0705 NGUYEN MINH HAI slides for presentation investigation of dome collapse of steel plant

INVESTIGATION OF DOME COLLAPSE OF STEEL PLANT Nguyen Minh Hai, Ph.D AGENDA Introduction Soil Data Investigation of Dome Collapse Back Calculation of Dome Collapse Conclusion INTRODUCTION Mississippi River The steel plant was located on the north bank of the Mississippi River Project Site The geology in this area is characterized by thick deposits of soft deltaic soil The steel plant consisted of three iron ore storage domes The domes INTRODUCTION Each dome was comprised mainly of five substructures: the shell/ring beam, the slab, the lean concrete ring foundation, the stacking tube and an underground tunnel H = 124-ft Shell The shell of the dome was made of reinforced concrete with a thickness varying from 18-in at the bottom to 10-in near the upper vertical portion, increasing to 13-in close to the apex The shell was connected to ring beam Slap Stacking Tube Tunnel Ring Beam D = 248-ft INTRODUCTION One of three domes was collapsed in 2013 The scope of this investigation is to evaluate the cause of the dome collapse and recommend the remedial solution SOIL DATA SOIL STRATIGRAPHY Stratum No Range of Depth, ft I – 58 FAT CLAY, gray, brown, reddish brown, dark brown, with root fibers, decomposing organics, organic debris, shell, gravel, moist (CH) II 58 – 63 SILT, brown, with sand partings, moist (ML) III 63 – 69 LEAN CLAY, gray, brown, with silt, sand, moist (CL) IV 69 – 158 FAT CLAY, gray, greenish gray, dark gray, brown, with silt, shells, iron staining, sand, organic debris, moist (CH) V 158 – 178 SANDY SILT, gray, moist (ML) VI 178 – 200 SILTY SAND, medium dense to dense, gray with silt (SM) Soil Description* SOIL DATA BEFORE COLLAPSE 25 50 75 100 50 100 150 0 25 25 25 50 50 50 75 75 75 DEPTH (ft) DEPTH (ft) DEPTH (ft) UNDRAINED SHEAR STRENGTH (ksf) DRY UNIT WEIGHT (pcf) WATER CONTENT (%) 125 125 150 150 150 200 B-5 B-6 B-7 175 200 100 125 175 Organic Soils 100 100 B-5 B-6 B-7 175 200 B-5 B-6 B-7 INVESTIGATION OF DOME COLLAPSE SOIL DATA AFTER COLLAPSE WATER CONTENT (%) 100 200 300 50 100 150 0 10 10 10 20 DEPTH (ft) DEPTH (ft) DEPTH (ft) UNDRAINED SHEAR STRENGTH (ksf) DRY UNIT WEIGHT (pcf) 20 30 30 40 40 40 Boring R-1 => R-6Organic 50 50 20 30 Boring R-1 => R-6 Soils 50 Boring R-1 => R-6 10 Soil Classification Based on CPTu-8 AND CPTu-15 Robertson, et al.1986, Updated by Robertson, 2010 1,000 Normalized Cone Stress CPTu-8 100 CPTu-15 9 10 Sensitive, Fine-Grained Soils Organic Soils and Peat Clays [Clay to Silty Clay] Silt Mixtures [Silty Clay to Clayey Silt] Sand Mixtures [Sandy Silt to Silty Sand] Sand [Silty Sand to Clean Sand] Dense Sand to Gravely sand Sand/Clayey Sand to “very stiff” sand Very Stiff, Fine-Grained, Overconsolidated or Cemented Soil 0.1 1.0 Friction Ratio (%) 10.0 12 12 COMPARISON OF CPTU-8 & CPTU-15 WITH CPTU-16, CPTu-17 & CPTu-18 CONE STRESS, qt (tsf) 50 100 150 CONE STRESS, qt (tsf) 200 0 20 DEPTH (ft) DEPTH (ft) 150 10 20 30 CPTu-17 40 CPTu-15 30 CPTu-18 Organic Soils 40 CPTu-15 50 CPTu-8 60 100 10 Soil strength increased by construction activities of Dome and loadings 50 Soil strength increased by construction activities of Dome and loadings Soil strength increased by construction activities of Dome and loadings 50 CPTu-8 60 13 200 Diagrams of CPTU-8, CPTU-15, CPTU-19 and CPTU-20 CONE STRESS, qt (tsf) 50 100 150 200 0 10 10 20 20 30 CPTu-19 40 CPTu-15 Soil strength increased by construction activities of Dome and loadings 50 DEPTH (ft) DEPTH (ft) CONE STRESS, qt (tsf) 100 150 200 30 CPTu-20 Organic Soils 40 CPTu-15 50 CPTu-8 60 50 60 Soil strength increased by construction activities CPTu-8of Dome and loadings 14 SETTLEMENT MEASUREMENTS OF FOUNDATION AND TUNNEL 15 TIME (Day) 550 SETTLEMENT MEASUREMENTS OF RING BEAM FOUNDATION 570 590 610 630 650 0.0 Started Second Loading on September 23, 2013 2.0 100 200 300 400 500 0.0 SETTLEMENT (Inch) 6.0 700 8004.0 Started Second Loading on 6.0 September 23, 2013 2.0 4.0 600 SETTLEMENT (Inch) TIME (Day) Stacking Tube construction completed on July 27, 2012 8.0 Started First Loading on July 29, 2013 DMB-1 DMB-2 DMB-3 DMB-4 10.0 Last reading was taken on September 25, 2013 Dome was collapsed on September 26, 2013 at 3:10 PM 12.0 8.0 10.0 DMB-1 DMB-2 DMB-3 DMB-4 Started First Loading on July 29, 2013 Zero reading was taken on January 01, 2012 January 01, 2012 - September 25, 2013 12.0 Max Settlement of Differential Settlement of ring ring beam beam foundation between DMBfoundation (West) and DMB-3 (East) was measured was about 4-inch about 9-inch Ring beam foundation of Dome was inclined toward West 16 Collapse of Stacking Tube 125,000 MT Tunnel Max Consolidation Settlement = 1.2-ft Max Total Settlement = 2.5-ft Differential Settlement = 2.0-ft Max Immediate Settlement = 1.3-ft Settlement of Tunnel and Simulation of Dome Collapse 17 Overestimation of Soil Input Parameters Considering Shear Strength of Soil at 20-ft Depth Soil properties used for design at 20-ft depth c = 0.7 (ksf) f = 23 (0) As can be seen, combining cohesion of 0.7 ksf with a 23-degree friction angle equal to 1.8-ksf cohesion without frictional angle Shear Stress, t (ksf) c = 0.7 (ksf) -3 f = 23 (0) This responds to a cohesion of the analysis model that is 2.57 greater than the actual cohesion at this depth The safety factor calculated using the Dome model was 2.56 Mohr-Coulomb Failure Envelope of soil layer CH-1 used for stability analysis in the design period of Dome foundation Cu = 1.8 (ksf) Evolution of Mohr circle to reach failure envelope UU Test of Boring ESI-R5 at 20-ft Depth: Cu = 0.69 ksf 0 Normal Stress, σ (ksf) Combining cohesion of 0.7 ksf with a 23degree friction angle of for soil layer CH-1 indicates overestimating the soil bearing capacity In addition, the results of CPTU soundings indicate that the soil strength from through 21-ft depth did not increase when loading the Dome, suggesting that using friction angle in combination with soil cohesion to compute stability is not reasonable for the Dome analysis 18 Overestimation of Soil Bearing Capacity Bearing Capacity of Original Soil Cu = 0.5 tsf; ϕ = 00 , Nc = 5.14  Bearing Capacity of soil = 2.57 tsf Cu = 0.5 tsf Undrained shear strength obtained from UU and UC tests of Boring ESI-R-5 Safety of Factor (FS) At Center of Foundation: FS = 0.5 At Edge of Foundation: FS = 1.2 Undrained shear strength obtained from CPTu-8 and CPTu-15  Failure of Dome foundation occurred at center of foundation (at position of stacking tube foundation) 19 BACK CALCULATION OF DOME FAILURE 20 Soil Parameters Used for Back Calculation of Dome Collapse 21 CALCULATION MODEL 150 90-ft 32-ft ELEVATION (ft) 125 EL.+ 102' 100 75 EL.+ 66' Iron Ore Pile 50 Surchage Load: 25 ɣ = 137 (pcf) EL.+ 25' EL.+ 0.0' 0 31 62 93 124 155 186 DISTANCE (ft) 217 248 279 310 22 WEST CALCULATION RESULT FOR SHORT TERM CONDITION Safety of Factor = 0.41 23 CONCLUSION  The differential settlement between center and edge of foundation measured is about 2.5 times greater than the originally predicted differential settlement, which caused the stacking tube to tilt one way or the other and finally in a collapse of stacking tube and tunnel The underestimation of differential settlement in the Dome design is attributed to using an incorrect model (Assumption of the uniform load condition for the settlement calculation  Collapse of Stacking and Tunnel due to excessively differential settlement resulted in failure of Ring Beam Foundation  The assumption of uniform load condition in Dome design resulted in an underestimation of iron ore pile tress at foundation base The actual iron ore pile stress at foundation bottom center is about 1.5 times greater than the assumed and, the actual safety factor is significantly lower than assumed 24 CONCLUSION (Cont.)  The bearing capacity of soil was overestimated, which contributed into the failure of Dome The overestimation of bearing capacity can be attributed to use of overestimated soil strength parameters in Dome design For the actual soil condition and load, the estimated safety factor is about 0.5  Failed to identify and consider presence of organic soil layers below Dome Foundation  Back calculation indicates a safety factor is about 0.41 for short term condition 25 26 ... Introduction Soil Data Investigation of Dome Collapse Back Calculation of Dome Collapse Conclusion INTRODUCTION Mississippi River The steel plant was located on the north bank of the Mississippi... characterized by thick deposits of soft deltaic soil The steel plant consisted of three iron ore storage domes The domes INTRODUCTION Each dome was comprised mainly of five substructures: the shell/ring... Tunnel Ring Beam D = 248-ft INTRODUCTION One of three domes was collapsed in 2013 The scope of this investigation is to evaluate the cause of the dome collapse and recommend the remedial solution