SRW History Article Series: Specifications

Segmental Retaining Wall Systems  – What SRW specifications cover

Specifications are legal contract documents forming an agreement between the owner and the contractor regarding labor, material, costs, installation, equipment costs, inspections, and management.  It is a document defining the scope of work for the contractor.  The specification defines what is expected in terms of material requirements (color, strength, face type, etc.), construction and gives requirements for testing and performance specifications.

This article discusses the highlights what each section of the specification covers and what must be addressed when preparing specifications for segmental retaining walls.

CSI SPECIFICATIONS

The Construction Specifications Institute (CSI) is an organization that maintains and advances the standardization of construction language as it pertains to building specifications. CSI provides structured guidelines for specification writing in their Project Resource Manual.  All the sample documents in the NCMA Design Manual follow the CSI format and have the following three major sections: 1. General 2. Materials 3. Execution. In the following sections, we will describe briefly each section and the important items in each. For more information review the full article “NCMA SPECIFICATIONS – SRW Systems” and the SRW Best Practices Guide .

PART 1:  GENERAL

Description
This section describes the scope of work, references, approved systems, storage, and delivery.

Work shall consist of furnishing all materials, labor, equipment and supervision to install a segmental retaining wall (SRW) system in accordance with these specifications and in reasonably close conformity with the lines, grades, design and dimensions shown on the plans or as established by the owner or owner’s engineer.

The two most important items in the specification are for the contractor to supply all the materials and labor to install the wall as designed. It is equally important that the supervision and management costs be included in the quote. The final item in the scope of work is to have a defined wall area defined by the plans included with the project.  

Related Work

In large projects, there may be other contracts with related work that will be done under this section.  Coordination of work between contractors is important.  For work to proceed, the site will need earthwork (Section–Earthwork) and site preparation (Section–Site Preparation) that may be done by the contractor of this contract, or may be accomplished by the general contractor on the project.  Testing and Inspection may be included as part of the general contract, or could be required for the retaining wall contract alone.  A contractor may bid a project using on-site fill soils (no fill material costs) or if importing soils is required, will include fill soil costs and transportation.  If the wall contractor is waiting on the fill, there could be extra charges.

Referenced Standards

The owner and the contractor will be referencing published standards for design, testing and material properties for items supplied to the project.  The five major areas addressed are Engineering Design, Segmental Retaining Wall Units, Geosynthetic Reinforcement, Soils and Drainage Pipe. The NCMA Design Manual is the standard of practice for commercial projects, but there are other design guidelines for transportation structures.  Any items of design should be addressed in this reference and become part of the contract documents.

A full list of appropriate referenced standards can be found in the original article here.

Approved Segmental Retaining Wall Systems

The owner may have specific requirements as to the type of SRW system installed and may provide a list of approved retaining wall systems. SRW systems mentioned in this section list specify segmental retaining wall units and geosynthetic reinforcement combinations that cannot be exchanged without the approval of the owner’s engineer.

Suppliers of SRW material components shall have demonstrated experience in the supply of similar size and types of segmental retaining walls on previous projects, and shall be approved by the owner’s engineer. The owner may also require design submittals to approve unit face design, colors, etc.

Submittals

Material Submittals – No less than 30 days prior to the start of the project, the contractor shall submit manufacturer’s certifications, stating that the SRW units, geosynthetic reinforcement, reinforced backfill, and gravel fill meet the requirements of Part 2.0 of the specification.  The final part of the General section is the delivery and storage of the materials.

Delivery, Storage, and Handling

The contractor shall inspect the materials upon delivery, store and handle materials in accordance with manufacturer’s recommendations and shall protect the materials from damage. Damaged material shall not be incorporated into the segmental retaining wall. It is important that the materials delivered are the correct materials and while stored they are not damaged (cracked or broken) or discolored by placing the materials in soils.

PART 2:  MATERIAL

Part Two of the contract specifications are definitions of the materials to be used in the construction of the SRW project. There are four materials required for construction of an SRW wall: 1. Concrete SRW facing unit; 2. Geogrid soil reinforcement; 3. Drainage pipe; and 4. Fill soils and gravel fill. SRW products delivered to the site should meet at least the minimum requirements of the accepted industry standards as published in the referenced ASTM standard (ASTM C1372). For more information on SRW properties and freeze-thaw requirements, please review Section 2 of the SRW Best Practices Guide.

Reinforced backfill is the soil behind the wall face and is reinforced by geosynthetic or metallic reinforcements.  It is important that this material is a low plastic material (does not shrink or swell) and has well-defined strength parameters.  Residential and commercial projects could use the on-site or locally available fill soils but they must meet the industry recommendations. For more information on soil requirements please review Section 2 of the SRW Best Practices Guide.

PART 3 CONSTRUCTION

At this point in the project, a design has been prepared by a professional for the SRW wall.  Quality materials have been ordered and delivered to the site.  As the last step,  the construction is completed by an experienced qualified certified contractor. In the construction section, the specifications call out for verification of site parameters and materials delivered to the site.  It also defines each step of construction so that a quality product is installed.

Construction Observation

This portion defines what needs to be observed during construction and the frequency. For more detailed information on the inspection requirements see Section 3.8 and 7 in the SRW Best Practices Guide. 

As construction proceeds, the owner or owner’s engineer should verify the materials supplied by the contractor meet all the requirements of the specification. During this phase, it is also the contractor’s field construction supervisor to have demonstrated experience and be qualified to direct all work at the site to ensure proper wall installation.

The following sections briefly explain the construction procedure for installing a segmental retaining wall.

1. Excavation

Foundation trenches and cut walls will require site excavation.  This section highlights that the excavation must be completed to the lines and grades provided.  The provisions for soft soils, over excavation, and excavation support if necessary.  

2. Foundation Preparation

After excavation, it is important to examine the site and confirm the soils encountered are the soils used in design.  This section details the importance of making sure the surfaces are compacted and will meet the design requirements to support the SRW system and if the soils do not meet the design requirements, the geotechnical engineer will be needed to address the problem.

3. Leveling Pad Preparation

Facing units for SRW walls are precast concrete units. The leveling pad must be built to give the units a level base for construction and to provide a good bearing area for the units setting on soil foundation.  This section must cover the dimensions, location, and materials to be used for the construction of the leveling pad. The details should also highlight that the granular base shall be compacted to provide a firm, level bearing pad on which to place the first course of concrete segmental retaining wall units.

4. SRW and Geosynthetic Reinforcement Placement

The SRW facing will be seen for the life of the project and should be the most important step in construction.  This section must detail the construction sequence highlighting that the alignment of the wall should follow the proposed alignment so the top is straight and batter does not vary in and out. The geosynthetic reinforcement materials provide the horizontal support to the soil mass and provide the structural attachment of the SRW facing units to the compacted soil mass.  It is important the right material is used, the proper orientation is used and the materials are tensioned to remove any looseness from the layers.

5. Backfill Placement

Placed soils are the backbone of the system supporting the loads applied above the wall and holding the facing units in place.  The density of the soils and gradation are important as these parameters define the long-term performance of the wall.  Site drainage and control of surface water is important.  It is suggested that at the end of the day the construction site is graded to direct surface water away from the construction area.

6. Gravel Fill and Drainage Placement

Gravel fill shall be placed to the minimum finished thickness and widths shown on the construction plans and shall be appropriately compacted with a hand-operated compaction equipment to meet the specifications. The gravel fill shall fill any cavities in, between, and a minimum of 12 in. (305 mm) behind the units.

7. Cap Block Placement

The cap block and/or top SRW unit shall be bonded to the SRW units below using cap adhesive described in Part 2.01F. The block shall be dry and swept clean prior to adhesive placement.

PART 4 MEASUREMENT AND PAYMENT

Payment terms for the SRW wall may be on a lump-sum contract price or on unit price basis dependent on the projected face area of the wall. Any extras for the wall (e.g. over-excavation and replacement) should be detailed and included in the payment terms.

This abridged look at specifications emphasizes the need for clarity and details between the owner and contractor. A more detailed look can be found in the original article SRW History Article Series: SRW Specifications.