Engineered Segmental Retaining Walls - AEC Daily · 2016-03-29 · Engineered Segmental Retaining Walls ... retaining wall must be engineered and soil reinfo rced, and provides technical

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Engineered Segmental Retaining Walls

Unilock®

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Engineered Segmental Retaining Walls

Unilock®

401 The West Mall, Suite 610Toronto, Ontario M9C 5J5

Segmental retaining walls (SRWs) can strengthen steep slopes, hold back soil in grade changes, create useable land, and enhance the aesthetics of any landscape. This course looks at the site and application factors that determine whether a segmental retaining wall must be engineered and soil reinforced, and provides technical information about the components and construction of an SRW.

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The American Institute of Architects · Course No. AEC874 · This program qualifies for 1.0 LU/HSW Hour.

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Purpose and Learning Objectives

Purpose: Segmental retaining walls (SRWs) can strengthen steep slopes, hold back soil in grade changes, create useable land, and enhance the aesthetics of any landscape. This course looks at the factors that determine whether a segmental retaining wall must be engineered and soil reinforced, and provides technical information about the components and construction of an SRW.

Learning Objectives:At the end of this program, participants will be able to:

• discuss retaining walls in terms of their function, design, and application• discuss the site, application, and design factors that determine whether SRW must be

engineered• relay the sequence of SRW construction and the purpose of geogrid reinforcement• discuss the importance of subgrade stability, selecting the right backfill, soil compaction,

and water drainage, and• analyze examples of wall failure and explain the reasons behind it.






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Table of Contents

Introduction to Segmental Retaining Walls (SRWs) 7

SRW General Specification Guidelines 17

Geogrid SRW Construction 27

Case Study 73

Resources 85

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Introduction to Segmental Retaining Walls (SRWs)

Project: Whitby City HallLocation: Whitby, Ontario






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Retaining walls are strong, durable, vertical fronts that are used to:• strengthen steep slopes in grade changes• hold back soil to prevent it from slumping or

sliding• create flat usable space that would

otherwise be unusable because of the lay of the land

• serve as bridge abutments• contain and direct stormwater, and• protect waterfronts.

Retaining walls may look like simple structures, but in fact, they are carefully engineered wall systems that are complex to design, construct, and inspect. Poor retaining wall design poses serious safety hazards to people and property.

Retaining Walls






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In most U.S. states, retaining wall designs taller than about four feet must be designed by or approved by a qualified, licensed professional engineer. In Canada, retaining wall designs taller than one meter require a professional engineer’s approval.

It is important to check with and adhere to local building codes prior to any construction, even when walls are shorter than four feet or one meter, as local building code height restrictions can be different. Non-compliant retaining walls may collapse after a heavy rain event, after erosion undermines the base of the wall, or because the loads behind or on top of the wall are too heavy.

While retaining walls can be aesthetically pleasing hardscape features, they are load bearing wall systems that must provide structural, external, internal, and facial stability.

Retaining Wall Design






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Many different materials or a combination of materials are used to construct retaining walls, including brick, poured concrete, steel, wood, concrete block, and stone veneer. The building technique or type of wall selected may depend on many factors, including the application and location (commercial, residential, industrial), wall height, structural requirements, and soil conditions.

A professional engineer will:• evaluate a site’s stability (slope, soil, rock, etc.)• determine the location, size, and type of retaining wall required to meet the structural

requirements of the application• determine whether soil reinforcement is required• advise on the selection of materials • design the wall and provide drawings • provide specifications for the wall’s construction• monitor the construction process, and• inspect the wall.

Retaining Walls






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If a retaining wall project does not require engineering, a design and construction team should reference and follow the SRW installation guidelines and recommendations of individual manufacturers. Their proprietary wall designs utilize specific product lines, and substitution of any material or concrete block is not recommended.

Retaining Walls






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Segmental retaining walls (SRWs) are constructed of manufactured concrete modular blocks that are uniform in weight and dimensional tolerances. The blocks are dry-stacked (no mortar) and either hand- or machine-laid. Since the blocks in these mortar-less walls must not slide, they may be interlocked by the use of pins or clips, or a tongue and groove or rear lip molded into the blocks themselves.

The remainder of this course focuses on SRWs that utilize concrete blocks with a continuous tongue and groove system.

Segmental Retaining Walls (SRWs)

In the early 1970s, it was Angelo Risi, the founder of Risi Stone Systems, who designed the first SRW to use manufactured, high-strength concrete units that interlocked together with a continuous tongue and groove system. Since then, the industry has introduced a variety of SRW concepts and block designs.






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SRWs are gravity retaining walls that rely on their weight to resist lateral earth forces and surcharge loads. They are designed as conventional gravity retaining walls or reinforced soil retaining walls. The type specified commonly depends on the height of the wall being constructed, but there are applications where it is necessary to reinforce the wall regardless of its height, for example when the wall has surcharges, slopes, or is on difficult foundation soils.

Most block manufacturers offer assistance in determining the maximum height a retaining wall can be built to before reinforcement is required.

Segmental Retaining Walls (SRWs)






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The idea of reinforcing soil is not new. It can still be seen in structures constructed by ancient civilizations that are now thousands of years old and still standing today. For example, the: • ziggurats (terraced pyramids with a temple

at the top) of Mesopotamia were constructed of mud-clay bricks reinforced with woven mats of reeds laid on a layer of sand and gravel.

• Great Wall of China was built using a mixture of clay and gravel reinforced with red willow branches, and

• retaining wall structure built by the Romans for the wharf of the Port of Londinium(London) was reinforced with timber baulks placed perpendicular in the soil.

Soil Reinforcement






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SRWs use what is known as geogrids to reinforce and stabilize soil. Geogrids also serve to support separation, drainage, filtration, and erosion control.

Geogrids are sheet products, essentially flexible meshes commonly made from synthetic materials, e.g., polyester and polypropylene. They are placed between the layers of blocks at specific heights and extend back into the soil behind for specified lengths.

SRWs & Soil Reinforcement






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• SRWs can accommodate any wall design, including straight or curved (convex or concave) walls, tiered or terraced walls, corners, and stairs.

• SRW units are available in a wide range of sizes, shapes, textures, and colors.

• SRW individual units may range from 19 lb (8.6 kg) to 1,700 lb (772.7 kg), and some can be used to construct walls up to 40 ft (12.2 m) high.

• Solid concrete blocks are easy to split and modify on site with no risk to the integrity of the SRW being constructed.

• A solid body tongue and groove design of the blocks means shear strength is maintained along the entire length of a wall.

• SRW units with an interlocking mechanism molded directly into the block mean the units align and stack quickly and easily.

Advantages of SRWs






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SRW General Specification Guidelines

Project: Kinzie Street BridgeLocation: Chicago, Illinois

Designer: CDOT Bureau of Bridges






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The segmental retaining wall systems presented here are designed in accordance with the National Concrete Masonry Association’s (NCMA) design methodology for segmental retaining walls. TheirSegmental Retaining Wall Installation Guide and other educational materials are available on the NCMA website.

SRW Design






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The segmental retaining wall design shown here is a typical, non site-specific design. It is only provided to illustrate the general arrangement of the SRW structure. This drawing is not for construction.

Structures such as handrails, guardrails, fences, and terraces, and site conditions such as water applications, drainage and soil conditions, additional live and dead loads, etc. have significant effects on the wall design and have not been taken into account in this typical section.

SRW Design

Segmental Wall Unit

Perforated Drain with Filter Sock(connected to positive outlet)

Geogrid

Compacted Granular Base

Filter Cloth

Coping Unit

Infill Soil RetainedSoil

FoundationSoil

Typical Section: Not for Construction






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Backfill Soil: Soil material placed and compacted around the geogrids. Ideally free-draining granular material.

Base Course: The first row of blocks placed on top of the base. This unit or block is at or below grade. Some walls may have more than one course of blocks below grade.

Batter: Apparent inclinations of the retaining wall face due to the units’ setback, measured from vertical.

Compaction: The process of reducing the voids in newly placed soils by vibration, kneading, or tamping to ensure the maximum density and strength in the soil.

Coping: Top course of units on a wall. Provides a finished appearance and ties the wall together.

Dimensional Tolerance: SRWs are designed to be flexible structures that can tolerate deviations from construction drawing alignments. Established construction tolerances (e.g., variation in horizontal and vertical control [alignment], variation in wall rotation, and variation in settlement) cannot affect the stability of an SRW. The alignment of an SRW can often be corrected or modified during construction.

SRW Terminology






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Facia/Facing: The assembled modular concrete units that form the exterior face of the retaining wall.

Filter Cloth: A continuous sheet of flexible, permeable fabric used to separate, filtrate, and reinforce.

Proctor or Standard Proctor Density (SPD): The determination process for the moisture-density relationship in compacted soils. Generally, 95% is the goal.

Shoring: Temporary support to relieve the load on an SRW while it is constructed, reinforced, or repaired.

Surcharge: Loads or extra weight placed on the soil above and behind the retaining wall.

Wall Embedment: Depth of retaining wall that is buried. Distance from top of base to lower surface grade.

SRW Terminology






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As a general rule of thumb, the maximum height of a conventional gravity SRW is three times the block depth. This applies to any SRW wall or natural stone wall. Soils, slope, and surcharges will affect this ratio, but it is a good starting point for preliminary purposes.

For areas with poor soils, the engineer can introduce a granular wedge (a 45-degree wedge behind the facing) of high-quality soils like ¾-inch angular clear stone. This allows the designer to use this higher quality of material as the retained soils, therefore extending how high a product can go.

Gravity Walls: Height & Space Requirements

x

3x

Conventional Gravity SRWsMaximum Height = 3 x Block Depth






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NCMA allows a minimum base-to-height ratio of 60%; however, this does not account for poor soil conditions, slopes, surcharges, or the impact of seismic loading. For preliminary purposes, a higher ratio (70%–80%) will allow for some of these conditions.

Geogrid lengths may be significantly higher in some conditions. Areas with high seismic loading may require the upper geogrid lengths to be substantially longer than the minimum allowed by the NCMA.

Geogrid Walls: Height & Space Requirements

Geogrid SRWsDepth of Geogrid = 70% of Wall Height

0.7 x H

HPROPERT

Y LINE

Remember: Excavation cannot encroach property line






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Generally, local municipalities do not allow part of the retaining structure to extend over any property line. To save room and move the wall closer to the property line, consider a multi-depth gravity wall. The cost of material will be higher, but the room saved may more than make up for it. Remember, this does not account for excavation requirements. Should excavation not be allowed over the property line, then temporary shoring may be required.

Large Gravity Walls

PR

OP

ER

TY L

INE

PROPERT

Y LINE






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There is no way to quantify the effect of tree wind-loads on a wall. It is essential to plan the placement of a grid and tree(s) ahead of time. A tree must be staked until its root structure develops to stabilize itself. Where possible, a tree should be kept out of the reinforced zone.

The reinforced zone generally consists of a free-draining gravel, so any plants in this zone should be selected to perform in this type of environment.

Large Trees & Plants






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Not only do utilities make construction difficult to organize between contractors, but any failure of a utility structure, e.g., catch basin, may allow water to infiltrate into the reinforced zone (water causes wall failure), and future access may require dismantling of the wall itself.

If possible, suggest all utilities be placed outside the reinforced zone. Should utilities run in front of the wall, it would be suggested that the embedment depth be increased to allow for future access to them without undermining the wall’s integrity.

Other Structures & Utilities

UndergroundService

UndergroundService

Catch Basin






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Geogrid SRW Construction

Project: University of MichiganLocation: Ann Arbor, Michigan

Designer: Smith Group JJR






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While SRW systems offer many advantages, including design flexibility, a wide variety of aesthetics, lower costs, and rapid construction, it is important to pay attention to its construction and the factors that will affect its structural performance.

Build It Right






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An SRW is designed for internal and external stability, but if the existing foundation soil (the subgrade) is not capable of supporting the wall, then settlement and potential cracking or wall failure is possible.

The subgrade must provide adequate bearing capacity to support the wall without exceeding the allowable differential settlement.

Starting at the Bottom: Subgrade

Subgrade bearing capacity is critical.A compacted base is not the subgrade!






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SRWs are flexible in nature and can tolerate up to 25 mm in differential settlement. The site geotechnical engineer will have to determine if the site subgrade is suitable prior to construction. Should unsuitable soils be uncovered (those not capable of supporting the new load from the wall), then site improvements will be required.


Solid subgrade

Poor subgrade material






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Replacing the unsuitable/weak soil in the subgrade is not a solution! This approach supports only the wall face, leaving the reinforced zone to settle.

Differential settlement between the reinforcement and the wall facing can cause the geogrids to become over-tensioned and possibly tear along the back edge of the wall blocks.







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The entire wall needs to be resting on the same footing since it is a composite mass. If weak soil is to be removed, it is necessary to replace the entire wall footprint. Extend down at a 1:1 ratio to follow the line of influence (or as directed by site geotechnical engineer).







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For safety reasons, it is not possible to cut vertically when replacing an entire wall footprint. It is necessary to excavate out at an angle as directed by the site geotechnical engineer.







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The large red area in this picture represents the subgrade which needs to be replaced—the entire footprint of the wall which spreads out the line of influence at approximately a 1:1 ratio.

The excavation must be done safely in accordance with the occupational health and safety requirements.







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Once the subgrade has been approved, the wall units can be placed on a granular footing. This footing is to be a minimum 150 mm thick and must extend beyond the units by 150 mm in, from, and behind.

Since a segmental retaining wall is flexible in nature, it is not required to extend the wall down below the frost line. The minimum embedment depth required can vary depending on surrounding slopes, but the minimum allowable embedment depth is the greater of 150 mm or 10% of the wall height. Higher walls require more embedment to protect the footing. Slopes below the wall will also require additional embedment.







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If a lean mix concrete fill is used in the subgrade, it is not necessary to follow the influence line since concrete transfers a load straight down. Note that this may require shoring or a trench box in order to excavate vertically. The entire wall footprint will require the concrete.







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A geogrid reinforced wall, as shown on this slide, requires backfill to be placed around the geogrids themselves. In this example, native soils have been used (please refer to the NCMA guide for allowable material types). Since native materials generally have a high fine content (silts and clays particles), they are not considered to be free-draining. This means a drainage layer is required behind the wall face—a minimum 300 mm thick composed of a gap-graded material and wrapped in the appropriate filter fabric. Since a gap-graded material cannot be compacted to SPD (standard proctor density), it must be compacted to a “dense state.”

Backfill

Water is the leading cause of wall failures, so it must be directed out and away from the wall as fast as possible. Every wall requires a drainage tile to direct water to an appropriate outlet.

Drainage layerFilter fabric

Non-draining native soil backfill






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What is the right backfill?

Using site (native) materials for the backfill may be less expensive than bringing in imported materials (which generally implies removing the site material to make room for the imported materials), but this point of view considers only the cost of the soil. The following slides discuss how wall construction is impacted by an unsuitable backfill material and whether any savings are actually made by selecting an inexpensive soil.

Backfill






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There are several drawbacks to using native soils. With their high fine content, they are dependent on achieving and maintaining a suitable moisture content to achieve proper compaction. In dry environmental conditions, this may mean adding water. However, in conditions where too much moisture is present, the soil must be left to dry, which can take substantial time. Native soils are also generally more susceptible to long-term creep, which may result in slight movements over time.

Native materials commonly have parameters (e.g., internal friction angle, unit weight, etc.) less than that of imported gravel. They require not only longer geogrid lengths, but also tighter geogrid spacing, thereby increasing the material costs and the time to complete the wall installation.

Backfill






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What’s wrong with this picture?

Everything!• Contaminated drainage• No separation (filter fabric)• Inconsistent depth

Drainage






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This picture depicts the back of the wall, the reinforced zone.

From a drainage perspective, you can see that the gap-graded material is contaminated with the site soils. Once the fines get into this drainage layer, they will fill the voids and slow/prevent water from percolating to the drainage tile. A filter cloth is required to prevent the migration of these fines into the gap-graded drainage layer.

From a structural point of view, the gravel is not infilled to the top of the block, leaving a void for the grid to settle, stretch, and potentially tear. All material should be brought up to the top of the block before the geogrid is laid out on top of it.

Drainage






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What was the main cause ofthis wall failure?

Water is generally the largest contributor to wall failure. In this example, all of the snow from the cul-de-sac was piled at the end of the street, right above the wall. When the snow melted, water was introduced into the reinforced zone (the geogrid zone). The inadequate drainage measures led to a build-up of hydrostatic loading, which caused this large wall to collapse.

Drainage






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Well-graded gravel material with 8% fines or less is classified as free-draining. This backfill material will typically allow water to move through it faster than native material, thereby helping to prevent water build-up behind the wall and hydrostatic loading.

Drainage & the Right Backfill






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The only potential disadvantage to using well-graded gravel material with 8% fines is the extra cost. However, the extra cost may be offset by reduced construction costs due to the following:• The entire geogrid zone becomes the drainage

layer, which negates the need for the gap-graded drainage layer behind the wall facing.

• The material is easy to handle in all weather, which means compaction and performance is predictable.

• The geogrid lengths will be shorter and spaced farther apart (the NCMA recommends a maximum geogrid spacing of 610 mm [2 ft] to limit the impact of internal compound stability), and

• Less quality control is needed for inspection.

Drainage & the Right Backfill






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If a drainage outlet is to be installed through the wall face, native material (or less permeable material) should be placed below the drain to direct water to the pipe. Otherwise, the wall will fill with water before the pipe begins to drain the water out.

Drainage






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Outlet drains are suggested for every 45 feet on center (45 ft o/c).

Additional measures are required if additional water is anticipated.

Proper Drainage






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Why is this wall discolored?

Water is building up behind the wall and draining through the wall face. The white patches are ice.

One of the benefits of segmental retaining walls is that they are dry-stacked and will therefore allow water to drain. However, this is not always an adequate drainage measure and should not be relied upon to prevent hydrostatic build-up.

Proper Drainage






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Water always drains down and takes the path of least resistance. Typically, a 2% slope is recommended towards all outlets.

Ensure outlet drains and their locations are specified in the contract!

Proper Drainage






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It is sometimes difficult to fit concrete rectangular blocks around large round pipes without leaving any voids. If voids do occur, grout should be used to fill them and, if possible, filter cloth should be placed behind the concrete units to help prevent any gravel from washing out.

A precast or cast-in-place rectangular headwall can be used where possible to allow the units to abut flush and reduce the gapping formed at the intersection. Additionally, the retaining wall should abut the pipes/headwall with an expansion joint to allow for slight movements.

Proper Drainage






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The red oval in this photo highlights a slip joint. The walls on either side of this point are not attached to each other but are actually just abutting with an expansion joint.

The sidewall (to the left) and the headwall (above the box culvert) are free to move independently of each other without causing cracking at their interface.

Other Structures






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The side wall is sitting on its own granular footing (a typical wall detail) and may be at a different elevation from where the box culvert is resting. Since the two areas may not be sitting on the same material, they may move differently due to settlement or frost, etc. Without a slip joint, cracks will form at the point or in the area where a slip joint should be located.

Other Structures






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Temporary shoring may be required where property lines or other limitations exist.

Property Lines






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The strength of a geogrid reinforced segmental retaining wall lies in the quality and density of the backfill material. A wall design assumes an angle of internal friction* and a unit weight. These two factors are critical in the performance of the wall and must meet or exceed a design’s structural requirements.

Compaction reduces the number of voids in the soil, thereby increasing the density of the backfill material. Additionally, it also increases the friction angle. Proper compaction reduces settlement and creep and is achieved only with a suitable backfill with optimal moisture content. Compaction testing and monitoring is paramount to the long-term stability of the wall.

Generally, backfill lifts are limited to six to eight inches when fully compacted.

Correct Compaction

*Angle of internal friction(friction angle)A measure of the ability of a unit of rock or soil to withstand a shear stress. It is the angle (φ), measured between the normal force (N) and resultant force (R), that is attained when failure just occurs in response to a shearing stress (S).






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The majority of geogrids are not bi-directional and must be rolled out perpendicular to the wall face and then cut to length. They must be laid flat (horizontally) on top of the backfill that has been compacted right up to the top of the block, with no gaps or overlaps between adjacent pieces.

Construction machinery should be prevented from driving directly on the geogrid itself, but it is necessary to place a layer of soil on top of the grid as soon as possible to protect the geogrid and provide a layer of cushioning. If machines are required to move across the geogrid, they should be driven straight and wheel turns limited to prevent the geogrid below from being disturbed or moved.

Laying the Geogrids






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Commonly, stone SRW systems utilize polyester or polypropylene geogrids. A benefit of polyester grids is their flexibility and ability to bend around the shear keys to create a very high connection capacity to the blocks.

It’s important to note that specific industry tests are performed for each type of SRW system and geogrid used. The geogrid specified in the design must be the one used on site. Should an alternate geogrid be required, the engineer of record must be consulted, and full-scale testing may be required if no testing already exists.

Laying the Geogrids






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The tension of the geogrid is a critical factor to the performance of the SRW system. Any slack in the geogrid will result in the wall moving forward until the geogrid becomes taut. The geogrid should be tight, and it is suggested that the infill material be placed behind the wall face and pulled towards the back of the geogrid layers to help ensure all the slack is pulled out.

Laying the Geogrids






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This photo is an example of a contaminated backfill and a geogrid that is not tensioned. The upshot is a gapping grid with a void beneath the grid at the block which results in a down-drag force on the rear of the wall subjecting the grid to stress.

Laying the Geogrids






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As mentioned, water causes wall failures. Slopes and swales must be accounted for to prevent water from infiltrating the wall.

An eight-inch non-pervious layer of compacted native material is used to capthe top of a wall.

Finish Grading






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This photo shows a poorly graded swale. Any water that runs down the slope will hit the back of the wall, where it will infiltrate the reinforced zoneimmediately.

A site’s design/grading should prevent as much water as possible from infiltrating the reinforced zone.

Finish Grading






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Can SRWs be submerged in water?

Yes, if they are designed to be!

Water Applications






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To properly design an SRW for a water application, e.g., an embankment, both the normal water level and high water level must be known. The designer will use this information to assess the stability of an SRW design in a rapid drawdown analysis.

Water Applications






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Embedment depth (typically two feet) and drainage measures are generally required with any water application. Additionally, erosion control is required at the foundation to protect the footing. Wave and ice action cannot be accounted for in the SRW’s design as it is difficult to quantify their effects.

Water Applications






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Handrails, barriers, and fences all apply loads to an SRW and must therefore be planned ahead of time and accounted for in the design.

Handrails, Barriers, & Fences






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The term “hand-placed” blocks refers to the physical size of a concrete segmental retaining wall unit. Hand-placed units can be installed by hand, and each unit will typically weigh less than 80 lb.

Gravity walls (hand- or machine-laid) rely on the self-weight of the blocks to resist the soil forces that are exhibited on the wall. Typically, gravity walls cannot support any additional loads, so the barriers must be placed in separate foundations that extend below the base of the wall. The barrier must remain structurally independent and not transfer loading to the SRW.







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SRWs with geogrid reinforcement can support the added loads from fences, rails, and barriers. Each type of fence, rail, or barrier will apply a different load, and each one must be accounted for in the design of the wall itself. Typically, a sacrificial layer of geogrid (not required for the stability of the wall itself but specifically for the added loading) will be specified in the wall design to account for the loading from these objects.


A sonotube foundation must be positioned as the wall is constructed. The geogrid is cut perpendicular to the centerline of the foundation and wrapped up and around, and then laid flat on the back side. This maintains the structural capacity of the geogrids.






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Some machine-laid blocks may have enough mass to help support railings, and as such, the railing can be core-drilled directly into the wall. The depth of the core and the inclusion of any sacrificial geogrid will depend on the type of railing and type of wall system being installed.







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Core drilling directly into the block works in many applications, but it has to be done correctly.

The wall in this photo failed because water infiltrated the core hole and then froze. The mass of the wall was not able to withstand the forces and tension created by the ice build-up.







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Non-shrink grout is crucial to preventing problems due to the accumulation of water/ice around a post hole. The grout should be beveled to shed water and the hollow post pushed into the grout so it fills with grout. In addition, the post must be capped and a drain hole included to allow any water that may enter to drain away immediately.







Slide 69 of 87< >

If required, fences and barriers can be designed to be mounted on larger machine-laid SRWs, but this adds significant costs to a wall construction project.

Any specialty designs like the one shown on this slide need to be looked at on a case-by-case basis to determine if it is possible to secure a fence or barrier on top of the wall and what design elements are required to ensure it meets all required safety standards.







Slide 70 of 87< >

Proper methods and techniques of SRW construction are critical to the success of every installation.

Construction tolerances should be constantly monitored. The vertical and horizontal controls are +/- 1.25 inches over 10 feet. This means it is possible to have a variance in any 10-foot range. As a wall goes up, the planned alignment can be reset to correct a noticed deviance. For example, a wall to be built to 15 feet in total height could be out by up to 1.25 inches on the first 10 feet. The planned alignment can be adjusted over any 10-foot range so the total deviance at the top of the wall could be out 2.0 inches from the original alignment and still fall within the reset alignment and meet specifications.

Post Construction Management

planned alignment

reset planned alignment

actual alignment






Slide 71 of 87< >

Allowable wall deflection and tolerances are as follows:

• Vertical Control +/-1.25 inches over 10-foot distance

• Horizontal Control +/- 1.25 inches over 10-foot distance

• Rotation2.0 degrees from planned wall batter

• Bulging 1.0 inch over 10-foot distance

Post Construction Management






Slide 72 of 87< >

SRW design software, usually proprietary to individual manufacturers, allows wall designers to:• accurately display the true footprint of the wall• create elevation views• generate accurate reports and final design

submission drawings• import grading and layout information directly from

other CAD (computer aided design) software• run static, seismic, and ICS analysis in

accordance with industry standards, and• generate quantity estimates and project specific

reports.

Consult individual manufacturers for specific details pertaining to their SRW design and construction services.

SRW Design Software






Slide 73 of 87< >

Case Study

Project: West London Dyke RevetmentLocation: London, Ontario

Designer: Risi Stone Systems






Slide 74 of 87< >

Project:West London Dyke Revetment

Location: London, Ontario

Designer:Risi Stone Systems

Civil Engineer: Stantec, Consulting Engineers

Contractor:Ro-Buck Contracting Ltd.

Case Study






Slide 75 of 87< >

When the original concrete embankment that was installed to protect and contain the Thames River began to fail, the City of London decided to replace it with a retaining wall that would allow them to create a pathway to provide access to the adjacent parks. The proposed wall was to replace approximately 300 m of the existing dyke.

Case Study






Slide 76 of 87< >

Access to the site was limited, and all construction had to be completed from the high (retained) side of the wall to prevent contamination of the Thames River. The contractor set up a zone at each end of the build, and traffic flowed in one direction, taking in the materials as they were needed.

Case Study






Slide 77 of 87< >

Due to the limited space, coordination of the trades and each phase of construction (excavation, blockplacement, and placement of the infill material) was crucial to the success of the project.

Case Study






Slide 78 of 87< >

A reinforced toe wall that was constructed in the 1980s to stabilize the original dyke was left in place as it was still sound. It was used as a form of erosion protection for the proposed wall footing. It was cut to fit the new layout, and riprap was placed between it and the new wall.

Case Study






Slide 79 of 87< >

Since the entire wall had to be built from the high (retained) side, fall protection for the workers was mandatory to ensure no one could fall off the wall once the heights were deemed too high.

A fall arrest system with anchor wires secured to large blocks was utilized. The workers harnessed themselves to the anchor wire and allowed enough range of movement to construct the wall.

Case Study






Slide 80 of 87< >

To allow for the water to exit the infill zone as quickly as possible following a flood event, awall can be backfilled with a 10 mm gap-graded drainage layer. In this instance, the drainage layer was increased to 600 mm, and a smaller gap-graded material was used. Locally, it is called high-performance pipe bedding. Test results have shown that the migration of fines from a well-graded gravel with 8% fines is minimal.

The increase in the depth of the drainage layer allowed a transition zone for the fines to migrate into and create their own filtration zone. This, in turn, allowed the designer to forgo the filter cloth separation typically required in similar applications.

Case Study






Slide 81 of 87< >

Drainage tiles were installed at grade, for the two-year and the 75-year flood events. Drainage tile outlets were installed through the wall face at 15 m intervals.

Case Study






Slide 82 of 87< >

The south end of the wall was terraced to allow for a future pathway to extend under the Queens Avenue bridge.

Case Study

Please remember the exam password BRIDGE. You will be required to enter it in order to proceed with the online examination.






Slide 83 of 87< >

Finally, the pathway along the Thames River was incorporated into the new layout along the top of the wall.

Case Study






Slide 84 of 87< >

Case Study






Slide 85 of 87< >

Resources

Project: University of Michigan HelipadLocation: Ann Arbor, Michigan

Designer: Albert Kahn & Associates Inc.






Slide 86 of 87< >

• ConcreteNetwork.com. ConcreteNetwork.com, 2016. Web. Accessed January 2016. http://www.concretenetwork.com/

• National Concrete Masonry Association (NCMA). National Concrete Masonry Association, 2016. Web. Accessed January 2016. https://ncma.org/

• Inglesby, Tom. “Block On Block: A Brief History of Segmental Retaining Walls.” Mason Contractors Association of America (MCAA). Mason Contractors Association of America, August 18, 2004. Web. Accessed January 2016. http://www.masoncontractors.org/2004/08/18/block-on-block-a-brief-history-of-segmental-retaining-walls/

• Wendland, Steve. “When Retaining Walls Fail.” Civil and Structural Engineer.ZweigWhite LLC, June 2011. Web. Accessed January 2016. http://cenews.com/article/8330/when-retaining-walls-fail

Resources

http://cenews.com/article/8330/when-retaining-walls-fail

http://www.masoncontractors.org/2004/08/18/block-on-block-a-brief-history-of-segmental-retaining-walls/

https://ncma.org/

http://www.concretenetwork.com/






Slide 87 of 87< >

Conclusion

©2016 Unilock®. The material contained in this course was researched, assembled, and produced by Unilock® and remains its property. Questions or concerns about the content of this course should be directed to the program instructor. This multimedia product is the copyright of AEC Daily.

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Engineered Segmental Retaining Walls - AEC Daily · 2016-03-29 · Engineered Segmental Retaining Walls ... retaining wall must be engineered and soil reinfo rced, and provides technical