Construction of Embankment with Ground Improvement at By Pass of NH 34. Dilbag Singh Construction Head Transport Vertical HCC Ltd.

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1 Construction of Embankment with Ground Improvement at By Pass of NH Dilbag Singh Construction Head Transport Vertical HCC Ltd. Abstract The Baharampore Farakka corridor of the National Highway 34 (Km through ) is being constructed by Hindustan Construction Company Limited consisting of 10 Km green field bypass. This bypass height varies from 0.22 meters to as high as 17 meters demanding the need for the high embankment construction, for which estimating and checking of slope stability, settlement (quantum and time) and bearing capacity becomes important for ensuring specified pavement performance. In this paper work the embankment height more than 6 meters has been considered as the high embankment and is discussed therein. The obvious solution to such geotechnical problems is to adopt the Ground Improvement Technique i.e. soil replacement with sand with enhancing the soil bearing capacity by introducing the geo synthetic materials at regular intervals. With an objective of searching and adopting for an alternative technically superior and sustainable solution while being cognizant of the available construction time other resource schedules Stage Construction using consolidation has been adopted. It is adopted to replace the top soil at the Original Ground Level with freely draining, compacted sand and/or gravel shall bring the settlement within tolerable ranges. The adopted method is based on the philosophy of the Design-Build, as shown in the Figure 1 The principle on which this analysis design-construction cycle is based upon the design is not complete until the construction is. It is mainly adopted to increase the shear strength of the soft clay that would otherwise be inadequate carry the intended embankment load without failure. In this stage construction the advantage of increase in shear strength of sub soil strata due to consolidation by surcharge of embankment loading is taken into account. The gain in shear strength is a function of angle of shearing resistance increased in terms of effective stress parameters and degree of consolidation. 116

2 The construction of high embankment in stages is so that all its settlement takes place prior to the road pavement construction and post construction settlement is minimum and within tolerance limits as shown in the figure 2 below. The following is the process we had adopted for the construction of high embankment in stage wise. 1. In order to dissipate the pore pressure faster an additional drainage face shall be provided by replacing the top soil with sand which is called as Soil Replaced With Sand (SRWS). 2. The embankment construction shall take place in stage wise depending upon the height of the embankment with definite waiting period after every stage constructed. 3. The adopted method involves utmost careful instrumentation with piezometers for checking pore water pressure, settlement gauges for measuring settlement, inclinometer for measuring inclination, etc Figure 2.Stage Consolidation effect on Design Factors The constructed embankment shall be individually analyzed for bearing capacity, settlement, slope stability (local and global), lateral squeezing and long term stability. Assessment and analysis of the soil parameters and checking of slope stability, factor of safety against lateral squeezing, end of construction factor of safety under static and dynamic conditions for both drained and undrained conditions were done in this paper which shall be discussed further. INTRODUCTION: The NH34 Package III (KM through ) corridor being constructed by HCC Ltd. consists of Berhampur bypass (KM through ). The project is located on Gangetic plains and has clay deposits at the top in the form of silty (fine sandy)-clayey (varying stiffness) stratum, which in-turn are lying on medium to dense silty and strata. This bypass consists of embankment construction, including high embankments (height of embankment > 6 m), for which estimating and checking of slope stability, settlement (quantum and time) and bearing capacity becomes important for ensuring the specified pavement performance. All the embankments are geometrically proportioned to have one vertical: two horizontal (1V:2H) side slope. The intended pavement width for Stretch is 27.5 m, while the intended top width for both the approach embankments is 36 m. The height of embankments is ranging between 0.22 m to 17 m. 2. Geological Condition of Project Site: The general geology of the area indicates that the said stretch of pavement fall in the 117

3 Gangetic alluvium (flood plains) and stratigraphically represent Pandua formation. It consists of a thick river born sediments dominated with loose gravel, sand, silt and mixtures, sometimes intercalated with clay deposits. The spatially varying clay is soft to medium stiff and has considerable compressibility. Baharampur bypass is located in seismic Zone III moderate damage risk zone (zone factor 0.16). 3. Design Factors A preliminary analysis and design based on slope stability analysis (Bishop's modified method), settlement analysis (Terzaghi's theory of one-dimensional consolidation) and bearing capacity analysis (methods by Pilot, Silvestri, Meyerhoff) indicated that meeting the required factor of safety without any ground improvements was not possible either/or in static and/or dynamic conditions. To mitigate unsafe ground conditions, to make the slope stable and to bring the settlement within tolerable range, it was proposed to replace the top soil at the original ground level (OGL) with freely draining, compacted sand and/or gravel. The foundation embankment design is based on the principle of stage construction. Stage construction is employed mainly as a means of gradually increasing the shear strength of soft clay, which would otherwise be inadequate to carry the intended embankment load without failure. Due to the foundation consolidation (occurring during the waiting period) resulting from the embankment loading, and the pore water pressure dissipation, there is a gain in the shear strength parameters and hence the bearing capacity of the soil. The principle on which this analysis-design-construction cycle is based upon is the design is not complete until the construction is. The design will receive real-time data from the instrumentation done at site. The design and construction process shall be periodically fine-tuned based on real-time data. 4. Factors for Selection of Method Since the undrained shear strength of base soil is low initially, this restricts the construction of high embankments over soft clays in single stage or at a fast rate construction. These problems may be overcome by the method of stage construction with observational method. Stage construction is employed mainly as a means of gradually increasing the shear strength of soft clay that would otherwise be inadequate to carry the intended embankment load without failure. In stage construction, advantage of increase in shear strength of sub-soil strata due to consolidation by surcharge of embankment loading is taken into account. The gain in shear strength is a function of angle of shearing resistance improved in terms of effective stress parameters and degree of consolidation. The load settlement profile in response to staged construction is show below. The effects of stage construction are observed on other soil parameters and are summarized below Clause no of IRC: , Guidelines for the Design of High Embankments, allows construction of embankment in stages so that all its settlement takes place prior to road pavement construction and post construction settlement is minimum and within tolerance limits. Utilizing this principle, construction of these high embankments is in the following stages: 118

4 Initially, depending upon the thickness of soft clay, the soft clay at OGL shall be replaced with a suitable thickness of free-draining compacted sand. This layer is called as soil replaced with sand (SRWS). This sand will introduce an additional drainage face at the top. The pore pressure dissipation will be faster with an additional draining face. The embankment construction will proceed in stages. In the Ist and IInd stages, 5 to 6 m high embankments will be constructed and if required additional stages shall be adopted with commensurate to requirement waiting period. Instrumentation will be applied to measure the changes in the strains, pore-water and other movements. These constructed embankments shall be allowed to stand and settle for a monitored period of close to 90- days (actual period will be decided based on the changes in the soil parameters) Schematic Stage Construction. 5. Methodology 5.1. Equipment Required Following equipments are required for construction: - Site Clearance 1) Excavators / Backhoe JCB 2) Tipper /Dumpers 3) Crawler Dozer with ripper arrangement Embankment / Sub grade Construction-For Filling Section 1) Excavator 2) Tippers /Dumpers 3) Motor Graders 4) Vibratory Compactor 5) Tractors Dozer with Disc Harrows/ Spreading Blade / Ploughs 6) Water Bowser with Sprinklers etc. 7) Crawler Dozers 119

5 5.2. Clearing & Dewatering The ground level soil is tested for OMC and MDD before SRWS and /or embankment. Further the CBR and soil classification is carried out in lab. After stripping, if SRWS is required excavation and SRWS are done as follows or directly Embankment construction is followed 5.3. Excavation 5.5. Embankment The methodology for embankment construction is similar to that of normal road construction, however following points are added. Excavate ground to pre determined depth and width. Pl note that the top width of excavation is equal to embankment toe plus twice the depth of SRWS fill on either side. Remove unwanted material from the ground. Place geotextile layer upto top of ground level 5.4 SRWS Fill Fill the trench with free draining material - sand to the required height in layers Material for embankment has been specified is elaborated below: Materials for embankments must be free of vegetable and other deleterious materials, which may affect the performance of the embankment during its life. Clay may be used, provided it is workable and provide a stable embankment capable of supporting the road without excessive deformation. Unbearably expansive clays should be avoided. Materials excavated from borrow pits, or from the road prism, may be classified as suitable for certain horizons within the fill and may be stockpiled for this purpose, prior to placing them in their final position. The strength characteristics including cohesion and angle of internal friction should be frequently characterized during the filling of the embankment in order to cross-verify the design considerations. If any material is classified as unsuitable by this method, then that material should be tested by conventional shear strength testing methods before it is finally rejected. Embankment Width The stability of an embankment depends on the shear stresses developed in the material at the specified density. It is important this density to be achieved right up to the edge of the embankment, hence it is proposed to construct the embankment width minimum 500mm extra on either side than specified (drawing) width. The extra 120

6 extra width will be trim back during slope protection works. It is proposed to construct sand bands at specified intervals to drain out the water for faster consolidation. Layer thickness, Roller Compaction chart & Compaction Control Prior to start of the construction, field trial will be conducted to ascertain following aspects Optimum compacted layer thickness Required degree of compaction Number of roller passes Vibro roller speed Based on successful field trial, roller compaction chart is prepared and monitored layer wise by the supervisory staff. Construction Operation Sequence During embankment preparation following sequence to be followed 1. Ground preparation with SRWS already explained above 2. Construction of access road on either side width will be approx 7 to 10 m. 3. Anchorage mass After construction of access road in height of 1m, this activity will start. The width will be approx 10m from inner edge of access road toe to outer edge toe. It will be done in layers and total height will be 1m. (Though in figure it is triangle, during construction it will be trapezoidal shape with top width 7 to 10m) 4. Central Filling - It will be started after anchorage mass construction and continue in layer upto top reaches the level of anchorage mass. 5. Edge expansion same as anchorage mass 6. Completion of final section same as Central filling. The stage number 5 & 6 are repeated till the fill height reaches Stage 1 as mentioned in table 13 of Volume 1 (Annex A). Sand bands at specified intervals 5.6. Instrumentation schedule To monitor the settlement and consolidation process, various instruments are installed during the embankment construction. Geotechnical Engineer will guide the construction team during the placement of instruments. After construction of each stage in specified chain age, construction team will wait for the time duration. The geotechnical engineer will observe the readings of instruments and instruct for further action. Specifications of monitoring For the optical displacement monitoring a software package shall be used which allows a direct data flow. This software shall include features as follows. a. Calculation of 3-D coordinates and displacement of any desired point and its radial distance to the theoretical profile. b. Correction of errors based on physical effects. c. Transformation of coordinates after control measurements. d. Measurement results shall be tabulated and presented in graphs. The software processing the data shall include following features. a. Plot of displacement vectors with the cross section. b. 3D displacement vectors orientations. c. Assessment of displacements prior to zero measurement. d. Development of differences in displacement with time e.g. stage settlement minus settlement of top layer. e. Other evaluations as required. For the monitoring of geotechnical instruments a software package shall be used which allows a direct 121

7 data flow. This software shall include features as follows. a. Presentation of data related to the time and stage construction progress within one plot. as above. List of instruments required for High Embankment b. Multiple plot capabilities based. The first measurement (zero readings), for each measuring instrument, shall be made immediately after installation or as soon as the particular instrument may allow. The frequency of the further measurements or readings can be envisaged for each measuring section as follows: a. 1st week: daily b. 2nd week: twice a week c. 3rd & 4th week: once per week d. Later: monthly and bi-monthly e. The actual frequency of readings will however be influenced by the construction stages in the embankment Data of the optical displacement monitoring shall be processed on the same day and plots shall be available at alter afternoon. Other data from measurements must be processed with 24 hours after readings have been taken and must be plotted. Types of Instrumentation 1) Settlement: suitable temporary bench marks for the purposes of monitoring vertical movements. Such marks will be outside the zone of influence of construction 2) Piezometers (vibrating wire,) are to be provided in the ground for measuring changes in pore water pressure at different depths. 3) Inclinometers in the ground and within diaphragm walls (or retaining structures) or embankment slope are to be provided for measuring lateral displacements. The depth of instrumentation in the ground shall extend beyond the influence zone (to be proposed by HCC) of the works with respect to ground movement and shall be fixed at least 1 meter below the bottom of geotextile and/ or into a hard stratum Drainage and Slope Protection In view of the high rain intensity in this terrain, slope protection against erosion is considered to be an important aspect. The embankment wherever will be made up with clayey soil, simple vegetative turfing by planting locally available grasses and shrubs on side slope shall be adopted as an erosion control measure. In case of extremely dry weather conditions prevail during the growth period of plantation and/or due to nonavailability of artificial water sources required for the growth of plants, ready-made vegetative turfs will be substituted for the natural plantation cover. Stone Pitching and drainage chutes will be provided as per the high embankment guidelines. Longitudinal drains will be constructed to drain off the water to nearest water body. 4) Tilt meters shall be provided on embankments where tilt has been identified as being critical. Further stages will be constructed in the similar manner 122

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