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 2-06: 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

New 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.

New 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. Refer to Pavement Design below.

Alley Pavement Depth:

Pavement Thickness: For rigid pavements, a designed thickness of portland cement concrete is generally placed over a 6-inch deep base course of Type 2 mineral aggregate crushed rock. For flexible pavements, a minimum 2-inch thickness of asphalt concrete pavement is placed over a designed thickness of asphalt concrete Class E pavement over a 6-inch depth of crushed rock.

Pavement Design: Default Design Parameters for New Pavement

Pavement design for local streets in Residential and Neighborhood Commercial Zones: may be constructed using 3 inches of asphalt concrete over 6 inches of crushed rock or 6 inches of portland cement concrete.

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 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.

Subgrade testing and analysis: 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 during the course of pavement thickness design.

Pavement widening: When pavement widening is required, the width of new pavement plus any existing pavement shall yield a total width equal to half the roadway design width shown in Chapter 3.1 Overview of Requirements from the Land Use Code on the project side of the centerline, plus a travel lane and provision for drainage on the other side of the centerline.

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 sidewalk and pavement are detailed in the Street and Sidewalk Pavement Opening and Restoration Rules.

Additionally, when new street improvements are required and the existing pavement is in poor condition, it shall be replaced or restored with the following requirements:

• When the existing pavement cross slope is less than 1% or greater than 4%, the existing pavement shall be adjusted as necessary to provide a cross slope that falls within these limits.

• On Portland cement concrete streets, when the existing concrete pavement depth is less than the depth of the designed pavement, and when the panels are in poor condition, the existing pavement shall be replaced. If the existing panels are in good condition they may remain, with new pavement installed to the design depth. Tie bars for longitudinal joints and/or load transfer dowels for transverse joints may be required between new and existing pavement, depending on the condition of the existing pavement. In most cases, tie bars and dowels shall be required between new concrete panels on arterials, commercial access streets, and residential access streets used by trucks and buses.
  
• Existing and proposed concrete panel joints shall be shown on street improvement plans. The extent of panel replacement required shall be guided by the Street and Sidewalk Pavement Opening and Restoration Rules. The extent of pavement replacement shall be depicted on street improvement plans by shading panels, or portions of panels, to be replaced.

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.