STUDIES ON POST CONSTRUCTIONAL SETTLEMENT PROBLEMS AT A SITE IN COCHIN

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1 STUDIES ON POST CONSTRUCTIONAL SETTLEMENT PROBLEMS AT A SITE IN COCHIN 1 Alka Abraham, 2 Anitha Murali, 3 Anna Tomy Manavalan, 4 Mable Mathew, 5 Vedika V 1 Asst. Professor, Department of Civil, INDIA 2 VIII Department of Civil, INDIA 3 VIII Department of Civil, INDIA 4 VIII Department of Civil, INDIA 5 VIII Department of Civil, INDIA ABSTRACT This dissertation work deals with the study of causes of post constructional settlement at ICTT, Vallarpadam and remedial measures. The International Container Transshipment Terminal (ICTT) situated at Vallarpadam has been encountering soil settlement problems for the past few years which have adversely affected its transshipment effectiveness. Soil tests were carried out to identify the type of soil. The most probable reason for the failure was identified.a ground improvement technique known as URETEK deep injection method was suggested to recover from the settlement problems and prefabricated vertical drains (PVD) and stone columns to avoid post constructional settlement at areas for future expansion. Design of PVDs and stone columns are included in this paper. Index Terms: ICTT, Stone Columns, PVD, URETEK. INTRODUCTION Most of the greater Cochin area where ICTT is situated consists of marine clays which are known for its high compressibility and poor shear strength. The depth of the soft clay layer is considerable; often extending to meters. Hence it is not economically feasible to replace them by better material. There have been innumerable problems connected with settlement of structures founded on these soils and

2 with embankments and deep cuttings for both railways and highways. These problems have been extended to the areas of ICCT which has caused severe structural as well as geotechnical problems. The present work envisages a systematic study of the various settlement problems at ICTT and eradicatory measures. LITERATURE REVIEW Narain Jagadish and Ramanathan (1970) [1] were perhaps the earliest to observe the physical properties of marine clays in Kerala. According to them, the marine clays of this region undergo irreversible changes in plasticity characteristics. Air drying was found to cause formation of aggregates, which was considered responsible for the change in plasticity. Organic matter and carbonates are recognized as major cementing agents contributing to particle aggregation and in Atterberg limits upon drying Rao et al.(1989)[2]. Guetif et al., (2007) [3] reported based on improvement of a soft soil by stone columns is due to three factors. The first one is inclusion of a stiffer column material (such as crushed stones, gravel, and others) in the soft soil. The second factor is the densification of the surrounding soft soil during the installation of stone column. The third factor is the acting as vertical drains. So, the insertion of stone columns into weak soils is not just a replacement operation and stone column can changes in both the material properties and the state of stresses in the treated soil mass, Ghanti & Kashliwal, (2008) [4]. Prefabricated vertical drains are artificially created drainage paths, can be installed by several methods and can have a variety of physical characteristics. The vertical drains can reduce settlements from years to months. Due to this reason, most settlement occurs during construction phase itself, and post construction settlements are reduced to bare minimum, Nand Kishore, Railway Board Government of India,(2004)[5]. METHODOLOGY Any type of construction depends primarily on soil properties. A series of tests were conducted to identify the soil properties. The results obtained are given in TABLE I.

3 TABLE I. SOIL PROPERTIES SOIL PROPERTIES Specific gravity 2.72 Permeability (cm/s) 5.29x10 7 Max. dry density (g/cc) 1.96 Optimum moisture 25% content (%) Liquid limit (%) 48 Plastic limit (%) 29 Coefficient of 3.29x10 4 consolidation (cm 2 /sec) Compression index 0.66 Due to high compressibility of these soils, the consolidation settlements are of a very high order and from structural safety point of view, it would be better if major portion of this consolidation settlement takes place before/during construction phase itself. But it is of great concern that despite of being provided with drainage solutions like PVDs and stone columns, the settlement has become a great problem resulting in constant repair works and other structural problems as well. This has necessitated the need to look more into the design of PVDs and improvement methodology that were adopted at the site. From the studies, the reasons that were found which may have caused the present scenario at ICTT are listed below The depth of PVDs and stone columns provided at the site is 15m.from the bore hole details, it was observed that, the depth ranging from consists of blackish silty clay with organic matter. For proper drainage, the depth of PVDs and stone columns should extend below the highly compressible layer, preferably the thickness of clay layer which was not employed originally. Hence, improper drainage owing to the inadequate depth of PVDs and stone columns can be regarded as one of the reason for the continuous settlement of ground. A PVD of width 100mm,thickness 5mm was provided at the site for a spacing of 1.15m.considering the design of PVD provided at the site, Time taken for 70% consolidation with PVD =3 months. Total balance settlement after commissioning considering 70% consolidation = 407mm.

4 Instead of providing a period for 90% consolidation which is normally employed conventionally, the time given corresponded to 70% consolidation. The solution includes a new design for stone columns and PVDS that can be used at site which can reduce the amount of settlement at sites for future expansion. In the design of stone columns various trial designs were done according to the IS (part I) 2003.Among the trials, stone column which increases the bearing capacity of soil considerably was adopted as the optimum one. The PVD design also consists of various trials using various types of PVD drains locally available. The PVD design is an empirical method. The trial which gives minimum time for consolidation from the empirical equation is taken for using at site. URETEK geoplus. Improved ground bearing capacity is achieved by this patented technology that compacts and densifies foundation soils, by injection through tiny holes down to multiple depths in weak strata. Hence it can be effectively used in areas of ICTT. RESULTS AND DISCUSSIONS The variations in bearing capacity of soils with different diameters of stone columns as per the trial designs are given in TABLE.II TABLE.II CAPAVITY WITH DIAMETER Diameter (mm) VARIATION OF BEARING Spacing (m) Bearing capacity (kn/m 2 ) t= ln Advancements in the field of ground improvement have led to the fabrication of techniques that can be used in areas adversely affected by post construction settlement. One such technique is the non toxic mix of chemicals named From the table, it can be inferred that as the diameter of stone column decreases, the bearing capacity of stone column increases. Hence a stone column of diameter 0.7m at spacing of 1.3m was found to be the optimum one.

5 Details about the optimum design of PVD to be adopted at sites for future expansion are given below: Among the various trial designs done with different drain materials, SOLPAC was found to be the most effective one. The time for consolidation considering 90% degree of consolidation was the least for SOLPAC drain material. Variation in time taken for 90% consolidation for different drain materials with spacing is shown in FIGURE.1 time for consolidation in months ALIDRAIN 90%consolidation MD7407 MD7007 MD88H MD88HD SOLPAC 0 spacing 1 in m 2 3 FIGURE.1: Variation in time taken for 90% consolidation for different drain materials with spacing Specifications of SOLPAC: width 400mm, thickness 4mm.Triangular arrangement pattern was adopted since it provides more uniform consolidation between drains. CONCLUSION From the geotechnical experiments conducted on the sample of soil collected from the location and bore hole details, soil structure showed weak zones till a depth of 27.8m which are highly compressive in nature. The soil structure itself can define the reason for continuous settlement at ICTT. Moreover improper drainage owing to inadequate depth of stone columns and PVDs may have caused the present scenario at ICTT. URETEK ground improvement resin can be adopted at sites affected by post construction settlement. At areas for future expansion, PVDs and stone columns can be given for a depth of 30m including the weak zone of soft soil. REFERENCES [1] Narain J. and Ramanathan T. S., Variation of Atterbergs limits in relation to strength properties of highly plastic clay. Vol.9, 1970, page [2] Rao, S. M., Sridharan A. and Chandrasekharan S. Influence of drying on liquid limit behavior of marine clay, Vol 27, 1989, page [3] Guetif and Cabe. Bryan, Ground Improvement using the vibro stone

6 column technique", 2007 Department of Civil National University of Ireland, Galway. [4] Ghanti & Kashliwal, Analyisis of hydro test results for steel tank on stone column ground improvement, 2008,IIT Madras Chennai. [5] Nand Kishore, Railway Board Government of India, 2004 [6] IS 15284(PART 1): 2003, Design and construction for ground improvements Guidelines BIOGRAPHIES Alka Abraham Anitha Murali Asst. Professor, Department of Civil Vedika V mablemathew92@gmail.co m vedikavedu1991@gmail.co m Anna Tomy Manavalan annatomymanavalan@gma il.com Mable Mathew