What Happens When You Hit Rock During Construction? A Real Project Breakdown

Many landscape and pool designs are resolved before construction begins.

Levels are set.

Retaining systems are designed.

The layout works on plan.

But one of the realities of construction is that not all site conditions are fully known until excavation starts.

This project, located in Nulkaba in the Hunter region, is a good example of how actual ground conditions can differ from expectations — even when a design is well considered.

Initial Design Approach

The original landscape design included:

• Two retaining walls to manage level changes across the site

• A layout based on typical ground conditions for the area

• A construction method suited to standard excavation and installation

At the time, the retaining system was designed using a sleeper wall approach, which aligned with:

• Site conditions observed at surface level

• Expected excavation behaviour

• Typical cost and construction methods

From a design perspective, the solution was appropriate and buildable.

What Changed During Excavation

As construction began and excavation progressed, the ground conditions at one critical corner of the site changed significantly.

Rock was encountered:

• Exactly where one of the retaining walls required footing depth

• At a relatively shallow level

• In a location that was structurally important to the wall system

This was not something that could be fully confirmed during design.

Like many residential sites, the assumptions were based on:

• surface observations

• experience with nearby conditions

• standard construction approaches

Initial excavation often reveals ground conditions that cannot be fully confirmed at design stage.

Why This Matters for Construction

The presence of rock at that location introduced several challenges:

• The original sleeper wall system was no longer suitable

• Excavation for standard footings was not achievable using typical methods

• Construction time increased significantly

At this point, the project shifted from:

• a straightforward installation

  to

• a condition-driven adaptation during construction

Change in Retaining Wall Approach

Due to the ground conditions, the retaining solution needed to be revised.

This included:

• Moving away from the original sleeper system

• Adopting a blockwork retaining wall approach better suited to the conditions

• Adjusting installation methodology accordingly

While the revised design was still achievable, it required:

• different materials

• different construction techniques

• more time on site

On-Site Construction Reality

The most notable impact came from the effort required to continue excavation in the affected area.

To achieve even a relatively shallow depth:

• Heavy machinery was required

• Rock breaking was necessary

• Progress slowed considerably

In this case:

• Approximately three days were spent working a small section

• Solely to achieve around 400mm of excavation depth

In addition to the construction effort, the process also had a broader impact on the site and surrounding area.

Rock breaking over several consecutive days introduced sustained noise and disruption — both on site and for neighbouring properties and the immediate streetscape.

While this type of work is sometimes unavoidable when dealing with subsurface rock, it highlights another aspect of construction that is often not anticipated at the design stage.

This highlights an important point:

Even small areas of unexpected ground condition can have a disproportionate impact on construction effort

While this example relates to retaining wall construction, similar ground conditions are often encountered in pool excavation and can have the same impact on cost, timing, and construction approach.

Rock encountered during excavation can significantly change construction methods, timelines, and overall cost.

Cost and Time Implications

Although the overall project remained viable, the change in conditions resulted in:

• Additional construction time

• Increased labour and equipment use

• A measurable cost increase relative to the original scope

Importantly:

• This was not due to a design error

• Nor was it due to poor planning

It was a result of actual ground conditions only revealed during excavation

What This Project Demonstrates

This project reinforces a key reality of construction:

Design is based on informed assumptions — construction confirms what is actually there

Key Takeaways

1. Some Ground Conditions Are Only Confirmed During Construction

Even with experience and investigation, certain conditions — particularly rock — may only be revealed during excavation.

2. Localised Conditions Can Have Significant Impact

The issue occurred in a relatively small corner of the site, yet:

• It changed the retaining system

• It affected construction time

• It introduced additional cost

3. Construction Methods Must Adapt to the Site

The original design was not “incorrect” — it was based on reasonable assumptions.

But when conditions changed:

• The solution needed to change with it

4. Cost Changes Are Often Driven by Conditions, Not Design

Unexpected increases in time and cost are often linked to:

• excavation difficulty

• access

• material conditions

rather than the overall layout itself

Final outcomes often reflect adjustments made during construction to respond to actual site conditions.

A More Practical Way to Approach Projects

Rather than expecting a design to remain fixed throughout construction, it is more realistic to consider:

• Some elements will only be confirmed once work begins

• Flexibility may be required in how solutions are delivered

• Early awareness helps reduce friction when adjustments are needed

Where This Fits in the Design Process

This project reflects the construction phase of decision-making:

• After design is complete

• During excavation

• When real conditions are exposed

It complements earlier planning considerations such as:

• ground conditions → see Site Analysis and Soil Conditions

• slope and level changes → see Sloping Block Landscape Design

• retaining systems → see Retaining Wall Design

For broader context on how site conditions influence design decisions before construction begins, see:→ Where Should You Locate a Swimming Pool? Why Soil and Ground Conditions Matter More Than Layout

Final Thoughts

Not all challenges on a site are visible at the beginning.

Some only emerge during construction, when the ground conditions are fully exposed.

In these situations, the outcome is not determined by sticking rigidly to the original design —but by how well the project adapts to what is actually encountered.

And often, it is these small, unexpected moments on site that shape the final result more than the drawing itself.

If you are planning a pool and want to understand how your site conditions may affect location, design, or construction, see Landscape Design and Construction Considerations.