The role of the ground model in design of low cost slip repairs on rural New Zealand roads

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1 Watson, J. (2013) Proc. 19 th NZGS Geotechnical Symposium. Ed. CY Chin, Queenstown The role of the ground model in design of low cost slip repairs on rural New Zealand roads J Watson Engineering Geologist, Opus International Consultants, Hamilton, NZ. jonny.watson@opus.co.nz Keywords: landslide remediation, geological model, Waikato ABSTRACT There is high demand for low cost slip repairs on road networks throughout rural New Zealand as Local Governments balance the need to maintain access and services to rural communities against managing their limited budgets. The ground model is an essential tool in developing solutions for slip repair projects. The cost of determining the ground model is a good investment, as by understanding the geology and geomorphology of a slip site, a more appropriate and cost effective solution can be designed and constructed. Without one, the risk of increased costs or costly design changes, resulting from unexpected ground conditions, is high. Determining the ground model for slips on rural roads typically includes the following steps: a desk study of aerial photographs and geological maps, a site walkover inspection and engineering geological mapping, and some intrusive site investigations. Supplementary investigation may also be required at the start of construction. Designing remediation for 24 slip sites within Waikato district that resulted from an intense rainstorm during July 2012 for Waikato District Council has highlighted the importance and value of ground models based on previous and ongoing experience within a geographic area and sound engineering geology. It was essential for determining and clearly communicating risks and developing flexible solutions that can be easily modified during construction to cope with variable ground conditions. 1 INTRODUCTION One of the effects of the recent global financial crisis is that central and local governments have cut spending in order to limit debt. One area where provincial local governments have cut spending is on upgrading roads in rural areas. Upgrading unsealed roads to sealed ones has been dramatically reduced, if not cut altogether. In addition maintenance activities (planned or preventive) have been deferred. This may result in more reactive work such as slip repairs. However, provincial local governments still have an obligation to maintain access and services in rural communities. It is with this situation in the background that local governments are under increasing pressure to lower the cost of services, including slip repairs on rural roads. The following paper discusses the ground model; what it is, why it is important, and demonstrates the use of the ground model for the repair of slips in the Waikato District, highlighting the successful elements of the Waikato District example. 2 THE GROUND MODEL The following sections present aspects of a ground model with a focus on its use in engineering design. 2.1 What is the ground model?

2 The ground model presents all the available geological and geomorphological information about the site in a clear manner that shows the location of different features as well as soils and rock identified. Geological information should include rock and soil types observed as well as any information about groundwater. Geomorphological information should include alluvial, aeolian, colluvial, glacial processes, as well as all manner of landslide processes. Man-made changes to the environment should be included as well, as these are essentially geomorphological changes within a site. Man-made features include things like road embankments and fill material, cut slopes, open drains and culvert pipes, and even service trenches. Although sites can be similar with regards to geological and geomorphological formation processes, topography, aspect, or orientation of man-made features, it is unlikely that any two sites will be exactly the same. Therefore, a ground model should be unique and specifically constructed for a site. Although it should be noted that the ground model will never be able to achieve the same qualitative accuracy as the engineering design because of the complexity and the inconsistency of the ground. 2.2 Presentation of a ground model In its simplest form a ground model can be presented as a cross-section through the site, showing the condition of the site in the present. If there are significant changes or differences along a site then a series of cross-sections may be required. In geology literature, geological models are often presented as block diagrams (threedimensional sketches) and when stages of formation or development of a site are presented then a series of block diagrams are utilised. The ground model for use in engineering design is most useful if it is presented spatially accurate. In order to present a spatially accurate model it is essential that you have an accurate representation of the topography. A two-dimensional cross-section or series of cross-sections is therefore a minimum requirement. 2.3 How to construct a ground model? The ground model is usually something that is developed over stages of study and investigation. Initial development of the model should include a desk study of available aerial photos (often images from Google Earth have sufficient resolution), published geological maps and if available any historic records from the site, or others nearby. The site visit and mapping stage should come after the desk study. However, often one is called by a territorial authority to a visit a site before the desk study is carried out. This is where local knowledge and experience working in a specific geographic area is invaluable. This stage includes a site visit where engineering geological mapping can be carried out, that is identifying and recording the location and extent of geological, geomorphological and man-made features on the site as well as beginning to hypothesise on the mode of failure in the case of a slip. The site visit is an ideal time for scoping as well as considering the practical constraints of the next stage: a site investigation. A site investigation should include a topographical survey or measuring of topographical crosssections, intrusive investigations such as cored boreholes, excavated test pits, cone penetration tests, hand augers and Scala penetrometers, and laboratory testing of samples taken.

3 Although the engineering design may have been carried out, the ground model is not completely refined until construction when the actual ground conditions are revealed by supplementary testing, excavation or the driving of piles. Figure 1 (reproduced from Fookes 1997) shows the staged development of the ground model. It illustrates with this figure how a good understanding of geology aids the development of the ground model. With geology done well the intrusive investigation will be targeted to prove the model hypothesised based on the geology and may in fact reduce the scope of the intrusive investigation. Figure 1: Geological information anticipated during the stages of site investigation 2.4 Who should construct a geological ground model? A ground model is constructed using a thorough understanding of geological features, including soil and rock types, and geomorphological processes as well as the behaviour of soils and rock. It also requires skills and experience in the identification of these features and processes. All of this should typify the skills of an Engineering Geologist and therefore it is advisable to have one construct the ground model. Although it is advisable to have an Engineering Geologist, a Civil Engineer can carry out this work provided they have the suitable understanding of the areas highlighted above as well as experience in doing so within the geographical area. 2.5 What is the geological ground model used for? What a ground model presents has already been discussed, but why do we need to present this information? What is it used for? The ground model should be used as a basis for slope stability analysis and engineering design. Slope stability analysis is mainly used to investigate slope failure, determine a slope s sensitivity to different causal factors or triggering mechanisms, and quantify the probability of slope failure, usually expressed as a factor of safety. Without a ground model there are no meaningful inputs and slope stability analysis can be made to say anything.

4 At the most basic level the ground model should provide dimensions for engineering design through to the more complex level it ought to provide measured/tested or inferred strength parameters for engineering design. It must be stressed that understanding the mode of failure is key for both carrying out slope stability analysis and engineering design. With the availability of an array of slope stability software used for slope stability analysis it is easy to get the wrong answer even with the right ground model. Therefore analysis should follow observation of the site and endeavour to model what has been observed. Engineering design must address the mode of failure and the causes of that failure mode in order to be effective. 2.6 Why is the ground model important? Fookes (1997) notes that: the strength of the geological [ground] model is in providing an understanding of the geological [and geomorphological] processes which made up the site. This enables predictions to be made or situations anticipated for which explorations need to be sought in the geological materials, geological structure and the ancient and active geological processes in the area. It provides a rational basis for interpretation of the geology from understanding and correlation of observed geological features and exposures. Also it can provide an indication of the potential variation in the properties of the soil or rock mass and hence possible errors in calculations or assumptions, especially those assuming homogeneity. This statement from Fookes highlights the importance and usefulness of the ground model for any engineering project. The importance of the ground model is also highlighted by the consequences that can occur if you do not have a ground model, Design assumptions including strength parameters of soil and rock are subjective assessments, and therefore the design may have to be overly conservative, Even the presence and extent of soil and rock are often inferred An inappropriate solution may be proposed, Unexpected ground conditions are encountered in construction leading to changes in design or additional works, which are likely to incur additional cost. 3 AN EXAMPLE FROM WAIKATO DISTRICT The following sections provide some background about the Waikato District and describe a few of the slip sites in the Waikato District that occurred in July 2012, discussing different aspects of how the ground model was used for the repair of these slip sites, as well as highlighting the successful elements of this example. 3.1 Background The Waikato District covers approximately 418,893ha and has a population of 58,459. The road network is 2,364km; 1,683km are sealed while 681 remain unsealed (Waikato District Council, 2012). In July 2012 during a wet winter there was high intensity rainfall over two days which caused considerable flooding, damage and disruption to the road network within the Waikato District. The immediate response included the clearing of many over slips that had deposited debris onto the road and setting up of numerous warning signage and obligatory road cones around the under slips. This was capably handled by the Waikato District Roading Maintenance Team and their maintenance contractors.

5 As part of the immediate response a few of the slip sites that were considered by Waikato District Council to have an on-going high consequence of failure were inspected by the writer an Engineering Geologist to determine any immediate actions required to reduce the risk of further failure that may lead to significant consequences, e.g. road closures or damage to property. 3.2 Ponganui Road The slip at this site is an under slip that extended through one traffic lane of the road. The ground model at this site was developed initially through a site visit and mapping on site. Site observations were made on a rainy day so groundwater seepage was observed coming out of the cut slope above the road, over the surface of underlying mudstone. The second stage was a desk study of aerial photographs and geological maps. The desk study along with the site mapping was the basis for the intrusive site investigation which comprised several of topographical cross-sections measured using a tape measure and abney-level and a series of hand augers and Scala penetrometers to determine the extent of the mudstone which was observed in the cut slope above the road. The ground model was a simple cross-section through the site which showed the soils and the inferred extent of the underlying mudstone based on the site observations, refusal of the Scala penetrometers and scrapings from the end of the hand augers. This was the basis for the engineering design. The chosen remediation at this site was a rockfill slope to reinstate the road and shoulder, as well as being free-draining to address the significant seepages over the surface of the mudstone. The rockfill slope could be easily and cheaply adapted on site if the surface of the mudstone varied across the site. The ground model was completed when the slip debris was excavated and the mudstone was exposed, and as a result the configuration of the benches to key in the rockfill was modified on site during construction. Keys to success at this site were: A good geological ground model was able to be established with site observations and basic intrusive investigations, and The use of a flexible remediation meant that minor changes to design during construction did not result in increased cost or construction delays. 3.3 Te Puroa Rd This site is one of the large over slips which the writer inspected to provide immediate geotechnical advice for the clearance of the slip debris in order to re-open the road as quickly as possible and at the same time in a way that would reduce the risk of future movement given the ongoing wet weather. At this site the slip debris was 2m to 3m high over the road for a length of about 60m. With the tight timeframe the geological ground model and in particular the mode of failure could only be determined by site observations, including observations made by members of the Waikato District Roading Maintenance Team and maintenance contractor in the days leading up to the failure.

6 Figure 2: Photograph of the slip only hours after it occurred The depth of the slip was not known at first. It was not until the excavators clearing the slip debris uncovered the intact road surface beneath the debris, that we were able to determine that the failure was limited to the slope above the road. The temporary repair could only reinstated one lane to ensure some slip debris was left to buttress the remainder of the slope. The slip debris was also shaped to ensure that further rainfall would be shed and not allowed to infiltrate potentially triggering further movement. Figure 3: Photograph of the temporary repair of the slip The final repair at this site included battering back the upper slope, placing a rock buttress at the toe of the slope and installing horizontal bored drains. As the level of the groundwater was not confirmed and could not be observed with the slip debris remaining in place, the horizontal bored drains were included as a provisional item in the repair design. Once the excavation for the rockfill buttress was under way it became apparent that the drains were essential. Keys to success at this site were: Recognition that completely clearing the slip debris from off the road may have allowed further failure of the slope, Sharing knowledge; the observations of the Waikato District Roading Maintenance Team and the contractor were used in constructing the ground model, Use of provisional items in the design allowed the final design to be adapted based on the final ground model i.e. the actual ground conditions. 3.4 Ohautira Rd

7 The slip at this site is an under slip that extended through one traffic lane of the road leaving a near vertical scarp about 7m high and an approximately 80m long trail of slip debris down the vegetated slope. The ground model began its development with a site visit and mapping as well as a desk study. Based on the shape of the slopes on either side of the road it was clear that the road was formed by cutting the uphill slope and placing fill material to form the downhill side of the road. Therefore, it was initially hypothesised that the slip had occurred through embankment fill material, so the purpose of the intrusive investigation was to be focused on determining the extent of the fill material and to determine the soil type below the fill, which would be the founding conditions for rebuilding the road embankment. It must be noted that the uphill slope above the road was hummocky and considered to be prone to shallow instability so a cut retreat option, especially a temporary one, was dismissed. A topographical survey was taken using a reflector-less laser from the top of the slip scarp. But given the geometry of the slip scarp it was not considered safe to carry out exploratory holes within the slip as required to continue development of the geological ground model. Therefore, a second attempt at mapping the slip was undertaken by getting down the adjacent slopes and observing the slip from below. This proved successful as several small outcrops of in-situ material were observed within the slip. Figure 4: Photograph of the slip soon after it had occurred The ground model used for engineering design therefore was constructed using the site mapping and extrapolated using the topographical survey. The remedial option recommended by the writer and chosen by Waikato District Council was a 70 geogrid reinforced slope. The purpose of recommending this option was to allow flexibility during construction if the anticipated ground conditions were not realised. To manage this risk; the uncertainties of the ground conditions were communicated to and accepted by Waikato District Council; the engineering design assumed a pessimistic remedial slope height; and construction hold points were specified to inspect the actual ground conditions to allow design changes at the start of construction if required. The final outcome was that the slope height was 0.5m less than anticipated, however, the excavation revealed more than just fill material, as there were two buried topsoil layers the writer believes that there was also debris of a previous slip at this location that was subsequently buried in the formation of the road by the placement of a considerable quantity of fill material. Even though the actual ground model was more complex than first thought, the engineering design was sufficient to cope. Keys to success at this site were:

8 Mapping of the site was critical to the development of the ground model, Clear communication of risk with the client meant there were no surprises during construction, and The use of a flexible solution and construction hold points to finalise the ground model. 4 CONCLUSIONS In order to have cost effective lasting slip repairs for rural roads the development of the ground model is essential. Not only does the ground model enable a sound basis for engineering design and decision making, but without a ground model the risks of increased construction cost and delayed construction are increased as a result of unexpected ground conditions. Determining the ground model should at least include: aerial photo analysis, checking geological maps, site visit and engineering geology mapping, topographical cross-sections, and some intrusive site investigation. These are aided by an Engineering Geologist and their good knowledge of geomorphology, landslide processes, and local knowledge of the geology. The ground model is not final until the actual ground conditions have been revealed and it is advisable therefore to check the ground conditions during construction by means of construction hold points. Flexible remedial options that can be easily revised and modified during construction have proven to be worthwhile in order to cope with ground conditions that differ from the predicted ground model. 5 ACKNOWLEDGEMENTS I would like to acknowledge the Roading Operations Team at Waikato District Council for allowing me to use their slips as examples in this paper. I am also grateful for sage advice and input of Dave Dennison and the anticipated sponsorship of Opus International Consultants. REFERENCES Fookes, P.G. (1997) Geology for Engineers: the Geological Model, Prediction and Performance, Quarterly Journal of Engineering Geology & Hydrogeology November 1997 v. 30 no. 4 p Peck, R.B. (1969) Advantages and Limitations of the Observational Method in Applied Soil Mechanics, Geotechnique 19, No. 2, p Waikato District Council, 6 November 2012, District Overview, viewed 10 April 2013,