The Rio Grande Port breakwater, Brazil: geotechnical design

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1 Volume 6 Issue CE6 proceedings Proceedings of the Institution of Civil Engineers Civil Engineering Special Issue 6 November 213 Issue CE6 Pages Paper 136 Received /2/213 Accepted 1/6/213 Keywords: coastal engineering/embankments/geotechnical engineering ICE Publishing: All rights reserved 1 Fernando Schnaid PhD Associate professor, Department of Civil Engineering, Federal University of Rio Grande do Sul, Brazil 2 Gracieli Dienstmann MSc PhD student, Department of Civil Engineering, Federal University of Rio Grande do Sul, Brazil 3 Luiz Guilherme de Mello MSc Senior engineer, Vecttor Projetos, University of São Paulo, São Paulo, Brazil Sandro Sandroni, PhD Director, Geoprojetos Engineering; Associate research professor, PUC-Rio, Rio de Janeiro, Brazil Brazil is stimulating innovation in near-shore works design as part of its current effort to invest, extend and build new infrastructure facilities. The Rio Grande Port expansion project in the south is a strategic investment requiring the extension of the existing 2 m high, 3.2 km long parallel marine breakwaters that protect the entrance to the lagoon and port area by a total of over 1 km. The project involved an extensive geotechnical site investigation to characterise the site s soft clay deposits. Field performance during and after embankment construction was monitored to check the design hypothesis, evaluate uncertainties and ensure the work conformed to acceptable limits of behaviour. Numerical back-analysis of each construction stage enabled the design to be optimised during construction. The works were successfully completed in Introduction The port city of Rio Grande in southern Brazil is built on a peninsula connecting the main body of the Patos lagoon with the Atlantic Ocean. The port is protected by breakwaters constructed at both sides of the lagoon s access channel, built in 191 by Compagnie Figure 1. The eastern breakwater at the Rio Grande Port, Brazil has been extended by 37 m Française du Port Rio Grande. These run in parallel outwards from the coast into the ocean for 2 m. The 2 m high structures have now undergone 7 m and 37 m long extensions of the western and eastern sides respectively, accompanied by million m 3 of marine dredging to deepen the existing navigation channel from 1 m to 1 m depth, allowing access for larger ships (Figure 1). Rio Grande port is the second busiest port in Brazil and is part of the government s ambitious projects to deepen navigation channels, expand piers and construct or refurbish terminals. New developments comprise expanding the container traffic to 1 5 million TEU (twenty feet equivalent units) per year, putting in operation exploration rigs and shipyard facilities, as well as incrementally extending oil and gas port infrastructure. This is part of Brazil s federal government plan of investing in logistics and transportation (Pannett, 213). Breakwater design is an engineering challenge owing to the combination of difficult geotechnical and environmental conditions in the region, with strong currents from the lagoon system, severe winds and very high waves from the Falkland Islands. Designed to withstand the impact of overtopping waves, the breakwaters were constructed on thick, soft, sedimentary deposits, with 1:1 5 embankment slopes, protected by armour layers and submerged equilibrium berms for slope stability (Figure 2). The importance of modelling all construction stages by finiteelement analysis as an interactive process, supported by observations collected from instrumentation of the construction phases, is discussed in this paper. 56

2 Volume 6 Issue CE6 November 213 Berm Armour layer (tetrapods) Armour layer (rocks) Transition layer m Core 11 m 1 m Figure 2. Layout of a typical cross-section of the Rio Grande breakwaters 2. Site characterisation A comprehensive site investigation programme was carried out to evaluate site conditions with an average water depth of 1 m. A preliminary site investigation was conducted from a floating barge and included soil penetration tests and soil sampling. A detailed investigation was later performed using a seabed rig to perform piezocone and vane tests. Attention has been given to the properties of existing soft clay layers, with undisturbed 1 mm Shelby soil samples retrieved for laboratory tests, including triaxial and oedometer tests. A representative profile and the soil characterisation of the western breakwater are shown in Figure 3, revealing a sedimentary deposit with a 3 m sandy clay layer overlying a m thick, soft, clay layer. A superficial, thin, very soft, recently deposited, silty clay layer is frequently observed above the sandy clay layer. Triaxial test results were used to estimate soil parameters and to determine the general behaviour of the silty clay and clay layers. Later these results were used to calibrate the soil model to support the proposal of calibrated parameters for finite-element analysis. Unconsolidated, undrained, triaxial test results shown in Figure were performed as per IS 272 (part 11):1993 with a constant strain rate of A comprehensive site investigation programme was carried out to evaluate site conditions with an average water depth of 1 m Piezocone tests along western breakwater extension Sea Very soft clay 2 Sand 2 2 Very soft clay Dense sand (a) Corrected cone resistance: MPa (b) Soil type Thickness variation 1 2 very soft silty clay 2 m sand 2 5 m 7 9 very soft silty clay 3 m soft clay 1 m sand Weight (γ): kn/m 3 Water content, w; plastic limit, PL; liquid limit, LL: % LP LL w Undrained shear strength, s u /total overburden stress, σ vo Properties Compression index, C c ; static cone resistance, C r Cc C r Coefficient of consolidation, c v : cm 2 /s Initial void ratio, e Angle of friction, φ: deg c v e Friction angle Corrected cone resistance, q t ; pore water pressure, U: kpa U q t Figure 3. Soil conditions under the western breakwater extension: profile with piezocone results superimposed (a) and characterisation of soil properties (b) 57

3 Volume 6 Issue CE6 November mm/min. A load cell and linear variable differential transducers were used to measure the deviator load and vertical displacement respectively. All the tests were conducted up to a maximum axial strain of 15%. The stress paths and stress strain relationships show typical characteristics of natural soft clay deposits, reaching a peak strength and approaching critical state at large strains. The slope of critical state line, M, was determined to be 1 27, which corresponds to effective angle of internal friction f' = 26. Representative constitutive parameters assessed from both laboratory and in situ testing are summarised in Table 1. Deviator stress, q : kpa Pore pressure, u : kpa Deviator stress, q : kpa Axial strain: % Confining pressure 5 kpa 1 kpa kpa Confining pressure 5 kpa 1 kpa kpa 5 Confining pressure 35 5 kpa 1 kpa 3 kpa Effective mean stress, p : kpa Figure. Triaxial testing data 3. Instrumentation and monitoring This paper describes the construction of the extension of the breakwaters, which comprises underwater construction by ships and barges of a first mattress layer from the seabed at elevation 1 m to 11 m; raising the hydraulic breakwater up to elevation 5 m; enddumping construction up to elevation +2 m; and finishing at elevation +5 m with placement of armour layer and tetrapods. Seven open-ended, m diameter, steel casing towers deployed on the seabed, adjacent to the projected breakwater toe contour and embedded into the equilibrium berms, were used for installation of instruments. These consisted of inclinometers, settlement detection devices (magnetometers) and electrical piezometers (see Figure 5). A detailed description of instruments and installation procedures has been reported by Rabassa (21). Four monitoring towers were located along the western breakwater (MO1, MO2, MO3, MO) and three along the eastern breakwater (ML1, ML2, ML3). These monitoring towers enabled the instrumentation to be installed and provided protection to the instruments during actual construction. Inclinometer results from two instrumented locations have been selected for analysis in this paper: one on the western breakwater (MO3) and another on the eastern breakwater (ML1). Horizontal displacement plotted against depth curves measured at the axis perpendicular to the breakwater are shown in Figure 6 for a number of load increments recorded during breakwater elevation. Both inclinometers measured maximum horizontal displacements of about 1 mm at a depth of around 2 m. Overall settlements recorded during construction at the western breakwater are shown in Figure 7. Since the breakwater was built in stages, a large part of the settlements is due to consolidation effects. Maximum settlements measured at the depth of 19 m were of the order of 6 mm, at m depth were 1 mm and at m depth Western breakwater MO 1 MO 2 MO Channel MO 3 Piezometers ML 1 ML 2 ML 3 Piezometers Eastern breakwater Figure 5. Seven monitoring towers were installed around the extended ends of the breakwaters Sea Weight, : kn/m 3 Elastic modulus, E: MPa Poisson s ratio, ν Compression index: C c Recompression index, C cr Initial void ratio, e Angle of friction, f: deg Breakwater Sand Very soft silty clay Silty clay Soft clay Table 1. Design parameters 5

Volume 6 Issue CE6 November 213 were 2 mm. These observed settlements at the end of construction are about half the maximum predicted values after consolidation.

Note that the final construction level of the crest was elevation +5 5 m to provide an operational design hydraulic level of elevation +5 m.

Typical results from variation of pore-pressure measurements with time are illustrated in Figure for four piezometers placed on the same borehole, but at different depths below seabed level.

In the remaining time, there are periods of pore pressure increments (construction stages from elevation 5 m to +5 m), followed by pressure decrease due to consolidation.

There is no sign of instability emerging from measured data: increase in pore pressure at any construction stage is due to increasing normal stresses and is followed by consolidation.

Limits conceived to increasing rates of monitoring and implementing contingency plans were defined from both experience and numerical analysis: vertical deviation q greater than 15 mm/m per day and

Contingency actions planned to be triggered when monitoring values were outside acceptable limits comprised reducing construction rate, stopping construction and even modifying the layout of the

4 Volume 6 Issue CE6 November 213 were 2 mm. These observed settlements at the end of construction are about half the maximum predicted values after consolidation. Rates of displacements have decreased substantially at final construction stages but further consolidation and secondary settlements are expected in the future. Note that the final construction level of the crest was elevation +5 5 m to provide an operational design hydraulic level of elevation +5 m. To check the theoretical assumptions made during the design phase and to introduce possible consolidation effects on stability analysis, the breakwaters were closely monitored by piezometers. Typical results from variation of pore-pressure measurements with time are illustrated in Figure for four piezometers placed on the same borehole, but at different depths below seabed level. Within the clay layer, pore water pressures increased significantly during August and September 29, a period that corresponds to breakwater elevation from elevation 11 m to 5 m. In the remaining time, there are periods of pore pressure increments (construction stages from elevation 5 m to +5 m), followed by pressure decrease due to consolidation. Piezometer was placed at the deepest position, embedded in the sand lower layer, to record the hydrostatic pore pressure. There is no sign of instability emerging from measured data: increase in pore pressure at any construction stage is due to increasing normal stresses and is followed by consolidation. Acceptable limits of behaviour for staged construction close to undrained conditions were defined from results of numerical analyses of representative cross-sections (summarised by Dienstmann (211)). Limits conceived to increasing rates of monitoring and implementing contingency plans were defined from both experience and numerical analysis: vertical deviation q greater than 15 mm/m per day and variation of vertical deviation greater than 2 mm/m (e.g. Eurocode 7, BSI, 1997; Dunnicliff, 19; Ladd, 1991). Contingency actions planned to be triggered when monitoring values were outside acceptable limits comprised reducing construction rate, stopping construction and even modifying the layout of the designed cross-section. Whereas the two previous recommendations were implemented, the original design proved to be acceptable and there has been no need to reinforce the originally designed cross-section throughout construction, as discussed by Schnaid et al. (213). It is recognised that these acceptable limits should be viewed as reference values, because design was based on undrained loading conditions and it was not possible to anticipate the actual contribution of consolidation effects during construction (e.g. Lambe, 1973; Peck, 1969). Numerical analyses were thus performed during construction stages to refine stability analysis and to check corresponding settlements and pore pressures. (a) Displacements: mm (b) //29 2// Displacements: mm Figure 6. Horizontal displacements plotted against depth at stations (a) MO3 and (b) ML1 Displacement: cm Apr Apr May-29 -Jun-29 2-Jun-29 Breakwater Coast A Sea B+ B A+ 1-Jul-29 7-Aug Aug-29 -Sep-29 6-Oct Oct Nov-29 Figure 7. Vertical displacements plotted against time at station MO3 5-Dec Dec-29 1-Jan-21 3-Feb Feb Mar-21 -Apr-21 Breakwater Coast A Sea B+ B A+ 2-Apr-21 Depth 19. m Depth m Depth m 1-May-21 3-Jun Jun Jul-21 2-Aug Aug Piezometer pressure at 25. m Reference hydrostatic pressure at 25. m Piezometer pressure at m Reference hydrostatic pressure at m Piezometer pressure at 33. m Reference hydrostatic pressure at 33. m Piezometer pressure at 3. 5 m Reference hydrostatic pressure at 3. 5 m (a). 2. Pressure: kpa Jul Jul-29 1-Aug Aug-29 9-Sep-29 2-Sep-29 9-Oct-29 2-Oct-29 -Nov Nov-29 -Dec Dec-29 7-Jan Jan-21 6-Feb Feb-21 -Mar Mar-21 7-Apr Apr-21 7-May May-21 6-Jun Jun-21 6-Jul Jul-21 5-Aug-21. (b) m Displacements m Figure. Variation in pore water pressure with time Figure 9. Finite-element mesh (a) and total displacement calculations (b) 59

(stage 1) 29 Numerical (stage 1 to 2) Numerical (stage 2) 31 33 Stage 2 back-analysis Numerical (stage 3) 6//29 35 2//29 31//29 37 11/11/29 39 1//29 stage 2 3/3/21 1 1//21 3 1/5/21 5 1/6/21 5/7/21 7

5 Volume 6 Issue CE6 November 213 Figure 1. Comparisons between measured and predicted horizontal displacements Figure 11. Comparisons between measured and predicted vertical displacements Vertical displacements: cm Horizontal displacements: mm/m Stage 3 23 final construction Numerical (stage 1) 29 Numerical (stage 1 to 2) Numerical (stage 2) Stage 2 back-analysis Numerical (stage 3) 6// //29 31// /11/ //29 stage 2 3/3/21 1 1//21 3 1/5/21 5 1/6/21 5/7/ /7/21 stage 3 9 Vertical displacements: mm/m Stage 3 final construction Numerical (stage 1) Numerical (stage 1 to 2) Numerical (stage 2) Numerical (stage 3) //29 2//29 31//29 11/11/ //29 stage 2 Stage /3/21 back-analysis 39 1//21 1/5/21 1 1/6/ /7/21 11/7/21 stage Stage 1 Stage 2 Final stage 9-Apr Apr May-29 -Jun-29 2-J un-29 1-J ul-29 7-Aug Aug-29 -S ep-29 6-Oct Oct Nov-29 5-Dec Dec-29 1-J an-21 3-Feb F eb Mar-21 -Apr-21 2-Apr-21 1-May-21 3-Jun J un J ul-21 2-Aug Aug S ep-21 Depth 19. m Undrained predicted Drained predicted Figure. Comparisons between measured and predicted vertical displacements. Numerical simulation A numerical analysis was performed to compare the monitored results in order to evaluate the accuracy of design assumptions and the possible influence of consolidation effects developed during actual construction stages. Calculations have been carried out using the Plaxis finite-element code (Brinkgreve, 22) and comprise a series of two-dimensional plane strain models representing typical sections of the breakwater. The geometry, boundary conditions and details for finite-element mesh are presented in Figure 9. The discretised 5315 nodes and 77 stress nodes finite-element mesh (symmetric along embankment centreline) consist of linear strain triangular elements. For the embankment fill and sand layers, a simple, linear, elastic perfectly plastic, Mohr Coulomb model was adopted. The modified Cam-clay model (soft soil) was assumed for modelling the soft clay behaviour, as extensively used in the past for analysing the behaviour of soft clay embankments. Deformations were calculated on the basis of the fully coupled Biot consolidation model. At the design stage the minimum safety factor was of the order of 1 3 without considering any strength gain due to consolidation. This condition was assumed to give a sufficient safety margin provided that ground instrumentation was implemented and evaluation of factual conditions was also considered. Numerical analyses conceived to simulate the construction phases according to the actual construction stages were as follows n construction stage 1 placement of a first mattresses layer under undrained loading, followed by full consolidation (construction was halted after placement between 21 and 27) n construction stage 2 elevation of the hydraulic breakwater up to level +2 m; simulations comprise both undrained and partially drained conditions n construction stage 3 final construction at elevation +5 m, under fully undrained conditions. The load was increased incrementally by raising the height of the breakwater according to the described construction stages. Some consolidation was allowed during construction stage 2 to approximate the predicted displacements and pore pressures from the measured values and trends. Results are discussed herein. Predicted and measured lateral displacements and vertical deviation observed at MO3 are shown in Figures 1 and 11 for elevation of the hydraulic breakwater at level +2 m. The vertical deviation is defined as the increment in horizontal displacement divided by the distance between the measured points. The maximum horizontal displacements measured within the first 5 m below the seabed have been depicted by numerical predictions. Maximum vertical deviation of the order of 2 mm/m at 29 m below water level has also been represented with reasonable accuracy in the analysis. Since there is no sign of instability at this stage, the load was increased to the final construction level of +5 m, showing increasing lateral displacements with a trend of stabilisation at maximum values of the order of 17 mm (Figure 1). Maximum horizontal displacements later measured at elevation +5 m were of the order of 15 mm, slightly smaller than the maximum class A predicted values of 17 mm. Typical measured vertical displacements were greater than undrained predicted values (Figure ), which is consistent with the assumption that some consolidation takes place during breakwater elevation. Predicted drained displacements form a consistent upper bound to the measured 6

6 Volume 6 Issue CE6 November 213 values, suggesting additional 1 15 mm vertical settlement after completion of the work. Contribution of secondary consolidation in the overall displacement is disregarded in the present analysis. Finally, the variation of the induced pore water pressure with time is illustrated in Figure 13 for piezometers installed in one hole, enabling measurements at different depths. Within the highly compressible clay layer, pore water pressures increased significantly, showing pressures that were initially close to predicted undrained values. By the end of the second stage, a reduction in pore pressure is observed despite the soil s low permeability (confirming that some consolidation is taking place). The lower piezometer is showing the hydrostatic level within the sand layer. The numerical work was particularly useful in separating out the effects of drained, partially drained and undrained loading on predicted displacement patterns. This is considered to be fundamental in breakwater stability risk assessment because measured data cannot be interpreted in terms of pre-established limits of performance. Consider the example illustrated in Figure 1, in which vertical displacement is plotted against the rate of vertical displacement for measurements recorded at station MO3 at the depth of 29 m. Numerical predictions for undrained loading up to failure are compared to undrained loading followed by consolidation of the breakwater at an elevation of +5 m (predictions that correspond to observed field performance). In both cases the vertical displacement increases continuously to fairly high values of the order of 5%, irrespective of the drained path, indicating that measures of vertical displacement alone cannot be adopted as risk analysis criterion. On the other hand, the rate of vertical displacement seems to be a good predictor of instability given the fact that it increases considerably during undrained loading and reduces during consolidation. The combined analysis of vertical deviation and rate of vertical deviation gives the best approach to risk assessment, irrespective of the need for cross-correlating displacements to pore pressure measurements to depict signs of drainage. 5. Conclusions The paper describes a case study of a fully instrumented breakwater constructed on an offshore, compressible, sedimentary clay deposit. The work focuses on the numerical finite-element analysis of construction phases undertaken to compare the design assumptions with measured behaviour. A comprehensive site investigation was carried out to determine the characteristics and properties of the deposit. Calculations using measured design parameters showed that predicted pore pressures and vertical and horizontal displacements are in agreement with measured values provided that the effects of consolidation are taken into account. During consolidation pre-established reference acceptable limits of performance are no longer useful for risk assessment because displacements are a function of both shear and mean effective stresses. A space combining vertical displacement and rate of vertical displacement may provide a better method of anticipating slope stability problems during and after construction. Acknowledgements The authors would like to express their gratitude to the Consortium CBPO, Carioca, Pedrasul e Ivaí for permission to use the test data and collaboration throughout the work. Pressure: kpa Figure 13. Comparisons between measured and predicted pore water pressures Displacement rate: % per day Stage 1 Stage 2 Final stage Distortion: % References +2 m 11-Jul Jul-29 1-Aug Aug-29 9-Sep-29 2-Sep-29 9-Oct-29 2-Oct-29 -Nov Nov-29 -Dec Dec-29 7-Jan Jan-21 6-Feb Feb-21 -Mar Mar-21 7-Apr Apr-21 7-May May-21 6-Jun Jun-21 6-Jul Jul-21 5-Aug-21 Final construction height of +5 m Brinkgreve RBJ (22) Plaxis Finite Element Code for Soils and Rocks Analyses: Users Manual, Version. Balkema, Rotterdam, Netherlands. BSI (1997) BS EN :2: Geotechnical design Part 1 General rules. BSI, London, UK. Dienstmann G (211) Interactive Design of the Rio Grande Breakwater. MSc thesis, Federal University of Rio Grande do Sul, Brazil (in Portuguese). Dunnicliff J (19) Geotechnical Instrumentation for Monitoring Field Performance. Wiley Interscience, New York, USA. Ladd CC (1991) Stability evaluation during staged constructions: the twenty-second Terzaghi lecture. Journal of Geotechnical Engineering, ASCE 117(): Lambe TW (1973) Predictions in soil mechanics. Thirteenth Rankine lecture. Géotechnique 23(2): Pannett L (213) Brazil building the country of tomorrow. Proceedings of the Institution of Civil Engineers Civil Engineering 6(6): 3, org/1./cien Peck RB (1969) The advantages and limitations of the observational method in applied soil mechanics. Ninth Rankine lecture. Géotechnique 19(2): Rabassa CM (21) Geotechnical Monitoring of the Rio Grande Breakwater. MSc thesis, Federal University of Rio Grande do Sul, Brazil (in Portuguese). Schnaid F, de Mello LG and Sandroni SS (213) Observational method applied to the Rio Grande Port breakwater. Soil and Rocks, in press. What do you think? Measured ( 25. m) Measured ( m) Measured ( 33. m) Measured ( 3. 5 m) Predicted ( 25. m) Predicted ( m) Predicted ( 33. m) Predicted ( 3. 5 m) Failure loading Consolidation Figure 1. Vertical displacement plotted against rate of vertical displacement at 29 3 m depth (station MO3) If you would like to comment on this paper, please up to 2 words to the editor at journals@ice.org.uk. If you would like to write a paper of 2 to 35 words about your own experience in this or any related area of civil engineering, the editor will be happy to provide any help or advice you need. Undrained construction followed by full consolidation Undrained loading to failure Drained loading Field 61

Landslide FE Stability Analysis

Landslide FE Stability Analysis L. Kellezi Dept. of Geotechnical Engineering, GEO-Danish Geotechnical Institute, Denmark S. Allkja Altea & Geostudio 2000, Albania P. B. Hansen Dept. of Geotechnical Engineering,