Project Description

Caltrans proposes to upgrade the structure over U.S. 101 at Tamalpais Drive OC by replacing the existing nonstandard pedestrian facilities with ADA compliant features. The Project also includes seismic structural improvements, intersection modifications, reconfiguration of the U.S. 101 on/off-ramps, and repairs and maintenance of the existing structure (Project).

The Project includes seven alternatives being considered for construction: the No-Build alternative and six build alternatives. All six build alternatives include removal of the existing, non-ADA compliant, spiral pathways and staircases on the south side of the structure, and construction of a new ADA pathway that provides east-west access for pedestrians across U.S. 101 at Tamalpais Drive OC. Each build alternative will also include intersection modifications, and reconfiguration of the U.S. 101 on/off-ramps at Tamalpais Drive.

Click Here for a 3D presentation of the Alternatives

Click on the maps of the Project alternatives below to see a larger version.

Alternative 2A Impact Map

This is an impact map for Alternative 2B

Alternative 2B Impact Map

Alternative 3A Impact Map

Alternative 3B Impact Map

Alternative 4A Impact Map

Alternative 4B Impact Map

Project Features

ADA Improvements

ADA improvements within the Project area primarily include removal of the existing spiral walkway ramps located on the south side of the structure, and construction of an ADA pathway over U.S. 101 at Tamalpais Drive OC. Proposed ADA structure types for each alternative are described further in Section 2.4 of the Initial Study.

The Project will also remove and/or reconstruct existing pedestrian facilities within the Project area to tie in with the new ADA structure. All pedestrian facilities constructed as part of this Project will be ADA compliant.

Seismic Structural Improvements
The 2014 Bridge Inspection Report for bridge No. 27-0072 (Tamalpais Drive OC) noted that the existing restrainer cables in a slab of the bridge should be reconstructed with a new system that can be fully inspected during routine investigations.
The Project proposes reconstruction by installing a cable restrainer with concrete seat extensions at Abutment 1, Bent 2 and Abutment 10.

Intersection Modifications

Each of the build alternatives include intersection modifications of one or both of the U.S. 101 interchanges, located east and west of the U.S. 101.

Intersection modifications will be designed consistent with ADA requirements for the Project.

U.S. 101 On/Off-ramp Reconfiguration

All build-alternatives propose to alter U.S. 101 access through reconfiguration of the on/off-ramps at Tamalpais Drive. Reconfiguration of the on/off-ramps varies by alternative but would include a portion of the following work: removal of existing on/off-ramp(s), altering on/off-ramp access by adding new roadway(s), widening of existing on/off-ramp.

Existing bus bypasses on the U.S. 101 on/off-ramps will also be reconfigured and include removal of one or both bypasses, with relocation of the bus bypass within the Project area.

Repair and Maintenance

Additional repairs and maintenance of the existing OC will include:
 Replacing access doors.
 Repairing spalled areas on the bridge deck soffit.
 Cleaning deck drains.
 Removing all ivy on concrete surface of structure.

Right of Way Requirements

Most work is anticipated to be within the existing right of way, except for two temporary construction easements (TCEs) on Casa Buena Drive and northwest of the southbound off-ramp for U.S. 101. Right of way for this Project is associated with alternatives 3A, 3B, 4A, and 4B.

All relocation services and benefits are administered without regard to race, color, national origin, persons with disabilities, religion, age, or sex. Appendix A includes Caltrans Title VI Policy Statement.

Project Alternatives

Alternative 2A:

Remove the two existing pedestrian spiral walkway ramps and construct two new pedestrian loop ramps with stairways.

• Construct a new pedestrian sidewalk along southbound U.S. 101 off-ramp from the intersection at the Tamalpais OC to the existing southbound bus stop at the bus bypass.

• Installation of Class II bike lanes (5 foot shoulders), painted green, on north and south sides of the OC.

• Realign the northbound U.S. 101 on-ramp to a signalized intersection on Tamalpais Drive OC. The existing northbound U.S. 101 diagonal on-ramp will be converted to a bus bypass.

• The existing bus bypass on northbound U.S. 101 off-ramp will be removed.

• Estimated cost is $14,366,000.

Alternative 2B:

• Remove the two existing pedestrian spiral walkway ramps and construct two new pedestrian loop ramps with stairways.

• Construct a new pedestrian sidewalk along southbound U.S. 101 off-ramp from the intersection at the Tamalpais Drive OC to the existing bus stop at the bus bypass.

• Installation of Class II bike lanes (5-foot shoulders), painted green, on north and south sides of the OC.

• The existing northbound U.S. 101 diagonal on-ramp will be removed and realigned to be controlled at a new signalized intersection at Tamalpais Drive OC. The realigned northbound on-ramp will include a new bus stop and bus pullout.

• The existing bus bypass at the northbound U.S. 101 off-ramp will be removed.

• Estimated cost is $14,584,000.

Structure Work Description - Alternatives 2A/2B:

The structure work for alternatives 2A and 2B is identical. It is proposed to construct two single loop pedestrian ramp structures on the south side of the existing bridge to replace the existing spiral loop structures. Each of the two loop structures will be approximately 490 feet in length and are comprised of six spans each. The maximum longitudinal gradient will be 5%. The superstructure is proposed to be a cast-in-place reinforced concrete sections. Each of the bents will be a single column with a pile cap and driven Class 90 piles. Type 7 chain link fence will be constructed on either side of the 8-foot-wide superstructure. The existing sidewalk on the south side of the structure will be removed and replaced with a minimum six-foot-wide sidewalk and will tie into the new loop ramps. It is further proposed to construct two cast-in-place reinforced concrete staircases within the vicinity of the existing spiral structures as an alternative to using the loop ramps. Additionally, the existing bridge cable restrainers will be removed and replaced with steel support brackets at the abutments and the in-span hinge locations. A total of eight steel brackets will be installed at each location.

Alternative 3A:
• Remove the two existing pedestrian spiral walkway ramps and construct two new pedestrian ramps on the north side of the existing structure.

• Bridge widening and a new pedestrian/bike sidewalk at the southwest intersection on Tamalpais Drive to Casa Buena Drive.

• Work at the southbound U.S. 101 off-ramp intersection includes ramp widening for a new bus stop and bus pullout.

• Reconfigure the northbound and southbound U.S. 101 on-ramps to include signalized intersections (remove existing on-ramps, except northbound loop).

• The new northbound on-ramp will be widened for a new bus bypass/stop.

• The southbound U.S. 101 loop on-ramp and connected bus bypass/stop will be removed

• The northbound U.S. 101 off-ramp bus bypass will also be removed.

• Estimated cost is $23,510,000.

Structure Work Description - Alternative 3A:

In this alternative, it is proposed to construct two pedestrian ramp structures on the north side of the existing bridge to replace the existing spiral loop structures. The west ramp structure will be 466 feet in length and comprised of six spans. The east ramp structure will be 450 feet in length and comprised of six spans. The maximum longitudinal gradient will be 5%. The superstructure is proposed to be a cast-in-place reinforced concrete voided slab sections. Each of the bents will be a single column with a pile cap and driven Class 90 piles. Type 7 chain link fence will be constructed on either side of the 15-foot-wide superstructure. The existing sidewalk on the south side of the bridge will be removed along with the existing circular pedestrian ramps. It is further proposed to widen the existing bridge with a 17.42-foot maximum width precast, prestressed box girder structure on the north side of the existing structure. This widening will be adjacent to the existing bridge but structurally separate. The widening will vary in width as it approaches the existing bridge Abutment 10 and transition into a reinforced concrete slab structure. The pedestrian ramp structures will tee into this widening structure with a short cantilever span and expansion joint. Additionally, the existing bridge cable restrainers will be removed and replaced with steel support brackets at the abutments and the in-span hinge locations. A total of eight steel brackets will be installed at each location.

Alternative 3B:
• Remove the two existing pedestrian spiral walkway ramps and construct a new pedestrian OC for bicyclists and pedestrians on the north side of the existing structure.

• Bridge widening and a new pedestrian/bike sidewalk at the southwest quadrant of the intersection on Tamalpais Drive to Casa Buena Drive.

• Work at the southbound U.S. 101 off-ramp intersection includes ramp widening for a new bus stop and bus pullout.

• Reconfigure the northbound and southbound U.S. 101 on-ramps to include signalized intersections (remove existing on-ramps, except northbound loop).

• The new northbound on-ramp will be widened for a new bus bypass/stop.

• The southbound 101 loop on-ramp and connected bus bypass/stop will be removed.

• The northbound U.S. 101 off-ramp bus bypass/stop will also be removed.

• Estimated cost is $21,450,000

Structure Work Description - Alternative 3B:
In this alternative, it is proposed to construct a new single pedestrian OC structure to the north of the existing bridge that will tie-in near the existing abutments. This new structure will be 1,155 feet in length and 18 feet in width. The superstructure is comprised of both cast-in-place prestressed box girder spans and reinforced concrete slab spans. There will be a total of nine spans with closed bin-type structures on either end. The maximum longitudinal gradient will be 5%. Each of the bents will be a single column with a pile cap and driven Class 90 piles. The bin-type structures at the ends will have 18-inch reinforced concrete walls on narrow pile caps with a single row of Class 90 driven concrete piles. Type 7 chain link fence will be constructed on either side of the superstructure. The existing sidewalk on the south side of the bridge will be removed along with the existing pedestrian circular ramps. The existing structure will have a sliver widening at the northwest abutment location. This widening will be 5-feet-wide and will vary to match the existing reinforced concrete slab superstructure. The existing barrier rail at this location will be replaced with a Type 742 concrete barrier with Type 7 chain link fence mounted on top. Additionally, the existing bridge cable restrainers will be removed and replaced with steel support brackets at the abutments and the in-span hinge locations. A total of eight steel brackets will be installed at each location.

Alternative 4A:

• Remove the two existing pedestrian spiral walkway ramps and construct two new pedestrian ramps on the north side of the existing structure.

• Bridge widening and a new pedestrian/bike sidewalk at the southwest intersection on Tamalpais Drive to Casa Buena Drive.

• Work at the southbound U.S. 101 off-ramp intersection includes ramp widening for a new bus stop and bus pullout.

• Reconfigure the northbound and southbound U.S. 101 on-ramps to include signalized intersections.

• The new northbound on-ramp will include a new bus stop and bus pullout.

• The northbound and southbound U.S. 101 loop on-ramps and associated bus bypasses/stops will be removed.

• Estimated cost is $23,827,000.

Structure Work Description - Alternative 4A:

This alternative is identical to build alternative 3A except that it also removes the existing eastbound Tamalpais Drive to U.S. 101 northbound loop on-ramp.

Alternative 4B:

• Remove the two existing pedestrian spiral walkway ramps and construct two new pedestrian ramps.

• Bridge widening and a new pedestrian/bike sidewalk at the southwest intersection on Tamalpais Drive to Casa Buena Drive.

• Work at the southbound U.S. 101 off-ramp intersection includes ramp widening for a new bus stop and bus pullout. This option will reconfigure the northbound and southbound U.S. 101 on-ramps, and northbound and southbound U.S. 101 loop-ramps to signalized intersections.

• All existing bus bypasses/stops will be removed with this option.

• Estimated cost is $25,252,000.

Structure Work Description - Alternative 4B:

This alternative is similar to alternative 4A with the addition of a new southbound U.S. 101 on-ramp bridge and a new northbound U.S. 101 on-ramp bridge. These on-ramp bridges will be cast-in-place reinforced concrete slab superstructures and form a sort of side-hill viaduct with one side of the structure being cast at-grade. The northbound U.S. 101 on-ramp will be 245 feet long. The southbound U.S. 101 on-ramp will be 270 feet long. The width will be approximately 26 feet long and each bent will be comprised of 5 columns (Class 90 pile extensions). Type 742 concrete barriers will be placed on one side of each on-ramp bridge. As the on-ramp elevations touch down to existing ground elevation, the structure will transition to a reinforced concrete cantilever retaining wall on one side of the ramp only. This retaining wall will have a reinforced concrete pile cap foundation on Class 90 driven piles.