The document provides guidelines for specifying a mechanically stabilized layer (MSL) using Tensar SpectraPave software. It describes three specification options: 1a) using Tensar TriAx geogrids conforming to the design; 1b) using alternative geosynthetics meeting design requirements; and 2) a non-stabilized layer meeting thickness and life cycle requirements. Critical parameters like layer thickness and traffic loads are to be included. The conforming design from SpectraPave establishes performance targets for alternatives to meet.

1. Tensar model specification
MS/SpectraPave/TensarMSL
Issue date: 16.10.19
Tensar SpectraPave™ software Specification for a Tensar MSL™
(Mechanically Stabilised Layer) and conforming alternatives
Tensar model specification 1/3 MS/SpectraPave/TensarMSL Layer
Tensar Technology Guidelines for the Specification for Mechanically Stabilised Layers in Road
Design
1. Introduction
The following notes provide users of Tensar SpectraPave™ software with specification guidelines which, when
combined with the program output, enables the designer/specifier to develop the specification for the inclusion of a
generic mechanically stabilised layer, in unbound aggregate layers in road construction.
SpectraPave follows the AASHTO Guide for Design of Pavement Structures (1993) as augmented by the general
application of AASHTO Standard Practice for Geosynthetic Reinforcement of the Aggregate Base Course of Flexible
Pavement Structures: AASHTO designation R 50-09. By following this approach, it is ensured that critical parameters
in the SpectraPave pavement analysis are carried forward to the contract documentation and, in the case of Tensar
Technology, the overall methodology and the functional performance of Tensar TriAx geogrids in a mechanically
stabilised layer is carried from a performance in the analysis to its protection in its specification.
It is important to recognise that Tensar Technology is proprietary and has been developed exclusively on the
performance that has been measured for mechanically stabilised layers incorporating Tensar TriAx geogrids. However,
the specification approach is generic and does not reference proprietary products. It provides guidance on the
incorporation of the Tensar MSL™ performance into an analysis and hence into the contract documents by employing
generic terminology, (i.e. mechanically stabilised layer) and by also including the scope for an alternative solution,
(i.e. alternative stabilising geosynthetics solution or a non-stabilised layer).
The approach to specification is particularly appropriate for public projects, agencies and their designers where
alternative solutions are to be permitted in the contract. The three possible solutions are shown as options:
Option 1a shall provide the ‘Conforming Design’ on the tender drawings with a statement that the design is
satisfied by the inclusion of Tensar TriAx geogrids in MSL1 as the base and, if applicable, MSL2 in the sub-base or
the use of both MSL1 and MSL2.
Option 1b shall permit the submission of an alternative mechanically stabilised layer which meets the
requirements of the conforming design, apart from the layer thickness only.
Option 2 shall permit the submission of a non-stabilised layer which meets the requirements of the conforming
design, apart from the layer thickness only.
Specification for Option 1a
The requirements for Tensar MSLs is obtained from a pavement analysis that is carried out in SpectraPave™
software and provides the performance target for considering alternatives. The tender drawings and supporting
documentation adopts this option as the contract’s conforming requirements.
Specification Guidelines for Option 1b
As Option 1a provides the conforming design which has been carried out by the designer/specifier using
SpectraPave™ software with parameters specific to Tensar MSL incorporating TriAx geogrid, the following describes
how an alternative option for a mechanically stabilised layer incorporating geosynthetics other than TriAx geogrids is
to be submitted for Option 1b.
2 General
The specification covers the requirements for one or more mechanically stabilised layer(s) incorporating geogrid as a
base or as a sub-base or as both in a pavement structure. It applies to the use of geogrid stabilisation products in
association with an unbound aggregate such that the two components combine to enhance the performance of the
resulting composite layer by a suitable measure, (e.g. increased traffic life as a result of an enhanced resilient
modulus of the fill in the layer).

2. Tensar model specification 2/3 MS/SpectraPave/TensarMSL
2.1 Design and Bid Requirements
The design of the road section shall be in accordance with the project-specific guidelines, standards and procedures
deemed appropriate by the designer/specifier. The mechanically stabilised layer(s) shall be analysed by software
which includes the geogrid benefit appropriate to increased traffic life resulting from the enhanced modulus of the
layer as afforded by the inclusion of geogrid. The geogrid benefit shall be derived or validated from programmes of
full scale rolling wheel traffic testing. The method of derivation shall be endorsed by an independent third-party with
recognised relevant expertise.
The critical parameters employed in the analysis of the mechanically stabilised layer(s), are to be stated on the
relevant contract drawings and include the following:
Critical parameter
Mechanically stabilised layer
thickness, T mm
State the thickness of the layer
AASHTO ‘a*’ value Provide the enhanced modulus value
attributable to the inclusion of geogrid(s)
AASHTO analysis procedure State any exceptions in compliance with the
analysis procedure in Chapter 3
Life cycle cost analysis State whether the proposal has been the
subject of an analysis of life cycle costs
‘Carbon footprint’ analysis State whether the proposal has been the
subject of an analysis of the CO2e costs
3. Specification guidelines for Option 2
As Option 1a provides the conforming design which has been performed by the designer/specifier, the following
describes how an alternative option for a non-stabilised layer is to be submitted for Option 2.
3.1 General
The specification covers the requirements for one or more non-stabilised layer(s) as a base or as a sub-base or as
both in a pavement structure.
3.2 Design and Bid Requirements
The design of the road section shall be in accordance with the project-specific guidelines, standards and procedures
deemed appropriate by the designer/specifier. The non-stabilised layer(s) shall be analysed by appropriate software.
The critical parameters employed in the analysis of the non-stabilised layer, are to be stated on the relevant contract
drawings and include the following:
Critical parameter
Mechanically stabilised layer
thickness, T mm
State the thickness of the layer
AASHTO ‘a’ value Provide the modulus value of the proposed
aggregate layer
AASHTO analysis procedure State any exceptions to compliance with the
analysis procedure in Chapter 3
Life cycle cost analysis State whether the proposal has been the
subject of an analysis of life cycle costs
‘Carbon footprint’ analysis State whether the proposal has been the
subject of an analysis of the CO2e costs
4. Tender compilation notes
The following notes provide guidelines on how to compile tender drawings and contract documents.
 The MSL requirements for Option 1a, the conforming design, are shown on the tender drawings, typically as
indicated.

3. Tensar model specification 3/3 MS/SpectraPave/TensarMSL
Typical depiction of Option 1a on tender drawings
 The target design life of the road is to be provided along with a list of the fixed AASHTO input parameters and any
controlling geometry.
AASHTO 1993 input values Value Units
Target design life esal’s
Reliability percent
Standard normal deviate -
Serviceability loss -
Overall standard deviation -
Subgrade resilient modulus
 The conforming pavement parameters are:
AASHTO 1993 pavement parameters Value Units
Asphalt layer 1 ‘a’ value per inch
Asphalt layer 2 ‘a’ value per inch
Asphalt layer 3 ‘a’ value per inch
Base layer ‘a’ value per inch
Sub-base layer ‘a’ value, if used per inch
 The selected geogrids in the analysis are stated in the bill of quantities.
 Should the term, e.g. ‘Tensar®
TriAx®
TX...... geogrid or similar’ be used in the contract documents, it should be
modified to read, e.g. ‘Tensar®
TriAx®
TX...... geogrid or similar functional performance in the design’ to ensure
that there is an Engineering judgement on the preservation of the design life of the analyses.
 In-air index testing of quality control data, or product identification data of a geogrid are not acceptable as a basis
of proposing the product in the fulfilment of the design requirements.
 Option 1a carries the tender note, ‘The requirements of the mechanically stabilised layer(s) indicated in the
contract drawings are satisfied by the inclusion of Tensar MSL™ incorporating TriAx®
geogrids.’
 Option 1b carries the tender note, ‘The requirements of the mechanically stabilised layer(s) are to be
demonstrated to the Engineer that the conforming design is satisfied in terms of materials, their layer thickness
and the preservation of the target traffic design life, expressed in equivalent standard axle loads.’
 Option 2 carries the tender note, ‘The requirements of the mechanically stabilised layer(s) may be met by
conventional non-stabilised layer(s). It is to be demonstrated to the Engineer that the conventional design is
satisfied in terms of materials, their layer thickness and the preservation of the target traffic design life,
expressed in equivalent standard axle load
The information in this document supersedes any and all prior specifications for the product designated above, is of an illustrative nature and supplied by Tensar International
Limited free of charge for general information purposes only. Tensar reserves the right to amend product descriptions, properties and specifications at any time and without prior
notice. This document does not form part of any contract or intended contract. Tensar International Limited excludes to the fullest extent lawfully permitted any and all liability
whatsoever for any loss or damage howsoever arising out of the use of and reliance upon this information. It is your sole responsibility and you must assume all risk and liability
for the final determination as to the suitability of any Tensar International Limited product and/or design for the use and in the manner contemplated by you in connection with a
particular project
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