How to design a pavement using SAPEM, step by step

Pavement design using SAPEM, the South African Pavement Engineering Manual, can be followed in seven stages: investigate the road, classify the layers, estimate design traffic, record the road category, choose the methods, check capacity against traffic, and report. Structural design under SAPEM Chapter 10 (Pavement Design, 2nd edition, October 2014) takes the first four as its inputs and should not start until they exist (SANRAL, 2014b).

Key facts

• SAPEM, the South African Pavement Engineering Manual (2nd edition, October 2014), is a best-practice guideline that is not a specification and has no legal standing in a court of law.
• SAPEM Chapter 10 (Pavement Design) lists eight methods for estimating the structural capacity of a flexible pavement and makes the designer responsible for choosing the most suitable.
• The South African road manual TRH 4 describes design traffic as the cumulative equivalent 80 kN axle loads (E80) in the most heavily trafficked lane during the structural design period.
• SAPEM Chapter 9 section 15 assigns a design-equivalent material class to a layer of an existing pavement, and the layer need not satisfy the specification for that class.
• SAPEM Chapter 1 Table 1 pairs the TRH 26 functional road classes with the TRH 4 road categories A to D, so a design report should record both labels.
• SAPEM Chapter 6 section 9.2.2 lists what a Detailed Assessment and Design Report generally contains and recommends more than one design method for a rehabilitation.

What does SAPEM decide, and what is left to the client?

SAPEM, issued by the South African National Roads Agency (SANRAL) in a 2nd edition dated October 2014, is a best-practice guideline. SAPEM is not a specification, has no legal standing in a court of law, and does not replace the manuals it cites (SANRAL, 2014a). The client's terms decide the rest. SAPEM Chapter 11 says designs follow the client's codes of procedure, which specify the design inputs and describe the investigation and calculation procedures (SANRAL, 2014c). Chapter 10 adds that the design engineer stays professionally responsible for a design that reflects accepted practice (SANRAL, 2014b).

RoadSense does not design pavements. This guide follows the evidence a designer receives and the manual that owns each input.

What are the seven stages of pavement design using SAPEM?

TRH (Technical Recommendations for Highways) is a series of South African road manuals, and three older ones supply inputs: TRH 16 (Draft, December 1991) for traffic loading, TRH 26 (Version 1.0, August 2012) for road classes, and TRH 4 (Draft, 1996) for road categories.

Narrated film · 1:34

From test pit to pavement design

Two rails carry the seven stages, with design traffic and road category arriving from TRH 16, TRH 26 and TRH 4 to meet at SAPEM Chapter 10, then one synthetic section's traffic range is set beside the capacity check.

Read the film transcript

How do you design a pavement using SAPEM, step by step? SAPEM, the South African Pavement Engineering Manual, second edition, October twenty fourteen, is a best-practice guideline, not a specification. The client's terms decide the rest: codes of procedure, allowed methods and document editions. First, investigate and group the road into uniform sections, lengths similar enough for one design. Second, classify each existing layer by how it behaves, with a certainty, not by its specification. Third, estimate design traffic in E80, axle damage against one standard axle, as a range. Fourth, record the TRH 26 road class beside the TRH 4 road category. Design traffic and road category join the investigation and layer classes at SAPEM Chapter 10. Fifth, choose the methods the terms of reference allow, from the eight for flexible pavements in Chapter 10. Sixth, compare each method's capacity estimate with the whole traffic range. Seventh, report the road, its category, the investigations, methods and inputs. In this synthetic section, design traffic is five point five million E80, from four point zero to seven point five. Print each capacity beside that whole range, not the central value alone; the method's own table decides. These stages decide no thickness, material value or method, and RoadSense does not design pavements. Structural design should start only once the four inputs exist, with each source recorded. Next, see SAPEM Chapter 11 on documentation and tendering, and Chapter 14 on post-construction monitoring.

Stage	What it produces	Owning manual and section	What the report records
1. Investigate	Uniform sections (road lengths of similar structure and condition), pit logs, dynamic cone penetrometer (DCP) and laboratory results	SAPEM Chapter 6, sections 5 and 7	Positions, dates, methods, reason for each boundary
2. Classify layers	A design-equivalent class per layer, with certainty	SAPEM Chapter 9, section 15	Tests behind each class
3. Estimate design traffic	Cumulative equivalent 80 kN axles (E80), as a range	TRH 16 sections 3 and 4; SAPEM Chapter 10, section 4	Source, lane factors, growth, sensitivity runs
4. Record the road category	TRH 4 category beside the TRH 26 class	TRH 26 sections 3.6 and 3.14; SAPEM Chapter 1, Table 1	Class, category, who set them
5. Choose the methods	Methods the terms of reference allow	SAPEM Chapter 10, sections 5.3 and 7; TRH 4 section 8	Method, version, inputs
6. Check capacity	A capacity estimate per method	SAPEM Chapter 10, sections 6 and 7	Capacity beside the traffic range
7. Report	A design report a reviewer can follow	SAPEM Chapter 6, section 9	All of the above, cross-referenced

What does a pavement investigation hand to the designer?

A pavement investigation hands over uniform sections, the layers found in each and the tests behind them. A uniform section is a length of road whose condition and structure are similar enough for one design to suit all of it (SANRAL, 2014k).

For an existing pavement, SAPEM Chapter 6 (Road Prism and Pavement Investigations) follows the strategy in TRH 12, the South African manual on flexible pavement rehabilitation (Draft, 1997) (SANRAL, 2014k) (Department of Transport, 1997). On larger projects it advises at least three phases: non-intrusive measurements first, then test pits, cores and DCP probes placed using those results, then extra testing to fill gaps (section 7.2.2) (SANRAL, 2014k). It advises against delineating uniform sections from one parameter or statistics alone (section 7.2.2.1) (SANRAL, 2014k); the guide on segments, uniform sections and reporting intervals separates the lengths involved.

Deflection bowl parameters benchmark one section against another, and Chapter 6 section 7.3.4.1 takes much of its discussion from Horak (2008) (Horak, 2008) (SANRAL, 2014k). A new road follows the three-stage road prism investigation of Chapter 6 section 5.3 (SANRAL, 2014k).

For an existing pavement, SAPEM Chapter 9 section 15 derives a material class for each layer that can be used for design. A design-equivalent material class (DEMAC), written DE-G2 for a layer equivalent to a G2 material, means the layer behaves like newly built material of that class in shear strength, stiffness and flexibility; it need not satisfy the specification for that class (SANRAL, 2014n). The method weighs several test indicators, gives each a certainty from 0 to 1 and does not need every test (SANRAL, 2014n). For new work the centre-line soils investigation classifies the in situ subgrade (Chapter 10 section 5.1.1) (SANRAL, 2014b).

How is design traffic estimated and road category set before structural design starts?

Design traffic is the cumulative E80 carried by the most heavily trafficked lane during the structural design period (CSRA, 1996). An E80 expresses the damage of any axle load relative to one standard 80 kN axle (CSRA, 1991).

TRH 16 sets out the method: loading surveys by visual observation or weighing (section 3), then average daily E80, future loading, projection and a sensitivity check (sections 4.2 to 4.6) (CSRA, 1991). SAPEM Chapter 10 section 4 covers the same ground, and section 4.7.2 describes the sensitivity analysis: low, medium and high values for each input, recalculated in combination (SANRAL, 2014b). When the data cover the whole road, apply directional and lane distributions (sections 4.4.1 and 4.7) (SANRAL, 2014b), and record the range.

A road category groups roads by importance and level of service, and each carries an approximate design reliability (Chapter 10 section 3.1) (SANRAL, 2014b). TRH 4 defines four categories, A to D (CSRA, 1996). A register usually holds a different scale, the numbered TRH 26 functional class (COTO, 2012). SAPEM Chapter 1 Table 1 pairs the classes with TRH 4 categories (SANRAL, 2014a), although TRH 26 section 3.14 says the A to D scheme should in future link directly to the functional classes and should not be used (COTO, 2012). Write both labels in the report. The category also steers the scope of the investigation (Chapter 6 section 7.2.2) (SANRAL, 2014k) and is agreed with the road authority first (Chapter 10 section 5.1) (SANRAL, 2014b).

How are design methods chosen and capacity compared with traffic?

SAPEM describes the methods and the client's terms narrow the choice. Chapter 10 section 7 lists eight methods for estimating the structural capacity of a flexible pavement, says each suits only certain situations, and makes the designer responsible for choosing the most suitable. It also says inputs should come from the project's actual materials, not published data (SANRAL, 2014b). Section 7.2 describes the Pavement Number method and section 7.2.1 states where the method is not applied; RoadSense goes no further than naming it.

Chapter 6 section 9.2.2 records that deflection, DCP and the South African Mechanistic Design Method (SAMDM) have traditionally been used for rehabilitation, and recommends more than one (SANRAL, 2014k). TRH 4 section 8.1 advises using its catalogue as a guideline together with another method (CSRA, 1996), and a sidebar in Chapter 10 section 6 recommends the TRH 4 catalogues as a benchmark for any design (SANRAL, 2014b).

Capacity is an estimate: Chapter 10 section 6 says a pavement has no single structural capacity value, only a range from variable inputs (SANRAL, 2014b). A computed response is not a performance; the insight on response and performance models explains why.

What does one synthetic uniform section look like?

Synthetic example. Every value is invented. Synthetic Road S1 has an existing flexible pavement. Non-intrusive data divide it into four sections; this example follows section U3, chainage 14,150 m to 15,400 m (1,250 m).

• Investigation: five test pits at 250 m spacing and twelve DCP probes.
• Layers: the base class rests on four tests and the subbase class on two, so the report records a lower certainty for the subbase.
• Design traffic: a central estimate of 5.5 million E80, with low and high sensitivity runs of 4.0 and 7.5 million E80.
• Road category: TRH 26 class 3, with the TRH 4 category read from SAPEM Chapter 1 Table 1 and written beside it.
• Methods: the terms of reference name a mechanistic method and the DCP method, with the TRH 4 catalogue as a benchmark.

Each method returns a capacity in million E80, and the report prints both beside the 4.0 to 7.5 million range, not beside the central value alone. Whether a trial structure carries enough is for the method's table to decide.

What should the design report record, and what should you keep?

SAPEM Chapter 6 section 9.2.2 (9.1.2 for new roads) lists what a Detailed Assessment and Design Report generally holds: the road and its category, investigations with every testing position cross-referenced to plans, an evaluation by uniform section, the design methods with all their inputs, an economic analysis and drawings (SANRAL, 2014k). Chapter 10 section 5.1.2.2 adds a description of each section: layers, past and future traffic, moisture and temperature, performance, condition and cause of distress (SANRAL, 2014b).

Evidence to keep:

• Pit logs, photographs, DCP records and laboratory results, with chainage, date and method.
• The uniform-section list and the reason for each boundary.
• The design-equivalent class of each layer, the tests behind it and its certainty.
• The traffic source, survey date, lane factors, growth assumptions and sensitivity runs.
• The TRH 26 class, the TRH 4 category and who set them.
• Each method with its edition or version, inputs and capacity estimate.
• The terms of reference naming the documents, with editions.

What are the steps to design a pavement using SAPEM?

1. Divide the road into uniform sections from non-intrusive data, using more than one parameter.
2. Dig test pits, take cores and run DCP probes where those results point.
3. Assign a design-equivalent material class to each layer and record its certainty.
4. Estimate design traffic in E80 for the design lane and run the sensitivity cases.
5. Record the TRH 26 class and the TRH 4 category, and who set them.
6. Choose the methods the terms of reference allow, preferably more than one.
7. Compare each method's capacity estimate with the traffic range.
8. Write the report so a reviewer can follow every input to its source.

What do the seven stages leave undecided, and what is commonly misread?

The seven stages decide no layer thickness, material value, traffic class limit, reliability value or method. The final choice rests on the economic assessment of Chapter 10 section 5.4, which refers to TRH 4 and TRH 12 (SANRAL, 2014b).

• "Chapter 10 chooses the method." It describes eight for flexible pavements and makes the designer responsible for the choice (SANRAL, 2014b).
• "A design-equivalent class is a specification class." The class describes behaviour, and the layer need not satisfy the specification (SANRAL, 2014n).
• "A TRH 26 class is a TRH 4 category." They are different scales, linked by a table (SANRAL, 2014a).
• "Design traffic is a count." It is a projection with assumptions, tested by sensitivity runs (CSRA, 1991).

Which manual governs each stage

Stages of the journey, the manual that governs each one, and the evidence each stage should leave behind.

Stage	Governing manual	Evidence this stage produces
1. Investigate	SAPEM Ch. 6	Uniform sections, test-pit logs, DCP records and laboratory results, each with position, date and method.
2. Classify layers	SAPEM Ch. 9	A design-equivalent material class for each layer, with the test indicators and the certainty behind it.
3. Estimate design traffic	TRH 16	Design traffic in E80 for the design lane, with source, survey date, lane factors, growth assumptions and low-to-high sensitivity runs.
4. Record the road category	TRH 26	The TRH 26 functional class and the TRH 4 category read from SAPEM Chapter 1 Table 1, with who set them.
5. Choose the methods	SAPEM Ch. 10	The Chapter 10 section 7 methods the terms of reference allow, with edition or version, inputs and the TRH 4 catalogue as a benchmark.
6. Check capacity against traffic	SAPEM Ch. 10	A capacity estimate and range per method, placed beside the design traffic range.
7. Report	SAPEM Ch. 6	A design report with every testing position cross-referenced to plans, the evaluation by uniform section and the inputs of each method.

The journey, step by step

1. Divide the road into uniform sections

   Use the non-intrusive measurements, and more than one parameter, to mark section boundaries, because SAPEM Chapter 6 section 7.2.2.1 advises against relying on one parameter or on statistics alone. Keep: the section list and the reason for each boundary.

2. Dig test pits, take cores and run DCP probes

   Place the intrusive tests using the first results, in the phase SAPEM Chapter 6 section 7.2.2 describes, and test the samples (section 8). Keep: pit logs, photographs, probe records and laboratory results with chainage, date and method.

3. Assign a design-equivalent class to each layer

   Use SAPEM Chapter 9 section 15 to derive a material class for each layer of an existing pavement and record its certainty; for new work, classify the in situ subgrade (Chapter 10 section 5.1.1). Keep: the tests behind each class.

4. Estimate design traffic in E80 for the design lane

   Follow TRH 16 and SAPEM Chapter 10 section 4: apply directional and lane distributions when the data cover the whole road, project over the structural design period and run low, medium and high cases (section 4.7.2). Keep: the traffic source, survey date, growth assumptions and the range.

5. Record the TRH 26 class and the TRH 4 category

   Read the TRH 26 class from the register and agree the TRH 4 category with the road authority (SAPEM Chapter 10 section 5.1) using SAPEM Chapter 1 Table 1. Keep: both labels, who set them and the source of each.

6. Choose the methods the terms of reference allow

   Pick methods from SAPEM Chapter 10 section 7, preferably more than one for a rehabilitation (Chapter 6 section 9.2.2). Use the TRH 4 catalogue as a guideline beside another method (TRH 4 section 8.1). Keep: each method's version and inputs.

7. Compare each method's capacity with the traffic range

   Treat each capacity as an estimate with a range (Chapter 10 section 6) and compare it with the design traffic range from step 4, not with one central figure. Keep: both ranges and any disagreement between methods.

8. Write the design report so a reviewer can follow it

   Follow SAPEM Chapter 6 section 9.2.2 (9.1.2 for a new road) and Chapter 10 section 5.1.2.2: the road and its category, investigations cross-referenced to plans, evaluation by uniform section, methods and inputs, drawings. Keep: terms of reference and document editions.

Frequently asked

How do you design a pavement using SAPEM, step by step?

Pavement design using SAPEM, the South African Pavement Engineering Manual, can be followed in seven stages: investigate the road, classify the layers, estimate design traffic in equivalent 80 kN axle loads (E80), record the road category, choose the design methods, compare each method's capacity with the traffic range, and write the report. The first four stages produce the inputs, and structural design under SAPEM Chapter 10 (Pavement Design) should start only once they exist.

Is SAPEM a specification, and who decides which design method is used?

SAPEM, issued by the South African National Roads Agency, is a best-practice guideline. SAPEM is not a specification and has no legal standing in a court of law. SAPEM Chapter 10 describes eight methods for flexible pavements and makes the designer responsible for choosing the most suitable, while Chapter 11 says designs follow the client's codes of procedure. The client's terms of reference therefore narrow the choice.

What must exist before structural design under SAPEM Chapter 10 starts?

Structural design under SAPEM Chapter 10 (Pavement Design) needs uniform sections with test-pit and laboratory evidence, a material class for each layer, a design traffic estimate in equivalent 80 kN axle loads (E80) with its sensitivity range, and a road category. SAPEM Chapter 10 section 5.3 says structural design should start only after enough information has been gathered in the design investigation.

What is a design-equivalent material class in SAPEM?

A design-equivalent material class (DEMAC) is a class that SAPEM Chapter 9 section 15 assigns to a layer of an existing pavement, written DE-G2 for a layer equivalent to a G2 material. It means the layer behaves like newly built material of that class in shear strength, stiffness and flexibility, and it need not satisfy the specification for that class. The Chapter 9 method also reports a certainty.

How does a TRH 26 road class relate to a TRH 4 road category?

TRH 26 (Version 1.0, August 2012), the South African road classification manual, uses numbered functional classes. TRH 4 (also written TRH4; Draft, 1996), the structural design manual for flexible pavements, uses categories A to D. SAPEM Chapter 1 Table 1 pairs them, while TRH 26 section 3.14 says the A to D scheme should in future link directly to the functional classes and should not be used. Record both labels.

Does SAPEM give a single structural capacity for a pavement?

SAPEM gives no single structural capacity for a pavement: Chapter 10 section 6 says there is no single value, only a range from variable inputs, so every method returns an estimate. Compare the capacity range with the design traffic range from the sensitivity analysis, and record how any disagreement between methods was resolved. Each method's own criteria decide which structure is adequate, and the economic assessment in section 5.4 selects the final design.

Where this connects

The next two manuals to read are SAPEM Chapter 11 (Documentation and Tendering), which turns the design into contract documents (SANRAL, 2014c), and SAPEM Chapter 14 (Post-Construction), whose monitoring checks that pavements reach their design lives and feeds measured loads back into design (SANRAL, 2014f). Last reviewed on 21 October 2026 against SAPEM Chapters 1, 6, 9, 10, 11 and 14 (2nd edition, October 2014), TRH 4 (Draft, 1996), TRH 12 (Draft, 1997), TRH 16 (Draft, December 1991) and TRH 26 (Version 1.0, August 2012).

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. 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
3. Horak (2008). Benchmarking the structural condition of flexible pavements with deflection bowl parameters. Journal of the South African Institution of Civil Engineering, 50(2), pp. 2–9. Original source.Relative structural benchmarking with deflection bowls
4. SANRAL (2014a). South African Pavement Engineering Manual, Chapter 1: Introduction. Second Edition, October 2014. Original source.Sections, tables and clauses are cited inline in each guide
5. SANRAL (2014k). South African Pavement Engineering Manual, Chapter 6: Road Prism and Pavement Investigations. Second Edition, October 2014. Original source.Sections, tables and clauses are cited inline in each guide
6. SANRAL (2014n). South African Pavement Engineering Manual, Chapter 9: Materials Utilisation and Design. Second Edition, October 2014. Original source.Sections, tables and clauses are cited inline in each guide
7. 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
8. SANRAL (2014c). South African Pavement Engineering Manual, Chapter 11: Documentation and Tendering. Second Edition, October 2014. Original source.Sections, tables and clauses are cited inline in each guide
9. 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
10. CSRA (1991). Draft TRH16: Traffic Loading for Pavement and Rehabilitation Design. Draft, December 1991 (printed 1994). Original source.Sections, tables and clauses are cited inline in each guide
11. COTO (2012). TRH 26: South African Road Classification and Access Management Manual (RCAM). Version 1.0, August 2012. 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.