In Auckland, many sites have been developed over the past decades on fills of varying origin and compaction, from the volcanic ash and scoria fills of the isthmus to the engineered fills of the North Shore and South Auckland. The performance of shallow foundations on these materials depends heavily on the fill's composition, compaction history, and potential for long-term creep. A thorough foundations on fill analysis is essential before any structural design, as the variability of these layers often requires site-specific investigation rather than reliance on assumed bearing values. Complementing this with a resistivity survey helps map the lateral extent of the fill, while ground-penetrating radar can identify buried obstructions or utilities within the layer. The analytical process must account for both immediate settlement and time-dependent compression, particularly when the fill thickness exceeds two meters.
Uncontrolled fills in Auckland can exhibit up to 50 mm of long-term creep settlement over 10 years, demanding careful analytical treatment.
Methodology and scope
The volcanic-derived soils and alluvial deposits underlying Auckland's fill materials create a complex geotechnical setting. For foundations on fill analysis, the primary parameters include the fill's unit weight, moisture content, shear strength through consolidated-undrained triaxial tests, and oedometric compressibility. The NZGS guidelines for founding on fill require a minimum of one borehole per 200 m² when fill thickness is variable, with sampling intervals every 0.5 m within the fill layer. The analysis must also incorporate the influence of the underlying natural ground, as soft marine clays of the Waitemata Group can exacerbate differential settlement when fill is placed over them. A key output is the allowable bearing capacity at serviceability limit state, typically limited to 50–100 kPa for uncontrolled fills, rising to 150–200 kPa for well-compacted engineered fills placed under geotechnical supervision. The use of dynamic compaction as a pre-treatment method can significantly improve these values for deeper fills.
Technical reference image — Auckland
Local considerations
Auckland's rapid urban expansion since the 1950s has seen many gully heads and former quarries filled with heterogeneous materials, often without formal compaction control. The risk of differential settlement in such fills is high, particularly where buildings span the boundary between fill and natural ground. Foundations on fill analysis must evaluate the potential for collapse upon wetting, especially in fills derived from pumiceous or fine-grained volcanic sands that can undergo sudden volume loss when saturated. Additionally, the seismic response of these fills during a moderate earthquake (shaking intensity MM VI–VII) can amplify ground motions by a factor of 1.5–2.0, increasing the risk of foundation rotation or bearing failure. The presence of buried organic matter within uncontrolled fills introduces further uncertainty, as ongoing decomposition can cause localized voids and differential movement over the service life of the structure.
0.05–0.15 for granular fills; 0.2–0.5 for cohesive fills
Allowable bearing capacity
50–200 kPa depending on compaction control
Long-term creep coefficient (Cα)
0.005–0.02 (secondary compression)
Associated technical services
01
Fill Compaction Assessment & Bearing Capacity Analysis
In-situ density testing using sand replacement or nuclear gauge, combined with laboratory triaxial and oedometer tests, to determine the compaction level and allowable bearing capacity of fill layers. Includes analysis of immediate and consolidation settlement under structural loads.
02
Long-Term Settlement & Creep Evaluation
One-dimensional consolidation testing over extended periods (up to 7 days per load increment) to quantify secondary compression (creep) potential. Provides design parameters for post-construction settlement predictions over 10–50 years for lightly loaded foundations.
Applicable standards
NZS 3404:1997 (Steel Structures – foundation interaction with fill), NZGS Guideline for the Investigation and Assessment of Fill Sites (2019), AS/NZS 1289.6.3.1 (Standard Test Method for SPT – used in fill characterization)
Frequently asked questions
What is the typical cost of a foundations on fill analysis in Auckland?
The cost typically ranges between NZ$1,290 and NZ$4,610 depending on the number of boreholes, laboratory tests, and the complexity of the fill stratigraphy. A basic assessment for a single-lot residential site with 3–4 test pits and basic compaction testing is at the lower end, while multi-borehole investigations with advanced consolidation and triaxial testing for commercial sites approach the upper end.
How deep should boreholes extend into natural ground below fill?
Boreholes should extend at least 3 meters into the natural ground below the fill, or to a depth where the stress increase from the foundation is less than 10% of the overburden stress. In Auckland, where soft Waitemata Group siltstones or marine clays underly many fills, this often means drilling to 6–10 meters below ground level to capture the full settlement profile.
What laboratory tests are essential for fill characterization?
Essential tests include particle size distribution (sieve and hydrometer), Atterberg limits, standard Proctor compaction (NZS 4402), and consolidated-undrained triaxial compression (NZS 4402). For fills with organic content, the loss on ignition test is recommended. One-dimensional consolidation (oedometer) tests with long-term creep stages are critical when estimating differential settlement between fill and natural ground.
Can shallow foundations be used on uncontrolled fills in Auckland?
Shallow foundations on uncontrolled fills are generally discouraged unless the fill is less than 1.5 m thick and the settlement analysis shows differential movement below 15 mm. For thicker or variable fills, ground improvement (e.g., dynamic compaction, vibro-replacement) or deep foundations (driven piles to natural ground) are recommended. The NZGS Fill Guideline requires a minimum factor of safety of 3.0 against bearing failure for any foundation on fill.
Location and service area
We serve projects across Auckland and its metropolitan area.
