HomeMy WebLinkAboutStaff Report 2603-6052CITY OF PALO ALTO
Rail Committee
Regular Meeting
Tuesday, March 10, 2026
2:30 PM
Agenda Item
1.Review structural and design considerations for the Earthen Berm (retaining walls) and
long-bridge (Podium) options of the Meadow Drive Grade Separation Hybrid Alternative.
Staff Presentation
Rail Committee
Staff Report
From: City Manager
Report Type: STUDY SESSION
Lead Department: Transportation
Meeting Date: March 10, 2026
Report #:2603-6052
TITLE
Review structural and design considerations for the Earthen Berm (retaining walls) and long-
bridge (Podium) options of the Meadow Drive Grade Separation Hybrid Alternative.
RECOMMENDATION
Review and provide feedback to staff on structural and design considerations for the Meadow
Drive Hybrid Earthen Berm (retaining walls) and long bridge (Podium).
EXECUTIVE SUMMARY
On December 15, 2025, City Council directed staff to explore an Earthen Berm and Podium-style
Hybrid alternative for Meadow Drive grade separation. Caltrain evaluated these structural
variants and considered various components including structural systems, construction
challenges, maintenance requirements, implications for future Caltrain operations, and
qualitative cost factors for each alternative. This study session presents an analysis and seeks
feedback from the Rail Committee in advancing the design of alternatives.
BACKGROUND
The City of Palo Alto is pursuing a grade separation project at three existing at-grade crossings at
Charleston Road, Meadow Drive and Churchill Avenue. Caltrain is leading the current Preliminary
Engineering and Environmental Documentation phase of the project, with the City as project
sponsor. Preliminary Engineering Conceptual Refinement (15% Design) is anticipated to progress
through Summer-2026, followed by Preliminary Engineering (35% Design) and Environmental
Documentation under the California Environmental Quality Act (CEQA) and the National
Environmental Policy Act (NEPA), which is anticipated to conclude in Fall 2027.
On September 16, 2025, Rail Committee reviewed refined concepts that had been developed to
minimize private property impacts while maintaining and/or improving the traffic circulation and
enhancing pedestrian and bicycle crossings. On December 15, 2025, City Council (Staff Report
2506-4895)1 directed the project team to continue to advance the following alternatives:
Churchill Avenue Partial Underpass Alternative:
o Partial Underpass at Churchill without a landscaping strip
o Pedestrian/Bicycle Undercrossing at Seale Avenue using Alma Street ramp and
with attention to bicycle safety on the east side
Meadow Drive Hybrid Alternative
o Explore both an earthen berm and podium-style Hybrid alternative
Charleston Road Underpass alternative
o Underpass with Direct Access Ramp
ANALYSIS
Foundations: An Earthen Beam system requires MSE embankment walls that support the
soil and a short bridge segment that transfers load through supporting elements to deep
pile foundations and the soil. By comparison, a Podium system would require a concrete
superstructure that is generally more complex.
Settlement: Earthen Berms are subject to short- and long-term settlement, with potential
differential settlement across various segments or between the embankment and bridge
1 City Council, December 15, 2025; Item 26, Action Item, SR# 2506-4895
https://recordsportal.paloalto.gov/WebLink/DocView.aspx?id=84076&dbid=0&repo=PaloAlto
abutments. Therefore, to maintain rail operations, track performance monitoring is needed, and
tamping and re-leveling of track could be expected. For the Podium system, minimal total and
differential settlement is expected.
Construction Impacts: The construction sequence for Earthen Berm requires a relatively
large footprint compared to other retaining systems. By comparison, the Podium variant
generally provides for better constructability in constrained corridors, especially when
utilities are present. Transition zones for the Podium system require special detailing and
enhanced maintenance.
Future widening: Widening of an Earthen Berm system is less challenging and less
expensive than a Podium. In addition, the Earthen Berm system is more adaptable to
accommodating additional tracks, signal upgrades and electrification upgrades. By
comparison, widening of a Podium system is more challenging and more expensive.
Costs: Upfront and lifecycle costs are expected to be lower for an Earthen Berm compared
to a Podium system. Qualitative structural cost implications of each system are listed
below.
Table 1: Structural Cost Implications for Earthen Berm and Podium Variants
Aspect MSE Walls (Earthen Berm)Long Bridge (Podium)
FISCAL/RESOURCE IMPACT
3
3 City Council, December 9, 2024; Item 12, Consent Item, SR# 2408-3322
https://recordsportal.paloalto.gov/WebLink/DocView.aspx?id=6521&dbid=0&repo=PaloAlto
24000 (Meadow Drive and Charleston Road) and PL-24001 (Churchill Avenue Rail Grade
Separation and Safety Improvements). The secured $20 million is programmed to cover the costs
of the current 15% design phase as well as the subsequent 35% Preliminary Engineering and
Environmental Documentation.
STAKEHOLDER ENGAGEMENT
ENVIRONMENTAL REVIEW
ATTACHMENTS
APPROVED BY:
Caltrain Palo Alto Grade Separation Project
Memorandum: Evaluation of Structural Alternatives for
Meadow Drive/Alma Street Grade Separation
Date: 03/04/2026
1
Contents
List of Figures ............................................................................................................... 1
List of Tables ................................................................................................................. 1
1. Introduction ........................................................................................................... 2
2. Project Context and Requirements .......................................................................... 2
3. Structural Options .................................................................................................. 3
3.1. Alternative 1: “Earthen Berm” ........................................................................... 3
3.2. Alternative 2: “Long Bridge (podium)” ............................................................... 6
4. Structural Cost Implications (Qualitative) ................................................................ 8
List of Figures
Figure 1 - Existing site exploration at Meadow Drive ......................................................... 3
Figure 2 – 3D view of Caltrain corridor at Meadow Drive .................................................... 4
Figure 3 – Elevation view of railroad bridge at Meadow Drive ............................................. 4
Figure 4 – Elevation view of Earthen berm option ............................................................. 5
Figure 5 – Elevation view of long bridge (podium) ............................................................. 7
List of Tables
Table 1 – Qualitative cost implications ............................................................................ 8
2
1. Introduction
This memorandum provides a summarized technical evaluation of the two structural
alternatives for the hybrid underpass at Meadow Drive and Alma Street in Caltrain Palo Alto
Grade Separation (PAGS) Project. The analysis considers structural systems, construction
challenges, maintenance requirements, implications for future Caltrain operations, and
qualitative cost factors for each alternative.
For this evaluation, it is assumed that the removal and reinstallation of track systems and
traction power components will have comparable impacts for both alternatives.
2. Project Context and Requirements
The project involves raising the existing Caltrain track profile 16ft (max) over a distance of
approximately 6,400-ft, from Greenmeadow Way to Loma Verde Avenue. All existing
components of electrified tracks including Overhead Contact System (OCS), fiber lines and
other utilities, and power housing will be removed and reinstalled at a higher elevation to
maintain track operation. The design will need to provide flexibility for future widening of the
elevated segment within the Caltrain Right-of-way (ROW) to accommodate future Caltrain
operation and maintenance.
Approximately 7 ft of excavation is anticipated at the roadway crossings along Meadow Drive
to provide sufficient vertical clearance for the undercrossing and to connect east and west
sides of Caltrain corridor for the use of vehicles, pedestrians, and cyclists.
Although geotechnical study is still in progress, the preliminary indications suggest generally
favorable subsurface conditions, with no critical hazards identified, such as loose soils,
shallow groundwater table, liquefaction risk, or fault rupture within the footprint of the
structure. However, these conditions are to be further confirmed as additional geotechnical
information is available.
Existing underground utilities - including storm drain, sanitary sewer, water line, gas line,
fiber optic lines, and electrical lines – are present along both sides of the corridor.
Two structural alternatives are under consideration:
1- Short-span bridge over Meadow Drive with embankments enclosed by MSE walls,
referred to as the “Earthen berm”.
2- Continuous long bridge (podium) supporting the raised track profile.
3
This report focuses primarily on the structural evaluation of the alternatives, while also
considering construction logistics, durability, and cost implications as they relate to
structural design.
Figure 1 - Existing site exploration at Meadow Drive
3. Structural Options
3.1. Alternative 1: “Earthen Berm”
The Earthen Berm system consists of a reinforced soil embankment that supports the longer
portion of the raised track, along with a short, three-span bridge over the Meadow Drive
undercrossing to provide 16 ft–6 in minimum vertical clearance under the new railroad
bridge. The project involves raising the existing Caltrain track profile 16 ft (max) for a distance
of approximately 6,400 ft to eliminate at-grade crossings and improve mobility and safety.
The addition of soil mass at Caltrain corridor requires evaluation of extra excavation in the
next design phases.
4
Figure 2 – 3D view of Caltrain corridor at Meadow Drive
Figure 3 – Elevation view of railroad bridge at Meadow Drive
5
Figure 4 – Elevation view of Earthen berm option
For the embankment segment, the train loads would be transferred to the tracks and to the
embankment fill, MSE reinforcements, and supporting soil. For the short bridge segment,
the loads would be transferred through the middle bents and abutments (supporting
elements) to the deep pile foundations and the soil.
Two structural behaviors are utilized (1) the flexible embankment-supported tracks, (2) the
rigid bridge superstructure at Meadow Drive.
MSE wall embankment is typically compacted on native subgrade with facial panels
supported on shallow foundations; however, the bridge substructure will require deep
foundations, particularly if differential settlement tolerance is limited.
As mentioned in the previous section, MSE embankments are subject to short-term and
long-term settlement; and potential differential settlement at various segments of corridor
or between embankment and bridge abutments can be expected. Therefore, to maintain rail
operations and ensure track performance, tamping and re-leveling of track is expected. In
addition, transition zone performance should be monitored closely as approach slabs are in
the highest maintenance zone.
MSE walls perform well under seismic loading when properly designed; however, post-
earthquake serviceability may require embankment regrading or track adjustment. Track
operation is sensitive to rail alignment changes. Any potential soil mass movements under
the design earthquake may disrupt the operation of tracks.
The construction sequence of MSE wall embankment inherently requires a relatively large
footprint compared to many other retaining systems. In typical practice, the reinforcement
length ranges from approximately 0.7H to 1.0H (where H is the wall height), resulting in a
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substantial horizontal zone of influence. This reinforced soil mass must be constructed and
compacted in layers, generally in 8- to 12-inch loose lifts, in accordance with project
specifications and standard compaction requirements (often 90–95% of maximum dry
density per ASTM D1557).
Because MSE wall construction progresses from the bottom up, each lift of structural
backfill requires placement, spreading, moisture conditioning, and compaction using
vibratory rollers or plate compactors. The operation of this heavy compaction equipment
induces dynamic loads and vibrations that may adversely affect nearby utilities, particularly
the aging shallow-buried pipelines or conduits.
Drainage performance is a critical design consideration for the long-term reliability of the
MSE wall system and should be carefully evaluated during both design and construction and
closely monitored after completion. The reinforcement corrosion protection is critical for
long-term performance and is closely tied to the effectiveness of the drainage system.
Internal drainage elements must be designed to minimize clogging risk and mai ntain long-
term permeability, while surface water management should prevent ponding behind panels
that could increase hydrostatic pressure and accelerate corrosion of reinforcements. The
system should also provide accessible inspection points for drainage outlets and collection
features to support routine maintenance. Ongoing post-construction monitoring of drainage
behavior is essential to confirm proper function and to identify early signs of blockage or
distress before they affect structural performance.
The widening of this system is less challenging and less expensive compared to next
alternative as the widening would happen within the isolated Caltrain’s ROW. In addition,
this system is greatly adaptable in accommodating additional tracks, signal upgrades, and
electrifications upgrades.
The track electrification components shall be removed and reinstalled in this alternative.
3.2. Alternative 2: “Long Bridge (podium)”
Under the Long Bridge system, approximately 490 ft of the raised track (5 spans) may be
constructed on a continuous, long bridge (podium). The superstructure may consist of
concrete systems such as a cast-in-place slab on precast/prestressed concrete girders,
precast/prestressed side-by-side box girders, or cast-in-place/post-tensioned concrete box
girders, or equivalent steel girder alternatives that provide similar structural depth and
constructability. More complex structural systems - such as steel through-girders, truss
bridges, or architecturally constrained configurations - are discouraged because they
complicate future widening, track additions, and corridor expansion. Simpler girder-type
7
systems provide greater flexibility for staged construction and long-term modifications
within the Caltrain right-of-way.
Figure 5 – Elevation view of long bridge (podium)
The track loads would be transferred to the superstructure, bent caps, columns, and Cast-
In-Drilled-Hole (CIDH) pile foundations, and soil. This configuration offers a uniform
structural system with repetitive spans and consistent stiffness in the vertical and lateral
directions.
This system requires multiple columns and deep foundations (CIDH piles) at each bent.
Long bridge structures experience minimal total settlement and are less likely to experience
differential settlement affecting track geometry.
The seismic forces are carried through well-defined structural components (Superstructure,
bearings, columns, and foundations). The displacement-based seismic design can be
effectively implemented; and an easier post-event inspection and repair of discrete
structural elements can be carried out. In general, long bridges provide predictable seismic
behavior and clearer inspection and repair pathways. In contrast, to provide uniform
stiffness along the frames and reduce the likelihood of out-of-phase behavior of adjacent
frames, the need for isolation casing to extend the effective length of columns and provide
uniform mass to stiffness ratio along the long bridge makes the design and maintenance of
the bridges more complex.
Long bridge (podium) system generally provides better constructability in constrained
corridors, especially with utilities present. The transition zones require special detailing and
enhanced maintenance attention as mentioned for previous alternative. Higher likelihood of
long-term track settlement and geometry corrections exists. This system reduces ballast
maintenance and provides superior ride quality and long-term serviceability. By employing
8
direct fixation track, the ballast and its associated weight can be eliminated, further reducing
maintenance needs and allowing for a more compact, durable track structure. This
approach also improves track geometry stability and alignment over time, enhancing overall
operational reliability. Eliminating the ballast would significantly reduce the demands on all
components under service, strength, and seismic limit states.
The reinforced concrete bridge components have well-understood durability
characteristics, and individual components (bearings, joints) can be replaced. Longer
design life with predictable inspection and maintenance regimes is expected for this
alternative. Thus, this alternative offers lower long-term risk despite higher initial complexity.
The widening of this system is more challenging and more expensive compared to previous
alternative.
The track electrification components shall be removed and reinstalled in this alternative.
4. Structural Cost Implications (Qualitative)
The following table summarizes the qualitative structural cost implications for the two
alternatives under consideration: an earthen berm with MSE walls and a long bridge
(podium). This high-level assessment provides a framework for understanding how each
alternative may impact both upfront investment and lifecycle performance. The qualitative
structural cost implications are listed below.
Table 1 – Qualitative cost implications
Aspect Earthen berm Long bridge
(podium)
Initial structural cost Lower Higher
Foundation cost Lower Higher
Future Utility relocation risk Higher Lower
Long-term maintenance Lower Higher
Seismic retrofit flexibility Limited Higher
City of Palo Alto
Grade Separation Project
Churchill Avenue, Meadow Drive,
and Charleston Road
Rail Committee Meeting
March 10, 2026
City Council Direction and Project Timeline1
2
Meadow Drive: Hybrid Crossing Requirements2
Meeting Purpose: Review Considerations for Meadow Drive: Hybrid Options
Agenda
Retaining Wall (Earthen Berm) Option3
Long Bridge (Podium) Option4
Option Considerations5
City Council Alternatives Selected for 15% Design
3
ALTERNATIVE
CROSSING Meadow Drive Charleston Road
HYBRID UNDERPASSPARTIAL UNDERPASS
Churchill Avenue
Retaining Walls
(Earthen Berm)
Long Bridge
(Podium)
OPTIONS
Direct Access
Ramp
Vehicle Crossing
(without Landscape Strip)
Bike/Ped Crossing
(Alma Street Ramp)
Implementation study if
only Charleston Road
Underpass is constructed
Current Project Phase
4
2025 2026 2027
Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
Preliminary Engineering and Environmental
35% Design and Environmental 15% DesignConcept Development
and Refinement
City Council Direction #2:
Refine LPA selection to
advance to 35% Design and
Environmental Documentation
City Council Direction #1:
Identify alternatives to
advance to 15% Design
Final 35% Design and
Environmental Documentation
#1 #2
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~4,500 ft of Track Raised >1 ft
(of 6,400 ft of Track Modification)
Meadow Drive: Hybrid Crossing Requirements
5
Tracks raised 16 ft maximum
above existing ground
Meadow Dr lowered 7 ft
below existing groundImage for illustrative purposes – not to scale
16.5-ft clearance between
Meadow Drive and Caltrain Bridge
Bridge Width
55-ft minimum
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Meadow Drive Bridge Section
6
•Bridge Span: 125 ft
•Retaining Wall: 1,500 ft north
and south of bridge
•Bridge + Wall = 3,000 ft total
•Wall Height (approximate)
•Minimum: 3 ft
•Average: 10 ft
•Maximum: 17 ft
Retaining Wall (Earthen Berm) Option
7
Retaining Wall (Earthen Berm) Aerial View
8
ALMA ST
PARK BLVDPARK BLVD
ALMA ST
Bridge Extents
55 ft
125 ft
Retaining Wall
Extents
Retaining Wall
Extents
Retaining Wall (Earthen Berm) Section
9
Retaining Wall (Earthen Berm) Section
10
Retaining Wall (Earthen Berm) Examples
11
Railroad Tracks Parallel to Old County Road
San Carlos, CA
I-5 Parallel to Multi-use Path
Solana Beach, CA
•Bridge Span: 475 ft
•Retaining Wall: ~1,250 ft north and south of bridge
•Bridge + Wall = 3,000 ft total
•Clearance under bridge:
•8 ft at bridge abutments to 16.5 ft at Meadow Drive
•Ground under bridge is flush with surrounding ground
•Partially-lowered Alma St to east
•Existing backyards to west
Long Bridge (Podium) Option
12
Long Bridge (Podium) Aerial View
13
ALMA ST
PARK BLVDPARK BLVD
ALMA ST
Bridge Extents
55 ft
500 ft
Retaining Wall
Extents
Retaining Wall
Extents
Long Bridge (Podium) Section
14
Long Bridge (Podium) Section
15
Long Bridge (Podium) Examples
16
Railroad Tracks over Friars Road
San Diego, CA Railroad Tracks over Ralston Avenue
Belmont, CA
Retaining Wall (Earthen Berm)
•Less expensive
•Less time to construct
•Easier to widen in the future
Long Bridge (Podium)
•More expensive
•More time to construct
•More challenging to widen in the future
Construction Considerations
17
Retaining Wall (Earthen Berm)
•Less expensive due to minimal maintenance
•Regular track tamping and re-leveling
•Minimal fatigue due to earth pressure buffer
•Weather exposure on one side
Long Bridge (Podium)
•More expensive due to:
•Varied components
•More specialized repairs
•Higher labor and material costs
•Regular inspection of structural elements required
•Fatigue due to repeated dynamic loading (concrete cracking/spalling, joint/bearing degradation, steel corrosion)
•Weather exposure on all sides
•Specialized access equipment needed for inspections
Maintenance Considerations
18
Retaining Wall (Earthen Berm)
•Textured or treated wall facades
•Integrated planting and green
screening
•Potential mural or public art
treatments
Long Bridge (Podium)
•Landscape and integrated lighting
•Plantings only where sufficient daylight
•Architectural treatment of columns
Aesthetic Treatment Opportunities
19
Retaining Wall (Earthen Berm)Long Bridge (Podium)
Length 125 ft 500 ft
Height Wall Height: 3 ft to 17 ft
(10 ft average)Clearance: 8 ft to 16.5 ft
Construction Costs
& Duration Lower Higher
Maintenance Costs Lower Higher
Aesthetics
& Visual Impacts
Potential wall treatment options Shadows
Light and Air can pass under bridge Fencing and lighting may be needed
for Safety & Security
Future Corridor
Expansion Easier Harder
Considerations Summary
20
Rail Committee and City Council Engagement
Tasks Mar Apr May Jun Jul Aug Sep
15% Plan
Development
Construction Cost
and Schedule
Development
Rail Committee Meadow Drive
Options
15% Design/
35% Scope
No Rail
Committee
Special
Meeting:
15% Designs/
ICE & Schedule
City Council No Council
Direction for
35% Design/
Environmental
Documentation
21
Retaining Wall (Earthen Berm) Section
22
Long Bridge (Podium) Section
23