Roadway Pavement

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• Design Criteria

The most widely used pavement materials for Seattle streets and alleys are portland cement concrete (rigid pavement) and asphalt concrete (flexible pavement). Slag cement as a substitute for a portion of Portland cement in concrete may be allowed or, in some instances, required.

Standard Plan and Specification References

Standard Specification 2-09: Subgrade Preparation
Standard Plan 401: Residential Pavement Sections
Standard Plan 402: Commercial and Arterial Pavement Sections
Standard Plan 403: Cement Concrete Alley Pavements
Standard Plan 405: Types of Joints for Concrete Pavements

Design Criteria

Pavement Type: New pavement shall be of the same type (rigid or flexible) as the existing pavement when a street is being widened, extended, or replaced unless otherwise directed by Seattle Department of Transportation. Stormwater Code may require permeable pavement for alleys. See Drainage section for more information.

Pavement Depth: Pavement depth is determined by a pavement design and is based on the zoning, number and type of heavy vehicles per day using or expected to use the roadway, the strength of subgrade, and the type of pavement being designed. Required pavement sections are provided in the Right of Way Opening and Restoration Rules (ROWORR). The pavement sections provided in the ROWORR were developed to accommodate the varying design conditions (soil types, drainage conditions, etc) found throughout the City of Seattle.

If a project proposes to use a pavement section less than the one specified in the ROWORR, then a pavement design must be approved by SDOT. The design should be based on specific site criteria and the design parameters described below. For designed pavements, subgrade testing and analysis by a geotechnical engineer, a traffic analysis, and pavement design calculations are required. Subgrade strength tests (CBR, k-value, R-value, etc.) shall be performed by a qualified geotechnical engineer.

Alley Pavement Depth: Concrete is the standard material for alley construction in the City of Seattle (See Standard Plan 403). Concrete is the most suitable material to support solid waste trucks and similar heavy vehicles operating in alleys. Concrete pavement will be required any time a full alley is being constructed or where the existing alley being restored is concrete. Asphalt alley paving will only be approved when an alley is being partially restored and the existing alley pavement is asphalt or gravel.  All alley pavements are to be constructed over properly stabilized and compacted subgrade.

Pavement Design: Alley Pavement Depth

Land Use Zone	Pavement Type and Depth
1 or 2 new dwelling units	6″ Roadway Cement Concrete or 3″ Asphalt (HMA Cl 1/2″) over 6″ Crushed Rock (Type 2 Mineral Aggregate)
SF, LR1, LR2, LR3, MR, HR	6″ Roadway Cement Concrete or 3″ Asphalt (HMA Cl 1/2″) over 6″ Crushed Rock (Type 2 Mineral Aggregate)
NC1, NC2, NC3	8″ Roadway Cement Concrete or 9″ Asphalt (HMA Cl 1/2″ & 1″) over 6″ Crushed Rock (Type 2 Mineral Aggregate)
C1, C2, IB, IC, IG1, IG2 and Downtown	8″ Roadway Cement Concrete or 9″ Asphalt (HMA Cl 1/2″ & 1″) over 6″ Crushed Rock (Type 2 Mineral Aggregate)

Pavement Design: Default Design Parameters for New Pavement

Initial Serviceability Index (P i)	4.5
Terminal Serviceability Index (P t)	2.0
Reliability	90%
Asphalt Standard Deviation	0.45
Structural Coefficient Asphalt HMA Class ½” and Class 1”	0.39
Structural Coefficient Mineral Aggregate Type 2, Crushed Rock	0.13
Design Life	30 years- Flexible (Asphalt) 50 year- Rigid (Concrete)
Concrete Standard Deviation	0.35
Joint Load Transfer Coefficient	3.2
Modulus of Concrete Rupture	650 psi
Modulus of Concrete Elasticity	4.0 x 10^6 psi
Drainage Coefficient	.8 Flexible asphalt .9 Rigid concrete

The inputs in the table above should be used as a starting point for pavement design and adjusted as needed to reflect the specific project conditions. Pavement design reports should describe how each input value was developed. Pavement design on roadways that accommodate a high volume of heavy vehicles, including Major Trucks Streets, streets included in the Transit Classifications, and Downtown, Urban Village Main, Urban Center Connector, Neighborhood Corridor, and Industrial Access streets shall be designed using the 1993 AASHTO Guide for the Design of Pavement Structures, 4th Edition with 1998 Supplement.

Panel Layout: When new PCC pavement is proposed or required, the panel layout at all intersections and on arterial streets must be shown on the plans. Additionally, panel layouts may be required for non-arterial streets with non-standard street widths. Longitudinal joints must be placed so that they are not within the wheel path of vehicles or in an area used by bicycles. The joints, dowel bars, and tie bars shall be per Standard Plan 405a through 405d.

Pavement subgrade: The pavement shall be placed on a prepared subgrade of properly compacted suitable material as determined by Seattle Department of Transportation (SDOT).

Compaction of subgrade: The subgrade shall be thoroughly compacted to 95% of maximum dry density for all street and alley improvements, as outlined in section 2-09 of the Standard Specifications. Subgrade materials that cannot be compacted to this density shall be over-excavated (removed) and the subgrade replaced with acceptable material.

Soil tests: Soil density tests may be required during construction to show that the required degree of compaction has been obtained.

Limits of pavement replacement and restoration: The extent of new pavement to be installed on roadways with existing pavement depends on required pavement width and existing pavement conditions. Specific rules and requirements for new pavement are detailed in the Street and Sidewalk Right of Way Opening and Restoration Rules.

Existing and proposed concrete panel joints shall be shown on street improvement plans for all intersections, arterial streets and when the pavement restoration will exceed one block. The extent of pavement replacement shall be depicted on street improvement plans by shading panels, or portions of panels, to be replaced.