004 site & project evaluations.pdf

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Site & Project Evaluation;Project Consideration;and Geometric Design

PW Lesson 4

RM Chapter 2: Sections 2.6 to 2.8

Lesson 4 – Learning Outcomes

Participants will be able to:

• List typical MSEW & RSS exploration requirements

• List agency MSEW project criteria

• Discuss geometric design considerations of facing types

Soil and Site Investigation

• A MSE Wall is not an embankment

• A MSE Wall is a structure

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Article 10.4.2 (AASHTO, 2007)

• Subsurface explorations shall be performed to provide the information needed for design and construction. . .

• Borings should be in sufficient number and depth to establish a reliable longitudinal and transverse substrata profile . . . .

Site Evaluation Elements• Evaluate existing subsurface information

• Field reconnaissance

• Subsurface exploration– Drilling and Sampling, SPT, CPT, bulk samples– Borings spaced @ 100 ft along wall, 150 ft back

– Geophysics to reduce uncertainty between borings and planning of subsurface exploration

• Laboratory testing– Mechanical properties, grain size, index tests– Strength properties, shear (frictional)– Electrochemical properties, pH, resistivity, etc.

Typical Boring Layout

Wall boring1-2H

200 ft

100 ft150 ft

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Soil and Site Exploration• Flexibility has limitations

• Evaluate site stability

• Settlement/bearing resistance of foundation soils

• Location of ground water

• Need for drainage

• Availability of reinforced wall fill

Subsurface Exploration Requirements

Foundation Soil

Limits of Excavation

Reinforced Zone

Drainage

Retained

Backfill

Retained Backfill

Natural Soil

Primary Causes of Substructure Movement

• Post construction settlement of approach embankment or foundation soils

• Instability of foundation soils

• Loss of support due to lateral squeeze

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Foundation Soils Control Overall Stability

e.g., Bearing Capacity

Foundation Improvement Techniques

• Normal consolidation

• Surcharging w/ or w/o wick drains

• Stone columns

• Dynamic compaction

• Lightweight fill

• Deep foundations (e.g., piles &

drilled piers)

OWNER/AGENCY’S RESPONSIBILITY!

Ground Improvement

Testing• Identify & classify

– Mechanical properties, grain size, index tests

• Shear strength– Drained, undrained

• Consolidation

• Electrochemical

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Shear Strength of In-Situ Soils

• Laboratory Tests including triaxial (UU, , and CD) and direct shear

• In-situ tests including CPT and field vane test

• Consider appropriate drainage condition in the field and sample disturbance

CU

Reinforced Fill Properties• Gradation• Reinforced fill top size• Plasticity• Moisture – density relations• Shear strength• Soundness• Corrosion/Degradation

– Electrochemical (metals)– Durability (geosynthetics)

See RM - Chapter 3 and PW - Lesson 5

Retained Backfill Properties• Gradation• Plasticity• Moisture – density relations• Shear strength

See RM - Chapter 3 and PW - Lesson 5

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Selection Factors• Subsurface conditions

– bearing capacity

– settlement considerations

• Aesthetics – type of structure, MSEW or RSS

• Environmental conditions– aggressiveness of site

• Size of structure– ROW, technical limitations

• Risk

• Cost

Establishment of Project Criteria• Consider all possible alternatives

• Choose a system (MSEW or RSS)

• Choose facing system

• Identify performance criteria

• Assess environmental conditions

Performance Criteria• Load Factors

• Resistance Factors

• Lateral Deflection

• Embedment

• Design Life

• Joint Width

• Other (see manual)

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MSEW Load & Resistance Factors

• Strength Limits – see RM Chapter 4, PW Lessons 8, 9, and 10

• Serviceability Limits – see RM Chapter 4, PW Lessons 8, 9, and 10

• Extreme Limits – see RM - Chapter 7, PW Lesson 13

Nominal Reinforcement Strength

• Nominal steel tensile strength

• Nominal geosynthetic strength

• See RM Chapter 3, PW Lesson 7

RSS Factors of Safety

• PW Lessons 13 and 14, see RM Chapters 8 & 9

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Typical Deformation for 0.7H & H = 10’

Metallic reinf. ¾Geogrids reinf. 1.5Geotextiles 3

Lateral Deformation During Construction (Empirical) (RM Figure 2-15)

Project Criteria

Limits of Excavation

Wall EmbedmentSlope in Front of Wall

Minimum to Top of Leveling Pad

Horizontal (walls) H / 20

Horizontal (abutments) H / 10

3 Horiz : 1 Vert H / 10



• H is the total height of wall (for slopes can use H = hexposed + 2 ft)

• For sloping ground provide a bench 4 ft bench in front of wall

• Embed at least 2 ft below anticipated scour depth

• Embed below frost depth

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LevelingPad

Facing

Reinforcement (typ.)

Reinforced Fill

RetainedBackfill

Reinforced Zone

H

ds

dh

4 ft min.

dh-min = 2 ft

Required Design Life

Structure Classification Design Life (years)

Permanent structures 75

Abutments and critical applications

100

Temporary walls < 3

See RM Chapter 3, PW Lesson 7

Limiting Differential Settlements for MSEW Facing Systems

Type of Facing Limiting Differential Settlement

Joint Width

Precast ( 30 ft2) 1 / 100 ¾-inch

Precast(30 ft2 to 75 ft2)

1 / 200 ¾-inch

Precast Full-height 1 / 500 ¾-inch

Drycast MBW 1 / 200 N/A

Welded Wire 1/50 N/A

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Manifestation of Differential Settlements

Manifestation of Differential Settlements

Lesson 4 – Learning Outcomes

Participants will be able to:

• List typical MSEW & RSS exploration requirements

• List agency MSEW project criteria

• Discuss geometric design considerations of facing types

004 site & project evaluations.pdf

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