TRH 4 vs SAPEM Chapter 10: what each decides in a pavement design

In South African pavement design, TRH 4 (Draft, 1996) sets out a process and a catalogue of standard structures for a new flexible pavement (CSRA, 1996), while SAPEM Chapter 10 (2nd edition, October 2014) describes the wider design process and the methods that estimate structural capacity (the traffic a structure can carry) and points back to TRH 4 (SANRAL, 2014b). TRH (Technical Recommendations for Highways) is a series of South African road manuals, and SAPEM is the South African Pavement Engineering Manual, issued by the South African National Roads Agency (SANRAL).

Key facts

• The South African manual TRH 4 (Draft, 1996) sets out a design process and a catalogue of standard flexible and semi-rigid pavement structures for new interurban and rural roads.
• SAPEM Chapter 10 (2nd edition, October 2014) is a South African guideline describing design traffic, the investigation and design process and eight structural capacity methods for flexible pavements.
• SAPEM Chapter 10 Section 1 lists TRH 4 for new flexible pavement design and TRH 12 for rehabilitation design among its guideline documents.
• SAPEM Chapter 10 Section 6 describes the TRH 4 catalogues as tried and tested and recommends using them to benchmark any pavement design.
• The client's terms of reference decide which document or method governs a pavement design, and neither TRH 4 nor SAPEM Chapter 10 settles that choice.
• TRH 12 (Draft, 1997) and SAPEM Chapter 14 (2nd edition, October 2014) take over once a pavement exists and needs investigation or rehabilitation.

What are these documents, and what status does each carry?

TRH 4 is a draft: a note in its front matter says the text is circulated in draft form for a trial period (CSRA, 1996). SAPEM Chapter 10, Pavement Design, is a second edition dated October 2014, and its scope statement calls SAPEM a best practice guide that does not replace the many manuals and guidelines already available (SANRAL, 2014b). TRH 4 is titled Structural Design of Flexible Pavements for Interurban and Rural Roads, and its cover is marked Draft, Pretoria, 1996, published by the Department of Transport for the Committee of Land Transport Officials (COLTO) (CSRA, 1996).

TRH 12, Flexible Pavement Rehabilitation Investigation and Design (Draft, 1997), covers existing flexible pavements (Department of Transport, 1997). SAPEM Chapter 14, Post-Construction (2nd edition, October 2014), covers a pavement after construction (SANRAL, 2014f). Quote the edition and status each time: "TRH 4" or "SAPEM" alone does not show which text was used.

How do TRH 4 and SAPEM Chapter 10 compare at a glance?

TRH 4 sets out a catalogue and SAPEM Chapter 10 describes capacity methods; the table compares them, citing sections without reproducing their tables.

Question	TRH 4 (Draft, 1996)	SAPEM Chapter 10 (2nd edition, October 2014)	Where to look
What is its status?	Draft, circulated for a trial period	Second edition, a guideline	Covers; TRH 4 front matter; Chapter 10 scope, p. ii
What is it for?	Selecting and designing flexible and semi-rigid pavements	Overview of design practice for flexible, concrete and block pavements	TRH 4 Section 1; Chapter 10 Sections 1 and 2
How is a candidate structure chosen?	A catalogue of standard structures by road category, traffic class and base type	Lists catalogues among its methods and points to the TRH 4 catalogue	TRH 4 Section 8.4; Chapter 10 Section 5.3.1
How is capacity checked?	Names several methods and advises using the catalogue with one of them	Describes eight methods for flexible pavements; the designer selects	TRH 4 Section 8.1; Chapter 10 Section 7
What counts as design traffic?	Equivalent 80 kN axles in traffic classes	Estimation methods; recommends the TRH 4 classes	TRH 4 Section 4; Chapter 10 Section 4
What about an existing road?	Points to TRH 12	Sets out the rehabilitation investigation and points to TRH 12	TRH 4 Figure 1; Chapter 10 Section 5.1.2

Narrated film · 1:30

TRH 4 vs SAPEM Chapter 10

Two compare scenes set TRH 4 beside SAPEM Chapter 10, a bar chart sets synthetic demand against capacity, and hand-off arrows run from TRH 4, SAPEM Chapter 10 and Chapter 14 to TRH 12.

Read the film transcript

Is TRH 4 still used for pavement design, now that SAPEM Chapter 10 exists? TRH 4, a South African road manual, is a nineteen ninety-six draft with a catalogue of standard structures. SAPEM, the South African Pavement Engineering Manual, is a best practice guide, and Chapter 10 is a second edition, October twenty fourteen. For a new road, TRH 4 offers a catalogue, and advises using it beside another method. Chapter 10 lists that catalogue, and describes eight flexible pavement methods that estimate the traffic a structure can carry. Take a synthetic road, S1, with a demand of four point two million equivalent standard axles. A Chapter 10 method gives a trial structure a capacity of five point zero million, about nineteen per cent more. That ratio is not a reliability, since Chapter 10 treats capacity as a single estimate. Neither document decides which method a project uses, or which design reliability an authority accepts. Once a pavement exists, TRH 4, SAPEM Chapter 10 and Chapter 14 each hand over. They point to TRH 12, a nineteen ninety-seven draft on rehabilitation investigation and design. Chapter 10 does not replace TRH 4, and the catalogue is not a capacity check. A draft is not a withdrawn text; its front matter describes a trial period. Neither is the single governing method; the client's terms of reference decide which applies. Next, read TRH 12 and SAPEM Chapter 14, quoting each edition.

What does TRH 4 decide for a new flexible pavement?

TRH 4 arranges design as nine sections that lead from inputs to a selected structure. The first six gather the inputs: service objective, road category, design strategy, design traffic, materials and environment. The designer then reads candidate structures from the catalogue, estimates future maintenance and compares the candidates on the present worth of life cycle cost (CSRA, 1996).

Four road categories, A to D, express importance and level of service, and each carries an approximate design reliability. TRH 4 treats its category table as a guide, not a determinant: the road authority's service objective largely fixes the category (CSRA, 1996). Design traffic is counted in equivalent standard axles of 80 kN (E80) and grouped into classes. The RoadSense insight on E80 design traffic assumptions shows why that count is a scenario, not a measurement.

The catalogue lists structures by base type, with separate granular catalogues for dry and wet regions. Most were derived with the South African mechanistic design method (SAMDM) and then adjusted to practical layer thicknesses. Each is designed to carry the upper value of a traffic class, and the catalogue assumes every subgrade is brought to an equal support standard (CSRA, 1996). TRH 4 adds that the catalogue may not suit special conditions and works best beside another method (CSRA, 1996).

What does SAPEM Chapter 10 describe that TRH 4 does not?

Chapter 10 describes design principles, considerations such as analysis periods and life cycle strategy, design traffic estimation, an investigation and design process for new and rehabilitated pavements, and capacity methods for flexible, concrete and block pavements (SANRAL, 2014b). For all fourteen chapters, see SAPEM explained.

Section 6 sorts structural methods into empirical, mechanistic-empirical and catalogue methods. Section 7 describes eight methods for flexible pavements, among them the SAMDM (which Chapter 10 calls the South African Mechanistic-empirical Design Method), the Pavement Number method, the dynamic cone penetrometer (DCP) method and several based on measured surface deflection, and leaves the choice to the designer (SANRAL, 2014b). Section 7.2 sets out when the Pavement Number method applies, and this guide goes no further. The insight on Pavement Number method limits explains why the number means something only inside its own method.

Chapter 10 does not reproduce a flexible catalogue. Section 5.3.1 lists the TRH 4 catalogue and other industry catalogues among the methods for new design, and Section 6 calls the TRH 4 catalogues "tried and tested" and says they should be used to benchmark any pavement design (SANRAL, 2014b). That is a recommendation in a best practice guide; it does not make the catalogue the governing method.

Is TRH 4 still used, and which document governs a design?

TRH 4 is still named: SAPEM Chapter 10 names it for new flexible design and recommends its traffic classes, and the Department of Transport still hosts the 1996 draft (CSRA, 1996) (SANRAL, 2014b). The client's terms of reference decide which document governs; neither text settles it for a project.

TRH 4 says its procedures do not necessarily exclude other methods and that the best results probably come from using the catalogue with another method (CSRA, 1996). SAPEM Chapter 10 says the designer is responsible for selecting the most appropriate method or methods, and that responsibility for following accepted design practice stays with the design engineer (SANRAL, 2014b).

Read the brief for the document it names, the edition it cites and whether it asks for a check against the catalogue. If it says only "design to SAPEM", ask whether Chapter 10 alone is meant, because Chapter 10 itself points to TRH 4 for new flexible design.

What does a catalogue design look like beside a capacity check?

Synthetic example. Every value is invented. Synthetic Road S1 is a new rural road with a design traffic of 4.2 million equivalent standard axles of 80 kN (E80) in the design lane over its structural design period.

The catalogue route reads that figure as a traffic class. The structure listed for that class and road category is designed for the upper value of the class, which is at or above 4.2 million E80, so the designer gains margin without calculating it (CSRA, 1996). The class limits and structures belong to the TRH 4 tables, which this guide does not copy.

The method route starts from a trial structure with the project's own layer thicknesses and material inputs. Suppose the chosen Chapter 10 Section 7 method returns a structural capacity of 5.0 million E80. Check value: 5.0 / 4.2 = 1.19, so the trial structure carries about 19 % more than the demand (SANRAL, 2014b).

What the check does not tell you: SAPEM Chapter 10 treats a capacity as a single estimate taken from a distribution, so 1.19 is not a reliability; for new design, Chapter 10 says the reliability linked to the road category is usually introduced when the capacity itself is estimated (SANRAL, 2014b). Chapter 10 also notes that the catalogue designs were developed with a lower tyre pressure than present loading, a difference to record when comparing a design with the catalogue (SANRAL, 2014b).

Where do TRH 12 and SAPEM Chapter 14 take over for an existing road?

Once a pavement exists, the question changes from what to build to what the pavement is doing, and TRH 12 and SAPEM Chapter 14 take over. TRH 12 sets out a systematic approach to investigating, evaluating and analysing an existing pavement, designing the rehabilitation and appraising the options economically (Department of Transport, 1997). Its front matter adds that it does not cover design methods in detail. TRH 4 itself points to TRH 12 for maintenance and rehabilitation (CSRA, 1996).

Chapter 10 Section 5.1.2 says the rehabilitation investigation is more involved than for new design and is described in detail in TRH 12. Section 5.3.2 then covers structural design for each uniform section that needs strengthening, and Chapter 10 adds that once structural design starts, the differences between new and rehabilitation design are less significant (SANRAL, 2014b). SAPEM Chapter 14 covers monitoring and distress, and its Section 7.1 sends rehabilitation design to TRH 12 and to Chapter 10 Section 5, among other places (SANRAL, 2014f). For terminal condition, Chapter 14 refers back to the definition in TRH 4 (SANRAL, 2014f).

The guide to pavement design using SAPEM follows that sequence, and pavement management system steps covers the monitoring that raises the question.

What do the two documents leave undecided, and what is commonly misread?

Neither document decides which method a project uses, or which design reliability a road authority accepts: both give approximate reliabilities by road category as a guide (CSRA, 1996) (SANRAL, 2014b), and TRH 4 says management may choose a higher or lower one (CSRA, 1996). TRH 4 has no section on investigating an existing road (CSRA, 1996), and Chapter 10 defers to TRH 12 for the detail (SANRAL, 2014b).

Common misreadings:

• "SAPEM Chapter 10 replaces TRH 4." It names TRH 4 for new flexible design, and SAPEM says it does not replace the manuals and guidelines already available (SANRAL, 2014b).
• "The catalogue is a capacity check." It is a set of derived structures for standard conditions, and TRH 4 says special conditions can need other methods (CSRA, 1996).
• "Draft means withdrawn." The front matter describes a draft circulated for a trial period and does not say withdrawn, and Chapter 10 still cites the 1996 text in 2014 (CSRA, 1996) (SANRAL, 2014b).

What should a design record keep?

• The terms of reference that name each document, with the edition and status printed on its cover.
• The design traffic estimate in E80 per design lane, with its scenario assumptions.
• The road category, who set it, and the approximate design reliability assumed.
• The catalogue structure read, or the method, its version and its inputs.
• The benchmark comparison: which catalogue structure the design was placed beside, and why any difference was accepted.
• The present worth comparison of the candidates.

Frequently asked

Is TRH 4 still used for pavement design in South Africa?

TRH 4 is still named: SAPEM Chapter 10 (South African Pavement Engineering Manual, 2nd edition, October 2014) names it for new flexible pavement design and recommends its traffic classes. TRH 4 (Technical Recommendations for Highways number 4, also written TRH4) is a South African road manual, Draft 1996, for the structural design of flexible pavements. Whether a project follows it is set by the client's terms of reference.

What is the difference between TRH 4 and SAPEM Chapter 10?

TRH 4 (Technical Recommendations for Highways number 4) sets out a design process and a catalogue of standard structures for new flexible and semi-rigid pavements. SAPEM Chapter 10 (South African Pavement Engineering Manual) describes the wider design process, design traffic estimation and eight methods that estimate the structural capacity of flexible pavements, plus concrete and block methods. Chapter 10 points to TRH 4 for new flexible design and to TRH 12 for rehabilitation.

Does SAPEM Chapter 10 replace TRH 4?

No. SAPEM, the South African Pavement Engineering Manual, describes itself as a best practice guide that does not replace the existing manuals and guidelines. Chapter 10 names TRH 4 (Technical Recommendations for Highways number 4) for new flexible pavement design, recommends its traffic classes and describes its catalogues as a benchmark for any design. TRH 4 remains a separate Draft 1996 document with its own catalogue.

What does SAPEM Chapter 10 say about the TRH 4 catalogue?

SAPEM Chapter 10 (South African Pavement Engineering Manual, 2nd edition, October 2014) Section 6 describes the TRH 4 catalogues as tried and tested and recommends using them to benchmark any pavement design. Section 5.3.1 lists the TRH 4 catalogue among the methods for new design, and Chapter 10 does not reproduce a flexible catalogue. A benchmark is a recommendation in a best practice guide, not a rule that the catalogue governs.

Which pavement design method should a South African project use?

The project's terms of reference decide. TRH 4 (Technical Recommendations for Highways number 4) says its procedures do not necessarily exclude other design methods, and SAPEM Chapter 10 (South African Pavement Engineering Manual) says the designer is responsible for selecting the most appropriate method or methods. A design record should cite each document by edition and status: TRH 4 as Draft, 1996 and SAPEM Chapter 10 as 2nd edition, October 2014.

Where does rehabilitation design of an existing road start, TRH 4 or TRH 12?

It starts with an investigation of the existing pavement, which TRH 12 (Technical Recommendations for Highways number 12, Draft 1997) sets out, rather than with the TRH 4 catalogue. SAPEM Chapter 10 (South African Pavement Engineering Manual) Section 5.1.2 and SAPEM Chapter 14 Section 7.1 both point to TRH 12 for the detail, and TRH 4 itself points to TRH 12 for maintenance and rehabilitation of existing roads.

Where this connects

The two manuals to meet next are TRH 12, which carries an existing flexible pavement from investigation into rehabilitation design (Department of Transport, 1997), and SAPEM Chapter 14, which covers monitoring, distress and the decision to rehabilitate (SANRAL, 2014f). Last reviewed on 21 October 2026 against TRH 4 (Draft, 1996), SAPEM Chapter 10 (2nd edition, October 2014), TRH 12 (Draft, 1997) and SAPEM Chapter 14 (2nd edition, October 2014).

References

1. CSRA (1996). TRH4: Structural Design of Flexible Pavements for Interurban and Rural Roads. 1996 edition. Original source.Design framework; confirm project applicability
2. SANRAL (2014b). South African Pavement Engineering Manual, Chapter 10: Pavement Design. Second Edition, October 2014. Original source.Sections, tables and clauses are cited inline in each guide
3. Department of Transport (1997). TRH12: Flexible Pavement Rehabilitation Investigation and Design. Draft TRH12, Pretoria: Department of Transport for COLTO. Original source.Chapter 1: rehabilitation design process; investigation, evaluation, analysis and economic appraisal
4. SANRAL (2014f). South African Pavement Engineering Manual, Chapter 14: Post-Construction. Second Edition, October 2014. Original source.Sections, tables and clauses are cited inline in each guide

Use the manual edition and project requirements that apply to your work. Worked examples use synthetic values and are not site-specific design advice.