Acta Polytechnica CTU Proceedings doi:10.14311/APP.2018.16.0011 Acta Polytechnica CTU Proceedings 16:11–17, 2018 © Czech Technical University in Prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app ANALYSIS OF THE TIME COURSE OF A BRIDGE ABUTMENTS SETTLEMENT Luboš Hruštinec Slovak University of Technology, Faculty of Civil Engineering, Department of Geotechnics, Radlinského 11, 810 05 Bratislava, Slovak Republic correspondence: lubos.hrustinec@stuba.sk Abstract. The paper deals with the time course of the bridge abutments settlement (consolidation of the subsoil) on the motorway D4 in Stupava, Slovakia. The bridge abutments are founded on an earth embankment 5.5 meters in height and a group of piles. Over 6 years of geotechnical monitoring after the construction of bridge abutments, there were measured settlements from 102 to 106 mm. The measured settlement of intermediate bridge piers was only up to 16 mm. Geotechnical calculations and analysis are focused on the comparison of the final settlements prognosis and its time course with the real measured values. Keywords: Bridge abutment, geotechnical calculations, settlement, consolidation. 1. Introduction Bridge objects No. 211-01 and 211-02 are situated at the crossroads “Stupava” of the D4 motorway, which is bridging the D2 motorway in the direction from Bratislava to Stupava. A three-span bridge structure of total length 100.9 m and width 26.15 m was put into operation in 2010. Bridge abutments are founded on piles in an embankment 5.5 m high. On the bridge supports, vertical deformations (settlements) have been continuously measured. In the time period from March 2010 to September 2016 (an overall period of 6.5 years) settlement of the internal piers was mea- sured to be from 12.7 to 16.0 mm and settlement of the bridge abutments from 102 to 106 mm. Such an uneven settlement (maximum angular deformation is ∆s / L = 0.0034) causes a significant increase of internal forces in the bearing elements of the bridge structure (especially in the bridge deck) due to the de- formation load. In order to avoid failure of the bridge structure, an over-limit uneven settlement had to be eliminated by inserting compensating steel plates into the bridge bearing of the abutments. The paper deals with prognosis of the time course of the subsoil con- solidation of the bridge abutments and the causes of their over-limit uneven settlement. This problem is solved in detail in [1]. 2. Engineering and geological characteristics of the territory under study Engineering and geological conditions of the studied territory and soil properties of the subsoil have been assessed on the basis of archive documentation [2]. The subsoil is formed by quaternary and neogene soils. Quaternary soils are represented by deluvial, eolian, proluvial and fluvial sediments. Neogene sediments are located in the whole area under quaternary sed- iments in the form of alternation of the clayey and sandy soils. In the location of bridge structure No. 211 the JM-5 to JM-8 exploration boreholes were con- ducted to a depth of 15 m. The layers of quaternary fluvial non-cohesive sediments occur in the form of sandy soils. In accordance with STN 72 1001 [3] soils belong to classes: silty sand (S4-SM) and clayey sand (S5-SC). The thickness of the quaternary layer ranges from 0.7 m (borehole JM-5) up to 1.7 m below the surface (borehole JM-6). Under the quaternary sandy soils there are neogene clay soils. The clays belong to certain classes: clay with medium plasticity (F6-Cl) and clay with high plasticity (F8-CH). The consistency of the clay changes with the increasing depth, from firm to stiff. In the JM-8 borehole, the clay consis- tency is soft to firm. The groundwater level was found at 0.7 to 1.0 m below the surface. The above men- tioned information and knowledge have been taken into account when determining the subsoil model and soil properties used in the geotechnical calculations. 3. Determination of the subsoil models and soil properties For bridge abutments No. 1 and No. 4 the subsoil mod- els were defined based on the evaluation of JM-5 and JM-8 boreholes, respectively. Computational models of subsoil and bridge abutments used in the geotechni- cal calculations are shown in Figure 1. The depth of the exploration boreholes (JM-5 and JM-8) was 15 m under the original terrain. Geotechnical calculations showed that the deformation zone (HA) extends to a depth of 22.3 m below the terrain. Based on the eval- uation of the results of the geodetic measurements of the settlements and geotechnical calculations, it was assumed in the subsoil models that from the depth of 15 m to the depth of the deformation zone “HA” 11 http://dx.doi.org/10.14311/APP.2018.16.0011 http://ojs.cvut.cz/ojs/index.php/app Luboš Hruštinec Acta Polytechnica CTU Proceedings there are soils of class F8-CH with soft to firm con- sistency. The soil properties used in the geotechnical calculations are shown in Table 1. 4. Technical parameters of the bridge structure Information about the technical parameters of the bridge structure were taken from [4]. The bridge load- bearing structure (bridge deck) is a monolithic struc- ture made of pre-stressed concrete of class C35/45 with total height of 1.8 m. The three-span bridge beam has a range of fields 26.5 + 36.5 + 26.5 m. The total length of the bridge structure is 100.9 m (Fig- ure 2). In the transverse direction, the bridge consists of two separate structures. For the right traffic lane, the bridge is marked as No. 211-01 and for the left traffic lane No. 211-02 (Figure 3). In the cross-section, two pre-stressed concrete beams are connected with a reinforced concrete slab (bridge deck). The total width in the transverse direction of the bridge is 13.75 m (structure No. 211-01), and 11 m (structure No. 211- 02). Geometric boundary conditions of the abutment structures No. 1 and 4 (including embankment and backfill) for structures No. 211-01 and No. 211-02 have been taken from the project documentation[4]. Bridge abutments are deep founded on piles and on the em- bankment (Figure 2). The foundation consists of a group of piles with a diameter of 0.9 m, a length of 14 m (No. 211-01 group of 7 piles and No. 211-02 group of 6 piles). Computational models of bridge abutments No. 1 and 4 are shown in Figure 1. 5. Determination of load states for geotechnical calculations The loads due to the bridge structures were taken from the static analysis mentioned in [1]. The total loads acting on the bridge abutments and subsoil are shown in Table 2. In the geotechnical calculations, the most significant loads of the subsoil from the individual structures (embankment, abutment and the backfill behind the abutment) and the technological progress of the construction of bridge structures No. 211-01 and 211-02, were considered. Three following load states were defined: • 1st Load state (1st LS): construction of the em- bankment below the abutments. Under the abut- ment No. 1 the height of the embankment is hn1 = 5.5 m, which causes a uniform plane load qn1 = γ × hn1 = 20 × 5.5 = 110 kNm−2. Under the abutment No. 4 the height of the embankment is hn4 = 4.8 m, which causes a uniform plane load qn4 = γ × hn4 = 20 × 4.8 = 96 kNm−2. • 2nd Load state (2nd LS): construction of piles, abut- ments and bridge decks (Table 2). • 3rd Load state (3rd LS): construction of a backfill be- hind the bridge abutments. Behind the abutments the height of the backfill is hz = 4.3 m, which causes a load of size qz = γ × hz = 20 × 4.3 = 86 kNm−2. From the defined load states it follows that the total load of the subsoil at the level of the terrain from embankments and backfills, causes the load of up to q1 = 196 kNm−2 (for abutment No. 1) and q4 = 182 kNm−2 (for abutment No. 4). The total vertical load acting locally below the abutments (at the foundation gap) has a size q211−01 = 220.9 kNm−2 (for abutment No. 211-01) and q211−01 = 232.5 kNm−2 (for abutment No. 211-01). From these values of load intensity it is obvious that the large embankment and backfill will have a significant impact on the final settlement of the abutments and cannot be neglected in the calculations. 6. Results from geodetical measurements of the bridge supports settlement The vertical displacements (settlements) of the bridge supports have been continuously measured using the geodetic method of a very precise levelling. In the period from July 13, 2010 to September 3, 2016 (i.e. over 6 years), a total of 23 stage measurements have been performed. The detailed results of the geodetic measurements are given in [1]. Graphical evaluation of the time course of settlement of supports No. 1 to 4 of the bridge structure No. 211-01 and No. 211-02 is shown in Figure 4. The measured maximum set- tlements of the middle piers were 12.7 to 16.0 mm and the settlements of the edge bridge abutments were 102.0 to 106.0 mm. From the static analysis of a bridge structure, the maximum value of the settle- ment difference of the neighbouring supports ∆s = 20 mm [1] resulted. The measured maximum settlement difference of the neighbouring supports is ∆s = 106– 16 = 90 mm and maximal uneven settlement is ∆/L = 90/26500 = 0.0034. The above-mentioned over- limit uneven settlement causes a significant increase of the internal forces in the bridge bearing elements (especially in the bridge deck) due to the deforma- tion load. To compensate for the over-limit uneven settlements of the bridge supports, the bridge deck structures at the place of abutments were lifted and steel washers with a thickness of 45 mm were inserted on November 15, 2010. 7. Evaluation of geotechnical calculations The geotechnical calculations were focused on the de- termination of the rate of the final settlement and the time course of settlements of the bridge abutments No. 1 and 4. The calculations were performed in ac- cordance with valid standards [5, 6]. The final settle- ment was calculated using the finite element method (planar task) and the time course of settlements was analyzed in terms of the theoretical assumptions of 12 vol. 16/2018 Bridge abutments settlement Geological period Soil Class-symbol Consistency γ [kN/m3] ν [-] E [MPa] cv [m2·s−1] Quaternary Silt with low plasticity F5-ML firm 20.0 0.40 4.0 5.0E-06 Silt sand S4-SM fine fraction is firm 18.0 0.30 8.0 7.5E-05 Clayey sand S5-SC fine fraction issoft firm 18.5 0.35 8.0 3.8E-06 Neogene Clay with medium plasticity F6-CI soft to firm 21.0 0.40 3.0 5.4E-07 Clay with high plasticity F8-CH soft to firm 20.5 0.42 2.0 2.2E-07 Clay with high plasticity F8-CH firm 20.5 0.42 3.0 2.2E-07 Clay with high plasticity F8-CH firm to stiff 20.5 0.42 4.0 2.2E-07 Clay with high plasticity F8-CH stiff 20.5 0.42 6.0 2.2E-07 Table 1. Soil properties used in geotechnical calculations. Figure 1. Computational models for abutments No. 1 and 4 used in geotechnical calculations. 13 Luboš Hruštinec Acta Polytechnica CTU Proceedings Figure 2. Longitudinal cross section of the bridge structure [4]. Figure 3. Cross section of the bridge structure [4]. Bridge structure Abutment Type of load Vz[kN] Hx [kN] Hz [kN] Mx [kNm] My [kNm] 211-01 1 and 4 Dead weight 7,870.5 0.0 0.0 0.0 2,245.0 211-02 1 and 4 Dead weight 6,685.3 0.0 0.0 0.0 1,952.3 Table 2. Loads for the bridge abutments used in geotechnical calculations [4]. Figure 4. The measured time course of settlements (si) of the bridge supports (piers and abutments) and settlements difference (∆si) of neighbouring supports. 14 vol. 16/2018 Bridge abutments settlement Figure 5. Calculated time course of settlement of the bridge structure No. 211-01, abutment No. 1. Bridge structure Abutment Calculated final settlement s Measured settlement (3. 9. 2016) Settlements (difference calc.-meas.)Abutment Embankment Backfill Completely (embankment, abutment, backfill) sa [mm] se [mm] sb [mm] sc [mm] sm [mm] ∆sc [mm] 211-01 1 26.65 92.00 35.45 154.10 102.00 52.10 4 34.45 86.60 40.70 161.75 106.00 55.75 211-02 1 28.30 93.15 36.20 157.65 104.70 52.95 4 32.90 85.30 38.25 156.45 104.00 52.45 Table 3. Comparison of calculated and measured settlements with the prognosis of further settlement ∆sc [mm]. one-dimensional consolidation [7]. Geotechnical cal- culations yielded many quantitative and qualitative results. The detailed results of the calculations can be found in the author’s archive records. This article presents only some representative results of geotechni- cal calculations and their processing in a clear tabular and graphical form. Graphical evaluation of the cal- culated time course of settlement of the individual structural parts (embankment, abutment, backfill, and the complete structure) of the abutment No. 1 (bridge No. 211-01) is shown in Figure 5. The table comparison of the measured (as of Septem- ber 3, 2016) and the calculated values of the final settlements of the bridge abutments with the prog- nosis of expected increase of settlement ∆s is given in Table 3. The relative percentage comparison with the calculated final settlement is presented in Table 4. The graphical evaluation of the comparison of the measured settlements and the calculated final settle- ments of bridge structure No. 211-01 for a period of 0 to 10 years is shown in Figure 6. From the evaluation and analysis of the results of the geotechnical calcula- tions and a comparison with the measured settlements, the following facts and knowledge follow: • Calculated values of the final settlements (aver- age values in the centre of abutments) range from 154.1 mm (bridge No. 211-01, abutment No. 1) to 161.75 mm (bridge No. 211-01, abutment No. 4). From the total value of the final settlements, the effects of loads due to the individual structural parts is the following: due to the embankment up to 59.7 % (from 54.5 to 59.7 %), due to the back- fill behind the abutment it is up to 25.2 % (from 23.0 to 25.2 %) due to the subsoil loading by the abutment structure it is to 21.3 % (from 17.3 to 21.3 %). From the above mentioned it follows that the final value of the settlement is mainly affected by loading of the subsoil by the embankment and the backfill behind abutment. The final settlement caused by the embankment and backfill causes up to 82.7 % (from 78.7 to 82.7 %) of the total final settlements of the bridge abutments. The size of the calculated settlements is also significantly affected by the presence of clays soils with soft consistency in the bridge abutments subsoil. • The calculations results of the time course of settlements (under the assumed boundary condi- tions) showed that the subsoil consolidation will be finished (consolidation degree U = 100 %) after 51.1 years (Figure 5), or 80 % degree of consolida- tion will be achieved after 10 years (Figure 6). • From the comparison of the measured settlements (as of September 3, 2016) and the calculated final settlements, there is the result that the measured values reach up to 66.5 % of the final settlements, i.e. approximately 2/3 of the final settlements value (Table 4). The expected next settlements ∆s of the bridge abutments are in the range from 52.1 mm to 15 Luboš Hruštinec Acta Polytechnica CTU Proceedings Bridge structure Abutment Calculated final settlement s Measured settlement (3. 9. 2016) Settlements difference (calc.-meas.)Abutment Embankment Backfill Completely (embankment abutment, backfill) so [%] sn [%] sz [%] sc [%] sm [%] ∆sc [%] 211-01 1 17.3 59.7 23.0 100.0 66.2 33.8 4 21.3 53.5 25.2 100.0 65.5 34.5 211-02 1 18.0 59.1 23.0 100.0 66.4 33.6 4 21.0 54.5 24.0 100.0 66.5 33.5 Table 4. The percentage comparison of calculated and measured settlements. Figure 6. Time course of settlement of the bridge structure No. 211-01, abutments No. 1 and 4 – comparison of measured and calculated values over a 10-year period. 55.75 mm (Table 3), i.e. 33.5 to 34.5 % of the final settlements (Table 4). The above findings must be taken into account in the complex assessment of the reliability of the bridge structures No. 211-01 and No. 211-02. 8. Conclusions Based on the analysis and evaluation of the geotech- nical calculations undertaken it can be stated that the over-limit settlements of the bridge abutments occurred mainly due to the following factors: • Failures during the execution of the geotechnical survey (mainly insufficient depth of survey boreholes that did not reach the depth of the deformation zone; ending boreholes in cohesive soils of soft consistency; insufficient determination of geotechnical data valid for soils in the subsoil). • Failures during the execution of the geotechnical calculations and design of the bridges abutments (especially neglecting the significant part of the load- ing of the subsoil due to the embankment and the backfill bridge abutments; neglecting the consolida- tion processes in the subsoil). To ensure the reliable operation of bridge objects No. 211-01 and No. 211-02, the following recommen- dations and proposed measures were formulated: • At regular intervals, to visually check the bridge structures and their load-bearing elements. Dur- ing the bridge checking it is necessary to focus on deformation of the structures (including the em- bankment and the surrounding terrain), the origins of cracks and other disproportional phenomena (de- formation of the terrain, inadequate deformations and rotation, etc.). • To perform ongoing geodetic measurements of ver- tical displacements (settlements) of bridge supports No. 1 to 4 (including internal piers No. 2 and No. 3) at a minimum of twice per year. • After each periodic measurement it is necessary to graphically evaluate the measured values of set- tlement and to compare them with the calculated values. If the measured settlement values are not 16 vol. 16/2018 Bridge abutments settlement in accordance with the expected values, it is nec- essary to take measures to eliminate the uneven settlements and their effects on the reliability of the bridge structure. This involves following the recommendations of the static analysis of the bridge structure, such as e.g., inserting washers into the abutments bridge bearings. • If the measured settlements are higher than the expected ones, or they progressively rise, it is neces- sary to carry out an additional geotechnical survey to the depth of the deformation zone. • In case of another disproportional progressive in- crease of the settlements, it is necessary to take more complex measures, such as e.g., pre-injection of the subsoil using jet grouting technology. List of symbols γ Unit weight with natural moisture [kN m−3] ν Poissin ratio [–] Edef Modulus of deformation [MPa] cv Coefficient of consolidation [m2 s−1] V y Vertical force [kN] Hx Horizontal force in the x-axis direction [kN] Hy Horizontal force in the y-axis direction [kN] Mx Moment in the x-axis direction [kNm] Mx Moment in the y-axis direction [kNm] s Settlement; vertical displacement [mm] ∆s Settlement difference [mm] U Degree of consolidation [%] Acknowledgements The author is grateful for support from the Grant Agency VEGA of the Slovak Republic, project No. 1/0412/18. References [1] J. Halovonik. Assessment of stress in the bridges structures no. 211-01 and 211-02 – crossroad Stupava. Bratislava, 2016. [2] I. Modlidba. Crossroad stupava – south on the D2 motorway. A detailed engineering-geological survey. Final report. TERRATEST. Bratislava, 2006. [3] STN EN 72 1001: Classification of soil and rock in engineering geology and geotechnics, 2010. [4] M. Sloboda. Crossroad Stupava – South on the D2 motorway. Documentation of actual construction. STRABAG – DOPRASTAV, 2011. [5] STN EN 1997-1 Eurocode 7: Geotechnical design. Part 1: General rules. 2005. [6] STN 73 1001: Geotechnical structures. Foundation, 2010. [7] J. Jesenák. Soil mechanics. STU Bratrislava, 1994. 17 Acta Polytechnica CTU Proceedings 16:11–17, 2018 1 Introduction 2 Engineering and geological characteristics of the territory under study 3 Determination of the subsoil models and soil properties 4 Technical parameters of the bridge structure 5 Determination of load states for geotechnical calculations 6 Results from geodetical measurements of the bridge supports settlement 7 Evaluation of geotechnical calculations 8 Conclusions List of symbols Acknowledgements References