GROUND IMPROVEMENT USING DYNAMIC REPLACEMENT FOR NCIG CET3 COAL STOCKYARD

Transcription

1 FOR NCIG CET3 COAL STOCKYARD Charles CH Chua 1, Ming Lai 2, Graeme Hoffmann 2 and Brett Hawkins 1 1 Connell Hatch, Australia, 2 Keller Ground Engineering Pty Ltd ABSTRACT This paper presents an overview of the geotechnical design considerations and various analytical work undertaken for the development of the 30 mega-ton per annum (Mtpa) coal stockyard area at the Coal Export Terminal 3 (CET3) on Kooragang Island, NSW. The coal stockyard of CET3 covers an area approximately 1.2 km long by 300 m wide. The site is underlain by soft compressible soil strata of variable thickness. To support the 21m high coal stockpiles and the stacker-reclaimer machinery loads, the upper ground required stabilisation/consolidation improvement. The original design of the proposed ground improvement was to use dredged sand fill won from the proposed berth and wharf facilities constructed for the project to progressively preload the ground and eliminate the majority of the stability concerns and consolidation settlements during the construction period. However, due to programme constraints and availability of dredged sand material, ground improvement by dynamic replacement (DR) was employed to reduce construction time and need for preload material. The project, believed to be the largest DR project completed in the Southern Hemisphere to date, was contracted to ground improvement specialist, Keller Ground Engineering on a design and construct basis, based on performance criteria to limit the post-construction settlements and satisfy the settlement criteria for machinery operation considerations. The process of the dynamic replacement work and the construction constraints of the ground improvement technique are presented. 1 INTRODUCTION The Coal Export Terminal 3 (CET3) development on Kooragang Island, NSW, consists of a stockyard area measuring approximately 1.2 km in length and 300 m in width. The 30Mtpa coal stockyard area requires ground improvement to treat the subsurface soils for the future loadings. The coal stockyard will need to support 21 m high coal stockpiles and also coal stacker-reclaimer machinery traversing on 4.5 m high machinery berms. Dynamic replacement was employed to improve the ground characteristics for the future operational loads. The layout of the various elements of the development is shown on Figure 1, comprising three coal pads stockpiles, two machine berms and one containment berm. The area of ground improvement is divided into 30L, 30C and 30R as shown on Figure 1. During the feasibility design stage, the proposed ground improvement to the foundation soils involved preload treatment of the soft soils to manage foundation stability and induce consolidation settlement under controlled surcharge. Due to the construction program constraints and availability of fill material, the preloading option was deemed unsuitable. Preloading the stockyard in stages and other alternative methods of ground improvement were examined. However this preloading process had its drawbacks in that not only did it involve massive earthworks to place the preload material but the potentially costly and time consuming task of removing the preload material also needed to be carefully considered to render it practical. Various types of ground improvement techniques were examined with specialist ground engineering contractors to identify a technique most suitable for the stockyard area. Dynamic replacement was accepted to be the solution to improve the upper soft strata to nominal depths of 6m. Due to the presence of a thick sand stratum of varying layer thickness from 10 m to 30 m, ground treatment beyond this sand stratum was not considered practical. The total and differential settlements of the lower clay have been evaluated and found to be within the design criteria. In addition, these settlements will be managed via regular re-ballasting after the coal export terminal turns operational. This paper presents a general description of the treatment method, which comprises the adopted design philosophy, the verification and construction work and post construction testing. Australian Geomechanics Vol 43 No 3 September

2 Figure 1: NCIG CET3 Stockyard 2 GROUND CONDITIONS A series of cone penetration testing (CPT) and borehole investigation was carried out by Connell-Hatch prior to the awarding of the contract. This was supplemented by additional CPT testing after the awarding of the contract to Keller. These soil investigations indicate that soft compressible soil strata of variable thickness underlie the site. A typical result of pre-construction CPT testing is shown in Figure 2. Figure 2: Typical Pre-Construction CPT Result The existing ground levels vary between RL +4 on the east to RL +1 on the west. The ground conditions of the eastern half of the stockyard area generally consist of 2m of very loose to medium dense sandy fill overlying 2 m to 3 m of soft clay. Underneath the soft clay, medium dense to dense sand is intersected before rock is encountered at approximately RL Australian Geomechanics Vol 43 No 3 September 2008

3 For the western half of the stockyard, the thickness of sandfill diminishes and soft clay was intersected at the surface in some locations. The rockhead dips significantly towards the westerly direction. Below the medium dense to dense sand underlying the soft clay layer at the top, a layer of stiff to very stiff clay members is encountered at approximately RL -30, which extends to a depth of nominally RL -40. The groundwater table is generally at 1 m to 4 m below the existing ground surface at the 30R and 30C areas. At 30L, the groundwater is 0 to 2 m below the existing ground surface. Generally the DR columns are expected to be 4 m to 6 m in length extending up to 1 m into the medium dense to dense sand layer. 3 STRUCTURES AND LOADINGS It was proposed that sand material be placed onto the improved ground to form a platform up to nominally RL +7 at the eastern end (30R) to nominally RL +4 at the western end (30L) before stacker reclaimer berms are constructed. The construction of the platform will involve placement of sand by hydraulic means and then, once the design levels for the top of platform are achieved, compacted by numerous passes of impact rolling equipment. On the platform, stacker reclaimer berms of typical height 4.6 m will be installed using geotextiles and sand material. At the stacker reclaimer berms, dead and live loads from the machinery (amounting to 120 kpa) need to be considered. The improved ground needs to support up to 21 m of coal stockpiles in the coal pads during operation. A construction surcharge of 20 kpa was considered in the design. 4 CRITERIA After the dynamic replacement, the improved ground is required to remain serviceable during construction and operation by limiting settlements and ground movements to tolerable limits, as specified in the Table 1 below: Table 1: Limiting Criteria for Improved Ground after Treatment. Coal Pad (Load=210 kpa) & Limiting Value Stockpile Containment Berm Vertical settlement (mm) 200 Horizontal displacement (mm) 25 Stacker Reclaimer Berm (Load=120 kpa) Limiting Value Vertical settlement (mm) 150 Horizontal displacement (mm) 25 Differential settlement V: 1/500 over a 25 m chord H: 1/1500 over a 8 m chord Tilting (%) 0.3 The stability of the ground including berms in the stockyard needed to achieve minimum factors of safety of 1.5 and 1.1 in the long-term static and seismic load cases respectively. For the short-term, a minimum factor of safety of 1.3 was required. 5 GROUND IMPROVEMENT 5.1 DYNAMIC REPLACEMENT METHOD It was expected that, without treatment, the resultant settlement from the soft clay layer within the first 5 m could be excessive and there is potential for slip failure due to the loadings from the Coal Pads and Stacker Reclaimer Berms. Due to the shallow treatment depth, dynamic replacement has been chosen as the most suitable treatment method for the soft clay layer on site. Dynamic replacement is a technique that combines the features of dynamic compaction (dropping of a heavy weight (pounder) from a substantial height to cause deep compaction of the ground) and vibro-replacement (commonly referred to as stone columns where gravel columns are installed using a vibrating probe to increase the stiffness and strength of the ground). In the dynamic replacement process, compacted columns made of stone or sand, are installed into the ground by a pounder dropped repeatedly onto a stone/sand layer. The craters created as a result of the impact are backfilled with stone/sand during the installation process. A typical installation process is shown in Figure 3. Australian Geomechanics Vol 43 No 3 September

4 Figure 3: Dynamic Replacement Upon completion of the installation process, a compacted column of stone/sand is left in the ground, surrounded by a soil/stone/sand matrix of increased density. The columns and the in situ soils form an integrated system having low compressibility and high shear strength. The excess pore water pressure can dissipate through the column, which also acts as a vertical drain. The settlement expected for the treated soil is reduced while the rate of settlement is increased when compared with the untreated soils. The degree of improvement (settlement reduction and increased shear strength) achieved by the dynamic replacement process depends on the soils being treated, the impact energy, the diameter of the columns installed and the installation spacing. 5.2 DESIGN The aim of ground improvement using dynamic replacement is to: a. Improve the deformation modulus of the treated soils to reduce the potential settlement within tolerable limits; b. Increase the shear strength of the treated soils to ensure stability of the proposed coal pads and stacker reclaimer berms during construction and operation. To achieve the above objectives, the design process was carried out with the assistance of: i. Greta assessment of ground improvement parameters; ii. SlopeW assessment of the slope stability of the proposed geometric configurations; iii. Plaxis finite element method to assess overall settlement effects of various loadings and tilt. 66 Australian Geomechanics Vol 43 No 3 September 2008

5 5.2.1 Greta Improvement Analysis The Greta software program is the ground improvement program developed and used by Keller. It uses the theory presented in the Priebe s method (1) to assess the degree of improvement achieved by the DR method. The method has been well accepted in Europe and Asia for many years and proven to provide a suitable and reliable ground improvement analyses. The design method refers to the improving effect of the columns in a soil which for design purposes is considered to be unaltered from its initial state. Firstly, an improvement factor is established based on the improved performance of the subsoil in comparison to the state without columns. According to this improvement factor, the deformation modulus of the composite system is increased and settlements are reduced. All further design steps refer to this basic value. The typical basic improvement factor n o, as a function of replacement ratio (A/A c ) and friction angle of the column material φ c, is shown on Figure 4. A is defined as the tributary area for each DR column while A c is the cross-sectional area of DR column. With the various adjustments that take into account the compressibility of the columns, the density of the surrounding soils and the columns and the founding layer of the column, a final improvement factor is established whereby the soil modulus is increased resulting in reduced settlements. Figure 4: Improvement Factor versus Area Ratio for different Friction Angles. (µ s is the Poisson s ratio of the column material) In many practical cases the reinforcing effect of the columns is superposed with the densifying effect of dynamic compaction, i.e. the installation of the columns often densifies the soil in-between. However, this effect is conservatively ignored in most cases. Using the above design concepts, the adopted dynamic replacement configurations can be summarised in Table 2. Table 2: Configurations for Compacted Sand Column and proposed Structural Elements Description Diameter (m) Spacing (m) Sand Columns 2.5 m 5.0 m to 6.0 m SlopeW analysis The shear performance of ground improved by dynamic replacement is most favourable. While under shear stress more rigid elements may break successively, sand columns deform under load and any overload is transferred to neighbouring columns. The columns receive an increased portion of the total load, which depends on the area ratio of the proposed Australian Geomechanics Vol 43 No 3 September

6 configurations. According to the proportional loads on columns and soil, the shear resistance from friction of the composite system can be readily averaged. Similarly, the cohesion of the composite system is also obtained from the proportional loads. The calculated composite parameters of friction angle and cohesion are input parameters for SlopeW, a limit equilibrium analysis utilising the Bishop Simplified method, to assess the stability of two configurations: a. Internal Stacker Reclaimer Berm and Coal Pads; b. External containment berm and coal pads. For the consideration of the earthquake condition, an acceleration coefficient of 0.16g has been used in the stability analyses of the improved ground. The region is an area of low to moderate seismicity and lies within an intra-plate area. A significant earthquake occurred in December 1989, which registered approx. 5.6 on the Richter Scale and was assessed to have a return period of 500 years. Figure 5 shows a typical result of the slope stability analyses through the most critical section. Figure 5: Slope Stability Analysis using SlopeW Plaxis Analysis The improved engineering parameters such as strength and elastic modulus of the treated ground are used in a finite element program, Plaxis, to assess the overall interaction on settlement of the various coal pads and stacker reclaimer berms. The results show that the specified criteria are satisfied with the adopted dynamic replacement configurations. For the 30R area, the predicted maximum vertical settlements are approximately 170 mm and 40 mm, occurring under the triple cone coal pad and stacker-reclaimer berms respectively. For the sensitive stacker-reclaimer berms, the expected differential vertical settlement over a 25m chord is 1 in 1470 with tilt of 0.26%. The expected differential horizontal settlement over a 8m chord is 1 in VERIFICATION As part of the specification for the improvement work, verification testing is to be carried out. The aims of the verification tests are: 68 Australian Geomechanics Vol 43 No 3 September 2008

7 a. To confirm the parameters adopted for the design, or to use the achievable parameters to revise the design accordingly; b. To establish construction configurations that will allow the objectives of the project to be met. The variables in the construction parameters include: i. Pounders dimensions of size and weight; ii. Number and height of drops; iii. Phasing of the energy input and requirement of an ironing pass. The construction parameters of the dynamic replacement have been varied to arrive at an optimum configuration to achieve the objectives of the project in terms of technical criterion and program. To determine the achieved end products and to establish the optimum construction parameters, the following verification tests have been carried out: i. Post treatment Cone Penetration Test, CPT to determine the column soil parameters and diameter; ii. Plate Load Test, PLT to assess the load carrying capacity deformation of the columns under uniaxial load; iii. Dilatometer Testing, DMT to determine the constructed elastic modulus of the columns and to establish a correlation with the CPT results Post treatment Cone Penetration Test, CPT Post treatment CPTs have been carried out on selected columns as varying distances from the surveyed centre of the columns. Typical results of two columns are shown on Figure 6. Based on the results of these tests, it can be concluded that a column diameter of at least 3.3 m is achieved to a depth of 3 m. Beyond this depth, a column diameter of nominally 2.5 m had been measured. Correlations as proposed by Jamiolkowski et al. (1985) and Schmertmann, (1978), have been used to assess the friction angle of the constructed columns, using the post treatment CPTs. The results indicate that a friction angle of 36 o to 40 o can be expected. This result is confirmed by the Dilatometer test (DMT) detailed later. Australian Geomechanics Vol 43 No 3 September

8 5.3.2 Plate Load Test Figure 6: Cone Penetration Test results at column locations before and after DR treatment. Plate load tests (PLT) were carried out on selected columns. The diameter of plate used is 1.2 m diameter. Working loads (WL) of the columns for the plate load test are determined as: Stacker Reclaimer Berm 185 kpa = 209 kn or 21 ton Coal Pads and Containment Berm 270 kpa = 305 kn or 31 ton The maximum loads tested for the columns were 1.1 to 1.5 times the working load of the column. Counter-weight for the tests was provided by the dynamic replacement crane (weighing 110 ton). Results of the plate load tests are presented in Figure 7, which was tested to a maximum pressure of 405kPa, or an equivalent load of 46 ton. 70 Australian Geomechanics Vol 43 No 3 September 2008

9 Figure 7: Plate Load Test results While the plate load can only test the column to a depth of about 2 times the diameter of the plate, it can be used to approximate the stiffness (elastic modulus) of the column. For the calculation of the stiffness of the columns, the slope up to 100% WL, has been assessed as this represents the serviceability condition of the columns, in which settlements are estimated. Together with the CPT testing carried out on the completed columns, a preliminary correlation is established between the results of CPT and PLT, which gives the following approximate relationship: E = 3q c (MPa) (1) The estimated column stiffness from the above equation is presented in Table 3. Table 3: Interpreted Elastic Modulus of Dynamic Replacement Columns Depth Soil Condition Average q c (MPa) Interpreted E (MPa) 0 m 2 m Sand m 4 m Clay m 5 m Sand Dilatometer Test Dilatometer tests (DMT) were carried out to determine or confirm the following: i. The friction angle of the completed column; ii. The constraint modulus (D s ) and hence the elastic modulus (E = 0.8D s ) of the column; iii. A correlation between the elastic modulus determined from DMT with the cone penetration resistance q c. Figure 8 shows selected dilatometer tests with the calculated friction angle. It can be seen that the measured/correlated friction angles are generally higher than 36 o as assumed in the design. Australian Geomechanics Vol 43 No 3 September

10 Column 1998 Column 2023 Figure 8: DMT results for Columns 1998 & 2023 Selected results of the elastic modulus estimated from the DMT and the associated correlated from CPT are presented in Figure 9. The figure shows that the elastic modulus estimated from DMT tests are generally in good agreement with that estimated from PLT tests and that the adopted values as shown in Table 3 are generally on the conservative side. 72 Australian Geomechanics Vol 43 No 3 September 2008

11 Figure 9: Comparison of Elastic Modulus using Correlated DMT and CPT results Construction Methodology Based on the results of the above tests, the construction methodology to be adopted for the dynamic replacement work comprises: i. Primary/penetration phase this is carried out with a square penetrating pounder, weighing 26 ton. A 20 m drop height is used. The aim of this concentrated pounding is to create a column, reaching the intended depth with the specified diameter. ii. Ironing/compaction phase this is carried out using a octagonal compaction pounder weighing 24 ton with three drops from a reduced height of 10 m. The parameters established for the dynamic replacement sand columns are tabulated in Table 4. Depth Soil Condition Table 4: Design Parameters adopted for Dynamic Replacement Columns Expected Diameter (m) Friction angle, φ (deg) Interpreted E (MPa) Remarks 0 m 2 m Sand Compacted layer 2 m 3 m Clay Compacted column 3 m 4 m Clay Compacted column 4 m 5 m Sand Compacted layer 5.4 CONSTRUCTION The dynamic replacement has continued since March The construction of the Dynamic Replacement work is divided into three areas namely 30R, 30C and 30L. Each of the 30R and 30L covers an area of approximately 30 0m x 300 m, while 30C covers an area of 600 m x 300 m. At the time of preparing this paper, the installation work at 30R is already completed and work is proceeding in the 30C area. Three Keller 120 ton capacity rigs are currently working on site. Australian Geomechanics Vol 43 No 3 September

12 Post compaction testing comprises cone penetration testing, dilatometer testing and plate load test on selected columns as in the verification trial. These testings form the main quality control of the ground improvement work. The regular testing also allows for the calibration of the adopted compaction methodology throughout the improvement process, and to make the necessary adjustments to ensure that the objectives of the ground improvement are satisfied. Though the loadings from the stacker berms and coal pads have yet to be applied, the post construction testings have shown that the ground treatment by dynamic replacement can meet the movement limits set out in the technical specification. Figure 10 shows the completed improved ground with the column prints. Figure 10: Completed Improved Ground by DR. 6 CONCLUSION The design and construction of the dynamic replacement works for the coal stockyard involved many challenges posed by highly variable and difficult ground conditions. The dynamic replacement works at the coal stockyard were in progress at the time of writing this paper. After construction of the sand bench, geotechnical instruments were to be installed. The design and performance of the improved ground will be closely checked and monitored using valuable data collected from the proposed instrumentation schemes. 7 ACKNOWLEDGEMENT The authors are grateful to the developer, Newcastle Coal Infrastructure Group, for their kind permission to publish this paper. Coffey Geotechnics Pty Ltd is acknowledged in their capacity as technical reviewer of the dynamic replacement works design carried out by Keller Ground Engineering Pty Ltd. 8 REFERENCES Priebe H.J. (1995), The Design of Vibro-Replacement, Ground Engineering, December Schmertmann, J.H. (1978). Guidelines for cone penetration test: performance and design. US Department of Tranportation, Federal Highway Administration, Offices of Research and Development, Washington (DC), Report FHWA-TS July Jamiolkowski et al (1985). New developments in the field and laboratory testing of soils. Theme Lecture, 11 th International Conference on Soil Mechanics and Foundation Engineering, San Francisco, Chan, K. F., Raj, D., Hoffman, G. and Stone, P. (2007), Designing stone columns to control horizontal and vertical displacements. 10 th Australia New Zealand Conference on Geomechanics, Brisbane, October Australian Geomechanics Vol 43 No 3 September 2008