Retrofitting Hostile Vehicle Mitigation into Existing Streetscapes

The Foundation Depth Problem in Retrofit Schemes

Conventional PAS 68 and IWA 14 rated bollards were historically developed with foundation depths of 800mm-1200mm or more, depending on the vehicle mass and speed rating being tested against.

In an existing streetscape this creates several practical obstacles:

• Utility strikes: Existing footways and public realm areas are typically dense with gas, water, electricity, telecoms, and drainage infrastructure, often at depths of 300-600mm. A deep foundation dramatically increases the risk of utility conflicts, which adds cost, delay, and diversion works.
• Existing structural buildups: Retrofitting into a footway with an existing sub-base, ducting, or basement/vault below can make deep excavation impractical or impossible without structural engineering input.
• Heritage and conservation constraints: Historic paving, listed streetscapes, and conservation areas often restrict how much of the existing surface can be broken out and reinstated.
• Programme and disruption: Deep excavation in a live pedestrian or retail environment extends the works programme and the disruption to trading, footfall, and traffic management.

Shallow mount HVM products are designed specifically to address this gap, typically requiring a foundation or recess depth of around 100mm rather than the 800mm+ associated with deep-mount systems.

How Shallow Mount Products Still Achieve a PAS 68 Rating

A reasonable question when specifying retrofit HVM is how a shallow foundation can achieve the same impact performance as a much deeper one. The answer lies in how the product distributes and dissipates impact energy.

Shallow mount PAS 68 rated bollards, blocks, and planters typically achieve their rating through one or a combination of the following engineering approaches:

• Wide baseplate or footprint design: Rather than relying on depth to resist overturning moment, the product spreads the load across a wider surface area at, or close to, ground level.
• Controlled deceleration/energy absorption: Some systems are designed to allow a degree of controlled movement or deformation, absorbing the vehicle's kinetic energy over the impact event rather than arresting it instantly.
• Product mass and ballast: Concrete blocks and planters in particular can use mass at surface level to resist displacement, rather than depending on a buried foundation.

It's worth noting that PAS 68 and its successor testing framework under ISO 22343 do not specify a required foundation depth. A product either passes the physical impact test at a given vehicle mass, speed, and angle, or it doesn't. Foundation depth is simply how the manufacturer has engineered the product to pass. This is why shallow mount and deep mount products can carry the same PAS 68 rating despite very different below-ground requirements.

When specifying, it's important to check the specific test certificate for the exact foundation depth and specification that was used during the accredited test, since this is what must be replicated on site for the rating to be valid.

Shallow Mount Product Types for Retrofit Schemes

Security Bollards

Shallow mount bollards are typically the first product considered for retrofit schemes because they offer a familiar visual language to existing streetscape furniture and can be spaced to match sightlines, pedestrian routes, and existing bollard rhythms. For retrofit work, bollards with a 100mm foundation depth are particularly relevant on footways with limited depth available above existing services.

PAS 68 Rated CT Blocks

Concrete security blocks are often used where a continuous line of protection is needed, such as along a building frontage, event perimeter, or pedestrianised zone. Shallow mount blocks are commonly specified for retrofit schemes, including event security and phased masterplan delivery, since they can be installed much more easily than a deep-mount system.

HVM Planters

Planters rated to PAS 68 combine the security function with a softer, greener streetscape appearance, which is often preferred in retrofit schemes where the objective is to protect a space without it visually reading as "security infrastructure." Shallow mount planters are especially relevant in town centre and public realm retrofit projects where landscaping and place-making objectives sit alongside the counter-terrorism or vehicle mitigation requirement.

Retrofit Installation Considerations

Site Survey and Utility Mapping

Before specifying any retrofit HVM product, a detailed utility survey should be carried out to confirm what is buried beneath the proposed line of protection. Even at a 100mm recess depth, this step should not be skipped, as shallow services and ducting can still be present close to the surface.

Existing Surface Reinstatement

Retrofit schemes need to account for how the surrounding paving, kerb lines, or surface finish will be reinstated once the shallow foundation is installed. This is particularly relevant in conservation areas or where matching historic paving materials (e.g., natural stone setts or flags) is a planning requirement.

Vehicle Dynamics Assessment (VDA)

Before selecting a specific product and rating, a Vehicle Dynamics Assessment should be carried out to establish the realistic threat vehicle, approach speed, and angle of attack for the specific retrofit location. This determines which PAS 68 or ISO 22343 rating is required, rather than defaulting to the highest available rating, which can add unnecessary cost and increase the visual and physical footprint of the scheme.

Integration with Existing Streetscape Elements

Consideration needs to be given to how new shallow mount products integrate with:

• Existing street lighting columns and sightlines
• Existing drainage falls and gully positions
• Pedestrian desire lines and accessibility requirements (including DDA/inclusive access)
• Cycle lanes and active travel routes running alongside the protected area

Procurement and Regulatory Context

Retrofit HVM schemes delivered by public sector bodies will typically need to be procured in line with the Procurement Act 2023, and in many town centre and public realm contexts, the requirement for HVM at all is increasingly driven by the Terrorism (Protection of Premises) Act 2025 (commonly referred to as Martyn's Law).

Since procurement routes and the specific thresholds and duties under Martyn's Law are subject to ongoing guidance updates, it's worth confirming current requirements against the latest government guidance rather than relying on a fixed reference point, given how recently this legislation has come into force.