en.565.745 term project paper_final

EN.565.745: RETAINING STRUCTURES AND SLOPE STABILITY

5/5/2015 TERM PROJECT: “EXTERNAL STABILITY EVALUATION OF DESIGN ALTERNATIVES FOR CULVERT REPLACEMENT”

Shayan Amin

John O’Donald

The purpose of this report is to detail the slope stability evaluation of two

different design alternatives in a hypothetical situation where a culvert and

supported roadway have failed due to a historic storm event.


Table of Contents

1. Project Background………………………………………………………………………………………...1

2. Discussion of Site Parameters……………………………………………………………………………...2

3. Discussion of Design Alternatives and Slope Stability Evaluation.........................................................2-11

a. Precast Dual Cell Concrete Box Culvert with Cast-In-Place (CIP) Cantilever Wing Walls………..2-8

b. Geosynthetic Reinforced Soil – Integrated Bridge System (GRS-IBS)……………………………8-11

4. Conclusion………………………………………………………………………………………………..11

5. References………………………………………………………………………………………………...12

6. Appendices…………………………………………………………………………………………....13-18

a. Appendix A: Cantilever Retaining wall Stability Calculations…………………………………...13-15

b. Appendix B: MSE Wall Design Calculations…………………………………………………….16-17

c. Appendix C: MSE Wall Design Drawings………………………………………………………...…18


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EN.565.745: RETAINING STRUCTURES AND SLOPE STABILITY T E R M P R O J E C T: “ E X T E R N A L S TA B I L I T Y E VA L U AT I O N O F D E S I G N A LT E R N AT I V E S F O R C U LV E R T R E P L A C E M E N T ”

1. Project Background

A hypothetical project, the site is located in Central Maryland. Due to a historical flood, an

existing state-owned 20’-7” x 13’-2” corrugated steel structural plate pipe arch (SPPA) culvert that

facilitated a major stream flow under a moderate-use rural collector roadway has washed out, destroying

the structure and collapsing the supported roadway with it. The image below depicts a similar failure of

a culvert under New York’s I-88 caused by a record rainfall in June 2001. Immediately following the

hypothetical event, the roadway was shut down and

detours were temporarily set as a quick fix until a

proper solution could be found. A Hydraulics and

Hydrology (H&H) analysis was performed at the

site to determine the suitable alternatives to replace

the failed culvert. The analysis revealed that a wider

opening would be required to facilitate the design

storm volumetric discharge. With this information,

it was determined that two alternative structure concepts would be weighed against one another and

presented to the client to opt from. These options will be discussed in further detail in Section 3.

2. Discussion of Site Parameters

Due to the washout of the existing culvert, the existing roadway structurally supported by the

culvert has also collapsed. It is the intention of the project to restore the roadway to its previous

Image 1: Culvert washout on I-88, Delaware County, NY


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functionality, replacing the damaged portion in-kind. The existing and proposed roadway typical section

is comprised of two 12’-0” lanes (one in each direction) and two 8’-0” shoulders, for a total of 40’-0”.

The streambed invert elevation is located 20’-0” below the top of roadway elevation. Limited ROW is

available outside the boundaries of the roadway.

A geotechnical investigation has been conducted to determine the site soil conditions and

how they may affect the possible solutions. The investigation has yielded the following soil parameters

for a silty sand classification:

, 120

, ∅ 30°

, 0

, 9

3

3. Discussion of Design Alternatives and Slope Stability Evaluation

The two design alternatives as determined by the H&H analyses, geotechnical investigation,

and a structural consultant are as follows:

a. Precast Dual Cell Concrete Box Culvert with Cast-In-Place (CIP) Cantilever Wing Walls

The focus of this section is to detail the external stability methodology of this option.

Therefore, internal stability of structures will not be covered, i.e. steel reinforcement design of concrete

sections. The first option utilizes the implementation of a 12’-0” x 12’-0” dual-cell concrete box culvert

that meets the hydraulic requirements of the site with cast-in-place cantilever wing walls to retain the

backfilled soil and roadway above. All four wing walls will abut the proposed roadway and run directly

parallel to it. Since the drop from the top of the roadway to the stream invert is 20’-0”, it was determined

that the length of retained soil for each wall would be 30’-0” to allow for a 2:3 slope along the front face

of the wall down to the stream invert. A wall shorter in length was considered, but the resulting steeper

slope was undesirable. Due to the large length of the walls, each will be split into two 15’-0” long


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segments utilizing stepped footings in order to save materials and costs, resulting in one tall wall section

and one short wall section. Plan and elevation views of the proposed layout are shown below.

The dual box culvert has been adequately designed to resist the loads and it has been

determined that the global stability of the box section is not a concern. It remains to be seen how the

wing wall sections must be proportioned in order to satisfy all external stability requirements. The

following will concentrate on these requirements.

Image 2: Alternative 1 – Plan View of 12’-0” x 12’-0” Dual Cell Box Culverts with CIP Wing Walls

Image 3: Alternative 1 – Elevation View of 12’-0” x 12’-0” Dual Cell Box Culverts with CIP Wing Walls


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Proportioning the retaining wall is the first step after the site conditions have been

established. Certain assumptions per common practice must be made about the dimensions of the wall as

a starting point. For instance, one assumption will be that the wall stem thickness will be no less than

12”. Below is a summary of proportioning assumptions used to develop the dimensions of the proposed

retaining walls, as adapted from Das, 7th. Let it be known that these do not have to be followed verbatim,

but rather used as a guide.

Dimension Criteria Wall Stem Thickness 12" min

Depth to Bottom of Footing, D

2'-0” min and below seasonal frost line

Base Width, B 0.5 to 0.7H Toe Width 0.1H

Footing Thickness 0.1H

As an example, the preliminary wall proportions are formulated for the tall wall section as follows:

, 12"min→ 1′ 6"

, 2 0"min→ 3′ 0"

0.5 0.5 22 11 ; 0.7 0.7 22 15.4 → 14 6"

, 0.1 0.1 22 2.2 → 3 0"

, 0.1 0.1 22 2.2 → 1 6"

Again, the proportion criteria are to be used as a general guideline. Although the footing

thickness calls for 0.1H, it is the designer’s experience that a concrete section that large will not be

needed. Therefore a 1’-6” thick footing is selected and the external stability checks may commence with

all the preliminary dimensions proportioned. The short wall dimensions were formulated in a similar

fashion but are not shown here. The design sections to be evaluated for external stability are depicted

below.


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For the purpose of this report, the external stability of only the “tall retaining wall” section

will be analyzed in detail, as the “short retaining wall” section will follow a similar methodology. Four

external stability criteria are to be investigated: sliding, overturning, bearing capacity, and global

stability. For the sliding criterion, the wall must resist forces that may cause the structure to slide along

its base. To satisfy the overturning criterion, the wall must resist moments that may cause the wall to

rotate about its toe. The foundation soil must be able to support the base of the wall in order to fulfill

bearing capacity standards. Global stability concerns the overall stability of the slope as a whole and its

ability to resist a deep-seated shear failure. Traditionally in stability checks, the resisting forces and

moments have been compared to the driving forces and moments in order to procure a Factor of Safety

(FOS) value that must be greater than a predetermined minimum FOS value in order to be considered

safe. This is in line with Allowable Stress Design (ASD) methodology. More recently, the American

Association of State Highway and Transportation Officials (AASHTO) has incorporated Load and

Resistance Factor Design (LRFD) into the stability checks for sliding, overturning, and bearing capacity

to account for uncertainties in applied loads and the risk associated with the particular form of

Image 4: Alternative 1 – Design Sections. Tall Retaining Wall (left) and Short Retaining Wall (right) to be connected by stepped footings


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resistance. The following investigates the stability of the “tall retaining wall” section using LRFD

methodology in accordance with the AASHTO LRFD Bridge Design Specifications, 7th. The general

design equation for LRFD is: Σ , where

, , ; 1.0

;

;

The force effects considered for this retaining wall design include DC – Dead Load of

Components, EV – Earth Load Vertical, EH – Earth Load Horizontal, and LS – Live Load Surcharge.

These load effects were considered at three different limit states: Strength I-a, Strength I-b, and Service

I. See the table below for the load factors applied to the load effects at each limit state.

DC includes the dead load of the concrete wall section. EV includes the vertical loads from

the retained soil weight. EH includes horizontal loads resulting from lateral earth pressure. This load

was calculated using Coulomb’s Lateral Earth Pressure Theory, which accounts for wall friction at the

soil-wall interface. The angle of wall friction, δ, can be estimated by 30° 20°. All other

calculated parameters are detailed in Appendix A – Calculations. LS includes live load surcharge

DC EV LS EH

Strength I‐a 0.90 1.00 1.75 1.50

Strength I‐b 1.25 1.35 1.75 1.50

Service I 1.00 1.00 1.00 1.00

Load Factors

Limit

State


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Limit State

Factored

Resisting

Force, φFr

(kips)

Factored

Driving

Force, FD

(kips) Status

Strength I‐a 17.96 14.75 1.22 OK <

Strength I‐b 23.48 14.75 1.59 OK

Service I 17.45 9.59 1.82 OK

Category Strength 1‐a Strength 1‐b Service I

Sl iding

(Friction) 0.8 0.8 0.8

Sl iding

(Pass ive

Pressure) 0.5 0.5 0.65

Bearing 0.45 0.45 1.00

imposed on the retained soil from the roadway above. This imposes a direct downward force as well as

manifesting itself in lateral earth pressure – the forces derived from live load horizontal earth pressure

and live load vertical earth pressure are denoted PLSH and PLSV respectively. EH accounts for forces

derived from lateral earth pressure. This includes the resultant force Pa. Refer to Appendix A for all

force calculations. The following resistance factors, φ, were applied to the nominal resistances in order

to fulfill the general LRFD design equation.

It should be noted that passive pressure above the toe of the wall was conservatively neglected in

stability calculations. In the case that a shear key is used at the base of the footing, passive pressure may

be incorporated. At this site location, the geotechnical engineering consultant has suggested that a

minimum 6” bed of aggregate be used under the wall foundations in order to increase sliding and

bearing capacity. The sliding friction angle for this material is ∆ 31° and the coefficient of friction is

tan∆ tan 31° 0.600. For all three limit states, the design equation has been satisfied for sliding

stability and the factored resistances are larger than the factored loads. Refer to Appendix A for

verification.

All eccentricity criteria regarding the overturning stability of the wall have been evaluated and

also pass the requirements as depicted. The maximum allowable eccentricity is not exceeded in any of

the three limit states.


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Limit State Pv (k) B‐2eB (ft)

Factored

Bearing

Stress,

quniform (ksf)

Factored

Bearing

Resistance qR

(ksf) Status

Strength I‐a 41.56 12.41 3.35 4.05 OK

Strength I‐b 53.04 13.45 3.94 4.05 OK

Service I 38.70 13.82 2.80 3.00 OK

The service bearing pressure has been specified to be 3.0 ksf. The unfactored bearing resistance

was determined to be 9.0 ksf from the geotechnical investigation. With the resistance factor of 0.45 for

Strength I-a and Strength I-b, the factored bearing resistance is 4.05 ksf. All bearing requirements have

also been met, as the factored bearing resistances for all three limit states exceed the factored bearing

stresses, as shown.

The global slope stability of the wall was evaluated using GeoStudio Slope/W software using Spencer’s

method. Spencer’s method proposes a method that rigorously satisfies static equilibrium by assuming that the

resultant interslice force has a constant but unknown inclination (Abramson). The minimum FOS for global

stability of the wall was deemed 1.50. The calculated FOS = 1.977 > 1.50 and therefore the design is

acceptable.

b. Geosynthetic Reinforced Soil – Integrated Bridge System

The second method of construction selected was a bridge superstructure supported by a

Geosynthetic Reinforced Soil Integrated System (GRE-IBS). This is a type of Mechanically Stabilized

Earth (MSE) wall that uses layers of reinforcement within the soil to internally stabilize the soil. The

three main components of a GRS-IBS are the reinforced soil foundation, the abutments and wing walls,

and the integrated approach slab (FHWA 51). The GRS mass is a flexible composite material and it not

supported externally. Therefore the facing of the MSE wall is considered a non-structural component

Limit State Pv (k)

Factored

Driving

Moment, MD

(k‐ft)

Factored

Resisting

Moment, MR

(k‐ft) X0 eB (ft) emax (ft) Status

Strength I‐a 37.36 117.64 349.55 6.21 1.04 3.63 OK

Strength I‐b 48.84 117.64 446.12 6.72 0.53 3.63 OK

Service I 36.30 75.72 326.52 6.91 0.34 3.63 OK


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and the lateral forces within the soil are resisted by the internal reinforcement layers. This allows us to

select different types of facing systems such as a precast concrete panel, a modular block facing or a

natural vegetative facing with geotextile wrap around. For this design, the client has selected a modular

block wall facing because of its lower cost per square foot and constructability. Another benefit of

selecting a MSE wall is it does not require skilled labor or specialized equipment to complete the soil

stabilization process. However it does require quality backfill granular material that can be more

expensive and sometimes difficult to obtain.

MSE walls are based on several design considerations and assumptions have to be made in order

to perform an external analysis of the MSE Wall. The spacing of the reinforcement (12 in or less) is a

principal factor in the performance of GRS-IBS. A minimum wall embedment of H/10 is recommended

for abutments, and a minimum soil reinforcement length of 0.7H is recommended for an initial design.

The MSE wall was checked using Rankine Theory under the assumption that there is a wall

friction angle equal to 0. Calculations are shown in the attached Appendix B for reference. The

controlling limit state was determined to be global stability with a Factor of Safety equal to 1.67. FHWA

requires a minimum factor of safety greater than 1.3-1.5 for global stability concerns, therefore the

structure is deemed to be stable. The internal stability of the MSE mass depends on the length, quantity

and strength of the geotextile soil reinforcement. GRS-IBS using modular block walls typically have

geotextile reinforcement because it is much easier to construct with connection requirements, and cheap

to use and install. A design check of the soil reinforcement was not prepared for this report, but it is

expected that the wall will be constructed with 8” CMU block and geotextile fabric reinforcement at

each course. This will satisfy the 12” max spacing requirement for GRS-IBS (FHWA).

Water infiltration into the quality backfill can cause excessive settlement and erosion to occur

jeopardizing the stability of the MSE wall over time. The design of the MSE wall will need to focus on

controlling water above and below grade in order to maintain the long term stability of the structure. A


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subsurface longitudinal drainage pipe will be placed at the base of the wall near the wall facing to collect

water draining through the aggregate above and filter it out of the soil through drainage pipes. The

proximity of the major stream and recent flood event make this cite prone to future inundation. To account

for this in our design, it is reccommended that #57 coarse aggreggate be placed within the reinforced backfill

below the 100 year historic flood event water elevation. Below is a typical example of a MSE Wall cross

section that incorporates inundation considerations into the design for wall stability.

Depending on the true flood level elevation, the MSE soil mass is recommended to be consist

and the reinforced material above may also match the material below and the requirements of AASHTO

Section 518.03, Concrete Repair Materials, and Designation SP-2-89. The reinforced backfill should be

compacted to a minimum 95 percent of maximum dry density according to AASHTO T-99. And the top

5.0 feet of the abutment shall be compacted to 100 percent of maximum dry density according to

AASHTO. The top of the GRS-IBS features an integrated approach slab system that also consists of

granular fill and geotextile reinforcement layers. This reinforcement zone supports the roadway and

seamlessly transitions into the main MSE wall mass as it reaches the bridge structure. Since the rigid

Image 5: Alternative 2 –Wall Inundation Detailing Considerations, (FHWA Figure 5-11)


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bridge superstructure rests on the same reinforced soil it will have a very similar settlement compared to

the roadway and a differential settlement bump commonly found in other bridge approach slabs will not

occur. This is a big selling point for the use of GRS-IBS as it increases the aesthetics of the bridge

structure.

4. Conclusion

The results of this report have highlighted the advantages and disadvantages of both the Concrete

Box Culvert with CIP Cantilever Wing Walls and bridge structure with GRS-IBS. CIP walls have increased

durability and higher factors of safety for design, while the GRS-IBS has better constructability. To expand

on this report an in-depth cost analysis could be used to determine the most effective site specific option for

the design. Below is a graph provided by the FHWA comparing typical costs based on overall height.

In addition to cost are the considerations of performance, construction, aesthetics, and environmental

impact. All of these factor need to be considered along with the estimated cost of each option in order to

determine the appropriate design to implement.

Image 6: Soil Retaining Structure Cost Comparison, (FHWA Figure 6)


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References

AASHTO LRFD Bridge Design Specifications, Customary U.S. Units, 7th Edition, with 2015 Interim Revisions

Abramson, Lee W., Thoms S. Lee, Sunil Sharma, Glenn M. Boyce. Slope Stability and Stabilization Methods. 2nd ed. New Jersey: Wiley, 2002. Print.

Berg, R.R., Christopher, B.R., and Samtani, N.C. (2009) Design of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, U.S. Department of Transportation, Federal Highway Administration, Washington DC, FHWA NHI-09-083 and FHWA GEC 011, 668 p.

Breskin, P.E., Kalia. "State Route 32 Strut Bremen, Maine." Geotechnical Design Report (2012): 50.

Contech Engineered Solutions Structural Plate Design Guide, 5th Edition

Das, Braja. Principles of Foundation Engineering. Seventh Edition ed. Global Engineering, 2011. 794. Print.

FHWA-NHI-05-094-LRFD for Highway Bridge Substructures and Earth Retaining Structures, Reference Manual, FHWA, Revised 2006

"GRS-IBS." Accelerating Innovation. Federal Highway Administration, 12 May 2012. Web. <http://www.fhwa.dot.gov/everydaycounts/technology/grs_ibs/>.

Von Handorf, P.E., Chris. "GRS-IBS: The 5-Day Wonder." National Precast Concrete Association, 9 Apr. 2013. Web. <http://precast.org/2013/04/grs-ibs-the-5-day-wonder/>.

Yamin-Garone, Mary S. "Record Rainfall Destroys Section of New York's I-88." Construction Equipment Guide, 12 July 2006. Web. 2 May 2015. <http://www.constructionequipmentguide.com/Record-Rainfall-Destroys-Section-of- New-Yorks-I-88/7191/>.


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Appendix A: Cantilever Retaining Wall Stability Calculations


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Appendix B: MSE Wall Design Calculations


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Appendix B: MSE Wall Design Calculations


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Appendix C: MSE Wall Drawings

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