Analysis of a Failed Port Slope and Stabilization

Transactions of Tianjin UniversityISSN 1006- 4982pp381- 385Vol. 11No. 5Oct.2005Analysis of a Failed Port Slope and StabilizationYAN Shuwang(闫澍旺) , SHU Yang(舒阳)", CHUJian(楚剑)尸( 1. School of Civil Engineering, Tianjin University, Tianjin 300072, China;2. Nanyang Technological University ，Singapore )Abstract :A nonmetal transportation port with soft soil foundation of Tianjin New Harbor failed duringconstruction in 1997. The soil properties and geological changes before and after landslide were in-vestigated by in situ test means. The stability of the slope was estimated with several calculationmethods, such as the simplified Bishop method and the non-circle analysis method. According tothe investigation and analysis results, the factors that may cause the slope failure can be deter-mined as follows: excessive excavating， violating the normal construction procedure, excessivepore water pressure during earlier piling and sudden water-level fluctuation in front of the slope.Measures to improve the soil foundation for reconstructing the port include strengthening the failedslope with vibro sand piles, fling up the crushed stone layer，dividing the excavation into four stepsand driving piles by larger intervals. The port now has been successfully reconstructed and begunoperation.Keywords:slope failure; back analysis ; stabilization; soft clayA slope in Tianjin New Harbor， China， col- the slope and the construction management were pro-lapsed in 1997. The slope was for constructing a port vided. The stability of the slope was back analyzed .for nonmetal material transportation. The sliding body with in situ vane test results.of the soil slope was about 210 m long and 190 mAfter the slope failed， the sliding area waswide. Thousands of tons of soil as well as some tem- strengthened with vibro-sand piles. Other measuresporary shelters for the construction workers slid into for construction were also taken in reconstructing thethe sea. Fortunately， no life loss was claimed, be- port. With greater care, the port for nonmetal materi-cause some deep cracks had occurred a few hours beals was reconstructed successfully in 1999 and now infore the slope collapsed, giving an alarm to people service.working there， who managed to escape just in1 Background of building the portThe slope sliding started at about 10:00 a. m.The collapsed slope was formed for a port con-and lasted for about 40 min. It was a typical progres-structed for transporting nonmetal materials, such assive failure case. A smaller sliding body first col- coal. It took a high pile form, which is the commonlylapsed and rushed into the sea, which immediately in- adopted type of design for ports established in Tianjinduced another part of the remained slope to be in a New Harbor area. The construction work commencedlimiting equilibrium condition and slide successively. in 1997 after the hydraulic flling had been finished toThis process continued until an appropriate mecha-provide a cargo stacking yard for the port. The recla-nism was established in the remained soil.mation soil that was mainly the soft clay was supposedIn this study，the failure process was described. to be strengthened with the vacuum preloading tech-The details for constructing the port， including the nique,中国煤化工: stabilizing soft claysoil properties, the designed and the actual profiles of foundatiMYHCNMHGlaid on the surface of米Accepted date:2005- 01- 10.YAN Shuwang, born in 1950 , male , M , Prof.E-mail: Profyan@ eyou. com.Transactions of Tianjin UniversityVol. 11 No.5 2005the soft clay and the drainage tapes were being driven for vane shear tests after landslide were chosen closeinto the soil. But the vacuum preloading was not car- to those before landslide. Fig. 1 shows typical resultsried out.of a pair of adjacent boreholes for vane shear. It canAccording to the initially designed construction be seen that below a certain depth, the strength doesprocess, piling work for building the port should be not change much. But above this depth, the strengthperformed after the completion of the soft clay stabili- changes dramatically and the strength after landslidezation work. But a decision was made to drive piles becomes only about one third of the original strength.before schedule to catch up with the best construction This means that the soil above this depth has beenseason. In order to keep the pile-driving vessel in po- greatly disturbed. By comparing the vane shearsition，excavation work had to be carried out, which strengths before and after landslide, the slipping sur-should be done after soil strengthening.face can be determined, upon which the soil has beenThe excavation was completed and a slope con- disturbed by sliding. The slipping surface in this casetour was formed which was stiffer than the designed is almost a straight line with a inclination of about 6slope contour. More than 40 piles had been driven in- degree to the original ground surface. Cracks devel-to the slope before the slope failed.oped in the remained soil body, implying that isolatedwedges was formed but stopped by the earlier sliding2 Failure descriptionbody from further moving.The slope failure took place on September 16,1997, when the highest level and the lowest level ofthe tide appeared on the same day. The head differ-●Before slidingence between the highest water level and the lowest+ After slidingwater level of the tide was about 3 m. Both experience-4-and theoretical analyses indicate that the most unfa-vorable condition for the slope stability is the suddenwater-level fluctuation in front of the slope, which isone of the fatal factors to start the slope failure.-1652040s0The first wedge sliding into the sea happenedShear strength/ kPanear the tip of the slope where the slope was over ex-(a) Borehole No. 4.cavated. The sliding of the first wedge gave disturb-ance to the remained soil and put another part of theslope in an unstable condition, inducing the succes-&sive wedge to slide. The progressive failure lasted forabout 40 min until a new limiting equilibrium mecha-i -4nism was established. The next sliding wedge couldnot move across the earlier one and had to stay in aplace closer to the remained soil body.-12-The contours of the collapsed slope were clearly6(80three-dimensional. The actual sliding surface alongShear strength 1 kPawhich the failed wedges were riding cannot even be( b) Borehole No.7seen because they were covered by the successiveFig. 1 Vane shear strength tested before and after slidingsliding wedges. But the bottom of the sliding body canbe estimated by comparing the vane strength beforeThe elevations of the slope surface and the nearerand after landslide. Vane shear test is a kind of insi- area "中国煤化工rafter landslide.tu soil strength investigation method, which can give 3 STYHCNMH Gmore reliable results than the indoor tests because dis-turbance can be avoided during sampling ，transporta-The constructed port was located in Tianjin Newtion and testing. Vane shear tests were arried out be- Harbor, China. Boreholes with 15 m spacing and 30fore and after landslide. The locations of the boreholes m depth within the port area were driven to investigate一-382YAN Shuvang et al :Analysis of a Failed Port Slope and Stabilizationthe soil profile, as described in Tab. 1. It can be seen large void ratio, great compressibility and low shearthat beneath a 1.0 m hard shell, there exists a layer strength. The physical and mechanical properties areof about 18 m of soft clay consisting of mud and silty given in Tab.2 and Tab. 3, respectively.soil, which are characterized by high water content ，Tab.1 Soil stratum descriptionsSoil stratumElevation/mDescription4.4-3.4About 1. 0 m thick hard shel, formed by air-dry and sun shine.3.4--5.0Soft clay with some sandy pockets, in soft or flow plastic state.-5.0--11.0Soft clay mixed with black organie matter, upper middle or high plasticity, in soft or flow plastic state.-11.0--14.0Soft clay with thin sily sand and organic interlayer, high plasticity, in soft plastic state.-14.0--17.0 .Silty clay ，middle plasticity ，plastic state, continuous distribution, mixed with sand and broken shells.-17.0-- 20.0Clay，middle to high plasticity ，in plastic state, mixed with some silty sand and shell debris.-20.0--23.0Clayey silt, low plasticity, hard plastie ，discontinuously distributed, mised with silty sand.below -23. 0Dense sand.Tab. 2 Physical properties of soils of the failed slopeSoil stratum Water content W/% Unit weight γ/( kN●m-3 ) Void ratio eLiquid limit W/% Plasticity index Ip Liquidity index 18.643.722. 40.3241. 717.91.1530.714.41.738.216.1. 6649.325.61.3846. 617. 51.3042.922. 01.1725. 419.00.7528. 812.70.7334.718.70.9841.020. 70.7121.919.50. 6225. 96.10. 32Tab. 3 Mechanical properties of soils of the failed slopeDirect shear, consolidatedTriaxial shear, unconsolidatedTriaxial shear, consolidatedUnconfinedSoil straumand undrainedcompressionq/(°)C/kPa4/(°)9./kPa12180.80517.0618.50.931113.016.01:1.2716.8i239.825.74. 8021.0122. 62:1. 803316.597.028. 3The upper 18 m soft clay strata were formed by face methods. The results are given in Tab. 4.recent natural sedimentation as well as by hydraulicfilling that had been finished about two years beforeTab.4 Safety factor of the failed slopeconstructing the port. Investigation shows that theseMethodsCaleulated with vane shear Calculated with C。strengthstrata were under consolidation.Fellenius0. 7891.335Bishop0.7891.4274 Back analysis .Kre中国煤化工1. 3791.4214.1 Safety of factors:fYHCNMH G-The safety factors of the over excavated slopeFrom Tab.4, it can be seen that since the soilwere calculated with various kinds of strengths accord-consisting of the slope is underconsolidated， usinging to the Bishop approach and other circular slip sur-consolidated undrained strength parameters from direct383--Transactions of Tianjin UniversityVol. 11 No.5 2005shear tests will overestimate the safety of the slope. influence on the stability of the slope.The safety factor is only 0. 8 with the vane shearIt is realized that any single factor describedstrength. The true safety factor should fall in between above was not able to cause the slope failure. It wasthe figures calculated with these two methods.the combined effects of all these factors that induced4.2 Progressive failure simulationthe large-scale landslide.There were large curved fissures on the remainedReconstruction of the failed portland , indicating that some soil bulks had isolated fromother soil bodies and that these bulks would go onThe failed slope was reconstructed in 1999. Insliding without the obstacle of earlier sliding bodies.this process, the following steps of construction workThis implies that the slope had undergone a progres- were included. .sive failure. This process can be simulated by slope( 1 ) Strengthening of the failed slope with vibro-stability analysis5-7l. A soil body with a minimum sand-pilessafety factor of 0.67 was found to perform slope stabil-The landslide area was smoothed and strength-ity analysis, representing the first sliding body of the ened with the vibro-sand-piles. The sand piles wereslope. When this soil body was slipping away, the formed from -. 14.0 m ( elevation of the bottom of thenext soil body with a safety factor of0.74 was formed soft clay layer) to -3.7 m( the average elevation ofand collapsed. And so did the successive sliding the remained ground surface of the failed slope ), withbodies.a diameter of 0.5 m and a spacing of 1.0 m or 1.44.3 Factors that may cause the failuren. The quality of the sand piles was controlled withThere are many reasons causing the landslide ,the SPT number greater than 18 or the internal frictionincluding the following factors.angle not less than 35°. This work started on June( 1 ) Excessive excavating of the slope21，1998 and finished on November 6，1998.The slope was excessively excavated for the pile-( 2 ) Filling up the crushed stone layerdriving vessel to reach its position， which increasedFrom December 3 to 22，1998, a crushed stonethe instability of the slope.layer of up to +3.0 m was laid on the strengthened( 2 ) Violating construction procedureground surface. The crushed stone layer acted as aPiling and excavating activities were carried out stacking load that accelerated the consolidation of thebefore performing soil-strengthening work, which vio- subsoil. Settlements at several places were observedlated the designed constructing procedure and wor- and the results are given in Tab. 5. The observed set-sened the instability of the slope.tlement finished 85% of the estimated ultimate consol-( 3 ) Excessive pore water pressure during piling idation settlement, which can be taken as the degreePile driving work would create increased exces- of consolidation.sive pore water pressure in the slope, which reduced( 3 ) Excavatingthe soil strength according to the effective principle.The excavation work then was performed in four( 4 ) Sudden water-level fluctuation in front of the steps, i. e.，from the surface to - 4. 0 m，fromslope-4.0 mto -10.0 m, from -10.0mto -11.5 m,According to the tide observation record, the and from -11.3 mto -13.8 m. The flowed soil washighest tide and the lowest tide happened on the same cleared during excavation and the slope was kept to beday when the slope failed. The sudden water-level 1:2.8.fluctuation in front of the slope exerted a considerable .(4 ) PilingTab. 5 Observed settlementGauge No. 1Gauge No. 2中国煤化工Gauge No.4DateSettlement/mDatdTYHCN M H GDate1998-12-1701998-12-241999-01-051998-05-250. 486.199905-250.3721999-05-250. 6580.4701999-10-010.5131999-09-260. 4341999 08-130.7221999-09-250. 506- 384YAN Shuvang et al:Analysis of a Failed Port Slope and StabilicationPiling work was carried out with greater care.ReferencesPiles were driven in an interval three times as that oftwo adjacent piles, in order to slow down the develop-Craig R F. Soil Mechanics[ M ]. 5th ed. Chapman &ment of excess pore water pressure. The onshore piles[2] Balasubramaniam B I. Performance of test embank-Hall, London, 1992.268.were driven during higher tide period, and the off-ment constructed to failure on soft marine clay[ J ].shore piles were driven during lower tide period. TheJournal of Geotechnical Engineering,1995,121 (4):piles were framed as a unity with the upper structure305- - 315.as soon as possible to increase the stability of the [3] Low BK, Gilber R B, Wrights G. Slope rliabilityslope.analysis using generalized method of slices[ J ]. Jour-By taking the above remedy measures, the failednal of Geotechnical and Geoenwironmental Engineer-slope was stabilized， and the port has been rebuilting, 1998 , 124( 4 ):350- -363.scesfully. The displacement of the port has been [4] Anderson M C, Richards K s. Slope Stabiliy[ M ]John Wiley & Sons Inc,New York, 1986. 160.monitored and no serious problems occurred ever[ 5] Azouz A Z, Baligh M M , Ladd C C. Three-dimension-since.al stability analysis of four embankment failures[ A ]. .6 ConclusionsIn: Proceedings of the 10th International Conference onSoil Mechanics and Foundation Engineering[ C ].The instability of the failed slope may be causedVol.3. Publication Committee of ICSMFE, Balkema ,by several factors, such as excessive excavating， vio-1981.343- -346.lating the normal construction procedure， excessive[ 6 ] Justo J L,Saura J. Behavior of V enemo dam by three-pore water pressure during earlier piling and suddendimensional FE[ A ]. In: Procedings of the 10th Inter-water-level fluctuation in front of the slope. The insta-national Conference on Soil Mechanics and FoundationEngineering[ C ]. Vol. 3. Publication Committee of IC-ble slope will undergo a progressive failure.UsingSMFE, Balkema, 1981. 449- -452.consolidated undrained strength parameters from direct[7] Leroueil s, Vaughan P R. The general and congruentshear tests will overestimate the safety of the slope.effects of structure in natural soils and weak rocksThis case may give valuable information for building[ J]. Geotechnique, 1990, 40( 3 ): 467-488.new ports of this type on soft soil foundations.中国煤化工MHCNMH G