Expressway Embankment Landslide Treatment Technology for Peat Subgrade

Transcription

1 J. Civil Eng. Architect. Res Vol. 1, No. 3, 2014, pp Received: June 30, 2014; Published: September 25, 2014 Journal of Civil Engineering and Architecture Research Expressway Embankment Landslide Treatment Technology for Peat Subgrade Cui Songming 1, Zhang Qi 1, Cao Yang 2 and Yan Xia 1 1. CKE Project Office, No.97 Awarakotuwa, China MCC20 Group Corp. Ltd. Kerawalapitiya Wattala, Sri Lanka 2. No.777 Pangu Road, Baoshan district, Shanghai, China Mcc20 Group Corp. Ltd., China Corresponding author: Cui Songming ( @qq.com) Abstract: It's extremely important for the professional expert at Expressway to search, find and evaluate a repair proposal at economical cost for the solution of a landslide happened to the embankment of an expressway which had been built on the soft ground consisting layers of peat. This paper focuses to provide the examples and the references for the embankment failure occurrence to any project of the similar nature, as well as focuses to the significance for working out a restoration solution plan by expounding the Cause Analysis of an embankment landslide. Stability Analysis of a treatment proposal starting from a serial of activities such as an investigation or investigations after the occurrence of the landslide at physical site, complementary geological investigation, sampling and data collection and so on, also attested by the data obtained from the observation and monitoring. Key words: Expressway peat subgrade, embankment landslide treatment technology. 1. Introduction An Expressway was built in the soft ground foundation consisting of peat layers, while the length of such peat soft ground subgrade is Km, covering more than 50% of the total length of the entire Expressway. In the above mentioned project totally 3 sections had flaw incidents of embankment failure (somewhere we are calling landslide), all of these landslides happened in the zone of peat areas. Directing at the embankment failure occurrence, this essay discusses and studies several possible destructive mechanism of the land filling failure occurrence to highway embankment and displays the technical treatment solutions searched out by several analyses and evaluations as well as including the influences from failure occurrence to highway embankment to project progress, cost. Therefore, the technology for the embankment stability control and preventive measures to be taken is a proactive and effective method to overcome such a default through optimal design prior to failure, if any embankment failure happens to the project, it is a must to keep monitoring. The practical example is taken from K in the captioned project where embankment failure happened for the cause analysis and study to determine rational economical treatment solutions. 2. Site Investigation After the embankment failure occurred to expressway project, site investigation is very necessary for cause analysis and to be used as the basis for making out a reasonable repair proposal. (1) Identify the exact location (Chainages) and determine the failure width and length, plot Layout Plan for the defaulted section of embankment, then make an investigation to the surrounding of site and environmental condition of the engineering situation such as inspecting whether there is any crack outside of the failure scope or not, whether appears any bumpy or swelling from the existing topography around the incidental area so on and so forth.

2 204 Expressway Embankment Landslide Treatment Technology for Peat Subgrade (2) Carry out site survey to the slope failure area, then plot general layout plan of slope failure and need to do the horizontal cross sectional surveying every 10 m of the land failure subgrade so that the analysis for stability can be done. Cross section should be extended beyond the ROW land acquisition lines, with clear marks on ROW in order to analyze the possibility for slope improvement with protective berms. (3) Collect engineering geological data of landslide sections, engineering geological logs/histograms and layout plan Drawings. (4) Soft ground treatment methods, Plane drawings for soft ground treatment, longitudinal sectional drawings for soft ground treatment. (5) Monitoring data such as settlement observation data and displacement observation data of the default section before the subgrade landslide. (6) Data and documentation for subgrde filling up period of time. The landslide section was built in the soft peat subgrade, so the said section was treated for improvement prior to embankment filling through high density sand compacted piles designed and constructed, the soft soil consolidation was expedited and improved then respectively the bearing capacity was increased greatly; while the diameter of density sand compacted piles is 0.5 m and the center spacing of SCP piles is 1.4 m, arranged in the shape of regular triangle. Embankment filling was over at the design level and surchage pre of 1.8 m high was completed when slope landslide happened. 3. Geological Prospective Investigation The parameters need to be obtained from site geological investigation such as the thickness of each layer at various sectional stratum of different textures under the ground surface, especially we need to obtain the Cu Value of undrained shear strengths of residual soil layers at the section of slope landslide in the weak clay soil section and how much is the thickness of peat layer. The purpose of vane shear test (VST) is to directly obtain the CU of soil USS at site and the purpose of Standard Penetration Test (SPT) is to obtain inner friction angle of drainage for non-sticky soil. Cone Penetration Test (CPT) helps to obtain measured value the tip resistance q c and side frictional resistance f s and work out by calculation the Cu (undrained shear strength). Tests of three kinds were taken for the failed section of landslide area: 3 (CPTs), 4 (SPTs) and 4 vane shear tests (VST). One thing we confirmed by the test result of core sampling from bored hole #01 and #02 is that two failed sections had thicker peat layers. Based on the geological investigation and the probe tests, we studied the 5-meter filling of sea sand under the natural ground surface, which was the first exposed layer from surface cleaning after landslide as embankment filling material. There were peat layer and peat clay layer of 6m under sea sand filled layer and 1.0 m of fine sand, the last layer we saw is weathered rock. See attached profile section for stratum placing Fig. 1. The test results of VST show the USS (undrained shear strength) of peat layer ranging from 30 KPa-35 KPa. Residual shear resistance could not be clearly defined from the field vane shear test reports due to the characteristics of extreme compressive stress, thus it is shown from the vane shear test the minimum range of (USS) scope is 6-20 KPa, referring to graph in the Fig. 1, while (SPT) N30, the N Value of SPT of the fine sand layer under shows 30; next N Value of SPT of the weathered rock is more than Cause Analysis for Embankment Landslide Default The sand filling material was in concentration piled at the shoulder of failed side during backfilling period of time, further spreading was not done in a timely manner. That means the construction method is not correct.

3 Expressway Embankment Landslide Treatment Technology for Peat Subgrade 205 Fig. 1 Stratum layer profile, SPT hammer blows and vane shear test and minimum undrained shear strength. There is a river at the failed side of embankment slope, moreover, there had been disturbed phenomena in the said riverbed, prior to the occurrence of embankment slope landslide at this section, the thickness of soft layer was 11 m under the riverbed. The status of surface soft layer is largely different from what was supposed for the design because the weak soil layer is much thicker than the design thickness. The filling rate is 30 cm/d, obviously it is too quick to be accepted for such a situation, the filling rate had to be controlled sternly during surcharge pre. The discrepancy happened between height of filling and trend of settlement, there appeared delayed reaction phenomena of curve of settlement. The cause of such a delayed reaction phenomena of settlement curve is in this clue: 1.4 m is the interval space of density sand compaction piles (SCP) as soft ground treatment method, comparatively this value is larger. The soft soil strength of weak soil between SCPs had not been enhanced within big scope, plus load came from filling material from above work platform directly to the SCPs; when further filling load was applied, SCP hardly borne the load from above, they were naturally to transfer excessive load to the soft soil around which strength had not been improved, once soft soil had no way to bear the above load, the incident of slope landslide then happened. The actual embankment slope ratio is 1:1.3 less than design slope ration 1:2. 5. Re-Analysis for the Stability of Subgrade Filling Up The logging presents the USS values are widely different before and after the landslide of the undrained resisting shear strength of peat and peaty clay (USS) The average initial value is 12 kpa, that is to say: the average strength of CKE Project peat layer alongside the line is 12 kpa. Therefore, such a value i.e. 12kPa is considered as the USS of peat where landslide happened. Taking account of the N Value of (SPT), the inner friction angles are 30 & 34 for first layer of sand and sand layer under the first layer. Table 1 displayed all the characteristic parameters of filling material used in slope stability analysis, we

4 206 Expressway Embankment Landslide Treatment Technology for Peat Subgrade may notice in Table 1 the consolidation status of embankment under filling construction was given but there was no any increase of USS of the peat layer. A computer program Slope/W developed by Geo-slope International; Canada based on Limit Equilibrium concepts were used for the analysis. The results of slope stability analysis were obtained from circular slip surfaces using the Morgenstern-Price, Bishop s, Janbu and ordinary methods. The minimum FoS from those 4 methods is taken as the FoS against slope failure. Mohr Coulomb model was used to analyze the soil material behavior. There was no any effectiveness being taken into consideration for stress concentration, thus we have to use the down limited (minimum) figure as the factor of stability worked out from the simulation model. As far as the embankment stability analysis for period of refilling after landslide is concerned, 2 phases were considered during the rebuilding time range: one is FOS of embankment stability under (equally loaded) condition; the second is FOS of embankment stability under surcharge load condition. 5.1 FOS of Embankment Stability under no Any Treatment Condition for Soft Ground Gained Strength Parameter of Geotechnical Layers Analyzed through the monitoring data from settlement plates observation, the peat had gained 60% of consolidation degree when embankment failure occurred for this section of subgrade which had been filled for 4 months, with filling height of 3.5 m; under such an imposed load the peat had already gained more than 65% of consolidation degree according to (Asaoka) calculation method [1-3]. Let s suppose peat had already gained more than 65% of consolidation degree after landslide of this said section. That is to say, during such period of time, the said weak soft soil had already obtained certain degree of consolidation strength. Using the experienced co-relation formula proposed by Skempton [4] [5] [6] [7] as below, soft soil gained effectiveness can be worked out: C u = Ip. (1) This experienced co-relation formula in Table 2 shows gained strength factor of estimated soft layers: Refer to the estimated factor of gained geotechnical strength given in Table 2, but we take conservative factor 0.35 for estimated value. According to the stress imposed by embankment onto the soft soil and the actual situation of the sand compaction piles installed into soil during the subgrade treatment period, 4 zones can be divided in the scope under that failed subgrade soft soil section, they are Zone1, Zone2, Zone3 & Zone 4, refer to the distribution arrangement in Fig. 2 for the details Density SCP had been constructed in Zone1, bearing all the weight of embankment of 3.5 m high; The weight borne by Zone 2 is exactly equal to that weight on Zone 1, but SCP are not able to function because this Zone 2 is the center of landslide; Zone 3 Table 1 Strength parameters & thickness of layers based on samples taken from bored holes and VST. Thickness of Undrained Layers in layer (m) Friction φ shear strength stratum Bored Bored (degrees) c u, (kpa) Hole 01 Hole 02 Sea sand (Layer I) Peat ( (Layer II) Fine sand/ sandy soil (Layer III) Table 2 Gained strength factor of estimated soft layer at various depth of failed slope. Ip/(%) Depth/m Cu v Max. Min. Average Average 0.50 v

5 Expressway Embankment Landslide Treatment Technology for Peat Subgrade 207 is the foundation for embankment slope, while the height of embankment is of 3.5 m. Considering in this Zone, SCP are still not able to function because this Zone 3 is the side slope of landslide part. Now come to Zone 4: Zone 4 is the improved zone under the function of berm load, which basically consists of the first layer and as well as the Part of Road. Table 3 indicates the actuality of estimation for USS (undrained shear strength) of every zone being improved. Table 4 gives result on stability analysis. Under the impose of embankment weight, all of the FoS are all less than 1.3. After imposing surcharge, such FoS reduced less than 1.0, therefore, subgrade filling construction could not be fulfilled without any soft ground treatment. Elevation /m MSL Layer I Zone 1 Zone 2 Design surcharge level Design pavement level Section after failure Existing 3.5 m high embankment Layer II Zone 3 Zone 4 Layer III Fig. 2 Distance from centre line /m Soft soil distribution of averaged stress of 4 zones under 3.5 m high embankment. Table 3 USS estimated parameters of improved combined soft soil. Zone 1 Zone 2 Zone 3 Zone 4 c u φ c u φ c u φ c u φ Table 4 Factor of Safety (FoS) obtained from analysis. Loading status Morgenstern Janbu Bishop Ordinary FOS when fill to pavement Level FOS after surcharge Stability of Rebuilt Sand Compaction Piles According to the Original Design Stability Analysis only Considering Rebuilding the Sand Compaction Piles Due to the fact that requirements for stability shall not be satisfied if there is no any treatment to the soft soil ground, thus construction of rebuilding sand compaction piles was done according to the original design within the scope of Zone 2 and Zone 3, but still without any consideration of gained strength of installed SCP piles prior to landslide of this embankment. The characteristics parameters used for above analysis are exactly the same as that of filled material used for stability analysis of Zone 1 and Zone 4. But the great changes had taken place to the characteristics of material filled into the newly installed SCP piles in Zone 2 and Zone 3. The characteristics parameters of material filled into the newly installed SCP piles are given in Table 5. Table 5 gives result on stability of embankment of different strength at different division zones when embankment was filled up at the equal load weight at all the zones, all of the factors of safety obtained are shown in Table 6. Although the stability factor of safety (FoS) obtained from the analysis methods of Morgenstern & Janbu shows strength increases of geotechnical soil, but the minimum FoS could not satisfy the required

6 208 Expressway Embankment Landslide Treatment Technology for Peat Subgrade Table 5 The characteristics parameters of improved peat stated in 5.2 (* Zone division See Fig. 2). Zone 1 Zone 2 Zone 3 Zone4 c u φ c u φ c u φ c u Φ Table 6 -Factor of Safety (FoS) obtained from analysis. Loading status Morgenstern Janbu Bishop Ordinary FOS when fill to pavement Level limit. Obviously embankment landslide might happen again under imposing the surcharge. Conclusion could be made from above analysis and facts: soft ground treatment for Zone 2 and Zone 3 was by adopting triangle arranged SCP piles with 1.4m intervals. Without considering soft ground increased strength results in the Zone 2 and Zone 3 by adopting such a soft ground SCP installation treatment prior to the embankment landslide, the embankment is hardly to bear all the surcharge load weight The Stability Analysis for Constructed SCP Piles Pattern Prior to landslide of this section, the constructed SCP piles had imposed on to the peat for enhancement, this analysis is to estimate the actual effect of installed SCP piles, so as to make out more rational remedial methods. After calculation, we know the area replacement ratio [2] [3] in failed zone is of the SCP piles installed in shape of regular triangle, the inner friction angle was supposed at 22 at that moment, which is the low boundary limitation, so this value (22 ) is assumed as the contribution to embankment landslide. The SCP piles arrangement in the remedial measure proposal was the same to that of the installed piles (i.e. in shape of regular triangle with interval space of 1.4 m), the area replacement ratio of the SCP installed piles in the zone affected by landslide was 0.116, thus the total replacement area ratio of the installed SCP piles was including old and new installed SCP ones. But when the comprehensive strength of soft ground foundation was in conservative estimation calculated after treatment with SCP piles, the replacement area ratio was only taken as 0.2. The increased strength for soft ground foundation was calculated when the embankment was filled up to level of 3.5 m high. Analysis by impose method was adopted after the consolidation reached 65%, when the embankment was filled up to level of 3.5 m high, then later on applied surcharge. Table 7 shows the characteristic parameter, after calculation, of the material used for filling; while analysis results are shown in Table 8. It is seen from Table 8 the FoS of embankment stability is still not satisfactory. Foundation increased strength when the embankment was filled up to the designed level of pavement. The embankment consolidation reached at 80%, when the embankment was filled up to the designed level of pavement. That means USS must be increased. Fig. 3 shows the Zones being divided based on the strength increase level under different function, the methods for analysis are the same, the improved filling geo-material characteristics after calculation is as shown as in Table 9. Table 7 The analysis characteristic parameters of improved peat under the load of embankment filled up to level of 3.5m high. Zone1 Zone 2 Zone 3 Zone4 c u φ c u Φ c u φ c u φ Table 8 Factor of Safety (FoS) obtained from analysis. Loading status Morgenstern Janbu Bishop Ordinary FOS when fill to pavement Level FOS after surcharge Table 9 Analyzing characteristic parameters of improved peat under the load of embankment filled up to level of finish pavement level. Zone1 Zone 2 Zone 3 Zone 4 Zone 5 c u φ c u φ c u φ c u φ c u φ

7 Expressway Embankment Landslide Treatment Technology for Peat Subgrade 209 Completed level 3.5m high embankment Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 Fig. 3 Soft soil division zones of average stress under 3.5m filled embankment. Elevation/m Fig. 4 Distance/m Circular failure surface after surcharge load with 2 geogrid. The analysis results shown in Table 10, apparently the stability of embankment are not satisfactory, so we have to add geogrid for enhancement to solve this problem thoroughly. 5.3 The Stability of the Treatment for Failed Embankment with Rebuilding SCP Piles according to the Original Design and Adding Layers of Geogrid As per analysis of 5.2, the stability safety factor FoS of the embankment under its gravity is 1.0, so it is easily to see if apply any surcharge on it, the embankment is under safety margin, not safe at all. Therefore, the stability safety factor FoS of the embankment was analyzed respectively at two location: one is 1.1 m under the pavement top level with first layer of geogrid; the other is 0.7 m under the pavement top level with the second layer of geogrid. The propose for the abovementioned two levels where we chose for installation of two layers of geogrid was to reduce the un volume during the time for embankment repairing construction. The strength for such geogrid is 200 kn/m, and the FoS of creep deformation and the destruction during the construction period of time takes 2.0 [8]. The characteristics parameters of the geogrid under surcharge are the same to that shown in Table 9. The analysis results are shown in Table 11. The critical slip slope of landslide is indicated in Fig. 4.

8 210 Expressway Embankment Landslide Treatment Technology for Peat Subgrade Table 10 Factor of Safety (FoS) obtained from analysis. Loading status Morgenstern Janbu Bishop Ordinary FOS after surcharge Table 11 Factor of Safety (FoS) obtained from analysis. Loading Morgenstern Janbu Bishop Ordinary No. status Geogrid FOS after surcharge FOS after surcharge It is seen that the FoS had reached to 1.2 from the analysis for stability after installed two layers, considering the factors such as time and cost, we can get the conclusion that the repair proposal is the most economic solution for remedial measures by arranging regular density SCP piles in interval of 1.4 m then install two layers of geogrid with 200 kn/m as the strength. 5.4 The Special Treatment Proposal for Embankment Landslide Proposal on using the density sand compaction for treating the failure embankment was optimized and finalized through comparison among various kinds of alternatives, the remedial proposal included two layers of geogrid to be added on the top of the road bed for rebuilding the failed embankment: a. Adopting triangle arrangement for constructing the density sand compaction piles with interval spacing of 1.4meter; b. Installing displacement stakes and settlement plates for data monitoring, after installation of density sand compaction piles; c. When embankment was filled near the designed level for pavement, 1.1 m below this level, install first layer of geogrid, then at 0.7 m below second designed level of the pavement, install the second layer of geogrid. 5.5 Settlement Monitoring after Embankment Repair It took 4 months for repairing the failed embankment construction, when time came to the 7 th month period after pre, surcharge of 1.5 m thick was still required according to the analysis for settlement from monitored data. Once the landslide has come to the embankment, an immediate treatment is needed. The remedial repair and rear stage observation directly influenced the project general construction progress. 6. Conclusion It is unavoidable to have landslide of embankment occur to a section of a Highway Project which is built on the soft soil ground if construction method is not properly applied or due to the characteristics of peat ground foundation being compressed during the embankment filling or if the design is too much economical. Filling rate must be controlled for embankment filling construction on the soft ground foundation with optimized design at rational and economical cost; Stress must be put onto the settlement monitoring and stability monitoring during the period of material filling; Furthermore, an extra cost for engineering works has to be reduced. This landslide treatment technology for the said failed section of embankment had been applied in other projects of the similar nature. References [1] K. Terzaghi, R. Peck, G. Mesri, Soil Mechanics in Engineering Practice, 3rd ed., John Wiley & Sons Inc., New York, [2] P.P. Raj, Ground Improvement Techniques, 1st ed., Laxmi Publication Ltd, New Delhi, [3] B.T. Bergado, L.R. Anderson, N. Miura, A.S. Balasubramanium, Soft Ground Improvement in Low Lands and Other Environments, ASCE press, New York, [4] B.M. Das, Principles of Geotechnical Engineering, 5th ed., Thompson Canada Limited, Toronto, 2006 [5] B.M. Das, Advanced Soil Mechanics, 3rd ed., Taylor and Francis Group, New York, [6] J.E. Bowles, Foundation Design and Analysis, 5th ed., Macgraw and Hill publications, New York, 1997 [7] W. Lambe, R.V. Whitman, Soil Mechanics, 1st ed., John Wiley & Sons Inc., New York, [8] R.M. Koerner, Designing with Geosynthetics, 5th ed., Pearson Prentice Hall, New Jersey, 2005.