Bridge Approach Embankments on Rigid Inclusions

Transcription

1 International Conference on Geotechnics, July, 2018 Yogyakarta, Indonesia Bridge Approach Embankments on Rigid Inclusions M. Rizal Rekakarya Geoteknik, Jakarta, INDONESIA, K. Yee Regional Synergy Consulting, Kuala Lumpur, MALAYSIA ABSTRACT Rapid development in Indonesia calls for a new highway to be constructed in Central Java. Along the highway alignment, bridges are to be constructed over rivers and existing local roads. Based on the soil conditions, performance requirements, construction schedule and project budget, ground reinforcement using Controlled Modulus Columns (CMC) was adopted to support the bridge approach embankments to minimise post construction settlements; and to improve bearing capacity and slope stability. The CMC system consists of vertical cylindrical grout columns installed in a predetermined grid spacing using displacement auger. Typically, the CMC column was terminate at stiff layer, which found at depth 1m to 24m. Due to the huge loading and thick compressible cohesive soil, selection of CMC column spacing and length is importance to ensure CMC capacity within design. To confirm design termination depth of CMC column a set of drilling instrument were fitted into the drilling rig. In this paper, 2D numerical modelling were verified by the 3D model and the results are presented. Also presented in this paper is a brief description of the installation method used in soft ground condition together with a description of the quality control procedure and acceptance testing. After completion of the CMC works, approach embankments up to 11m were constructed. Keywords: ground reinforcement, rigid inclusions, embankment, numerical modeling. 1 GENERAL INFORMATION 1.1 Project Background In the north-central Java, a new highway of 39 km is being constructed linking Pemalang and Batang. This new highway forms part of the Trans Jawa Highway. The project site is a paddy field of flat terrain. Figure 1 shows the project location and the ground improvement areas. It is necessary to ensure smooth transition between flexible pavement and rigid bridge structure. With a thick compressible soft cohesive soil deposit, excessive post construction settlement is a major concern. Also there is potential instability during embankment filling works if the bearing capacity is exceeded. The excessive differential settlement at the transition area between flexible pavement and rigid concrete bridge abutment will cause abrupt bump which will cause discomfort to road users and endanger lifes. Long term maintenance works is required which disrupt the smooth operation of the highway and it is a costly affair. Structural solution using RC piles and concrete slab is expensive. Geotechnical solution using ground improvement is a viable solution. Since the bridge abutments have already been constructed before commencement of any ground improvement work, the choice of ground improvement technique is limited to techniques that are environmental friendly i.e. techniques having minimum vibration and minimum lateral soil movement during construction works to avoid potential damage to the completed bridge abutments and the foundations. The solution of Controlled Modulus Columns (CMC) was selected. CMC columns were installed at nine different locations adjacent to the bridge abutments (Figure 1). Figure 2 shows the location of CMC. A typical CMC treatment area is 60m by 30m. The base width of the embankment is about 60m and the treatment covers a distance of 30m from the bridge abutment. The embankment height varies from 6m to 11m. 1.2 Ground Conditions For each treatment area, pre-treatment site investigation was carried out to ascertain the ground conditions. Two numbers of deep boreholes with standard penetration tests (SPT) and one number of in-situ cone penetration test (CPT) were carried out. Figure 3 shows the typical SPT N-values and cone resistance (q c ) values. 1

2 International Conference on Geotechnics Generally, the ground condition can be described as an upper layer of 6m thick soft alluvium (N SPT ) of grey colour low plasticity marine origin overlying firm clay layer (average N SPT = 7) to depth of 18m. Below, a layer of hard clay (average N SPT = 20) to depth of 28m. Following layer is back to firm silt layer again (average N spt = 7) to maximum drilling depth borehole of 40m. Pekalongan City To Pemalang Project location To Batang Figure 1 Project location and ground improvement areas where CMC columns are installed CMC diameter 0.42m, length 1m to 24m, Spacing 1.9m to 2.2m Pile length up to 0m Figure 2 Typical cross-section of CMC treatment area. 2

3 Depth (m) International Conference on Geotechnics, July, 2018 Yogyakarta, Indonesia Nspt Figure 3. Typical SPT N-values and cone resistance (q c ) values. 1.3 Performance Specifications The performance specifications are as follow: (1) The maximum allowable residual settlement shall be less than 100mm after the 10 years; (2) The factor of safety against slope failure shall be not less than 1.. The traffic loading shall be 1 kpa. q c (MPa) at 0m to 18m q c <1 MPa 2 CHOICE OF GROUND IMPROVEMENT TECHNIQUES The choice of suitable ground improvement techniques is governed by environmental constraints, a tight construction schedule and the soft ground conditions. Since the bridge abutments have already been constructed before commencement of any ground improvement work, the choice of ground improvement technique is limited to techniques that exhibit minimum vibration and minimum lateral soil movement during works to avoid potential damage to the completed bridge abutments and the foundations. Vibro stone columns or any casing driven granular columns will cause excessive ground vibration during installation works. Also, due to the low shear strength of the underlying soft soils, there will be excessive column bulging and possible column failure during loading. Vertical drains and surcharging requires sufficiently long consolidation time and time for stage loading of the embankment and surcharge fill construction. With time constraint, this solution is not feasible even with close spacing of vertical drains and with the addition of reinforcement geotextile at the base of the embankment. The rapid loading placed on the soft soils below will cause excessive lateral movement of the underlying soft soil deposit during placement of embankment and surcharge fill which may cause deflection of the installed piles supporting the abutments. Based on all the above constraints, an environmental friendly solution of ground reinforcement using Controlled Modulus Columns (CMC) was considered most suitable. The CMC columns are installed by a non-vibratory soil displacement augering process. The columns are cement-grouted columns and hence, have no column bulging problem and they are having higher load bearing capacity than any other granular columns. The columns are 42cm in diameter with a cement grout compressive strength of 20MPa. The column spacing varies from 1.9m to 2.2m square grid subject to the embankment height. 3 CONTROLLED MODULUS COLUMNS 3.1 Concept of CMC system The components in a CMC system consist of a load transfer platform (LTP) of 1.0m thick compacted sand or gravel to facilitate the transfer of fill load on to the columns uniformly. Two layers of reinforced wire mesh are placed inside the LTP layer to provide traction reinforcement. Cylindrical vertical grout columns (or also known as inclusions) are installed below the LTP using displacement auger. The process of load sharing mechanism in CMC is illustrated in Figure 4. Since the ratio of stiffness between CMC and the soil is between 1:1,000 to 1:10,000 it is necessary to consider the vertical deformation separately for the CMC and the soil. The deformation of a point inside the CMC at a given initial depth is different from an adjacent point at the same depth in the soil. In other words, there exists a different field of deformation between the CMC and the surrounding soil as explained below: Stage 1: Due to the transfer of imposed stress to the soil ( soil ) through the load distribution layer (sand blanket), vertical deformation (settlement) of the soil ( soil ) occurs due to consolidation. Stage 2: As a result of consolidation settlement, stress is transferred from the 3

4 International Conference on Geotechnics surrounding soil to the CMC. The deformation at the same given depth (except at neutral plane) in the soil ( soil ) is different from the CMC ( CMC ) due to different stiffness (E CMC > E soil ) and that soil > CMC, negative skin friction is developed in the CMC. Stage 3: At greater depth, the point deformation CMC > soil resulting in a stress transfer from the CMC back to the competent soil. This induces positive skin friction and base resistance. Stage 4: Overall, an equilibrium state of load distribution is achieved where the tip resistance, friction resistance and soil resistance is equals to the total load. Figure shows the locations of the neutral plane where point deformation of CMC and soil is the same. At this location, the CMC column carries the maximum stress. Figure. Graphs of vertical displacement, shear stresses and vertical stresses 3.2 CMC Design Numerical analysis using Plaxis 2D was carried out to estimate the deformation and slope stability. The 2D analysis was checked against 3D model. The results show minimum difference. Axisymmetric 2D model with long-term stiffness material is used to determine maximum vertical settlement and 2D plane strain model with short-term stiffness is used to excess slope stability. The analysis is carried out using three different models that is, a) drained and undrained axisymmetric b) single plane strain d) full plane strain. Results of SPT and CPT tests were used for the soil properties. They are compared with the laboratory test results Long-term settlement For long term settlement, axisymmetry model was used. CMC was modeled as a soil volume and inside the CMC a dummy plate was assigned. In doing so, the result of axial and shear forces on the CMC can be accessed. Figure 4. Design concept for CMC In the field, CMCs were installed in square grid pattern while in the axisymmetry model, it is circular. Hence, necessary correction is made based on area ratio. The result of stress inside the CMC and stress at CMC head were extracted and compared with plane strain model. To be conservative strength increase due to installation effect to the adjacent soil is not taken into the design. Maximum stress inside the CMC occurs at the neutral plane, and this value was used to determine the compressive strength of the cement grout. Typical example of settlement obtained from an axisymmetric model is shown in Figure 6. 4

5 Elevation (mrl) International Conference on Geotechnics, July, 2018 Yogyakarta, Indonesia CMC as plate Fictious CMC CMC as Soil Volume CMC model as embedded beam row Neutral plane Figure 7. Single unit plane stain with fictive extension at CMC head Figure 6. Vertical displacement result CMC axisymmetric model Short term for stability Step 1: Model Single plane strain Stress Inside CMC - 2,000 4,000 6, CMC was modeled as embedded beam row (EBR). EBR is not fixed or attached to the soil which allows the soil to flow through. EBR can carry axial force and bending moment value unlike soil volume. The top and bottom of the CMC was set to be free to move. The shaft resistance value was pre-defined based on initial soil stress. τ s = 2πrt max (z) (1) where s is shaft resistance, r is the column radius, t max is shear resistance t max = Rinter [c +σ (z) tan θ ] (2) Since the EBR is a single line it cannot capture load transfer to the column surface to account for arching. To simulate this, a fictitious CMC head was modeled by extending the top of the CMC (Figure 7). At the top t skin was pre-defined based on value obtained from previous axisymmetric model. In EBR CMC spacing, axial skin friction was modelled in an elastic-plastic behaviour. The stress inside the CMC was then compared with the axisymmetric model as shown in Figure neutral plane Figure 8. Comparison stress to CMC in Axisymmetric and Plain strain model In this model the position of the neutral plane was located at elevation -6.0mRL. Step 2: Full plane strain model Axisymmetry Model Plain Strain Model CMC After calibrating the single plane strain model with the axisymmetric model, a full plain strain model can be made with actual embankment shape. In this model steel wire mesh and CMC with steel bar were included. For the steel bar inside the CMC, a limit plastic moment, M p of 33 KN.m was assigned based on steel bar size and the numbers of bars used. For CMC without steel bar, a very low value of M p was used. Figure 9 belows a typical full plane strain model.

6 International Conference on Geotechnics 1 KPa Fill F H Two layer steel wire mesh on LTP R t;t Figure 11. Results of vertical deformation CMC without steel bar inside CMC with steel bar inside Figure 9. Full plane strain model Due to the lesser load below embankment slope CMC length is shorter than the centre. Due to the higher tensile force at the embankment edge, steel reinforcement bar was placed inside CMC at a certain depth. The stress distribution below the embankment is gradually reduced with depth, taking an example at 10m depth, the stress reduces 30% and at 2m depth, stress is reduced by more than half (Figure 10). Due to this, CMC length at the edge is shorter and wider. Figure 12. Results of axial force inside CMC Figure 10. Stress distribution below embankment without CMC The results of a full plain strain analysis are shown in Figure 11 to Figure 16 Figure 13. Results of bending moment inside CMC The results show maximum vertical and horizontal settlement of 8.cm and 3.0cm respectively (Figure 11) 6

7 International Conference on Geotechnics, July, 2018 Yogyakarta, Indonesia the requirement of the allowable wire mesh strength greater than the lateral active force, (R tt > F H ). F H = K aγh 2 2 (3) where F h is the lateral active force, k a is the coefficient of active earth pressure of the embankment, is the unit weight of the embankment fill and H is the height of the embankment at the crest. And Figure 14. Axial force of the steel wire mesh R t;t = f e. n. πd 2 y β 4 (4) where f e is the steel yield stress, β is the reduction factor for the wire mesh soil interaction (= 1.2), n is the number of steel bars per meter width of mesh, d is the diameter of steel bar in the longitudinal direction. Figure 1. Slope displacement pattern CMC with steel reinforcement bar CMC without steel reinforcement is good in compression but not in tension. When subject to embankment loading CMC will experience both axial and bending moment at the same time. Thus, it is necessary to check the capacity of the CMC column in resisting lateral load. In cases where the lateral force is large, tensile reinforcement is needed in the CMC columns. Steel reinforcement is incorporated into the CMC columns. Excessive lateral force in CMC columns is normally found near the toe of a high embankment slope. Calculation for the resistance capabilities of CMC on lateral loads can be done according to BS EN a) Calculate the axial design load and design moment. N ed = N plx sγ G () Figure 16. Factor of safety for slope stability Steel wire mesh Figure 9 also shows the load transfer platform layer reinforced with steel wire mesh. The wire mesh consists of transversal and longitudinal steel bars. The longitudinal steel bars enable to absorb the lateral forces caused by the active earth pressure of the embankment. The longitudinal bars also serve to limit lateral soil displacements and thus any lateral displacement of the CMC. The transversal bars enable to mobilize friction at the soil-wire mesh interface. Wire mesh also enhances the efficiency of load transfer distribution to the CMC thus will minimize the arching effect. The wire mesh design shall meet M ed = M plx sγ G (6) where N ed and M ed is the axial and bending moment acting on the CMC respectively. N plx and M plx is the axial and bending moment obtained from Plaxis calculation results. S is the column spacing and G is the factor of safety between 1.2 to 1.4. b) Eccentricity e (Figure 17), with the following equation e = N ed M ed (7) 7

8 International Conference on Geotechnics If reinforcement is required, then the CMC reinforcement need to be designed using M ed. Figure 17. Illustration of eccentrical load distance c) Calculate resistance area (A ref ) with following equation (Figure 18) A ref = R 2 (2θ sin2θ) (8) θ = arccos ( e R ) 4 CONSTRUCTION OF CMC CMC columns are constructed by soil displacement using a displacement auger. During auger penetration drilling, the lower screw section which has a conical screw-bit shape with variable auger flight pitches (Figure 19) will cut and loosen the soil and transport the soil to the displacement body section. The displacement body is a cylindrical shape with the same diameter as the lower screw section which prevents soil from passing through and thus, pushing (or displacing) the soil towards the borehole wall. The counter screw section above the displacement body has opposite direction flight. Soil collapsed from the above during drilling is brought downward to the displacement body and pushed towards the borehole wall. With this technique, there is minimum spoilt at the ground surface. where R is the radius of CMC Counter screw section Displacement body Lower screw section End cap Figure 19. Full displacement auger Figure 18. Illustration of resistance area (A ref ) d) Calculate axial resistance (N rd ) with the following equation N rd = A ref f cd (9) where A ref is the area resistance and f cd is the grout strength e) Check if reinforcement is required: if N rd >N ed, reinforcement is not required. If N rd <N ed, reinforcement is required. When the drilling auger reaches the design depth, grout is pumped through a flexible rubber hose and through the hollow steam attached to the displacement auger. The grout pumping pressure is monitored and auger-lifting speed is controlled by the CMC rig operator. Electronic and mechanical sensors are fitted to the CMC drilling rig to ensure a good column installation. Figure 20 shows the on-board computer monitoring system fitted with various sensors. A monitor display is installed inside the cabin to display real-time monitoring installation parameters. These parameters include: a) Depth of installation (m) 8

9 International Conference on Geotechnics, July, 2018 Yogyakarta, Indonesia b) Installation and extraction time (sec, min) c) Penetration rate (m/hr) d) Rotational torque (Bar) e) Injection grouting pressure (Bar) f) Auger lifting speed (m/hr) g) Grout volume (m 3 ) h) Computed CMC column profile deeper or shallower than initial design. CMC auger drilling is work by mean of penetration so it can give verification to the in situ test. The actual CMC depth is determine by live display drilling record by using penetration rate and torque. Before commencement of full production work, trial installation of CMC columns is carried out to calibrate the operation parameters. The optimum grouting pressure and the ideal speed of auger retraction during grout pumping are determined from the trial installation. Fast auger retraction causes necking of columns and slow auger retraction causes grout blockage in the rubber hose. Adequate grouting pressure recorded indicates the lateral resistance from the surrounding soil and that the grout has filled the entire augering and drilling volume. To ensure flowability of the fresh grout during auger retraction and grout pumping, high slump grout is needed. To prevent blockage during auger retraction each truck delivery is tested for slump value. In this project slump value between 23± 2cm is adopted. For the reinforced CMC columns, the reinforcement steel bars were placed into the columns shortly after completion of the grouting works. Figure 20. Onboard computer monitoring system fitted to the CMC rig For a 12m length CMC column, it takes about 11 minutes that is, about mins for drilling and about 6 mins for grouting. Shorter columns will take shorter construction time. The average production is about 600 to 1,400 linear meters per 10-hour working day per rig. The supply of cement grout and the condition of the working platform greatly influence the production rate. Figure 21 shows a typical installation record. The drilling rig was operated with torque 0 to 3 bar at depth 3m and at constant maximum torque of 40bar to 1m. The penetration rate was constant at 400m/hr from beginning till 13m and gradually drops to 7m/hr at 1m depth. At 13m below, it was substantial drilling resistance cause auger rotation decrease substantially and reducing penetration rate. This is consistence with the borehole result which indicates at the first 3m is soft soil (N SPT 4), and continue with firm soil do the depth 12m. At the depth 13m below, borehole indicate a layer of very dense sand which make auger difficult to penetrate. The CMC length is pre-determine from N spt, it is likely during installation the termination of CMC is Figure 21. Typical CMC drilling record 9

10 head displacement (mm) International Conference on Geotechnics PLATE LOADING TEST Plate load tests (PLT) were carried to maximum load of 7 ton or 110% of maximum stress induced in the CMC column. The tests were carried out after 28 days. A total of nine PLT tests was carried out; one for each area of treatment. The objectives of the carrying out the tests were to obtain the load bearing capacity of the columns; compare the PLT results with those obtained from Plaxis analysis; and back calculation of design values to refine further the numerical analysis. A typical PLT results is shown in Figure 22. PLT was carried out for a 42cm diameter column and column length of 20m. The column was installed through a layer of soft to firm silt layer and terminated at hard clay with N spt = 20. Since the test column was not tested to failure, Chin Method was used to determine the ultimate load capacity. An ultimate load capacity of 88 tons was obtained. The PLT test was modelled using Plaxis with a soil stiffness modulus E y equals to 70 N spt and soil-cmc interface factor of 0.8. The numerical analysis indicated an ultimate load capacity of 83 tons. The difference between the two methods is deemed acceptable Load (tons) FEM PLT Figure 22. Load -Settlement result from PLT The bearing capacity is also compared alpha-c u method and the result is summarised in Table 1.The alpha -c u give very close to PLT value. Table 1. Bearing capacity comparison 6 CONCLUSION Presented numerical modelling gives the possibility to correctly design 3D problem using 2D plane strain model. The result of the back-calculation PLT using FEM gives a gain in confidence to the predict CMC bearing capacity. Even though no settlement instrument installed, observation shows no indication of the differential settlement, soil heaving near the slope toe and instability when embankment rises rapidly. This project has demonstrated the successful application of CMC column to treat thick layer compressible soil and high fill embankment close to the newly constructed bridge abutment within given performance specification. The project also successfully completed within given time frame and budget. 7 BIBLIOGRAPHY ASIRI. (2011). Soil improvement with rigid inclusions. Paris: IREX. Chin, F. (1970). Estimation of the Ultimate Load of Piles from Tests not Carried to Failure. Southeast Asian Geotechnical Conference. Chin, F. (1983). Bilateral plate bearing tests. Proceedings of the International Symposium on in situ, (pp ). Paris. Combarieu, O. (1988). Amélioration des sols par inclusions rigides verticales application à l'édification de remblais sur sols médiocres. Revue Française de Géotechnique, (pp. 44, 7-79). Plomteux, C. a. (2000). Embankment construction on extremely soft soils using. Proceedings of the 16th Southeast Asian Geotechnical Conference. Kuala Lumpur. Yee, K. (2012). Controlled Modulus Columns (CMC): A New Trend in Ground Improvement and Potential Applications to Indonesian Soils. ISSMGE Technical Committee TC 211 International Symposium on Ground Improvement. Brussels, Belgium: ISSMGE. Ultimate bearing capacity (ton) Plate Load test FEM Plaxis alpha - c u Zhao, R. F. (1982). Estimation par les paramètres pressiométriques de l'enfoncement sous charge axiale de pieux forés dans des sols fins. Bulletin Laison Laboratoire Central des Ponts et Chaussees, (pp. 119, 17-24). 10