100+ Free S+SNZ Professional Exam — Land Development Engineering Practice Questions

Explanation: NZS 4431 (Code of Practice for Earth Fill for Residential Development) specifies that for cohesive soils, the air voids ratio of the compacted fill should generally not exceed 10%. Maintaining air voids below this threshold is critical to limit post-construction settlement and prevent water logging within the fill structure. Exceeding 10% indicates insufficient compaction effort or incorrect moisture content during placement.

Explanation: Under NZS 3604, 'good ground' is defined as soil or rock capable of supporting an ultimate bearing capacity of 300 kPa (which corresponds to an allowable bearing pressure of 100 kPa). A Scala penetrometer penetration rate of at least 5 blows per 100mm (down to a depth of twice the footing width below the proposed footing) is generally accepted as indicative of satisfying this requirement in cohesive soils. If the blow count is lower, the ground is considered soft, and a specific engineering design (SED) or remediation like undercutting is required.

Explanation: Auckland Council's GD05 (Erosion and Sediment Control Guide for Land Development) specifies that Sediment Retention Ponds must be sized based on the slope of the contributing catchment. For slopes less than 10%, the pond volume must be at least 2% of the catchment area (200m³ per hectare). For slopes of 10% or greater, the volume must be at least 3% of the catchment area (300m³ per hectare) to account for faster runoff and sediment transport.

Explanation: The bulking factor describes the volumetric expansion of soil when it is excavated from its in-situ state (bank volume) to a loose state. Excavation introduces air voids between soil particles, causing the volume to increase. For example, clay soils typically bulk by 10% to 30%, which must be accounted for when estimating truck capacities for off-site disposal. Conversely, shrinkage occurs when loose soil is compacted, often resulting in a final compacted volume slightly smaller than the original bank volume.

Explanation: A relative compaction of 92% fails to meet the structural specification of 95% MDD, which increases the risk of excessive settlement. The correct remedial action is to scarify (rip up) the failed lift, adjust its moisture content to bring it closer to its optimum moisture content (OMC) if it is too wet or dry, and then re-compact the layer with suitable compaction equipment. Proceeding without rectifying the layer is a serious breach of NZS 4431 standards.

Explanation: Auckland Council GD05 limits the maximum contributing catchment area for a standard Decanting Earth Bund (DEB) to 0.3 hectares (3,000m²). DEBs are smaller, temporary erosion and sediment control measures designed for localized catchments. If the contributing catchment area exceeds 0.3 hectares, a Sediment Retention Pond (SRP) must be designed instead to manage the higher sediment load and runoff volumes.

Explanation: The Plasticity Index (PI) is the numerical difference between the Liquid Limit (LL) and the Plastic Limit (PL) of a soil. A high PI indicates that the soil has a broad moisture range where it behaves plastically, which is characteristic of active clay minerals (like montmorillonite). These soils are highly cohesive, have low permeability, and exhibit significant volume changes (shrinking and swelling) in response to seasonal moisture variations.

Explanation: NZS 4404 generally recommends that permanent unreinforced cut or fill batters in cohesive soils should not exceed a slope of 1 Vertical to 2.5 Horizontal (1V:2.5H) to ensure long-term stability and allow for revegetation and maintenance (mowing). Steeper slopes are subject to erosion and shallow slip failures, and cuts steeper than 1V:2.5H normally require specific geotechnical stability analysis or retaining wall designs.

Explanation: GD05 guidelines require that the bottom edge of the silt fence fabric must be buried in a trench that is at least 200mm deep and 100mm wide. Backfilling and compacting the soil over the fabric in this trench is critical to prevent muddy runoff from undermining (flowing under) the fence during heavy rain. Failure to key in the fabric properly is the most common cause of silt fence failure on construction sites.

Explanation: Flocculation is used in New Zealand (particularly on clay sites in Auckland and Northland) because colloidal clay particles remain suspended in water indefinitely due to negative electrical charges. Adding a chemical flocculant (like Alum or PAC) neutralizes these charges, allowing the fine particles to bind together into larger 'flocs' that settle rapidly out of suspension. This significantly improves the sediment removal efficiency of the retention pond, reducing discharge turbidity.