Application of the Good Cheap Fast Rule in Geotechnical Engineering

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Published 16 December 2017
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Application of the Good Cheap Fast Rule in Geotechnical Engineering

Abstract

There is a commonly known golden rule regarding services to clients under the title Good- Cheap-Fast. The rule is that services provided can ultimately comprise two of the options at any one time. For example, if the client wants the service good and cheap it won’t be fast and vice versa. This rule can also be applied to geotechnical investigations.

The main purpose of geotechnical investigations regardless is to obtain and provide data sufficient to characterise site soil conditions and to provide parameters for design. A geotechnical investigation will also reduce uncertainty about ground conditions and reduce risks associated with foundation performance and cost variations in construction for a client.
When the client’s expectation of cost of the geotechnical investigations lead to the reduction in scope of the investigations, it can potentially lead to uncertainty in ground conditions and increased risk of unforeseen ground conditions, which in turn could potentially lead to unsatisfactory foundation performance and/or increased cost in construction and future repairs.

This paper discusses the challenges faced with small to medium scale site investigations in the wider Auckland area regarding the tensions between client expectation of costs of ground investigation and what is considered by the geotechnical profession as good practice. It also discusses common issues facing geotechnical professionals during the entire phase of the project between the initial site investigation through to construction based on the author’s local experience of projects within the Auckland region in the last 5 years.

1 INTRODUCTION

A geotechnical investigation is the assessment of ground conditions within a development and surrounding land. A geotechnical investigation and the understanding of ground conditions on a development site is an essential part of any project (MBIE 2016) whether it is the construction of large scale commercial buildings, subdivisions, public transport networks, public services or small scale private dwellings and other structures. In fact, the largest risk to a project often lies in the ground (GBICE 1991).

The two main purposes of geotechnical investigations are to identify geotechnical conditions of the ground on a site to provide geotechnical data for analysis and design and to reduce uncertainty about ground conditions to reduce and manage the risk of significant variations in the cost of a project (MBIE 2016, AGS 2006, CGIS 2016).
What is considered good practice for geotechnical services for a development project is summarised in Figure 1.1. It outlines the ideal sequence of events in an investigation and the monitoring and certification that follows.

The ideal service would begin with a study of the proposed development plans and a desk top study of existing information about the site and the surrounding land. A basic geological model of the ground conditions would then be developed and a scope of work together with a cost and time estimate would be formulated.

Figure 1.1: Flow Chart of ideal geotechnical service for projects

Once the proposal is accepted then the investigation would commence. The results of the investigation would either confirm the original geological model or lead to the development of a new model. Geotechnical recommendations for the proposed development would then be given in the geotechnical report. Although not essential, if the same geotechnical professional is included through to the construction phase of the project they would also review the final plans to be submitted for building consent to ensure that the plans are in accordance with the recommendations made in the report. The geotechnical professional would then go on to monitor earthworks and construction to confirm that the original geological model is valid. During the construction stage changes to the model and geotechnical recommendations may be made if ground conditions encountered during excavation and construction turn out to be different than what was initially anticipated.

The risk of issues occurring during construction due to unforeseen ground conditions depend on the quality and scope of the initial site investigation and whether the recommendations made in the geotechnical report are followed by the developer and contractor. As will be shown in the following sections below best practice above may not always be followed for a variety of reasons.

2 UNDERINVESTIMENT IN INVESTIGATIONS

Even though site investigation costs are a very small proportion of the overall cost of a site development (Clayton et al 1982), tensions between geotechnical professionals wanting to provide adequate data for foundation design and the developer, builder or owner wanting to minimise cost early on in a project are very common (MBIE 2016). If unexpected and undesirable ground conditions are encountered, which have a large impact on the cost and success of a project, it is often because of underinvestment in geotechnical investigations of a site (CGIS 2016, MIBE 2016). This underinvestment could lead to inadequate site investigations (CBICE 1991) There is strong evidence of a direct link between underinvestment in geotechnical investigations and large overruns in project costs (Figure 2.1) (CGIS 2016, MBIE 2016, McDonald 1994). It must be noted that significant variations in claims by piling contractors arise more from poorly known ground conditions than most other causes (Clayton et al 1982).

Figure 2.1: Impact of site investigation (SI) expenditure on UK highways contracts (MBIE 2016, McDonald et al 1994)

A possible cause of underinvestment in geotechnical investigations could be clients accepting proposals for investigations based on lowest price not best value or scope (CGIS 2016, MBIE 2016). This puts pressure on a Geotechnical Professional to push the scope of the investigation down in order that the job is won. Such situations can arise in a day to day practice. There is a possibility that these investigations would not achieve either purpose of an investigation, as stated above in the introduction. In contrast, cost savings in a project can be achieved if site investigations are carried out that have adequate scope to reduce uncertainty in the ground conditions (AGS 2006, MBIE 2016, CGIS 2016).

As well as underinvestment in geotechnical investigations, in some cases geotechnical reports have also been used for building consent applications that had not been issued for that purpose (Price et al 2015). It appears to be common to have contractors requesting site inspections of house construction where building consent have been issued and the only reports for the sites were reports that had been issued for the subdivision resource consent. In some situations, it was also found that in these reports there had been recommendations of further site-specific investigation that apparently were not picked up or not considered during the building consent application process. Usually when this occurs the recommended action is to insist on drilling additional boreholes on the site to confirm that the ground conditions within the specific site were as anticipated in the original report.

3 ISSUES DURING CONTRUCTION

The writer has worked as an Engineering Geologist for 14 years around the Auckland region. In that time, some projects have experienced problems during the earthworks and construction phase of a project (Figure 1.1) because the client, project manager or builder have tried to fast track to save time and to achieve savings. This has occurred by either not following specific recommendations provided for the work for a variety of reasons including; having not read or understood the recommendations, to finish the work quickly, they are unaware of their obligations due to inexperience, incompetence or a combination of the two, or they think the recommendations are unnecessary. In some situations, where no geotechnical investigation was carried out on a site the builder has noticed a potential problem with the ground and sought geotechnical advice and assistance.

The above situations have occurred on more occasions than they should and has often resulted in significant costs added to a project. Some typical examples are provided:

Figure 3.1: 3m deep unsupported cut near a boundary on an Auckland Property.  Figure 3.2: Slope failure on the cut slope after heavy rain.

3.1 Case Example 1

Figure 3.1 shows an excavation up to 3m deep located within about 1m of the property boundary, which was excavated vertically without proper sequencing or temporary support. The geotechnical recommendations were to either excavate the cut to construct the wall in stages or use temporary support to keep the cuts stable. The contractor decided to ignore the recommendations or were unaware of them and they excavated the cut all at once without temporary support and without any monitoring and inspection by a geotechnical professional. When this was discovered they were strongly recommended to install temporary support for there was insufficient room to batter the slope back to a safe temporary angle. The temporary support they did install was completely inadequate nor properly designed.

Unfortunately, a large rain storm event occurred a few days later and as a result a significant part of the cut slope failed. Figure 3.2 shows the failure of the cut and the inadequate nature of the temporary support the contractor installed. Fortunately, the slip did not propagate to the boundary and a large temporary timber retaining wall was designed and installed by the structural engineer to stabilise the cut. However, this wall added significant cost to the project and caused a delay in construction.

Figure 3.3: 3.5m high cut along the boundary of a property supported by a gravel toe bund.

3.2 Case Example 2

Figure 3.3 shows an excavation of up to 3.5m high where a slip occurred. Two geotechnical reports were written for this site prior to the earthworks being carried out. The second report, requested by Council, focused on the stability of the proposed cut and included a review of the structural engineer’s excavation methodology. Part of the cut was located at the bottom of a relatively steep slope and it extended along two boundaries where there was insufficient room to batter the slope back. Geotechnical recommendations were for the contractor to initially cut the slope at 1V:1H, install the retaining wall poles and then excavate behind the poles and install the wall in sections of about 3m lengths. However, the earthworks contractor did not follow these recommendations and excavated the cut vertically and all at once. To make matters worse they did not inform the geotechnical professional nor the structural engineer that this was taking place. Unfortunately, a large storm event occurred causing part of the cut along the steepest part to fail. They attempted to place a temporary retaining wall along the part of the cut that failed.

However, the regulators placed a stop work notice on the site and ordered them to place a temporary gravel toe bund as seen in Figures 3.3. At this point the geotechnical professional and the structural engineer who initially designed the retaining wall were reengaged.

A concrete pile retaining wall was constructed to retain the slip. As a result, what was initially going to be a 3.5m high timber pole retaining wall with 5m embedment became a 4m high concrete pile wall with 10m embedment to retain the slip (Figure 3.4). The cost of this wall is estimated to be at least 4 times what the original cost would have been. This situation also resulted in significant delays in the project.

Figure 3.4: Concrete Pile Wall Installed to retain the slip.

3.3 Case Example 3

Figure 3.5 shows footings for a house addition and extension. This project had been given a building consent for construction without a geotechnical investigation. The site was relatively level and located within an area that published geological maps showed was underlain by basaltic tuff. It appears that a geotechnical investigation was considered unnecessary as a result. However, the project experienced a significant delay when the builder noticed a potential problem with the ground conditions at the bottom of the excavated footings. The builder then sought the advice and assistance of a geotechnical professional to determine the size of the problem. Hand augers drilled on site indicated low strength volcanic ash soils over the upper 1.5m of the ground. As a result of this site investigation the shallow foundations had to be redesigned taking into consideration the lower strength soils using a reduced bearing pressure.

Figure 3.5: Footings for a house extension where low strength soils were encountered.

3.4 Case Example 4

In contrast to the above examples positive outcomes can occur if the client/developer gives due consideration to carry out site investigations that have adequate scope to reduce the uncertainty in ground conditions. One site investigation for a large warehouse, that had large floor and foundation loads on alluvial soils, required hand augers and machine drilled boreholes to assess both the shallow and deeper soils. This information was sufficient to provide adequate data on ground conditions to estimate settlement. During the detailed design process, additional CPTs were requested by the developer to better understand the consistency and variability of the ground conditions for the design of piled foundations.

4 Conclusion

The problems encountered during construction are caused either by fast tracking during construction, not following the specific geotechnical recommendations, by reducing the scope of investigations due to pressure to save on cost and time or by not carrying out a geotechnical investigation at all.

Contractors may face pressure by clients to keep cost down so they take on more risk by either not following recommendations made by geotechnical professionals or by not communicating their intentions with the geotechnical professionals. As the rule implied by the title above often they try and carry out a project cheap and fast. However, in many cases the end-product turns out not to be good or it is significantly more expensive than was originally planned. Local authorities may also be under pressure to process consents quickly to speed up the building process and either due to lack of resources or inexperienced staff are sometimes issuing consents based on inadequate of inappropriate geotechnical reports. There are concerns that the pressure to build houses faster in the Auckland region to meet the demand of a rapidly growing population is leading to an increase in problems caused by the above issues. It is obvious that houses need to be built faster to meet demand but projects should also be carried out properly with due consideration to best practice to reduce the risk of construction delays, budget blowouts and adverse effects on neighbouring properties.

It is essential that geotechnical professionals provide a service based on good practice. It may be necessary for the geotechnical professional to educate clients in what is considered best practice and the reason behind it. They should also endeavour to maintain their professional integrity by resisting the pressure to reduce the scope of investigations to win jobs based on the lowest price. Clients and developers need to be educated that awarding jobs based on lowest price will not necessary lead to cost saving. It is also highly recommended that clients are encouraged to use the geotechnical professional used for the initial site investigation throughout the entire life of the project to ensure that best practice is achieved and continuity of service.



Local Authorities should ensure that the appropriate geotechnical reports are used in building consent applications and that proper consideration has been given by the developer in all recommendations made by the geotechnical professional.

Clients, developers, earthworks contractors and builders should also be aware of their obligations under the resource and building consents. They should be aware that communication with geotechnical professionals is essential. They should also be educated in the risks involved and implications of not following through with geotechnical recommendations. Developers should also be careful in what contractors they use for a project. The cheapest contractor will not often be the best and may prove costlier in the long run. The geotechnical profession should also communicate better with developers and builders that trying to save money in the initial site investigation stages of a project runs a significant risk of increased costs and time delays further along in the process. The fact should be promoted that significant cost savings can be made in the project if site investigations have adequate scope like in example 4 above to reduce the uncertainty in ground conditions and good practice is carried out through an entire project. In turn, it should be emphasised that a “good” work may not be cheap and a “cheap” work may not be wholly successful.

REFERENCES

  1. Association of Geotechnical & Geoenvironmental Specialists (2006) AGS Guidelines for Good Practice in Site Investigation. Issue 2 Retrieved from http://ags.org.uk/item/ags-guidelines-for-good-practice-in-site-investigation-version -2/
  2. Ground Board of the Institution of Civil Engineers (1991) Inadequate site investigation. Thomas Telford Publishing, London.
    McDonald, M., Soil Mechanics Ltd (1994) Study of the Efficiency of Site Investigation Practices. Transport Research Laboratory, TRRL Project report 60.
  3. MBIE. (2016) Practice Advisory 17. Well-planned ground investigation can save costs. Retrieved from https://www.building.govt.nz/assets/Uploads/building-code-compliance/b-stability/b1-structure/practice-advisories/practice-advisory-17.pdf
  4. Price, C. & Alexander, G, (2015) Inappropriate use of geotechnical reports. Geomechanics News. New Zealand Geotechnical Society INC.
  5. Roberts, R. (2017) Comment on the Ground Investigation Specification Volume 0-Commentary, Introduction and Guidance. MBIE, New Zealand Geotechnical Society INC, New Zealand Drillers Federation INC, EQC & Auckland Council, Auckland Transport & Watercare. Retrieved from http://www.nzgs.org/library/nz-ground-investigation-specification/
  6. Simons, N.E. & Matthews, M.C. (1982) Site Investigation A Handbook for Engineers. CRI Clayton.


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