GROUND RISK Volume 2
All those familiar with development will be acutely aware that there are a myriad of challenges throughout a project lifecycle that present themselves during a construction project.
One of the more significant challenges that presents itself early in the construction delivery phase is ground conditions, with significant risk associated.
We, accordingly, believe it to be of benefit to canvas this risk item in brief, to ensure ground risk elements do not put a project’s success in jeopardy.
What Comprises Ground Risk?
- Underground obstructions
- Contamination
- Poor substrate materials such as peat, or material with a propensity for liquefaction
- High density substrate, such as rock
- High ground water table
- Underground streams
- Steep site inclines with limited stability
Why is it such a risk?
It’s the ‘unknown’ element which creates the risk, as even with geotechnical advice, simply not all ground risk can be identified and quantified. One of the only ways to fully appreciate what the ground beneath a development site is made up of is by completing close proximity investigative boreholes, down a significant depth, over the entire site. As you will appreciate, this can equate to a significant cost. Accordingly, this level of investigation is normally undertaken on larger scale developments, where it would otherwise can be uneconomic to undertake such extensive testing on a smaller scale project.
Traditionally, boreholes and related investigative measures are undertaken only to selected zones of a site, to obtain a high-level understanding of the likely make-up of the substrata of the ground. The results are generally modelled and transposed to all areas of a site, based on a subset of testing.
The main reason why the risk is so great is that the true extent of the grounds geological composition is only established during the excavation of the site, the discovery phase. Traditionally, the project budget is set up prior to this point with only identified risks attracting contingency allowances, without the geological makeup of the site having been fully established and costed. This leaves room for significant cost and programme omissions.
In addition, the risk is heightened by the ground conditions during the various seasons. For example, in winter, the water table of a site may be higher than in the summer months, potentially creating issues on site.
We wish to also note that plant and machinery costs are high, which adds to the cost risk element.
Can it be mitigated or managed?
There are ways to manage ground risk; however, it is extremely difficult to mitigate the risk in a substantial way, bar costing the relative risk accordingly. The key to these costings is the really gaining a comprehensive understanding of what ground conditions that are likely to be encountered. Risk management strategies can be effected, the better this is understood.
As always, the earlier the ground conditions are established, the better that mitigation measures can be effected. For example, there are some development sites where there is substantial risk associated with underground conditions which attracts a cost premium to remediate. This must be considered when negotiating purchase prices for high risk land parcels. Such should be dissuasive for developers to proceed without getting a true handle on geological conditions.
How can it be mitigated or managed?
Management and mitigation techniques are usually focused on having the best understanding possible of the site conditions by conducting a reasonable level of investigation and obtaining the associated reports. Therefore, a geotechnical investigation report with a reasonable volume of borehole investigation and analysis would be obtained. In this report, the groundwater water table height expectations would also be available. Further comprehensive contamination reports would be recommended that look into the background of what the site has been used for to assess risk and undertake testing. Traditionally, we would apportion specific additional contingency budget allowances based on the known risk elements. For example, if there is a certain zone of a site that had a dense rock sub strata we would include an allowance for rock breaking to that zone of the site. Likewise, typically winter works related allowances are made, such as the provision of lime for fill drying, should a larger project be proceeding through winter.
What developments are affected by ground risk more than others?
Traditionally, we find the following developments more susceptible to ground risk:
- Civil works subdivisions where there is a significant volume of earth being removed
- Residential sites with poor substrate
- Sloping sites
- Sites near rivers, or areas that have a high water table
- Sites in zones where the ground has a high peat content.
- Sites near old abattoirs
- Sites in volcanic rock zones
- Sites which where industrial chemicals were stored, or services stations
- Sites that were formally orchards, that may have a high arsenic content in the soil
- Sites that had old buildings with asbestos content in them that were demolished with potential asbestos contamination in the soil
- Former landfill sites
What reports are available that review ground elements and what do they cover?
There is a wide range of reports available in the geotechnical investigation field, with the most common reports being:
- Geotechnical Investigation Report – This report will provide the geotechnical engineers findings after an investigation on the site. The key information that would traditionally be included would be the previous history of the site usage, where the boreholes were taken during the fieldwork completed on the site, what material was found, and general ground condition commentary, as well as the water table level. Suitability of the ground for the proposed building loadings, development/foundation recommendations, among other things, are also found within these reports, which inform potential construction methodology.
- Contamination Reports – This report would provide a summary of findings with regard to any contamination on the site. The report would highlight the testing area, and what the testing results were. Chemicals that would be traditionally tested for would be pesticides, arsenic, lead, and petrochemicals.
- Asbestos Reports – This is generally the same as the contamination report above, aside from being asbestos specific
- Detailed Site Investigation (DSI) Reports – This includes a summary of the geotechnical history to date, site investigation fieldwork results/samples, and sample zones. The report will either state that the soil is in accordance with the residential contaminant standards/ building code standards, or exceeds the standards, with remediation required.
- Remedial Action Plan (RAP) & Site Management Plan (SMP) – In the event the DSI report noted above results in site soil remediation being required, an RAP and SMP would need to be completed. These reports will provide further testing and detail on what the remediation work will comprise, how it will be undertaken, and what the end results will yield, as well as specific recommendations.
- Site Validation Report (SVR) – Once the remedial works are completed in accordance with the RAP and SMP noted above, a site validation report will need to be submitted to the relevant local authority that would include further site fieldwork testing confirming the required contamination has been removed, in accordance with the RAP and SMP reports.
- Groundwater & Dewatering Reports – If there is high groundwater encountered a consent for groundwater take and diversion may be required, therefore further reports may be made available for this scope of work.
How are poor development sites treated, and what are some of the more common remedies?
There are remedies to address problematic sites, with the more common occurrences as follows:
- Soil Removal – Removal of contaminated earth materials from the site (this may result in additional fill required in the site).
- Piling – Piles are typically utilised in lieu of significant ground remediation, to transfer the building loads down to where the ground is stable enough to accommodate the load.
- Palisade Walls – This system is effectively akin to a retaining wall under the ground where either reinforced concrete piles, or timber poles are bored deep into the ground at close centres to stop soil creep on a sloping site. This contains the land within the zone the piles are situated.
- Shear Keys – This system is used on particularly sloping development sites, where there is risk of a landslide or significant horizontal movement of the ground. The process typically involves the excavation and removal of soils susceptible to movement. The area where the excavated spoil is removed is replaced with suitable fill material, in some instances hardfill, that is compacted. This gives the remedied area more stability during movement events.
- Dewatering – Where sites have high ground water tables, dewatering could either be undertaken in a simple or more complex fashion, depending on the volume of water that needs to be removed.
- Pre-Loading – Where the ground would be susceptible to sinking, a common method is placing a dense material, such as Gap 65 basecourse metal (typically used for roading subgrades), in a high stockpile over the affected area, and leaving this for a six-to-twelve-month duration (although, this is project dependent). This slowly compacts the soil to a level where it would be suitable for construction loadings. This is typically used on high concentration peat, low density soils.
- Mass Concrete filling to voids – Where there is volcanic rock substrata material that has fissures or voids, a typical methodology is filling the voids with a lower strength concrete mixture, which will enable the loads required on the site to be accommodated. There has to be careful consideration on this method as some of the rock caverns can be quite large and mass filling may not be the most cost effective solution.
Does the weather/ seasonal conditions affect a development?
The short answer is yes, sometimes materially.
The summer season, which comprises the earthworks season between 1 October to 30 April of a given year is the time when it is considered the best window to undertake significant earthworks.
When works occur outside of this timeframe, or the summer suffers unseasonal inclement weather events, the impact is usually additional costs. These additional costs will typically comprise:
- Additional removal of wet or unsuitable materials, resulting in additional excavation and removal of materials off site.
- Cost associated with drying out of clay materials which can require combining lime with the clay or laying the material in thin layers across the site to dry out sufficiently.
- Programme delays associated with the inclement weather, where works are unable to be undertaken for a period of time, or when waiting on material to dry.
The above is most applicable to civil works subdivisions on a larger scale; however, can also affect smaller development sites as well, albeit, to a lesser extent.
What does White Associates look for when assessing ground risk?
There are numerous ground risk elements associated with developments which vary, oftentimes significantly, between the North and South Island, between cities, even between suburbs. As a general rule, particular areas are typically well known to have a propensity for a certain risk, such as the volcanic substrata risk in some Auckland Areas, or liquefaction risk in some Christchurch zones.
The information available holds the key to the potential risk associated with a particular development. We would obtain as many reports as we can (which are made available to us) which would normally comprise the geotechnical site investigation report and contamination reports, which sometimes extends to site validation reports, and the like.
In the geotechnical report the findings are key. Typically, we would look at what date the report was completed, how may boreholes were completed (if there are few, this testing may not provide a comprehensive overview of what may be beneath the ground). In addition what the anticipated ground conditions would be, is the groundwater level low enough so it doesn’t create any issues?
Where there is potential risk specifically noted or implied in the geotechnical report, or other pertinent reports, we would include a specific ground risk allowance against that item, which will form a special budget item. For example, in the past we have allowed for rock breaking costs for sites in volcanic rock zones, where it was identified the rock was close to the required excavation level. In addition we have allowed for specific contaminated soil removal where there is a high probability this will occur.
Conclusion
As you will appreciate from the above there are many moving parts associated with ground conditions, that may heighten ground risk for a particular site. The risk can be identified and managed to a point with effective geotechnical consultant input and sufficient soil testing, that captures all areas of the development site.
As with all effective risk mitigants, it is all about obtaining as much information to make an informed decision. Where, for some reason or another, information is not available or is unable to be obtained prior to key decisions being required, the risk needs to be priced the best it can, to assist with any cost impact that may occur as a result of the risk.
In mention of this, ground risk is never able to be fully mitigated, and remains a component of a build project where all parties breathe a sigh of relief once the civil works phase completion milestone of the project has been achieved
Darin Bayer
Director, Project Finance Representative
dbayer@whiteassociates.co.nz
021 128 7363
Justin Bearne
Associate, Project Finance Representative
jbearne@whiteassociates.co.nz
021 667 551