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

Introduction Soil Data Investigation of Dome Collapse Back Calculation of Dome Collapse Conclusion 2... of Dome and loadings Soil strength increased by construction activities of Dome

INVESTIGATION OF DOME COLLAPSE OF STEEL PLANT

Nguyen Minh Hai, Ph.D.

Introduction

Soil Data

Investigation of Dome Collapse

Back Calculation of Dome Collapse

Conclusion

2

The steel plant consisted of three iron ore storage domes The domes

Mississippi River

Stacking Tube

H = 124-ft

D = 248-ft

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

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

One of three domes was collapsed in 2013

The scope of this investigation is to evaluate the cause of the dome collapse and

INTRODUCTION

SOIL DATA

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SOIL STRATIGRAPHY

Stratum

No.

Range of

I 0 – 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 DATA BEFORE COLLAPSE

0 25 50 75 100 125 150 175 200

0 25 50 75 100 125 150 175 200

UNDRAINED SHEAR STRENGTH (ksf)

Organic Soils

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INVESTIGATION OF DOME COLLAPSE

Boring R-1 => R-6

SOIL DATA AFTER COLLAPSE

Organic Soils

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to CPTu-15.

5 4

CPTu-15 CPTu-8

1 Sensitive, Fine-Grained Soils

2 Organic Soils and Peat

3 Clays [Clay to Silty Clay]

4 Silt Mixtures [Silty Clay to Clayey Silt]

5 Sand Mixtures [Sandy Silt to Silty Sand]

6 Sand [Silty Sand to Clean Sand]

7 Dense Sand to Gravely sand

8 Sand/Clayey Sand to “very stiff” sand

9 Very Stiff, Fine-Grained, Overconsolidated or Cemented Soil

Soil Classification Based on CPTu-8 AND CPTu-15

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of Dome and loadings

Soil strength increased

by construction activities

of Dome and loadings

Diagrams of CPTU-8, CPTU-15, CPTU-19 and CPTU-20

CPTu-20Organic Soils

Soil strength increased

by construction activities

of Dome and loadings

Soil strength increased

by construction activities

of Dome and loadings

SETTLEMENT

MEASUREMENTS

OF FOUNDATION AND TUNNEL

on July 27, 2012

Started First Loading on July 29, 2013 Zero reading was taken on January 01, 2012

Started Second Loading on September 23, 2013

Differential Settlement of ring beam foundation between DMB-

1 (West) and DMB-3 (East) was about 4-inch.

Ring beam foundation of Dome was inclined toward West.

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Settlement of Tunnel and Simulation of Dome Collapse

0 1 2 3 4 5

Mohr-Coulomb Failure Envelope

of soil layer CH-1 used for stability analysis in the design period of Dome foundation

Evolution of Mohr circle

to reach failure envelope

UU Test of Boring ESI-R5 at 20-ft Depth:

Cu = 0.69 ksf

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.

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.

Combining cohesion of 0.7 ksf with a degree friction angle of for soil layer CH-1 indicates overestimating the soil bearing capacity.

23-Considering Shear Strength of Soil at 20-ft Depth

c = 0.7 (ksf) f = 23 (0)

Soil properties used for design at 20-ft depth

In addition, the results of CPTU soundings indicate that the soil strength from 9 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.

Overestimation of Soil Input Parameters

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Cu = 0.5 tsf; ϕ = 0 0 , Nc = 5.14

 Bearing Capacity of soil = 2.57 tsf

Bearing Capacity of Original Soil

Overestimation of Soil Bearing Capacity

Undrained shear strength

obtained from CPTu-8 and

CPTu-15.

Undrained shear strength obtained from UU and UC tests of Boring ESI-R-5.

BACK CALCULATION OF DOME FAILURE

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Soil Parameters Used for Back Calculation of Dome Collapse

CALCULATION MODEL

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CALCULATION

RESULT FOR SHORT TERM

CONDITION

Safety of Factor = 0.41

WEST

 The differential settlement between center and edge of foundation measured is about 2.5 timesgreater than the originally predicted differential settlement, which caused the stacking tube to tiltone way or the other and finally in a collapse of stacking tube and tunnel The underestimation ofdifferential settlement in the Dome design is attributed to using an incorrect model (Assumption ofthe uniform load condition for the settlement calculation

 Collapse of Stacking and Tunnel due to excessively differential settlement resulted in failure ofRing Beam Foundation

 The assumption of uniform load condition in Dome design resulted in an underestimation of ironore pile tress at foundation base The actual iron ore pile stress at foundation bottom center isabout 1.5 times greater than the assumed and, the actual safety factor is significantly lower thanassumed

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 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 soilstrength parameters in Dome design For the actual soil condition and load, the estimatedsafety 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

(Cont.)

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