The Use of Piled Plate to Support airport runway Embankment on Very Soft Soils Foundation

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1 The 1 World Congress on Songdo Convensia, Incheon, Korea, August 7-31, 1 The Use of Piled Plate to Support airport runway Embankment on Very Soft Soils Foundation *Paulus P. Rahardjo 1) and Andy Sugianto ) 1), ) Department of Civil Engineering Universitas Katolik Parahyangan, Bandung, Indonesia 1) rahardjo.paulus@gmail.com ABSTRACT Construction of embankment on peats and soft soils have always been faced by the problems of instability and large settlement. The conventional system conducted in practice by Indonesian engineers is the use of bamboo pile raft system (known as cerucuk ), owing to its availability and low cost. This system might have deficiencies in the future due to durability of the bamboo/bacau or gelam and the prolem of long term settlement. The new airport in East Kalimantan was inevitably constructed on very soft clay deposit which is either lake deposit or suspected as dump material from ex-coal mining through deep water. Ground improvement was conducted by using PVD combined with preloading as initial design. During embankment work, slope failure occurred in the April which caused half of fill material in runway area damaged. Slope reinforcement was highly required to accelerate the runway construction, otherwise will delay the operational target. Mini pile plate system was selected as slope reinforcement as well as settlement reducer. Finite element approach has been conducted to simulate the condition. Finite element analysis showed that mini piles sq. xcm with 1m of effective length ctc 1.m still appropriate to retain m height of total embankment load above mini pile raft system level plus the load from the the airplane. The recalculated factor of safety of embankment slope after reinforcement increased to.. As settlement reducer, mini piles transsfer the embankment load through skin friction along pile shaft. Concrete plate were placed on top of each mini pile to distribute the load and connected as a system to gain more stiffness.the reduction of settlement can reach % to 7% of total settlement without the pile plate system. To increase the ability of the soil to distribute load, compacted sand blanket is placed under concrete plate. This system has been proved successful based on its performance measured by geotechnical monitoring system as well as from the numerical simulation using finite element analysis. Keywords: soft soils, ground improvement, pile plate embankment, settlement reducer 1) Professor, Universitas Katolik Parahyangan, Bandung - Indonesia ) Formerly Graduate Student, Universitas Katolik Parahyangan, Presently at PT GEC Consultant, Bandung

2 The 1 World Congress on Songdo Convensia, Incheon, Korea, August 7-31, 1 1. INTRODUCTION For many years,bamboo or gelam piles with raft have been used by Indonesians in the construction of embankment on soft soils. The main consideration is for stability reason and in certain cases to reduce the settlement (Rahardjo, ). Recently Pile Plate supported embankment has been used in practice by Indonesian engineers since 199s as repalcement of the conventional bamboo or gelam piles to stabilise fill on top of soft to very soft soils or organics. These mini concrete piles can be used with or without cap. The use of pile cap is more effective if upper soil layer is sufficiently stiff or a blanket of sand is used under the pile cap. The caps will be resting on the stiffer layer so that a portion of the embankment load is carried by the pile caps. Figure 1 and figure show example of this system in north Jakarta area. Figure 1 : Mini pile with cap used to stabilise embankment on Soft Soils (courtecy of PT JHS Indonesia) Figure : Pile Plate Embankment construction on Soft Soils (courtecy of PT JHS Indonesia)

3 The 1 World Congress on Songdo Convensia, Incheon, Korea, August 7-31, 1 Figure 3 : Three different Pile Supported Embankments (Rahardjo, ) The runway of an airport project in East Kalimantan lay is constructed on very soft deposit which is suspected as dump material from ex-coal mining as a former. Runway is proposed with m width and approximately m in length based on ministry of transportation requirement. Initial design for ground improvement by using prefabricated vertical drain combine with preloading up to m height were proposed in certain area dominated by soft clay layer. In, slope failure occurred which caused half of embankement in runway area damaged during fill placement Landslide Area Area Longsoran Landslide body Figure : Landslide Area on Runway Plan In order to accelerate the construction work, the other part of runway was constructed partially. Somehow, the project must be completed on scheduled as requested by government even though slope failure will delay the whole operation of the airport. Additional work of slope repair and ground reinforcement were proposed to speed

4 The 1 World Congress on Songdo Convensia, Incheon, Korea, August 7-31, 1 up the runway construction. PVD with preloading as initial design was no longer possible consider to the consolidation period after completion level of preload. Mini pile plate system was designed as ground reinforcement to support embankment, pavement, reduce the settlement, and as slope stabilization. Mini pile raft system consists of sq. Pile x cm connected with concrete cap which is placed sand blanket as platform.. SOIL CONDITION Soil investigation were conducted on site in three period of time, in 1, 1, and such as CPT, Nspt, and laboratory test. In fact, the data were not consistent compare with each other. In 1, more comprehensive soil investigation performed at the whole area of airport not only in air strip area, but also at the side slope of embankment. Soil investigation consists of piezocone, 39 (mechanical) CPT, and Borholes with SPT Beside soil investigations, existing topography prior construction also used to determine the soil condition. Based on topography data, there were a cut and fill area to reach the level of pavement. In principal, cutting area will not be an issue considering the soil condition is favorable. The critical part was in embankment on ex-dumped area, because in previous time the dump area was not conducted with engineering fill which was compacted properly. Error! Reference source not found. shows the slope failure area was in ex-dumped area comprise of soft clay and any other soft material deposit. Slope Failure Area Fig..Existing Topography Data Superimposed with Airport Layout Soil condition on slope failure area dominated with very soft to soft clay layer until 1m to m depth. The underlying layer is medium to stiff clayey silt material. Very soft to soft material are very sensitive due to construction especially after failure occured. Sliding plane still in there and progressive movement can be occurred any time triggered by external factors.

5 <- Depth in m to reference level Elevation [m] The 1 World Congress on Songdo Convensia, Incheon, Korea, August 7-31, Fig.. Soil Condition in Slope Failure Area Typical CPTu data at this station is shown on figure 7. The data shows that the whole depth of CPTu consisting of very soft to soft soils randomly placed through water with 3 m fill shown on top. Excess pore pressure response is high producing high value of pore pressure ratio (Bq >.7) indicating the soils is underconsolidating (Rahardjo, 1) Soil Behaviour Type after Robertson,19 Cone resistance,qc(mpa) Sleeve resistance,fs(mpa) Pore pressure, u ( MPa) Friction ratio,fr(%) Pore pressure ratio,bq () Silty clay to clay () Silty clay to clay Lapisan tanah lunak yang masih berkonsolidasi Figure 7 : Typical CPTu data in landslide area

6 The 1 World Congress on Songdo Convensia, Incheon, Korea, August 7-31, 1 For analysis purposes, the topography after failures were measured and soil investigastion along the section were used to determine the stratigraphy. Figure shows this stratigraphy. There are some spots of organics soils and peats and the peats heaved as high as - m at the toe of the embankment. The cross section is used to do back analysis Figure : Stratigraphy of the soils after failures (PT Erka, 1) 3. INITIAL MONITORING OF SLOPE Geotechnical monitoring system including inclinometers, piezometers, and settlement plates were used during construction to monitor performance of soft foundation soils and the influence of embankment work. Fortunately, inclinometers were installed nearby slope failure area. It has provided information on lateral movement after slope failure in. The cummulative lateral movement reach mm to mm only in a couple months from October to December and the depth of sliding planes were found m and 17 m from ground elevation. These are very useful data to be used for back analysi later on. Most important information is also found on the rate of movement which were intially unpredicted and not clearly shown on the surface. Cracks were later on found after failures

7 Depth ( m ) Depth ( m ) Cum. Disp. B-B ( mm ) Cum. Disp. B-B ( mm ) The 1 World Congress on Songdo Convensia, Incheon, Korea, August 7-31, 1 INCLINOMETER IN-9 Resultant A-A & B-B Cummulative Displacement ( mm ) INCLINOMETER IN-33 INCLINOMETER IN- 9 Resultant A-A & B-B PLAN VIEW Cummulative Displacement ( mm ) INCLINOMETER IN PLAN VIEW // 1: 1// 1: 1// 11: 1// 1:33 3 1// 11:1 1/11/ :3 1/1/ 1: 1/1/ 1: 3 1/1/ 1: 1/1/ 9:31 1/3/ :37 1/3/ : 3 Cum. Disp. A-A ( mm ) Fig 9 Lateral Movement Recorded in Inclinometer prior to Slope Failure 3 Cum. Disp. A-A ( mm The settlement which was recorded on site by using settlement plate around 3.cm to.cm with height of embankment between m to m. Those monitoring result were used as initial condition after slope failure to simulate into finite element programme as back analysis. Back analysis was widely used accurately to predict the initial conditon based on actual monitoring data (Duncan and Wright, ). It has been used to obtain the model close with the actual condition (i.e. sliding plane, soil settlement, excess pore pressure, etc).. FINITE ELEMENT MODEL OF INTIAL DESIGN PERFORMANCE Actual condition at slope failure area was modelled and simulated by using finite element method with Plaxis D. Slope stability and settlement under slope reinforcement were performed to predict the future settlement and ensure the stability of side embankment during construction. By using mini pile system, preload was no longer needed. Filling work only constructed until subgrade level of runway pavement. Wooden pile Mini Pile Raft Runway Embankment Estimated Sliding Plane Minipile Sq. x cm Minipile Sq. 3x3 cm Fig 1.Sketch of Slope Reinforcement By Using FEM Soil parameters were derived from back analysis which obtained lateral movement, settlement, and pore pressure close with actual condition and monitoring results. Any other parameters related with soil model were determined using soil parameter correlations with piezocone. Finite element model simulated the behavior of soil below

8 The 1 World Congress on Songdo Convensia, Incheon, Korea, August 7-31, 1 embankment due to construction and operational loading. For slope stability analysis, FEM with Plaxis D were performed to predict potential of slope failure in the future. Settlement for both condition (i.e. combination of PVD and preloading, mini pile raft) were compared to find the impact of settlement reduction. To model load-settlement behavior of mini pile under loading, 3D Midas GTS-NX was simulated and verified with full-scaled pile loading test. 1mm 1mm 1mm 1mm 1mm 1mm Fig. 11. Illustration of Mini Pile Raft System Figure 1 Preparation of mini-pile load test Soft soil model was used to obtain better load-settlement curve under pile raft loading as well as settlement analysis. Parameter of soft soil model used compressibility data from soil laboratory consolidation results served by KSO report in. Elasticperfectly plastic model using Mohr-Coulomb was used for fill material and sand blanket.

9 The 1 World Congress on Songdo Convensia, Incheon, Korea, August 7-31, 1 Void Ratio, e Compression Index, Cc Compression Ration, Cc/(1+e o) Coefisien of Consolidation,Cv (cm /sec) E- 1.E-3 1.E Depth (m) Depth (m) Depth (m) Depth (m) Fig. 13. Parameter of Compressibility for Very Soft Clay Deposit (Source: SI Report KSO, ) Table 1. Soil Parameters Based on Back Analysis Name E [kpa] Soil Model ν γ [kn/m 3 ] γ_sat [kn/m 3 ] C [kpa] φ [ ] e_o k [m/s] OCR λ k K NC Fill MC E Sand Blanket 1 MC Very Soft Clay Soft Soil E Stiff Clay Soft Soil E Embankment comprise of 1m height of construction platform above existing ground level, 1m sand blanket layer as porous material, and m height of fill material until final level of construction.. PILE LOADING TEST RESULT AND ANALYSIS Mini pile sq. x cm with 1m of effective length and spacing ctc 1.m was design with 1 tons allowable bearing capacity connected with 1.x1.x.13m concrete plate at the pile head which lay on sand blanket layer. The pile was designed as friction pile because of the very thick soft layer. Especially at the edge of runway adjacent with non-treatment area, mini pile sq. 3x3 cm with 1m of effective length was needed to cut the estimated sliding plane which was deeper than runway area. In fact, the bigger pile functioned as barricade against future lateral movement. Slope stability analysis showed the slope reinforcement using mini pile raft system increased the safety factor to.. Initial condition for back analysis which proposed by Duncan and Wright () approximately 1. if any movement or sliding already occurred. Lateral movement generated around cm through slope failure area during piling work and embankment construction until final level (approximately m height above mini pile raft). Construction of embankment was limited into several layers with 3cm thickness for each layer and compacted properly. If embankment work was conducted swiftly, it can endanger the stability of slope. Analysis showed the maximum incremental of

10 Settlement [cm] The 1 World Congress on Songdo Convensia, Incheon, Korea, August 7-31, 1 embankment height around 1m for a month to prevent huge number of additional excess pore water pressure. Fig. 1. Lateral and Vertical Movement During Embankment Construction Settlement between embankment using PVD + preload system and embankment using mini pile raft system were compared using finite element simulation. There was a deviation between both reinforcement systems up to cm of total settlement. PVD combined with preload area has bigger settlement rather than mini pile raft system. Side embankment which had no treatment of base soil gave lower settlement because of no additional preload had been applied. Displacement of soil was effectively reduced which shown in Fig.. Reduction of settlement around % to 7% of total settlement. Settlement Profile at Slope Failure Area Side Embankment Minipile raft PVD + Preload Area Distance [m] Fig.. Settlement Comparison Between Initial Design and Mini Pile Raft System Soil stress under concrete plate can be shown to study the behavior of pile raft system under embankment loading. 3D Midas GTS-NX can simulate the construction stage from initial phase to final embankment work. Pile in a group tend to interact with each other depending on the spacing between them. The smaller the spacing, the greater interaction and the larger the settlement (Poulos, 199). The greater settlement occured, the greater soil stress under concrete mat also occurred. Small model was determined with mxm mini pile raft system to simulate embankment work. Interaction of concrete mat and mini pile along depth shown that soil pressure at mini pile raft system much

11 Soil Pressure [kn/m ] The 1 World Congress on Songdo Convensia, Incheon, Korea, August 7-31, 1 lower than the side embankment area which no treatment. Reduction of contact pressure similar with the settlement around % to 7% absorbed by the mini piles. Cross Section 1-1 Fig. 1. FEM Settlement Analysis Using Midas GTS-NX Cross Section 1-1 Fill Material Ground Improvement Zone Sand Blanket Back Fill for Platform -1 Very Soft to soft clay deposit Stiff Clay - - Without Mini Pile Raft With Mini Pile Raft Distance [m] Fig. 17 FEM Settlement Analysis Using Midas GTS-NX For the purpose of verification as regulated by ministry of transportation, loading test is required to verify the pile bearing capacity as well as combined of mini pile plate system bearing capacity. Full-scaled loading test regarding for both conditions were conducted on site started with indicator pile. Kentledge method was adopted for cyclic and monotonic loading test where the incremental load was applied up to %. Five indicator piles were tested for axial compression loading test with two conditions which were single pile and also as pile raft respectively. To save construction time, pile loading test was not conducted with cyclic incremental loading to all piles. Monotonic loading test (quick test) was performed to obtain the data immediately where the incremental load applied in one single cycle.

12 The 1 World Congress on Songdo Convensia, Incheon, Korea, August 7-31, 1 Counterweight Concrete Raft Figure 1 Arrangment of Pile Plate System Applied Working Load Concrete Mat Mini pile Sq. x cm Fig. 19 Loading Test on Mini Pile Plate System Loading test result showed all the piles still in elastic condition where the total settlement during test below 1 inch. To compare the behavior of single pile and mini pile raft respectively, SLT- was tested in both conditions. Concrete mat has function to distribute the load to the soil below the mat. However, the mini pile significantly contributed to support the mat and external load above it. Settlement of single pile recorded,1mm where the contribution of concrete mat reduce the settlement become 3,1mm.

13 Settlement (mm) The 1 World Congress on Songdo Convensia, Incheon, Korea, August 7-31, 1 No. Uji Table. Soil Parameters Based on Back Analysis Working Load [Ton] Settlement [mm] 1% Working Load % Working Load Rebound [mm] Remark SLT Pile Raft SLT Single Pile SLT Pile Raft SLT Single Pile SLT Pile Raft SLT Pile Raft Simulation of pile loading test using 3D Midas GTS-NX tried to extrapolated actual condition. Thus, ultimate bearing capacity of mini pile can be predicted close to reality. Soil stratification used the previous analysis by using soft soil model. Load settlement curve represented the actual condition by using soft soil model rather than Mohr- Coulomb soil model. In fact, Mohr-Coulomb model will give elastic-perfectly plastic behavior which was not precise. Result of single pile loading test showed the pile almost reach the ultimate condition even though the settlement still relatively small around,1mm. In the other hand, after concrete mat installed at the pile head, load-settlement curve showed the mini pile raft system can resist the same amount of axial load with smaller displacement. Unfortunately, the pile loading test stopped at the % working load as stated in the regulation. FEM simulation using 3D Midas GTS-NX try to approach the behavior of pile raft system under axial loading. The result showed that mini pile raft system was predicted can resist axial load until % working load. Therefore, pile capacity can be increased due to installation of concrete mat at the pile head. Load [Ton] Single Pile Pile raft FE Midas SC Model Fig.. Performance of Mini Pile Plate System and Mini Pile (sq. x cm)

14 The 1 World Congress on Songdo Convensia, Incheon, Korea, August 7-31, 1. CONCLUSION Soil condition at landslides area is practically soft to very soft and still consolidating (excess pore pressure still exist) consisting deposit through deep lagoon. This condition caused the soil to be very sensitive and small additional load can cause the soil to sliding The use of concrete plate on mini piles is effective to stabilize the slope against lateral movement caused by sliding during embankment work. This is in line with analysis based on finite element method to simulate and analyse the actual contidion. In this case, mini piles sq. xcm with 1m of effective length ctc 1.m still appropriate to retain m height of total embankment load above mini pile plate system level. With higher embankment load, it can give higher lateral pressure and pore pressure as well. Safety factor of embankment slope after reinforcement raised to.. As settlement reducer, mini piles transfer the embankment load to the soil below and the objective is to support the embankment load through skin friction along pile shaft. Concrete plate located on top of mini piles help to distribute the load and connected as a system to gain more stiffness and finally can reduce the settlement induced by embankment load and pile. The reduction of settlement reached % to 7% of total settlement as well as the soil pressure under concrete plate. The distribution of the stresses under concrete plate are better by compacted sand blanket below the plate Overall, the use of minipiles and concrete plate under the embankment is sucessful to avoid failures of the embankment to support the runway 7. ACKNOWLEDGEMENTS The authors would like to thank Departement of Transportation of East Kalimantan to provide fund for the research and allow the use of the data for publication REFERENCES Duncan, J.M., Wright, S.G., (). Soil Strength and Slope Stability. John Wiley & Sons. Poulos, H.G. (199). Pile Behavior theory and application. Geotechnique, 39(3), 3-. P.T Erka (1), laporan Analsis longsoran dan rekomendasi penanganan nya pada sta +1 runway BSB, report submitted to the Departement of Transportation East Kalimantan, Indonesia P.T. GEC (1), Final Report on Slope Stability Analysis and Design of Embankment for BSB Airpot (in Bahasa), Design Report submitted to the Departement of Transportation East Kalimantan, Indonesia Rahardjo, P.P. () The Use of Bamboo and Bakau Piles for Soil Improvements and Application of Pile Raft System for the Construction of Embankment on Peats and Soft Soils in Ground Improvement edited by Jian Chiu et al, Elsevier Rahardjo, P.P., (1). CPTu in Consolidating Soils, proceeding th International Conference on Site Characterization (ISC-), Gold Coast, Australia