Recommended structural designs use the basic assumptions and classifications discussed below.  The limitations and applicability of these assumptions and classifications must be understood before using the recommended structural designs contained in this section.


Traffic is classified in a broad sense only.  There are many different traffic classification schemes available and traffic can vary widely in both make-up and volume.  This design catalog uses the “vehicle manufacturer truck classification”, which broadly divides vehicles into the three classes listed in Table 1.

Table 1: Design Catalog Vehicle Classification

Vehicle Category Gross Vehicle Weight Range Assumed ESALs per Vehicle Representative Vehicles
Cars and Light Trucks 0 – 6,000 lbs. 0.0007 cars, SUVs, pickup trucks, ambulances, delivery vehicles
Medium Trucks and Buses 14,001 – 16,000 lbs. 0.25 city cargo van, beverage delivery truck, wrecker, school bus
Heavy Trucks and Buses 33,000 lbs. and over 1.00 semi tractor trailer, concrete mixer, dump truck, fire truck, city bus
Note: this classification is different from the FHWA vehicle classification system


Subgrade soil conditions vary widely across the State.  This design catalog section classifies soils into three broad categories based on strength and stiffness (CBR, MR) as listed in Table 2.  These categories are broad and should be used with caution.  A subgrade should be categorized based on its strength or stiffness and not its soil type alone.  For example, it is not enough to know that a subgrade is primarily classified as an SW in the Unified Soil Classification System (USC), something must also be known about its strength or stiffness.  Furthermore, although a subgrade may be reasonably stiff or strong, it may also be frost susceptible, which must be accounted for in pavement structural design.

A Note on Extremely Adverse Subgrades
The “Poor” classification shown in Table 2 is meant to describe subgrades with low strength and stiffness values and/or high fines content.  Extremely adverse subgrade conditions such as water saturated soil, standing water and organic peat provide extremely poor support and do not meet the “poor” classification in Table 2.  Paving over extremely adverse subgrades should be avoided if possible.  If paving is necessary, the pavement designer and a geotechnical engineer should collaborate to develop a feasible pavement design.

Table 2: Design Catalog Subgrade Classification

Classification CBR MR(psi) Typical Description
(by USC)
Frost Potential
Good ≥ 10 20,000 Gravels, crushed stone and sandy soils.  GW, GP, GM, SW, SP, SM soils are often in this category. Less than 6 – 7 percent passing
the No. 635 sieve (0.02 mm)
Essentially  Not Frost Susceptible
Fair 5 – 9 10,000 Clayey gravel and clayey sand, fine silt soils.  GM, GC, SM, SC soils are often in this category. 6 – 20 percent passing
the No. 635 sieve (0.02 mm)
Potentially Frost Susceptible
Poor 3 – 5 5,000 Fine silty sands, clays, silts, organic soils.  CL, CH, ML, MH, CM, OL, OH soils are often in this category. Over 15 percent passing
the No. 635 sieve (0.02 mm)
Highly Frost Susceptible

Pavement Structure

  • “Aggregate base” refers to crushed aggregate.  The WSDOT crushed aggregate definitions of “crushed surfacing top course” (CSTC) and “crushed surfacing base course” (CSBC) both meet the intended definition of “aggregate base”.
  • The minimum recommended crushed aggregate base thickness is 4 inches.  Aggregate layers less than 4 inches thick are possible, however (1) they do not add much strength to the overall pavement structure, (2) they provide little protection from frost heave, (3) fines from the underlying subgrade may contaminate a substantial portion of the layer and inhibit drainage, (4) they are difficult to compact, (5) they are frequently subject to quantity overruns and (6) it is difficult to construct and maintain smoothness in aggregate layers thinner than 4 inches.
  • The minimum recommended HMA layer thickness is 2 inches.  Pavement layers thinner than about 2-3 times the nominal maximum aggregate size may be difficult to compact, tear under the screed, and rollers may crush the larger particles during compaction.
  • For many of the recommended structural designs presented here traffic is not the controlling factor.  For lower volume roads, parking lots and recreational facilities considerations such as HMA aging, constructability and subgrade conditions are primary concerns.

Other Information

  • If a bus route is anticipated on a particular low traffic pavement such as a residential street or collector it may often control the structural design.  Therefore, pavements with bus routes or anticipated bus routes should be designed using an approved design procedure.  Otherwise, the heavy bus traffic may cause premature pavement failure.
  • All recommended designs in this section were checked using the 1993 AASHTO empirical structural design equation using the following values: Reliability = 75 %, So = 0.50, ΔPSI = 2.0, design life = 20 years.