A residential development in the northern Waikato requires the excavation and placement of 700,000m3 of earth fill to generate suitable building platforms for domestic dwellings and associated infrastructure. During the first season of earthwork operations the highly variable and sensitive nature of the volcanically derived alluvial silts and silty sands on site have presented unique challenges putting pressure on construction deadlines and the conventional earthwork methodology adopted. A high in-situ moisture content combined with the extremely sensitive nature of the soils requires careful planning and soil management. Excessive disturbance during excavation, transport and compaction of these soils could render initially very stiff silts to near liquid conditions. Conventional practices such as lime stabilisation to stiffen these soils are generally uneconomical and research suggests that over time the soils can revert back to the original strength due to their complex volcanic mineralogy. In this paper, I discuss the engineering characteristics of these soils, from onsite observations during the early stages of the earthworks operations. Along with the alternative methodologies employed to overcome the associated challenges to achieve the specified fill characteristics.
Large-scale developments generally consist of large cut to fill operations which rely on the reuse of site won material in order to generate suitable building platforms. Developers and contractors are therefore required to make the most of the soils available on site and do so within tight construction timelines.
In this paper, I describe a development in the northern Waikato where CMW Geosciences has conducted construction observations of the earthwork operation that consists of cut and fill of up to 4.0m and 8.0m in height respectively.
2 Ground Model
2.1 Regional Geological Setting
The northern Waikato Basin consists of up to 110m thick sequence of soils of the alluvial Tauranga Group (Nelson, et. al 1988) which is separated from oldest to youngest into the Whangamarino, Puketoka and Karapiro Formations, the Kauroa and Hamilton Ashes and the Hinuera Formation.
Deposits of the Tauranga Group as stated by Nelson et. al (1988) are characterised by fluvial systems and silica rich volcanic parent material sourced from the nearby Coromandel Peninsula and the more recent central north island volcanic source. Changes in sea level and volcanism created an ever-changing depositional environment ranging from high energy braided river systems to low energy brackish lakes resulting in a complex sequence of soil types and interbedding.
From the late Pleistocene the landscape has experienced a long period of erosion, sculpting the present-day rolling hill topography which the more recent volcanic ashes mantle.
2.2 Project Geology
The published geological map (Institute of Geological & Nuclear Sciences Limited, 2001) for the area shows the rolling hill topography where the deep cuts were undertaken to be underlain by alluvial pumiceous clays, silts, sand, gravel and lignite of the Late Miocene to Mid Pleistocene aged Whangamarino Formation. Nelson et. al 1988, describes the depositional environment for these soils to be controlled by meandering rivers which has led to a complex soil profile with high vertical and lateral variability. On the elevated areas these soils are typically overlain by several metres of deeply weathered younger volcanic ash beds which mantle the rolling hill topography.
This geology was confirmed by the findings of the site investigation with special mention of the localised concentrations of particularly halloysite rich soils throughout the site. The presence of a layered soil profile with contrasting permeabilities had created perched water tables and localised areas of increased moisture content.
2.3 Soil Mineralogy
Pumice rich alluvial soils common across the Bay of Plenty and Waikato regions are generally more difficult to work than other alluvial soils due to weathering and alteration of volcanic glass to allophane and halloysite. The typical weathering process first forms allophane to which further chemical weathering occurs to form halloysite (Wesley, 1973). As shown in Figure 1 this process also has an effect on the water holding capacity of the soil.
These minerals have unique structures which when present as a small fraction of the soil mass can control the overall soil behaviour. In this case halloysite mineralogy is more prevalent in the Whangamarino Formation soils due to the poorly drained and layered soil profile.
The allophane minerals consist of aggregations of very small spherical shapes generally formed in well drained environments with vertical water seepage (Wesley, 2010). Wesley reports void ratios within these soils to range from 1.5 to 8 therefore allophanic soils are typically characterised by extremely high-water contents although they do not display “quick” behaviour when disturbed. It is noted that this research had been carried out in Indonesia. Upon drying, these soils can undergo irreversible changes resulting in previously plastic soils to become non plastic due to the breakdown of the mineral structure and loss of permeability (Wesley, 1973).
Halloysite minerals are typically formed in poorly drained environments with microstructures such as hollow tubes, spheres, plates and books. The fine grain size and high porosity of these structures has been stated to create a strong capillary force holding large volumes of water within the clay structures leading to a high moisture content of the soil (Wyatt, 2009).
Upon disturbance of these soils the microstructures break down expelling the water. The expelled water dilutes the clay bonds causing a large reduction in strength resulting in high sensitivity and a loss of plasticity. In some cases documented by Wyatt, the water content exceeded the liquid limit of the soil following the disturbance of the soil.
2.4 Expected Geotechnical Issues
Based on the known depositional setting of the soils on the site, and the known weathering processes in the region, the geotechnical risks associated and anticipated with soils of the Whangamarino Formation included high sensitivity to disturbance and remoulding, and rapid vertical and horizontal variation.
3 On site Observations and testing
3.1 Earthworks Methodology
On large developments time and cost are important driving factors behind the construction methodology that is adopted. In this case the project required excavation and transport of up to 700,000m3 of material within tight construction deadlines. The contractor initially chose to adopt the use of large motor scrapers to excavate and transport material with heavy bulldozers to spread the fill and sheepsfoot rollers to compact the fill. The slower but more precise machinery such as excavators and graders were to be retained for trimming to design levels where precision was necessary.
3.2 Observed Soil Behaviour and Site Operations
3.2.1 Excavation of Sensitive Soils
Investigations identify the in-situ soils of the Whangamarino Formation to be generally stiff to very stiff. During the early stages of the earthworks operation, it became evident that the earthworks construction methodology and size of the machinery being used was too large for the highly sensitive soils.
The motor scraper excavation and transport were observed to remould the soils to a depths of up to 0.8m, greatly softening them and making excavation and placement difficult. On site testing demonstrated remoulded shear strengths up to 30 times lower than peak strengths in the sensitive soils of the Whangamarino Formation causing plant to get bogged down and severely rutting the soils in the cut areas posing a threat to the quality of the soils at final level as shown in Figure 2.
This was discussed within the project team and it was decided that the lost time and risk of disturbing the final foundation soils outweighed any increases in efficiency gained by the use of motor scrapers in these sensitive materials. Therefore, an alternative excavation methodology was recommended where motor scrapers would cut to approximately 0.5m above design level and the remaining material would be top loaded using excavators and dump trucks working on the raised surface to minimise disturbance of the final foundation soils.
Where areas of sensitive material had been identified during the site investigation and/or encountered during the construction process they were entirely block cut to avoid time lost retrieving bogged machinery.
3.2.2 Placing Fill Over Sensitive Soils
In low-lying areas of the site where fills of up to 6.0m were proposed the underlying impermeable lignite caused high groundwater tables. Excess pore water pressures generated during heavy plant operation could not dissipate, ‘pumping up’ the underlying soils causing large areas of soils to effectively liquefy making it impossible for the movement of heavy machinery.
With large areas of problem soils present the use of imported granular material to form a drainage blanket and act as a ‘starter layer’ proved to be too expensive.
An alternative method was employed whereby networks of subsoil drains were installed in the natural soils underlying the fill area in order to help dissipate excess pore water pressures developed during trafficking and fill placement. This was combined with a 0.5m thick clay starter layer which was given nominal compaction creating a raised working platform to reduce further disturbance of the natural soils.
3.2.3 Variability of Soil Types
The vertical and lateral distribution of the highly sensitive halloysite rich soils required specific investigation to delineate and quantify them, with remediation works assessed on a case by case basis. This reminds us that a ground model is never complete and requires to be constantly updated throughout the construction process.
4 Fill Stabilisation
4.1 Lime Stabilisation
A common method to aid the compaction of soils with high moisture contents is the addition of a binder such as lime and/or cement. The binder is mixed with the material as it is placed, with the binder and soils undergoing a chemical reaction forming cementitious bonds strengthening the soil.
The results of testing in natural and lime stabilised allophanic soils carried out by Kett et. al, 2010 is illustrated in Figure 3.
The results indicate that the addition of lime to a “low allophane soil” makes little difference to the soil strength compared to that described by the authors as a “common soil”. The authors suggest that by adding higher percentages of lime it may be possible to cause a strength increase although it is deemed this would not be economical. Investigations conducted by McNamara, 2003, on allophanic soils encountered in Fiji has indicated that soils stabilised with low proportions of lime can revert back to original soil behaviour between 6 to 12 months following treatment, likely due to the unique mineral structure.
Literature on the effectiveness of lime/cement stabilisation on halloysite soils is not readily available, although based on the similar mineral structures it is expected that halloysite soils would behave similarly. This hypothesis combined with the specialist equipment required and high costs made lime/cement stabilisation uneconomical for the large scale bulk earthworks operation.
4.2 Soil Mixing
The more cost-effective method of using these difficult soils is to blend multiple soil types and allow them to air dry prior to compaction. Due to the high moisture contents of halloysite and allophane soils this needs to be done carefully and in thin layers, with only a minor proportion of these soils in comparison to the other constituents. In this case halloysite rich soils were mixed with other clay-rich and sandy layers of the Whangamarino Formation and the clay-rich volcanic ashes. The mixing of multiple fill sources takes great skill and over working of the soil can still lead to the degradation of the soil structures.
Figure 4 indicates the compaction curve results for one of the cut soils blended and used as fill on this site. Note the defined peak in the dry density curve around 23% and the sharp decrease in peak vane shear strength of the soil sample tested at 25%.
This indicates the small margins that the contractor worked within and highlights the need for careful management of excavated materials within the fill area in order to meet the required fill compaction criteria of 120kPa undrained shear strength.
Following the first season of earthwork operations within a large-scale residential development in the northern Waikato, conventional excavation methods have struggled with the highly sensitive soils of the Whangamarino Formation, resulting in lost time and disturbance of soils exposed at design level. An alternative method using a combination of motor scrapers and excavators has since successfully been used reducing further remediation works at finished level.
Due to the unique mineralogy of these soils and scale of the earthworks operation, lime/cement stabilisation was not economical and carried significant technical risk. Soil blending/mixing was therefore adopted to re-use the sensitive soils. This relied on having large volumes of soil to mix problematic soils with and a skilled contractor. The soils available on this site have a small margin of error requiring careful management and planning in order to meet the required compaction criteria.
Institute of Geological & Nuclear Sciences Limited. (2001). Geology of the Auckland area: scale 1:250,000.
Kett, I., Evans, J., & Ingham, J. (2010). Identifying an Effective Binder for the Stabilisation of Allophanic Soils. International Journal of Pavement Engineering.
McNamara, G. (2003). The Presence of Allaphanes in Soils and Its Effects on Long Term Stabilized Strength of Fiji Roading Materials. Auckland: Materials and Stabilising Consultancy.
Nelson, C. S., Mildenhall, D. C., Todd, A. J., & Pocknall, D. T. (1988). Subsurface stratigraphy, paleoenvironments, palynology, and depositional history of the late Neogene Tauranga Group at Ohinewai, Lower Waikato Lowland, South Auckland, New Zealand. New Zealand Journal of Geology and Geophysics, 31, 21-40.
Wesley. (1973). Some Basic Engineering Properties of Halloysite and Allophane Clays in Java, Indonesia. Geotechnique, 23(4), 471-494.
Wesley. (2010). Geotechnical Engineering in Residual Soils.
Wyatt, J. (2009). Sensitivity and Clay Mineralogy of Weathered Tephra-Derived Soil Materials in the Tauranga Region. Hamilton, New Zealand: University of Waikato.