Pavement Structural Design Standards

PURPOSE

Pavement structural section design determines the surface course, base, and subbase layers best suited to a specific set of project conditions.  These standards present policies, procedures, and minimum design parameters and criteria for uniform development of flexible pavement structural designs, which enable preparers to present findings and recommendations in a systematic manner, based on sound engineering principles, consistent with the expectations and practice of Mohave County.  A standard methodology and criteria for pavement structural design provides a consistent evaluation of pavement structural capacity and design of structurally comparable pavement structures under varying traffic load, roadbed soil, and functional conditions.

FLEXIBLE PAVEMENT STRUCTURAL DESIGN POLICIES

The design method for flexible pavement structural design shall follow procedures set forth in the AASHTO Guide for Design of Pavement Structures, 1993 Edition, and the design shall assure adequate strength and durability to carry predicted traffic loads under prevailing roadbed soil conditions for the design service life of the pavement.  Standard procedures and criteria are specified for determining design parameters as required for input into the AASHTO design method.  These pavement structural design standards apply to all new construction and to reconstruction and improvement of Mohave County roads and streets.  No deviation from these adopted pavement structural design standards may be made without approval of the County Engineer.  

FLEXIBLE PAVEMENT STRUCTURAL DESIGN PROCEDURES AND CRITERIA

Geotechnical Investigation

Geotechnical tests shall be performed to adequately characterize all roadbed (subgrade) soil types and results presented in an engineering report.  Soil classification tests (i.e., gradation sieve analysis and plasticity index for cohesive soils) shall be performed for each roadbed soil type at minimum 1,000-foot project intervals.  Special tests to characterize roadbed soil strength or stiffness as part of the pavement structure and pavement materials shall be performed for each soil type and at minimum 5,000-foot project intervals.  These include tests to determine Resilient Modulus (MR), Resistance Value (R-Value), and/or California Bearing Ratio (CBR).

Traffic

Reference traffic impact analysis findings for estimating the traffic design load, represented by the number of 18-kip equivalent single axle loads (ESAL’s) over the design service life.

1.    Apply the following equation to determine design ESAL’s:

•       W18 = Wo(2-18) * GF * 365 * DD * DL 

Where,

Wo(2-18) = initial 2-way daily 18-kip ESAL’s

GF = traffic growth factor for analysis period (design service life)

DD = directional distribution factor

DL = lane distribution factor

 

2.    Determine initial 2-way daily traffic in 18-kip ESAL’s, W0(2-18)

•       Option A – vehicle classification data available (FHWA classifications)

Wo(2-18) = i [N1 * EF1 + N2 * EF2 + … + Ni * EFi] Where,

Wo(2-18) = initial 2-way daily 18-kip ESAL’s

Ni = number of vehicles per day within a given classification

EFi = Traffic Equivalency Factor for a given classification as listed below   

 

Vehicle Class	Description	Traffic Equivalency Factor
1	Motorcycles	0
2	Passenger Cars	0.0008
3	Other 2-Axle, 4-Tire Single Unit Vehicles	0.0122
4	Buses	0.6806
5	2-Axle, 6-Tire, Single-Unit Trucks	0.1890
6	3-Axle Single-Unit Trucks	0.1303
7	4 or More Axle Single-Unit Trucks	0.1303
8	4 or Fewer Axle Single-Trailer Trucks	0.8646
9	5-Axle Single-Trailer Trucks	2.3719
10	6 or More Axle Single-Trailer Trucks	2.3719
11	5 or Fewer Axle Multi-Trailer Trucks	2.3187
12	6-Axle Multi-Trailer Trucks	2.3187
13	7 or More Axle Multi-Trailer Trucks	2.3187

 

•       Option B – vehicle classification unknown

Wo(2-18) = [ADT * (% cars and pickups/vans) * 0.004] + [ADT * (% heavy trucks) * 1.0] Where,

Wo(2-18) = initial 2-way daily 18-kip ESAL’s o ADT = Average Daily Traffic (two-way) o % heavy trucks = percent ADT representing buses and trucks under Classes 4–13 o % cars = percent ADT representing passenger cars and other 2-axle, 4-tire single unit vehicles under Classes 2–3 = 1 – (% heavy trucks)

 

3.    Estimate growth factor, GF, for analysis period

•       GF = [(1 + g)n – 1] / g

Where,

g = annual traffic growth rate expressed as percentage

n = analysis period in years

4.    Assign directional distribution factor, DD

•       Use DD = 0.5 unless data/studies support weighting one travel direction over the opposite, particularly as it relates to the distribution of heavy vehicle traffic

5.    Assign lane distribution factor, DL

•       Use DL = 

1.0 for one lane in each direction of travel

0.9 for two lanes in each direction of travel

0.7 for three lanes in each direction of travel

Minimum Criteria for Structural Design

•       Design Service Life

Paved surface = 20 years

Aggregate surface = 15 years

•       Reliability Level = 80%

•       Overall Standard Deviation, So

So = 0.45

•       Total Change in Serviceability Index, PSI

PSI = po – pt o po = initial serviceability index = 4.2 o pt = terminal serviceability index = 2.0 

•       AASHTO Structural Layer Coefficients  Asphalt concrete layer coefficient, a1 o a1 = 0.44 for plant mix

a1 = 0.42 for recycled asphalt concrete o a1 = 0.30 for cold mix

Aggregate base course layer coefficient, a2 o a2 = 0.14

Design Structural Number, SN

•       Reference design chart in Figure 3.1, page II-32, of AASHTO Guide for Design of Pavement Structures, 1993 Edition

•       * SN = (a1 * D1) + (a2 * D2)

Where,

a1 = asphalt concrete layer coefficient

D1 = asphalt concrete layer thickness in inches

a2 = aggregate base layer coefficient

D2 = aggregate base layer thickness in inches

Structural subbase course not provided and base course drainage coefficient, m2 = 1.0

Layered Design Analysis for Surface Course and Base Course over Roadbed/Subgrade

1.    Determine design structural number, SN

•       Reference design chart in Figure 3.1, page II-32, of AASHTO Guide for Design of Pavement Structures, 1993 Edition

•       Use resilient modulus, MR, of roadbed/subgrade material

2.    Determine structural number, SN1, required over base layer

•       Reference design chart in Figure 3.1, page II-32, of AASHTO Guide for Design of Pavement Structures, 1993 Edition

•       Use resilient modulus, MR, of aggregate base course material

3.    Calculate minimum required asphalt concrete (surface course) layer thickness, D1, in inches   

•       D1 = SN1 / a1 

•       Round D1 to the next integer  

4.    Calculate minimum required base layer thickness, D2, in inches

•       D2 = (SN – D1 * a1) / a2 

•       Round D2 to the next integer

5.    Confirm [(D1 * a1) + (D2 * a2)] ≥ SN

 

MINIMUM DESIGN STRUCTURAL SECTION

Table 1 presents minimum pavement structural section surface course and base course composition and layer thickness specifications by road functional classification and roadbed soils type.  The County Engineer may at his sole discretion require the Engineer of Record provide a complete soils investigation and/or design calculations in accordance with these adopted pavement structural design standards to substantiate any pavement structural section design.

Table 1

Minimum Pavement Structural Section by Road Functional Class and Roadbed Soils Type

Design Condition/Threshold (Pavement Layer Thickness)				Traffic Load
			Residential Access (local or collector)	Distributor (collector)	Through (arterial)
			0.05 million ESALs	0.3 million ESALs	1.0 million ESALs
	Good (gravelly)	GC to GW (MR ± 25 ksi)	HMA surface = 2 in. Agg. base = 6 in.	HMA surface = 2 in. Agg. base = 6 in.	HMA surface = 3 in. Agg. base = 6 in.
	Average (sandy)	SC to SW (MR ± 15 ksi)	HMA surface = 2 in. Agg. base = 6 in.	HMA surface = 2 in. Agg. base = 8 in.	HMA surface = 3 in. Agg. base = 8 in.
	Poor (silty/clayey)	CL/ML (MR ± 7 ksi)	HMA surface = 2 in. Agg. base = 8 in.	HMA surface = 3 in. Agg. base = 8 in.	HMA surface = 4 in. Agg. base = 8 in.

  

PORTLAND CEMENT CONCRETE APRONS

Any new or improved site access driveway serving ten (10) or more semi tractor-trailer or load equivalent trips per day (total ingress plus egress) shall have an engineered portland cement concrete apron extending across any adjoining Mohave County maintained road or street to support the high stress and strain forces attributed to slow moving heavy loads.  The apron throat length shall equal the greater of (1) the driveway throat width or (2) a distance encompassing the ingress/egress truck turning paths to/from the adjoining road or street.