Navigating Your Geotech Report: A Comprehensive Guide

It is not unusual to think that geotechnical reports are just the collection of random soil tests, but that could not be further from the truth. The aim of a good geotechnical report is to interpret the soil tests results, not just to report on them.

As foundations are the means by which all of the forces a structure will experience are transmitted to the ground, their importance cannot be overstated. The stability of any construction depends on how firmly it is planted on the ground below it.

The basic content of a geotechnical report is comprised of:

• Reporting on history and geology of the site;
• Test carried out and reporting of the results;
• Interpretation of the tests with respect to the proposed structure.

Sometimes the scope of a geotechnical report can be extended to include other soil-related assessments and investigations. For example, a geotechnical report may sometimes reference nearby tectonic faults, whether a site is in a flood plain or if it had some possible chemical contamination. All these are sometimes deemed necessary not because they would influence the strength of the ground, but because they can restrict the range of possible options.

For example, a site on which contamination has been found, is preferable to not be excavated as that will require a whole array of environmental mitigation measures. It may be preferable on these sites to choose a foundation method which minimises soil removal (i.e. screw piles). This solution in itself may be more expensive than a classic one, but when the cost of environmental remediation is added, it becomes a more cost-effective option over-all.

It is important to keep in mind that a geotechnical report is primarily aimed at the other engineering specialties, and it is completely normal to feel a bit lost in all of the technical jargon. To use an analogy, a doctor’s referral to an MRI will rarely make sense to the patient, as it is aimed to communicate information between the two medical specialists, not to the patient. That is the case with soil reports. Engineers will be happy, if asked, to explain their findings to the non-specialist client, but generally speaking, the report will use the engineering vernacular.

For reading a soil report, it is best to start at the end, with the report’s recommendation section. It is the most important one, and even other engineers will jump straight to it. You can think of geotechnical reports as small scientific papers. We will first present the observations, then present and test the hypothesis and finally draw the conclusion. The previous sections of the report will therefore be supporting arguments, rather than conclusions themselves.

The size and weight of the proposed structure will directly dictate the types of soil investigations required for the project. A five storey building will need deeper soil investigations than a single storey dwelling.

As such the nature of the proposed soil tests will be tailored to the project. Some projects require deep testing while some will only require shallow tests.

Also of importance is the nature of the soil likely encountered at the site. If for example we need to investigate the soil at 5m depth but above that we have a layer of gravels 2m thick, a simple CPT test may not have the capacity to “punch through” the gravels and continue investigating the soils we target at depth. Because of this, other tests which will deliver equivalent results will need to be specified.

It is important to remember that different tests are aimed at providing physical information about some type of behaviour. For example, the tests aimed at checking the bearing capacity of a strip footing are not the same tests as those for deep bored piles and are different from those required to carry out a liquefaction assessment.

Some of the regular soil tests employed are:

Dynamic Cone Penetration (DCP):

Also commonly known as a Scala test it consists of an operator dropping a 9kg weight from a set height and recording the number of hits that are required to penetrate 100mm. In this way, the energy delivered to the cone at the tip of the rod is kept constant and empiric relations can then be applied to assess the Ultimate Bearing Capacity of a soil. This is considered to be a shallow test as it cannot realistically exceed 3m. Due to its very simplicity it is the most commonly carried out test.

Hand Augers (HA):

A usual 50mm scoop is advanced into the ground much like a screw and the soil is retrieved for identification. The soil will of course be broken up (disturbed sample) so the information provided will mostly serve at identifying the type of soil we are likely to encounter at a certain depth. It is mainly used at identifying shallow soils and very rarely goes deeper than 3m.

Test Pits (TP):

Rather than carrying out a hand auger investigation, in certain instances, we will recommend that a test pit excavated using a machine (excavator) is preferable. This may save valuable time for the operator and lower the overall cost of the testing procedures. Just like with the hand auger, information about the soils are recorded.

Shear Vane Testing:

Most commonly in New Zealand, this test is carried out using a handheld shear vane. Two vanes in a cross pattern are pushed into the ground and then rotated until they rotate freely. This test is only to be carried out on clay soils and is therefore mostly encountered in the upper parts of North Island. Silts and coarse sands yield erroneous results and the test is not reliable for these soils. The test is mostly carried out within the first 1m of soil, hence it can be considered to be a shallow type test. Machine fitted tests may be able to carry out the test at greater depth but these are highly specialised and not widely used around the entirety of New Zealand.

Ground Penetrating Radar (GPR):

This can be thought of as an ultrasound equivalent but for the soil. The device will emit a radio wave into the soil. As sound travels faster through dense material than through lower density ones, the reflected wave is measured and gives an indication of the change in layers. This is a shallow test and mostly used to check for pipes and other services before other soil tests are carried out rather than offering direct information about the in-situ soil.

Cone Penetration Test (CPT):

A cone is pushed into the ground by a hydraulic press at a constant rate. The cone is fitted with sensors measuring the resistance on the tip and on the sides of the cone. For example, when going through a very “sticky” clay, the cone resistance will be low but the skin friction will be high. Conversely, when going through a coarse sand layer, the tip resistance will be high and the skin resistance will be low. The relationship from these two readings is analysed and based on many empirical relations, other information about the encountered soils are derived. This test has become the industry standard for liquefaction assessment. With the ability to reach depths of up to 25m, this is considered to be a deep soil test and is carried out by specialised contractors.

Machine Drilled Borehole (BH):

This can be carried out with/without Standard Penetration Test (SPT). If soil sample retrieval is required from greater depth, a borehole may be the only way to do that. Different types of machines will advance a hollow tube through the soil (and even rock) to a great variety of depths (10, 20 or even 30m are common). The tube is pulled out of the ground at certain intervals and the soil samples are deposited into boxes for later reference and identification by experienced geo-professionals. At set intervals, the geotechnical engineer may order an SPT test be carried out. A smaller diameter hollow tube is then lowered and a standardised hammer is driving the tube into the soil, recording the number of blows it took to drive it a certain depth. Many empirical relationships are available to estimate the soil characteristics based on the number of recorded blows. This test is considered to be a highly specialised test and geotechnical consultants will generally hire specialised sub-contractors to carry out this deep soil test.

Clegg Impact Hammer (CIV):

This test is aimed at measuring the compaction of a given soil (quality control). The use of the test is not very well standardised and the results are generally considered to be indicative in nature and are correlated with other observations, rather then relied on implicitly. The test measures the acceleration of a hammer dropped onto a soil and based on that a unitless number is displayed. The higher the number, the better the compaction. This test is used to certify the quality of a fill rather than determining the in-situ soil characteristics.

Nuclear Densometer (ND):

A low radioactive source is used to read the density of the soil. The test itself is safe for everybody involved if used correctly and by accredited personnel. Much like the Clegg Hammer, the test is used for quality assessment of compacted soils. If a project calls for the use of compacted gravels under a foundation, this test is used to make sure the gravels have been properly laid.

There are a great number of other tests but they are not commonplace and are required by the geotechnical engineer depending on the project.

Executive Summary:

Provides an overview of the key findings, conclusions, and recommendations of the report. This is present only when the volume of information presented is considered to be too cumbersome for a normal presentation, and also the report is requiring non-engineers to make executive decisions.

Introduction:

Outlines the purpose, scope of the project, and the applied methodology of the geotechnical investigation.

Building Proposal:

Outlines the details of the proposed building work about the project.

Site Description:

Describes the location, topography, active faults, and geology of the site, as well as any existing structures or other site features.

Desktop Research:

Provides initial site understanding, identifies potential geohazards, supplements any nearby existing field data, and offers background information, working together with field investigations to inform site development and design decisions.

Site Investigation:

We present the detailed results about the hand auger tests, DCP, CPTs, test pits, and other field tests conducted during the investigation.

Geotechnical Analysis and Recommendations:

This crucial section discusses the potential geotechnical issues and provides recommendations for foundation design, excavation, slope stability, and other aspects of the project.

Generally speaking, the geotechnical report has to be proportional to the size of the project. While for a simple dwelling, a five page report will suffice, the same cannot be said for a bridge along a State Highway. Due to the more complex interactions between the structure and the soil, heavier structures require more testing and more analysis. This is generally reflected by the volume of the report.

For a regular dwelling, a geotechnical report should generally contain an adequate number of shallow tests, references to deeper tests, an analysis of the results of the soil tests and the final recommendations for the design of the foundations.

When reviewing the geotechnical report, focus on key information such as ground model, groundwater conditions, potential geotechnical hazards, foundation recommendations, and earthwork and excavation considerations. Look for any identified risks, such as soil liquefaction, ground settlement, expansive soils, or slope instability issues, and the recommended mitigation measures.

The geotechnical report represents a study on the ability of the soil at a specific site to support the intended structure. A geotechnical report will consider the loads it needs to carry (calculated by the structural engineers) and the ability of the in-situ soils to do that without too much deformation (settlement).

The geotechnical report is also aimed at identifying the future risks associated with the ground such as risk of liquefaction, presence of tectonic faults, slope stability, etc. All these risks are taken into account and the likely short and long term performance of supported buildings is then calculated.

It is therefore easy to see why a suitable geotechnical report is the foundation of every construction project.

Approximately 50% of the cost of a geotechnical report is represented by the cost of in-situ testing. Larger buildings will transmit their loads further down into the soil needing deeper tests. They are also more sensitive to potential settlements requiring additional analysis than a simple one storey dwelling.

Because of the diversity of soils, geotechnical engineering is not as standardised as other civil engineering disciplines. Structural engineers for example, will most of the time work with codes that dictate precisely what the loads taken into account shall be, what the maximum wind action at a location will be, what is the maximum earthquake force a building is supposed to resist. Soil however can vary significantly over a certain area and therefore such a flowchart approach is not possible. Instead, all the other civil engineering disciplines rely on geotechnical engineers to apply their knowledge of soils and supplement it with in-situ testing in order to calculate the potential behaviour of the proposed structures.

It therefore becomes easy to understand why a single storey dwelling located on thick gravel layers will not be equivalent to a multi-storey office building on complex soil strata having a likelihood of liquefaction.

The complexity and therefore the cost of a geotechnical report will be given by the following factors:

• Complexity of the intended structure;
• Complexity of the soils at the site;
• Number and types of soil tests required;
• Complexity of analysis needed to calculate the soil response to structural loads;
• Amount of coordination needed with other engineering specialities.

When a new building is planned for a site, a Building Consent is needed to be sought. The purpose of the Building Consent is to give the opportunity to the Local Authority (LA) to check that the proposed building will comply with the Building Act. As such, the representatives of the Local Authority will need to make sure that the designers of the building have taken all necessary measures to assess the potential risks associated with the proposed building and have taken action to limit the potential problems that may arise during the intended lifetime of the building (generally 50 years) to the maximum values prescribed in the Building Code.

One of the ways that this proof is met, is by ensuring that a competent and adequately sized geotechnical report and site investigations have been carried out and that the building is deemed to perform its function over its lifetime. Hence, geotechnical reports are requested by the Local Authorities in the process of authorising new works associated with a property (new build or additions).

In some cases, there are a minimum number of soil investigations required depending on the proposed project, but most of the time this is left to the care of the geotechnical engineer who will use their experience and knowledge to determine the minimum number of tests which will help with investigating these risks. It sometimes happens that the geotechnical engineer’s reasoning is not fully acknowledged by the representatives of the Local Authority and that additional tests or reasoning need to be presented in support of a proposed solution. These are known as Requests for Information (RFI) from the LA and need to be addressed by the engineers during the consenting process.

RFIs are one of the major contributors to additional fees charged for checking the Building Consent application and a source of frustration as it can lead to delays in approval of a project. An experienced geotechnical engineer will generally have a very good understanding of what the LAs want to see during the checking process and will tailor the report to present the data and recommendations in a format that leaves no room for interpretation and leads to a quick approval from the LA. That being said however, minor RFIs during the consenting process are not uncommon.

Although they can be perceived as a confrontation between the project engineers carrying out the design and the engineers checking the building consent application, we suggest that a thorough peer review is at the heart of any good and successful project. Hence, we see the RFIs as an integral part of a robust and engineeringly sound process, where different specialists look at the data presented and work towards a common goal, ensuring nothing was overseen and all the risks associated with a project have been fully understood and mitigated.

All constructions have some kind of interaction with the soil which provides stability to them. In order to establish if enough support is being offered, a geotechnical engineer will direct different tests to be carried out on the soil, followed by interpretation and recommendations. These are collectively known as a Geotechnical Report.

Geotechnical Reports are aimed at other engineering specialists who will rely on the interpretation offered by the geotechnical engineer to guide them through their own design and the interaction their part of the project will have with the supporting soil.

Geotechnical Reports also form part of the design documentation which needs to be submitted to Local Territorial Authorities (City, District or Regional Councils) as part of the Building Consent Application. The discussions about technical issues between the engineers of the Councils checking the work and those of the client carrying out the work, are known as Requests for Information (RFI). RFIs are fundamental in ensuring all risks to a project are understood and mitigated.

Eliot Sinclair have team of qualified Geotechnical Engineers who can assist with all aspects of geotechnical reporting. Contact our friendly team to see how we can help you.