Grouted Mattress Specification: Tender Document Guide

A grouted mattress specification needs five clauses: product type, thickness with its hydraulic design basis, fabric acceptance against GRI GT16, grout strength with a testing schedule, and an installation method statement. GRI GT16 covers the geotextile fabric only, so grout strength and installation must be written into the project specification. Missing a clause leaves the supplier room to deliver less than intended.

A well-written specification is the engineer's primary quality control tool: it defines what the supplier must deliver and gives the client the contractual basis to reject non-compliant material. A poorly written one allows substitution of inferior products, incomplete test programs and incorrect installation, all without breach of contract, because the contract did not clearly require better.

Grouted mattress specifications are often deficient in consistent ways: they specify the product by trade name only (no performance criteria), do not say which test documents the supplier must provide, and leave the grout strength requirement vague. This guide gives a model framework that closes those gaps. For background on the standard, see GRI GT16 Explained.

What Does GRI GT16 Cover, and What Does It Not?

GRI GT16 is a manufacturing quality control specification for the geotextile fabric used in grout-filled mattresses. It does not cover the compressive strength of the grout, the performance of the filled mattress, thickness selection, hydraulic design, installation or design life. Those belong to the project specification and the engineer's design. Keep this boundary in mind when reading the clauses below, because each clause draws on a different source.

The Five Essential Specification Clauses

Clause 1: Product Definition and Type

Specify the exact product variant required. The three main types have different performance characteristics and are not interchangeable:

• Standard (impermeable) grouted mattress: fully impermeable woven face. Specify for canal lining, river bank protection and slope protection where seepage control is required and rapid drawdown uplift is not a design scenario.
• Filter point grouted mattress: drainage openings at cell intersections. Specify for reservoir upstream faces, tidal zones, coastal revetments and any application where hydrostatic pressure reversal is a design scenario.
• Vegetated grouted mattress: large open cells for plant establishment. Specify for ecological slopes and amenity waterways where vegetation integration is required.

Clause 2: Thickness and Hydraulic Design Basis

Specify the design thickness with reference to the hydraulic design velocity and the design basis the engineer used. The thickness range is 75 to 200 mm, and the rated velocity rises with thickness: 75 mm for 2.5 m/s, 100 mm for 3.5 m/s, 150 mm for 4.5 m/s and 200 mm for 6.0 m/s (HydroBase design guidance). A model wording:

"The grout-filled mattress shall be [nominal thickness] mm as selected by the designer based on the design flow velocity of [X] m/s at the protected surface, in accordance with [the design basis stated in the specification]. The minimum filled thickness at any point shall be [value set by the designer]."

Clause 3: Fabric Acceptance Against GRI GT16

This clause sets the acceptance basis for the geotextile fabric. It should say what the fabric is accepted against and what documents the supplier must provide:

"The geotextile fabric shall be accepted against the requirements of GRI GT16 (Geosynthetic Research Institute specification for geotextile fabric used in grout-filled mattresses). The Contractor shall provide the manufacturer's quality control documents for the delivered lot, as listed in the specification. Any independent laboratory testing the Engineer requires shall be performed by a laboratory with ISO/IEC 17025 accreditation for the test methods concerned."

GRI GT16 provides for manufacturer certificates supplied on the buyer's request, so the tender should ask for them explicitly. Ask each supplier which test documents apply to your order before you finalize this clause.

Clause 4: Grout Strength and Testing

The grout requirement belongs to the project specification, not to GT16. State the required strength, the cement standard and the testing schedule, using values set by the project engineer:

"The grout shall be a fine-aggregate cement grout using Portland cement complying with [EN 197 / ASTM C150] and clean fine aggregate sized to pass the fabric weave and injection hose. The mix proportions and water/cement ratio shall be as approved by the Engineer. The 28-day compressive strength of the grout shall be not less than [value set by the project specification]. Test specimens shall be taken at the frequency stated in the specification, cured and tested in accordance with [test standard]."

Cylinders are tested under ASTM C39. If the project uses cubes, name the cube test standard used locally instead; do not describe cube testing as ASTM C39.

Clause 5: Installation Requirements and Method Statement

Require the contractor to submit an installation method statement for the Engineer's approval before work begins:

"Before commencing installation, the Contractor shall submit a method statement for Engineer approval covering: (a) subgrade preparation and compaction program; (b) panel layout and overlap plan; (c) grout mix design and batching method; (d) injection sequence and pump arrangement; (e) curing procedure and duration. Installation shall comply with the approved method statement. Any deviation requires the Engineer's written approval."

The method statement is also where the limits on placement conditions are set, so the specification should not carry fixed numbers for them.

Who Sets What?

Common Specification Errors and How to Fix Them

Specification references: GRI GT16 (Geosynthetic Research Institute) is the reference for fabric acceptance, ASTM C39 covers compressive strength testing of cylindrical concrete specimens, and ASTM D4439 provides standard terminology for geosynthetics that tender documents can use.

For installation context, read our grouted mattress installation guide and our articulated concrete mattress technical guide before drafting your specification.

What a Specification Cannot Do for You

The five clauses are a framework, not a finished document. Every bracketed value in the model wordings has to come from the project designer, and none of them can be copied from a supplier's brochure or from this guide. A specification also cannot repair a weak design basis. If the design velocity, drawdown behavior or subgrade conditions are unknown, no clause will make the tender reliable, and the hydraulic and geotechnical work has to come first.

This approach suits a fabric-formed, grout-filled mattress. It does not suit tenders that allow precast block systems, rock or other linings on equal terms, which need performance-based wording for each alternative. Contract wording on rejection, retesting and program impact belongs to your contracts specialist, and local standards or the client's own documents may override anything suggested here. Treat the model clauses as a starting draft for the engineer to adapt and approve.

What to Send HydroBase While the Tender Is Being Drafted

The best time to send details is before the tender is issued, while the thickness clause and the document list can still be changed. HydroBase makes the grouted mattress at its factory in Inner Mongolia, China, in thicknesses from 75 to 200 mm, with the velocity ratings given in Clause 2. Pricing is by project, so there is no list rate to place in a bill of quantities. Send the design velocity, channel or slope type, area, location and drawings through the contact page.

Also say which documents your specification will require, so the reply addresses them. For the program, stock sizes dispatch in 3 to 5 days and custom-size panels take 5 to 7 working days to produce, before shipping. The engineering team replies within 48 hours. The reply does not replace the designer's approval of thickness and fabric type.

Sources and Further Reading

Open each document to confirm its current edition and scope before you rely on it.

• GRI GT16: The manufacturing quality control specification for the geotextile fabric used in grout-filled mattress. Checked 2026-10-24 (link and title).
• Geosynthetic Institute specifications index: Index of GRI test and specification documents. Checked 2026-10-24 (link and title).
• ASTM geotechnical engineering standards: Catalogue page for ASTM geotechnical standards. Checked 2026-10-24 (link and title).
• CEN EN 13254, geotextiles for reservoirs and dams: European standard for geotextile characteristics in reservoirs and dams. Checked 2026-10-24 (link and title).

Frequently Asked Questions

Can I specify grouted mattress without a brand name?

Yes, and this is usually the better approach for competitive tendering. Specify the product by performance requirements: type, thickness, fabric acceptance basis and grout strength, rather than by brand. This opens competition to multiple suppliers. Include an "or equivalent" clause that requires bidders to submit their documents before the tender deadline, and evaluate equivalence against the specification clauses, not just price.

What should I do if the delivered product fails testing?

Notify the contractor in writing that the lot is non-compliant and quarantine the delivered material so installation does not proceed. Ask for replacement with compliant material. If the contractor disputes the results, the specification can provide for retesting by a second accredited laboratory on split samples from the same lot. Include a clause that allocates the program impact of a rejected lot.

Should I specify grouted mattress with a geotextile filter beneath it?

In many cases the mattress fabric itself can serve as the filter layer. A separate geotextile filter is needed where the fabric alone does not meet the filter criterion for the subgrade, for example with gap-graded or piping-prone soils. Confirm the criterion using the project geotechnical data, and include the separate filter layer only if the check requires it.

Drafting a tender document? Send the HydroBase engineering team your design velocity, channel or slope type and drawings and we will reply within 48 hours.