Designing Timber Compaction Piles using CPT

What are Timber Compaction Piles?

Loose, saturated sands often have low stiffness and strength, making them vulnerable to liquefaction. Before construction, these soils typically need improvement. One proven option is to install Timber Compaction Piles (TCP). Driven into the ground in a square or triangular grid, TCP densify the surrounding soil and extend slightly beyond the building footprint.

Although TCP are widely used across New Zealand, their design is still largely empirical. This can lead to layouts that are over-designed (unnecessarily costly) or under-designed (failing to achieve the required density on site). Both outcomes create uncertainty and potential delays during construction.

This article introduces a simple CPT-based design method, following Barounis & Philpot (2021), to more reliably achieve a target soil density.

When to Apply This Method

• Soil type: inorganic sands (not organics, peats, or uncontrolled fill).

• Groundwater: fully saturated zones confirmed by u₂ readings, dissipation tests, or groundwater monitoring.

• Mineralogy: assume typical sand with specific gravity Gs ≈ 2.65.

Step-by-Step Method

Step 1 – Estimate bulk unit weight (γ) from CPT

Use Robertson’s (2010) correlation:

where:

• γ = bulk unit weight

• γw = unit weight of water

• Rf = CPT friction ratio (%)

• qt = corrected cone resistance

• pa = atmospheric pressure

From this, initial soil density (ρi) can be estimated.

Step 2 – Estimate number of piles to reach target density

Once ρi is known, the number of piles (np) required to achieve a target density (ρtar) is:

where:

• Aimp = Area requiring improvement

• Dp = pile diameter

• ρw = water density

Example Results

For a sandy soil with an initial soil density: ri =1725kg/m3 and a target soil density: ρtar = 1800kg/m3, the pile-to-soil ratio becomes:

If the area requiring improvement is 300m2, then the number of piles can be determined for various pile diameters. Applying different pile diameters gives:

For Dp = 300mm:

For Dp = 450mm:

Why This Matters

With just one CPT and a straightforward workflow, engineers can:

1. Estimate soil unit weight.

2. Calculate how many TCP are needed for achieving a target density, assess feasibility of this method.

3. Optimise pile diameter vs. number of piles to balance cost and constructability.

4. Reduce the risk of surprises on site if the target soil density isn’t met post-installation, helping avoid costly redesigns or construction delays.

Key Considerations

• Verification: Post-improvement CPT testing should confirm soil density/stiffness meets design intent.

• Construction effects: Soil heave or material loss during pile driving can reduce effectiveness.

• Limitations: Best suited for fully saturated sands; results are less reliable in silts or partially saturated soils.

For further details, see Barounis & Philpot (2021), published for the 2021 NZGS Symposium.