A case study: Anchored Wall

Author: Adis Skejić, Faculty of Civil Engineering, Sarajevo A case study: Anchored pile wall 1. Introduction It is not a rare practice that retaining structures in BiH are built as piles additionally supported by ground anchors. These walls are often designed in road engineering to support cut slopes and in private sector for retaining of deep excavations. Designing the piles without anchors usually cannot provide an appropriate safety factor and cannot appropriately reduce displacements, since the piles flexural capacity is quite limited. In many practical examples it is not possible to perform large diameter piles in unstable areas (due to heavy equipment), so only small diameter piles (D < 600,0 m) must be used for remediation. The construction phases for this type of retaining structures were defined in a special chapter of this manual. One of the walls is selected as a case study for this manual. The details like construction sequence, geotechnical investigations, numerical analysis, monitoring results and interpretations along with the practical construction details are presented in order to get the insight to possibilities of these structures. On the other side, the shortcomings of these structures are highlighted to draw the attention to practical engineers that will build these walls in the future. 2. Location Selected wall is located at highway Vc route, section Gorica – Bilješevo. This area is known by the first modern age tunnel constructed in BiH (tunnel “1st March”). The tunnel exit at the north side is located near Gorica landslide that is partially remediated by drainage system under the viaduct located in the vicinity of the tunnel exit (figure 1). The anchored pile wall ( the subject of this section) is built to support the unstable reinforced soil slope (RSS) right at the tunnel exit (direction Zenica - Sarajevo shown in figure 3 as well). Bosnia river Putovići village Gorica village Tunnel exit Figure 1: Wall location, and indication of tunnel exit location The existing ground conditions are mixed of dumped material from tunnel excavation and the natural weathered and stiff rock material. The Existing ground slope ranges from 22˚ to 27˚, while flat area can be found at the RSS location next to the tunnel. Bosnia river bed is located about 50,0 meters downwards (figure 1). An indication of ground instability was found at the particular part of the location before the construction of RSS. The contour of the top of the landslide area is indicated in figure 2. The same figure shows the location of the tunnel, and the RSS is to be built next to it. This failure indicates that the stability of the existing slope is on the verge of failure and that the RSS should not be founded at the ground level. At this moment, it was unknown, whether the soil failure includes only dump material or the natural ground is sliding along with dump material. Author: Adis Skejić, Faculty of Civil Engineering, Sarajevo Figure 2: Ground conditions observed at the location before RSS construction Geotechnical investigation works along with the construction sequence will be presented below in order to understand the details of this case study. 3. Geotechnical investigations At the first phase of investigation works large trial pit was performed in order to reach the natural ground below the top dump material. The trial pit was about 8,0x10,0 meters in layout, and 3,0 meter deep (figure 3). Underground water was not observed during and after the excavation of trial pit at the particular location. However, water seepage at the bottom of the slope was observed and that information will be used for defining groundwater conditions along with the data observed during exploration drilling in the second phase of investigation works. The second phase of investigation works included the drilling of 15,0-meter deep borehole next to RSS foundation (figure 4). These investigation works were performed after the RSS was built and the cracks were observed at the facing of the reinforced slope (see details in “construction sequence” section). The stiff rock base (marl) was reached at the depth of 10,0 meters. Figure 3: trial pit at the location of RSS near the tunnel The water level was observed at the depth of about 4,0 meters, so the final soil and groundwater profile were adapted according to figure 4. An indication of borehole and trial pit location is presented in the same figure along with the cut and cover tunnel and reinforced soil slope geometry. The same figure indicated the location of Bosnia river bad downwards, as well as the flat area near the tunnel where RSS is to be built. Since the marl level was observed during constructing the cut and cover tunnel, a total cross section could be defined according to one borehole. Author: Adis Skejić, Faculty of Civil Engineering, Sarajevo Figure 4: Engineering geological profile defined after two phases of exploration works 4. Construction sequence Two characteristic phases are highlighted as specific for this case study:   Phase 1: Construction of RSS founded on natural ground (foundation depth of about 3,5 meters below ground level and failure of RSS facing few days after construction Phase 2: Construction of anchored pile retaining wall as a remediation measure 4.1. Phase 1 As shown in figure 4, in phase 1, trial pit excavation reached the depth of about 3,5 meters from the original ground surface. After that, the RSS was constructed according to the usual procedure described in a special chapter of this document. Few days after construction, excessive settlements of the ground occurred and the crack on RSS facing was observed. The details of observed cracks are presented in figure 5. Immediately after the cracks were observed, the monitoring consisting of a geodetic survey and inclinometer measurements was set up. Figure 5: Cracks observed after the RSS was constructed Author: Adis Skejić, Faculty of Civil Engineering, Sarajevo The monitoring results confirmed two important aspects of the displacement mechanism. First one is that the sliding rate reaches all most 1,0 cm per day. Figure 6 shows the measured magnitudes of lateral and vertical displacements 6 days after the cracks were observed. The second aspect is confirmed by inclinometer measurements and it concerns the depth of sliding. Namely, as indicated in figure 7, two sliding surfaces were observed. First one is at the contact of dump material and in situ weathered rock material, and the second one is at the contact of highly weathered rock and marl. Figure 6: Monitoring results after the cracks on RSS facing were observed (geodetic survey) Figure 7: Inclinometer location and monitoring results The observed failure plane will be later used for back analysis to determine the appropriate soil and weathered rock strength parameters. 4.2. Phase 2 The remediation design solution consists of piles additionally supported by anchors. In accordance with descriptions given in retaining wall section, 3 phases are highlighted as typical for this type of structures. Construction phases are illustrated in figure 8. In the first phase, the piles were installed according to layout plan described in section 5. In the second phase, the capping beam was constructed, and finally, the anchors were installed and pre-stresses in the third phase. Author: Adis Skejić, Faculty of Civil Engineering, Sarajevo Figure 8: Construction sequence after bored piles installation: a) capping beam construction, b) anchors prestressing 5. Wall stability analysis After summarizing the geometry and geotechnical investigation works results, it was necessary to define an appropriate design solutions. Since the depth of stiff rock material is relatively deep (about 10,0 meters below ground level) it was not possible to support RSS by counterforts. Other techniques of the deep foundation had to be introduced. Additional aggravating factor during the selection of design solution was the needs for the performance of construction remediation works in the landslide area. Namely, it is not possible to use heavy equipment in the active landslide area. Finally, design solutions with different diameter of piles and permanent geotechnical anchors are designed to support the existing RSS. Total bored piles length is 15,0 meters. Bored piles with 1200,0 mm diameter were constructed at the stable area, while piles 600,0 mm in diameters were installed in the unstable area since the heavy machine for 1200,0 mm diameter could not work in the unstable area due to safety reasons. After that, the excavation bellow Figure 9: design solution layout existing foundation of RSS was performed in order to construct the capping beam. The capping beam cross section dimensions are 1,9-meter width and 2,5-meter height. Opening in capping beam is provided by PVC pipes (160 mm in diameter) in order to enable the drilling of boreholes for anchors installation. Additional opening in capping beam is provided to enable the installation of replacement anchors in future. The installation of permanent geotechnical pre-stressed anchors was performed at 4,0-meter spacing. Total anchors length ranges from 20,0 to 25,0 meters (total fixed anchors length is performed in marl material). An installation of anchors is performed at different angles (15˚ to 25˚) in order to prevent its possible Author: Adis Skejić, Faculty of Civil Engineering, Sarajevo intersection. Four tendon cables made of 7 wire strands were designed. Pre stressed force of 250,0 kN is applied in order to reduce the lateral displacements of anchored pile wall and RSS. This design solution concept does not provide the stability of existing ground below the retaining wall. The possibility of failure of dump and natural material in that area will be analyses as special load case for design. Typical cross section of suggested remediation measures is shown in figure 10. Along with the details of retaining structures some details of RSS are presented as well. Biaxial PP geogrids with an ultimate strength of 40 kN/m’ were installed at 25,0 cm vertical spacing (ΔH). The cross section indicates the layout of geogrids bellow the ground surface as it was constructed before the failure occurred. Figure 10: Cross-section 1-1, design solution Characteristic calculation phases are in accordance with construction phases described previously. The first phase simulates the observed conditions identified after in situ observations. The second phase includes the stability and serviceability analysis of repaired reinforced slope after the designed landslide mitigation measures. Wall stability analysis was performed by using Plaxis 2D software. Mohr – Coulomb constitutive model was used for representing soil behavior. Critical failure mechanism was calculated by shear strength reduction technique (φ/c reduction method). Other details of numerical analysis performed for this case study will be presented in next section. 5.1. Back analysis of observed failure before and after RSS construction At the first calculation phase, the back analysis was performed in order to determine the failed soil strength parameters. Two sets of back analysis were performed. First one for determining dump soil parameters since the sliding was observed before RSS construction (figure 2), and the second one for determining weathered rock parameters (observed sliding after RSS construction, figure 5). The constant value of dump soil cohesion equal to 4,0 kPa (average depth of sliding surface) was used along with the groundwater level at depth of -2,0 meters in order to determine the internal friction angle that results in the safety factor equal to 1,0. The values of φ=26˚ were adopted as appropriate. On the other hand, cohesion value of 10,0 kPa (equal to average sliding depth) was used for determining internal friction angle od weathered rock material that failed after RSS construction according to results of inclinometer measurements. Named parameters along with other Mohr-Coulomb model parameters of characteristic geotechnical mediums are given in table 1. Geotechnical parameters of marl are adopted according to a back analysis performed during tunnel excavations and convergence measurements. The material used as backfill for RSS construction is crushed rock granular material. The compaction of this soil was performed in layers and achieved reference deformation modulus Author: Adis Skejić, Faculty of Civil Engineering, Sarajevo measured by static plate load test is about 40,0 MPa. The internal friction angle of 36˚ is adopted according to authors experience with this kind of material at the defined level of compaction. Table 1: Soil and rock parameters according to back analysis results and in situ observations Geotechnical medium Unsaturated unit weight, γunsat [kN/m3] 19,0 highly weathered rock 20,0 Saturated unit weight, γsat 20,0 21,0 21,0 21,0 Internal friction angle [º] 27 32,0 28,0 36,0 Cohesion [kPa] 4 10,0 50,0 0,0 0,0 0,0 1,0 6,0 7 000 10 000 50 000 40 000 0,3 0,3 0,3 0,3 Parameter [kN/m3] Dilatancy angle, φ [º] Reference Young’s modulus, Eref [kPa] Poisson ratio, ν [-] dump soil Marl (bedrock) 20,0 Backfill material for RSS 20,0 The critical failure plane as a result of the back analysis is given in figure 11. Figure 11: Critical failure planes, FS = 1,0 – back analysis a) before and b) after RSS construction The predicted failure planes are in accordance to in situ observations (see figures 2 and 5). The cracks location are confirmed by inspection of numerically predicted tension points. 5.2. Analysis of stability of repaired slope The numerical analysis of repaired slope is performed according to construction phases defined in section 4.2. The serviceability and stability of retaining wall are checked for all particular phases. Eurocode 7 is used for designing the reinforcement for preventing wall structural failure, and piles embedded depth necessary to reach appropriate global safety factor. The design solution is given in figure 10 for a typical cross section. The piles are embedded 5,0 meters in marl material. A total of 25,0 meters of the anchored wall is constructed to support existing RSS. Plane strain numerical model is analyzed by using 15 node triangular finite elements for soil and rock. Piles are modeled as beam elements that can sustain bending, tension, and compression. Three separate beam elements are defined since it is expected that piles will deform separately, as they are not designed as secant piles. This calculation approach is in accordance with the “in situ” conditions since piles in the group were constructed at about 70,0 cm axle spacing. Anchors fix length is a model by a slender geogrid element that can sustain only tension, while the free length is modeled by the node to note anchor element that can sustain tension as well as compression. Ultimate pullout strength of anchors fixed length is calculated according to recommendations (empirical charts). Pre stress force is applied for 1,0 meter of model width. The main piles reinforcement is adopted as 18ϕ25 for 600,0 mm piles and 32ϕ28 for 1200,0 mm piles according to design approaches proposed by Eurocode 7. The stirrups ϕ12/7,5/15 were adopted for both designed piles. Capping beam reinforcement calculated according to 3D numerical model is adopted as 30ϕ25 plus 18ϕ12 (BSt 500S reinforcement). Author: Adis Skejić, Faculty of Civil Engineering, Sarajevo The calculated wall deformed shape is shown in figure 12, along with the critical failure mechanism. The displacement magnitude and the appropriate safety factor are given in figure description. Calculated displacement due to the sliding of dump material that is expected in near future is calculated as 0,5 cm, while the safety factor of critical failure slope is predicted as 1,35 that is higher than 1,25 required by Eurocode 7. Figure 12: Predicted wall lateral displacement for possible load scenario (maxS = 0,5 cm) and critical failure plane (FS = 1,35) 6. Concluding remarks The details of practical landslide remediation measure are presented by the case study. The suggested solutions required special geotechnical construction equipment for construction of piles and anchors. One and a half years after the wall is constructed, no indication of extensive lateral displacement or sliding is observed at the location. Namely, geodetic survey and inclinometer casing installed in one of ϕ600 piles confirm that the designed solution provide appropriate safety against failure and extensive displacements that can threaten the cut and cover tunnel serviceability. The main shortage of presented practical solution is the durability of geotechnical anchors. Namely, it is usually only an illusion that ground anchors can be considered as a permanent structure like piles and capping beam for example. That is the way it is important to enable the continuation of monitoring so it is possible to predict period for replacement anchors installation.