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 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

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.

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 Pavement Opening and Restoration Rules (PORR).  The pavement sections provided in the PORR were developed to accommodate the varying design conditions (soil types, drainage conditions, etc) found throughout the City of Seattle and are fairly conservative.  

If a project proposes to use a pavement section less than the one specified in the PORR, 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:

Pavement Design: Default Design Parameters for New Pavement

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, Regional Connectors, Commercial Connectors, 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 must be compacted to at least 95% of maximum dry density for all street and alley improvements. 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 Pavement Opening and Restoration Rules.