipments can not be removed during the process of treatment In this regard, use of the ground improvement method may not be adequate because the ground slab may be partially lifted by the grouting and the equipments in the LWTP may be damaged accordingly The risk of damage to the equipment must be completely avoided As mentioned previously, the ground settlement of approximately 25 cm has been induced in this case The in-situ observation indicats that the settlement of columns supported by the piles is much less than the ground slab without the construction of foundation Fig 10 Flow chart proposed for forensic activity in the LWTP case

8 4 Kung et al, Sustain Environ Res, 22(5), 7-4 (2012) girders and piles In other words, use of piles can effectively reduce effects of the ground settlement Therefore, it may be feasible to construct extra piles and girder foundation to enhance the stiffness of structure against settlement In fact, such proposal is only feasible for part of the LWTP because the heary waste treament equipments are not allowed to remove Therefore, other methods to increase the capability of the waste treatment equipments against ground settlement must be considered, eg, the micropile may be one of choices Figure 9 only shows the principle of treatment for the LWTP case It is necessary to further conduct forensic activities to clarify the responsibility of the contractor, and then plan a strategy for effectively, practically treating the settlement-induced damage to equipments and facilities In this regard, this study proposes a flow chart to achieve this goal (Fig 10) In this flow chart, first two steps (investigate settlement induced damage to LWTP, examine structure safety of the LWTP) have been performed in this study Then, it is necessary to conduct the forensic investigation to clarify the responsibility of the design party and the contractor Subsequently, it is essential to conduct the advanced settlement analysis to predict the scenario of settlement-induced damage in the near future based on the existing data Another crucial task in this step is to implement the damage recovery in this plant, including ground slab repair, tank adjustment, and flexible piping replacement Such measures can enhance the capability of facilities against differential settlements Finally, it is suggested to conduct a 5-yr plan to monitor the settlement of ground and slab and reevaluate the potential damage to facilities regularly, eg, every six months CONCLUSIONS The issue of settlement-induced damage to existing buildings is common and many techniques or analytical methods have been developed and employed in the design phase to avoid such problem From geotechnical engineering aspects, it is not difficult to deal with the landfill and construction-induced settlement, which may cause the damage to facilities of the plant Based on the investigation of this study, the designer had made efforts to prevent the possible scenario of settlement, including employment of piles, flexible ground slab, and ground improvement However, this plant still encourntered the settlementinduced damage This is the main objective of this study to investigate the forensic issue in the history of this case In this case, the pile foundations designed in the LWTP to enhence the capability of structure against ground settlement, is considered reasonable, while the lack of foundation girders in y-direction of this case (Fig 5) causes the situation of hazard Essentially, the soft stratigraphy and the fill of 3 m are also the predominant factors This study further analyzed the settlement behaviour and proposed a procedure for mitigating the possible hazards It is suggested that enhancement of the capability of the LWTP against differential ground settlement is more desirable Lessons could be learned from this case ACKNOWLEDGEMENTS The study on which this paper is based was supported by the Environment Resource Management Research Center through Grant Nos H96-7 and H97- A06 The financial support is greatly appreciated REFERENCES 1 ERMRC, Report of Site Investigation and Analysis for Construction of Laboratory Wastes Treatment Plant Environment Resource Management Research Center, National Cheng Kung University, Tainan, Taiwan (2002) (in Chinese) 2 Holtz, RD and WD Kovacs, An Introduction to Geotechnical Engineering Prentice-Hall, Englewood Cliffs, NJ (1981) 3 US Navy, Soil Mechanics, Foundations, and Earth Structures NAVFAC Design Manual DM-7, Washington, DC (1971) 4 Boscardin, MD and EJ Cording, Building response to excavation-induced settlement J Geotech Eng ASCE, 115(1), 1-21 (1989) 5 Bjerrum, L, Discussion Conference on Soil Mechanics and Foundation Engineering, Vol 3, Norwegian Geotechnical Institute, Oslo, Norway, 1-3 (1963) 6 Son, M and EJ Cording, Estimation of building damage due to excavation-induced ground movements J Geotech Geoenviron, 1(2), (2005) 7 Schuster, M, GTC Kung, CH Juang and YMA Hashash, Simplified model for evaluating damage potential of buildings adjacent to a braced excavation J Geotech Geoenviron, 135(12), (2009) Discussions of this paper may appear in the discussion section of a future issue All discussions should be submitted to the Editor-in-Chief within six months of publication Manuscript Received: May 27, 2012 Revision Received: August 13, 2012 and Accepted: August 13, 2012