Structural design has been described by many as the use of materials not fully understood to make frames that cannot be accurately analyzed to resist forces that can best be estimated. Foundation design is no difference. In fact, even worse when you consider the highly probable nature of soils. In foundation design, accuracy is absolutely a fantasy, you cannot be a 100% precise. Hence, the need to exercise caution and sound engineering judgement when designing a foundation.
It is obvious that the function of any foundation would be to transfer the forces acting on a structure to the ground/soil supporting it. This, it must do safely within the allowable load-carrying capacity of the soil, else a failure would ensue. Foundation failures are mostly the most daunting and costly to remedy, and righly so, a damage to a foundation could mean damage to the structure in it entirety. Thus, it is extremely important to carry out a detailed site/soil investigation. This requires investigating the underlying soil layers to determine the soil condition, its properties and how it will probably react under loading. Without these pieces of information, it will be difficult and may not be possible to guarantee or even design a safe and economical foundation.
Indeed, the significance of quality information and understanding of the soil conditions on a site can’t be understated. The foundation choice and type chosen is completely reliant upon what the site examinations will uncover. It is commonly up to the structural engineer, with the guide of geotechnical designers and specialists, to decide the degree of this examination and decipher its outcomes. Site investigations just like X-rays and other tests on a sick patient is not an exact science. The investigation is laid down by geological activities, sometimes modified by previous construction work, mining etc. It requires detailed planning, testing and analysis and careful collection of information to be as reliable as possible. Most importantly it requires the application of sound engineering judgement, knowledge and experience.
Need for Investigation
A site investigation influences the choice and type of foundation, method of construction. It can also ultimately affect the design of the superstructure as an economic viable proposition. Hence, the designer, the contractor and the client have a “need to know”
The designer’s need to know is paramount and is at the top of the list because it is expected to reveal the suitability of the site for the proposed structure. Beyond the suitability of the site for the proposed structure, it must be proven if it can be built economically on the soil or if an alternative location is to be sought. The designers need to know to include but not limited to the following:
- The bearing capacity of the soil, settlement and characteristics of the soil
- Influence of new foundation on neighbouring foundations and structures
- Water table fluctuation, shrinkage and swelling characteristics etc.
Next on the list is the client’s need. A site investigation would afford the client the opportunity to know if a site is actually worth buying considering the issues likely to be encountered. For example,
- If the foundation will be slow and expensive to construct.
- Are the soil conditions complicated such that there are planning constraint on the proposed building.
- Will the site be subjected to flooding, subsidence or landslides etc.
The contractors need to know basically bothers on the need to have useful information that will assist in producing the most economical and safe construction. The need to know include:
- The stability of the soil during excavation and construction. For example soft mud and similar materials will not support heavy piling frames without matting.
- The need for geotechnical improvement processes such as chemical injection, stabilization or even dewatering.
- The presence of useful excavated material such as broken rocks for hardcore, sand for concreting and laterite as backfill material
- The need for any special health and safety precautions due to soil conditions.
Principles of Investigation
Before any site examination can be conducted, the extents of the investigation must be defined. This is dependent upon the sort of soil that is probably going to be encountered in the site just as the size of the structure that will be introduced there. For instance, the difference in the degree of site examination for a single-story residential dwelling when contrasted with a 20 story high business advancement is huge, yet the two structures require some comprehension of the strata they are being based upon.
It is for this reason that the site investigation is one of the most essential tasks during the design process, regardless of a new structure or the conversion of an existing one. Where loads to the substructure has been altered in any way, some understanding of the soil conditions is required.
Purpose of Site investigation
The primary purpose of carrying out a site investigation is to inform the structural designer of the substructure. It should aim at addressing the conditions of the site that may impact the type of foundations or substructure. This can be summarized as follows
Bearing Capacity: The bearing capacity of the soil/rock determines the size of the elements that make up a sub-structure, such as basements and foundations
Settlement: The amount of settlement a foundation is likely to be subjected to impacts on the choice of its solution. All foundations should be designed to limit settlement depending on the flexibility of the superstructure. New structures should be designed around a certain degree of uniform settlement, whereas adding to or refurbishing existing structures require knowledge of short and long term settlement criteria.
Soil composition: Properties of the soil within the site, such as the values of internal friction and cohesion are important to understand for the development of the foundation design.
Location of ground water level: The location of the ground water influences the bearing capacity of the soil, with granular soils being particularly affected by the presence of water as it reduces its strength significantly. The presence of water also influences displacement behaviour of soil when it is placed under load. In the case of basements, the ground water location is of paramount importance because if it is within the depth of the basement, buoyancy of the sub-structure needs to be taken into account, as well as the penetration of water through the walls of the basement.
Soil contaminants: If there are contaminants in the soil, these will have to be taken into account when designing the foundations. In some extreme cases the soil will have to be treated prior to any sub-structure works commencing. There is also the need to address the presence of gas in the soil, which may require the design to allow for venting of such gases.
Location of existing structures/services: Where sites abut other structures there is a need to determine what impact the proposed works will have on any existing foundations.
The scope of a soil investigation is dependent upon the size of the structure that is to be built. BS EN 1997-1 – Eurocode 7: Geotechnical Design – Part 1 General Rules defines three categories of structures that require increasing degrees of investigation
Scope of Site Investigations
GC1: Small and relatively simple structures e.g. single storey or retaining walls that are less than 1m in height.
GC2: Conventional structures to be founded on soil that is not deemed to be difficult, e.g. multi-storey residential property with no basement to be founded on uncontaminated gravel with a low groundwater level.
GC3: Unusual structures and/or soil that is difficult to use as foundation strata. Examples include sports stadia, very tall structures with unusual geometry and areas subject to frequent seismic action
It’s important to note that following these investigations it is possible for structures that were originally deemed to be within GC1 to become classifed as GC2 or even GC3. For example, the discovery of a previously unknown layer of silt within a site that was thought to be an area consisting entirely of stiff clay will make the foundation solution more complex. This revelation would push the proposed works into a higher geotechnical category.
Projects that fall into GC2 and GC3 do require thorough site investigations to be carried out prior to construction work commencing.
Prior to any investigations being carried out, a schedule of required tests and investigations needs to be developed. Both the UK specification for Ground Investigation by the Institution of Civil Engineers and Clause 188.8.131.52 of BS EN 1997-2 describes what the programme of works for the site investigation should include
Method of Investigation
Methods of site investigations are defined in both BS EN 1997-2 – Geotechnical Design – Part 2: Ground investigation and testing and BS 5930: 1999 (including addendum from 2010 Code of practice for site investigations).
There are five stages to site investigations. The first is a desk study of the site itself, which involves searching records for geological information, historical use and the location of existing services and underground structures such as mines and tunnels. This is normally carried out by the designer.
The next stage is the walk-over inspection of the site, commonly known as site reconnaissance. This is a visual inspection that determines the presence of any surface anomalies as well as the potential location of trial holes and other intrusive investigations and is typically carried out alongside the desk study. Both the site investigating contractor and the designer would carry out such a survey, as it informs the following stage.
The third stage is the intrusive investigations into the soil/rock within the site’s boundary. The extent of these investigations is defined by the designer, while the site investigation contractor carries them out. These investigations typically consist of relatively shallow trial pits and deep boreholes. The trial pits establish the condition of the uppermost part of the soil and also uncover any anomalies that have not been recorded previously, such as buried services and items of archaeological interest. Boreholes are deep cores that are sunk into the ground to determine the strata of the soil. They are located where the foundations of the proposed structure are to be placed, in order to ensure the most accurate and appropriate model of the soil is developed for that structure. This, however, is not advised for trial pits as they have a tendency to weaken the soil, hence they should be placed at a reasonable distance away from the intended location of the structure.
In addition to boreholes and trial pits, window samples of the soil and the testing of soil for groundwater and gas pressure are also carried out. Other methods of investigation such as drilling are detailed in BS EN 1997-2 – Geotechnical Design – Part 2: Ground investigation and testing.
Following these intrusive investigations, on-site tests can be carried out on the soil to determine its properties, all of which are conducted by the site investigation contractor. Examples are
Standard Penetration Test (SPT) – Measures the resistance the soil has to be penetrated by either a solid cone or split barrel sampler. Correlations are available for determining strength parameters for fine to coarse soils
Dynamic probing – Measures the resistance the soil has to be struck by a cone and provides the strength and deformation properties of soil
Cone penetration test (CPT) – Consists of pushing a cone into the soil via a series of rods at a constant rate. This test is used to determine the strength and deformation properties of both coarse and fine soil.
Pressuremeter test – Measures the deformation of soil via the expansion of a probe with a flexible membrane • Plate bearing test – This test measures the resistance the soil has against being subjected to an applied load from a steel plate. This can provide bearing capacity and deformation data on the soil
California bearing ratio – Used to determine the resistance a soil has against road traffic. It is a penetration based test and is described in BS 1377: Soils for Civil Engineering Purposes: Part 9, In-situ Tests
Field vane test – Records the resistance the soil has against the rotation of a vane to determine the soil’s undrained shear strength. This is typically used in clay soils
Dilatometer test – Measures the strength and deformation of the soil by inserting a steel probe into the soil and expanding a membrane that is attached to it. This test can provide shear strength, deformation and the current state of stress within the soil. Should only be used in clays and sands with small particles
Permeability test – Used to determine the ability of the soil to absorb and distribute water. This takes the form of a soakaway test that measures how long a volume of water takes to disperse within the soil. These tests provide the coefficient of permeability of the soil
Piezometer test – Determines the depth and condition of the water table within the soil. It is a key test whose results have a significant impact on the design of the substructure to a development
Gas test – In many instances it is possible for gasses to be produced and/or contained by soil. These can range from flammable forms such as methane to carbon dioxide, which can asphyxiate.
Table 2.1 in BS EN 1997-2 – Geotechnical Design – Part 2: Ground investigation and testing provide a summary of what applicable field investigation methods should be adopted based on the type of soil.
The final phase is lab testing, which is usually carried out by a separate laboratory on behalf of the site investigation contractor. The results of these tests should be read in conjunction with the field tests to create a complete picture of the make-up and condition of the soil/rock within the site. These tests can provide data on the following properties of Table 2.1 in BS EN 1997-2 – Geotechnical Design – Part 2: Ground investigation and testing provide a summary of what applicable field investigation methods should be adopted based on the type of soil.
• Natural moisture content • Liquid and plastic limits • Particle-size distribution • Unconfined compressions • Tri-axial compression (un-drained, consolidated un-drained and drained) • Consolidation • Swelling and suction • Permeability • Chemical analyses • Presence of gases including carbon dioxide and methane
The collation of this data is referred to as the Ground Investigation Report and its contents are defined in Clause 3.4 of BS EN 1997-1 – Eurocode 7: Geotechnical Design – Part 1 General Rules
The Ground Investigation Report provides the recorded observations that have been referred to previously, in order to describe the site conditions. This is normally carried out by a specialist geological engineer or engineering geologist who then passes this data on to the designer. The evaluation of this data is crucial in determining the properties of the soil and can be conducted by either the design engineer or the originator of the test data.
Note that as mentioned throughout this post, a site investigation is only an investigation of a small area of ground and therefore engineering judgement is very key to applying these results to the whole site.
Tomlinson, M. J (2001) Foundation Design and Construction 7th ed. New Jersey: Prentice-Hall
Institution of Civil Engineers (2011) UK Specification for Ground Investigation 2nd ed. London: Thomas Telford
The institution of structural engineers (2012)- Site investigations Technical guidance note, Level 2