A tunnel is not a flat panel.
The geometry changes everything.
Underwater tunnel acrylic panels — curved PMMA geometry, segment joints, installation sequence, and thickness range 150–400mm.
An underwater tunnel looks like a series of panels curved into an arch. It is actually a system — where panel geometry, joint design, installation sequence, and civil structure must all be resolved together before any fabrication begins. This note explains what that means in practice.
Tunnel joints need different preparation — and it matters before the panel goes in
Jointing between tunnel panels is not the same as jointing on a pool wall or a fish tank.
Before the sealant goes in, the acrylic surface needs specific preparation — the process is different from a standard installation, and if it isn't done correctly, the joint won't perform under sustained water pressure. This is one of those steps that looks minor in a specification document and becomes a site problem if it's skipped or done out of sequence.
We walk through the joint preparation requirements before fabrication is confirmed, not after the panels arrive on site.
What changes when you go from a flat panel to a curved tunnel segment.
Flat panels have two structural inputs and one perimeter interface to design. Tunnel segments have four structural inputs, a joint at every edge, and an installation sequence that is itself a structural constraint.
Two inputs, one interface
Thickness determined by span and depth. One perimeter interface to design. Installation is sequential and largely reversible if something is wrong.
Four inputs, joints everywhere
Thickness governed by radius, depth, arc span, and safety factor. A joint at every panel edge. Installation sequence is a structural constraint — not a logistics choice.
Sequence determines sealing
In a flat panel installation, sequence is about access. In a tunnel, sequence determines whether each joint can be physically reached to be sealed. Get it wrong and panels must be removed.
The four inputs that govern tunnel panel thickness and forming geometry.
| Parameter | What it means | Why it matters |
|---|---|---|
| Internal radius | Internal radius of the tunnel cross-section | Governs curvature of each segment — smaller radius limits maximum panel arc width and increases forming complexity |
| Water depth to crown | Depth from water surface to top of the tunnel arch | Crown carries the highest pressure load in a fully submerged tunnel — governs the upper segment thickness |
| Arc span between joints | Arc length of each segment between joint centre lines | Shorter segments reduce bending demand but increase joint count — each joint is a potential leak point |
| Safety factor | Design margin on characteristic PMMA tensile strength | Public walk-through tunnels typically require 3.5–4.5 — higher than residential pool wall applications |
Indicative only. Curved geometry affects structural behaviour — final thickness requires project-specific review for each tunnel configuration.
Every joint is a sealing problem. Resolve it in the drawing, not on site.
In a flat panel installation, there is typically one perimeter seal to design. In a tunnel, there is a joint at every segment edge — and at the base corner, where curved side panels meet the flat floor panel, the geometry is the most demanding on the whole project.
Rebate geometry at each joint
- Rebate depth and width at each joint line
- Panel edge clearance and spring-back allowance from forming
- Rebate in the civil ring — must be cast to acrylic panel tolerance
Compression seal continuity
- Seal type and compression across the arc
- Corner seal continuity at panel transitions — crown, haunch, and base
- Access for seal replacement after flooding — not guaranteed once assembled
Base corner — the hardest joint
- Where curved side panels meet the flat floor — geometry changes at this transition
- Requires a custom transition detail — not handleable with a standard rebate and seal
- Most common source of tunnel leaks when not designed explicitly in advance
The sequence is a structural decision — not a site logistics decision.
Each panel must be placed in a specific order so that the joint to the previous panel can be sealed before the next panel closes access. This sequence must be confirmed before fabrication — not worked out by the installer on site.
Sequence confirmed in the fabrication package
Installation sequence is documented and approved before panels are fabricated. Panel numbering, delivery order, and joint sealing access are all confirmed. The installer follows a defined sequence — no improvisation required on site.
Sequence resolved on site
Installer places panels in a logical order that is not the sealing order. A joint becomes inaccessible before it is sealed. The blocking panel must be removed — the sealant joint is damaged in removal, or the panel is. Rework at installation stage is the most expensive rework in a tunnel project.
Three parties who must be in the same design conversation before fabrication begins.
Acrylic fabricator
Provides panel dimensions, forming radius tolerance, edge rebate requirements, and installation sequence. Must know structural ring geometry before machining begins — the panels are machined to fit the ring, not the ring built to fit whatever arrives.
Civil / structural engineer
Designs the bearing ring that supports the panels. Ring geometry and surface tolerance must accommodate acrylic panel edges and rebate. Ring construction precedes panel installation — errors in the ring cannot be corrected after it is cast.
Installation contractor
Executes the confirmed installation sequence. Must have panel delivery schedule, lifting plan, and joint sealing access confirmed before work begins. Site improvisation in tunnel installation is a project risk, not a problem-solving opportunity.
Underwater tunnel — questions that come up on every project
How thick are acrylic underwater tunnel panels?
Typically 150–400mm depending on tunnel radius, water depth above crown, arc span between joints, and safety factor. Curved geometry provides structural efficiency — reducing required thickness compared to an equivalent flat panel under the same pressure. Final thickness requires project-specific calculation for each tunnel configuration.
Why is an underwater tunnel more complex than a flat viewing panel?
Each panel must be placed so that the joint between it and the previous panel can be sealed before the next panel blocks access to that joint. If the sequence is wrong, a joint becomes inaccessible — and the blocking panel must be removed to complete sealing. Removal risks damage to the sealant or the panel. The sequence must be confirmed before fabrication, not worked out on site.
What is the most critical step in underwater tunnel installation?
Confirming the installation sequence before fabrication begins. Each panel must be placed in a specific order so that the joint to the previous panel can be sealed before the next panel closes access. If a joint becomes inaccessible, the blocking panel must be removed to complete sealing — removal risks damage to the sealant or the panel itself. Sequence is a structural decision, not a site logistics choice.
How many segments does an acrylic underwater tunnel typically have?
A typical walk-through tunnel uses 6–12 curved segments per ring cross-section plus a flat base panel. Segment count depends on tunnel diameter, maximum panel arc width from the forming process, and access requirements for joint sealing during installation.
When is a tunnel project ready for technical evaluation?
When tunnel internal radius, water depth to crown, indicative segment count or arc span, and structural ring material are confirmed. A cross-section drawing — even at sketch level — is the single most useful input for technical evaluation of a tunnel project.
Three engineering problems. Three completed tunnels.
Each tunnel configuration presents a different geometry challenge. These completed projects illustrate how segment count, radius, and joint design vary across tunnel types — and why each requires a separate engineering resolution before fabrication begins.
PG Acrylic has delivered walk-through acrylic tunnel segments up to 25.8 m — including the Acuario Nautilus tunnel in Peru (70 mm curved cast-PMMA panels, 800 mm water depth). For integrator Forma Fabrica (Gdańsk, Poland), PG supplied a 20.4 m tunnel build in 100 mm cast PMMA. A 6.5 m tunnel section was delivered for an aquarium project in India.
Read the 25.8m Nautilus tunnel project review →
Which configuration applies to your project? Internal radius, water depth to crown, and indicative tunnel length are the three inputs that determine which geometry constraints apply — and what needs to be resolved before fabrication can begin.
View all installed projects →Material and quality documentation for tunnel-grade PMMA.
Tunnel projects involve multiple engineering parties. The following test reports and certificates are available for download to support your procurement and review process.
Related notes
Thickness range and what we need from you.
Typical thickness range
150 – 400mm
One-time cast PMMA curved segments. Radius, tunnel length, and crown-to-waterline distance all affect thickness and forming geometry.
What to send us
Tunnel panels require three inputs that flat panels don't need: radius, total length, and the distance from tunnel crown to water surface. Without these, segment geometry and thickness cannot be calculated.
- Tunnel internal radius
- Total tunnel length
- Distance from tunnel crown to water surface
- Segment joint type and spacing
- Civil structure drawings if available
Have a underwater tunnel project?
Send us your dimensions, water depth, and installation type. We'll confirm whether the thickness works, what's missing, and what needs to be resolved before fabrication.