Seismic Considerations and Design Methodology for Lightweight Cellular Concrete Embankments and Backfill

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1 Seismic Considerations and Design Methodology for Lightweight Cellular Concrete Embankments and Backfill STGEC 2018, Louisville KY Steven F. Bartlett, Ph.D. P.E Department of Civil and Environmental Engineering October 9 th, 2018

2 Low-density cellular concrete (LDCC0 is defined by ACI 523 as Concrete made with hydraulic cement, water and preformed foam to produce a hardened material with an oven dry density of 50 pounds (22.7 kg) per cubic foot or less. Preformed foam is created by diluting a liquid foam concentrate with water in predetermined proportions and passing this mixture through a foam generator.

3 LCC vs Flowable Fill Types of Foam Preformed Produced by Foam Generator Agitated Produced by the mixing action of a concrete mixer ACI 523 ACI 229 Cellular Concrete CLSM Cellular concrete can be flowable fill (ACI 229 Chapter 8) but flowable fill (CSLM) cannot be cellular concrete because of the density being higher than 50 pcf.

4 Pervious & Non-Pervious LCC Closed cell Open cell

5 Cellular concrete pore structure when cured Cementitious materials encapsulate the air bubbles, then dissipate leaving a void structure as a replacement to traditional aggregate Low Density Cellular Concrete differs from conventional aggregate concrete in the methods of production, the density of the material and the extensive range of end uses.

6 Topics Introduction Material Properties Seismic Considerations Design of Embankments and Backfill

7 Topics Introduction Material Properties Seismic Considerations Design of Embankments and Backfill

8 Unconfined Compressive Strength (psi) Unconfined Compressive Strength Unit weight (lb/ft 3 ) Source: Cell- Crete Corp. Qu = 120 psi for 25 lb /ft 3 Qu = 250 psi for 35 lb/ft 3 Qu = 400 psi for 45 lb/ft 3

9 Topics Introduction Material Properties Seismic Considerations Design of Embankments and Backfill

10 U.S. Seismic Hazard Source: USGS Many regions of the U.S. are exposed to relatively high earthquake hazard requiring seismic resilient design

11 Seismic Benefits Material Properties Lightweight = reduction in static and inertial loadings (4 to 5 times less weight than conventional backfill). F i = ma Relatively compressible = reduced stiffness and increased damping. This can be used to reduce horizontal earth pressure against buried structures. Predictable properties = manufactured material with known behavior.

12 Seismic Benefits Material Properties Ratio of Shear strength to Unit Weight Comparison Stiff Clay = 75 kn/m 2 / 18 kn/m 3 = 4.2 Med. Dense Sand at 5 m = 74 kn/m 2 / 22 kn/m 3 = 3.4 LDCC = 333 kn/m 2 / 3.9 kn/m 3 = 85

13 Topics Introduction Material Properties Seismic Considerations Design of Embankments and Backfill

14 Grade Separation for Union Pacific RR Mainline

15 Colton Crossing - Typical Cross Section (1) 8.5-foot wide concrete ties with ballasted track section [12 inches ballast/18 inches sub ballast], (2) 3-foot thick upper layer of Class IV cellular concrete, (3) variable thickness of Class II cellular concrete, (4) 2.5-foot thick Class IV layer of cellular concrete with a 4-foot deep shear key embedded in the foundation soils (at higher embankment sections), (5) vibro-replacement stone columns approximately 15 ft deep in the foundation soils.

16 Modes of Excitation / Failure for General Stability Interlayer Shear / Sliding Horizontal Sway and Overstressing Basal Sliding Rocking and Uplift

17 Design Spectra Colton Crossing Spectral Acceleration (g) Level 2 horz Level 2 vert Level 3 horz Level 3 vert Period (s)

18 Colton Crossing -Project Performance Criteria 1. Level 1 The embankment structure should remain intact with no permanent deformation (i.e. the seismic loads must remain within the elastic range of the stress-strain curve of the embankment). 2. Level 2 The embankment structure should be repairable, with only minor permanent deformation. 3. Level 3 The embankment structure must not collapse after experiencing permanent deformations. AREMA (2010)

19 Design Spectra and Spectral Matching

20 Example of FLAC Numerical Model 1D free field motion deconvolved (SHAKE) MSE reinforcement not modeled Mass is a relatively rigid, cohesive mass Free field boundary Convolved Motion Basal interface Free field boundary Quiet boundary Mejia, L. H. and Dawson, E. H. (2006)

21 Colton Crossing Design Findings Evaluations suggest that the LDCC embankment remained in the elastic range for AREMA Level 1 and 2 earthquakes and will not exceed the peak shear strength under any of the AREMA Level 1, 2 and 3 earthquakes. Reinforcement of the LDCC mass is recommended to prevent the potential for minor cracking resulting from excitation. Interlayer sliding and overstressing of LCC due to sway did not occur. Estimated basal sliding of the tallest section of the embankment is expected to range from 1 to 4 inches at the Level 2 earthquake, and from 4 to 7 inches at the Level 3 earthquake. The presence of basal shear key was integral to limiting basal sliding for the AREMA Level 3 event. Higher strength LCC is also recommended near the top and base of the embankment. Rocking mode is not significant and any minor overstressing from such should be addressed by higher strength LCC in basal layer.

22 Introduction Steps of Simplified Approach for Interlayer Shear and Sliding and Basal Sliding Calculate Develop Determine Determine Compare Fundamental period of the LDCC embankment Design acceleration response spectrum Design acceleration and inertial forces at the base and top of embankment Determine the internal sliding and shear forces within the embankment Internal sliding force and shear forces with available resistance Calculate Factor of Safety

23 Introduction - Simplified Approach (Single Degree of Freedom System - SDOF)

24 Simplified Approach Calculate Fundamental Period of Embankment as SDOF System flexural, shear and axial stiffness of the beam are considered in this equation TT 0 = 2ππ σσ vv 0 HH ggee ttii 4 HH BB νν To = fundamental period σσ vv0 = effective vertical stress H = height of embankment g = acceleration of gravity B = width of embankment EE ttii = Youngs modulus of LDCC νν = Poisson s ratio (Hotta. 2001)

25 Simplified Approach Develop Design Acceleration Response Spectrum

26 Simplified Approach Determine Design Acceleration and Inertial Forces the Base and Top of the Embankment 0.8 g at top 0.32 g at base

27 Simplified Approach Determine Design Acceleration and Inertial Forces the Base and Top of the Embankment m = lumped mass (i.e., mass of embankment above potential sliding plane) a = acceleration in embankment at potential sliding or shear plane with acceleration linearly interpolated from top of embankment to bottom.

28 Simplified Approach Compare Internal Sliding and Shear Forces with Available Resistance F r = Resisting Force F i = Inertial Force FS = F r / F i

29 Seismic Advantages Backfill for Buried, Non- Yielding Walls Mass Reduction Strategy Building LDCC LDCC Seismic thrust Seismic thrust greatly reduced due to low unit weight and compressibility of LCC

30 Seismic Advantages Lightweight Cover and Backfill for Pipelines Undergoing Vertical Offset Profile (Longitudinal) View Down dropped side of fault Diagram of Bending Moments in Pipe from 2 m offset

31 Seismic Advantages Lightweight Cover and Backfill for Pipelines Undergoing Vertical Offset Asphalt LDCC Pipe with Sand Lightweight-Cover System(X-sectional View) Displacement Vectors During Failure

32 Questions