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What are Rigid Inclusions

What are Rigid Inclusions?

Rigid Inclusions have become increasingly more common in UK ground engineering practice, the technique allows the use of an alternative ground improvement approach on sites and in ground conditions which are not best suited to more flexible techniques such as vibro stone columns.

Most often at present rigid inclusions are formed using displacement augered methods, as this system is very well suited to the nature of ground conditions where rigid inclusions offer their greatest benefit. Although, this need not always be the case, many other traditional piling methods can be used as part of a rigid inclusion scheme, for instance –

Soft soils overlain by very stiff soils, where premature refusal of a displacement auger might occur. In this case pre-boring could be carried out prior to installing displacement augered rigid inclusions, or alternatively, replacement augered piles (CFA) could be used to form the rigid inclusions.

Commercial floor slab replacement in restricted access, low headroom conditions. For a project of this nature, steel cased bottom driven piles could be used as rigid inclusions, avoiding the need for an expensive suspended piled floor slab.

Remote sites without a dependable supply of concrete. For sites outside of the serviceable radius of the nearest concrete plant, pre-formed driven displacement piles could be used as rigid inclusions, removing concerns over material conformity.

Many more examples exist, and the technique is readily adaptable to a wide range of foundation types and structures.

In this blog we will explore some of the benefits of the Rigid Inclusion technique, along with some of the key technical considerations.

Benefits of Rigid Inclusions

Key benefits of the technique can be listed as –

Use of Shallow Lightly Reinforced Foundations: When installed to an appropriate area replacement ratio, rigid inclusions below pad and strip foundations can successfully limit settlements to < 25mm under unfactored bearing pressures of up to circa. 250kPa. When detailing foundations over rigid inclusions, consideration must be given to load transfer platform thickness, inclusion spacing, and the maximum permissible load on individual inclusions. These points will be discussed in greater depth below.
Reduced Slab Thicknesses: For commercial developments, where the cost of the floor slab represents a relatively large proportion of the overall build cost, the use of rigid inclusions will help to minimise slab thickness and reinforcement content. The preeminent design guide for Rigid Inclusions – ASIRI (2012) – gives a lot of attention to the design of ground bearing slabs, and we will discuss some key points below.

Improved Bearing Capacity, and Load/Settlement Performance: When compared with vibro ground improvement alternatives, rigid inclusions can offer bearing capacities up to 100% higher, for a similar settlement performance. Or alternatively, rigid inclusions can offer improved settlement performance compared with vibro stone columns, at equivalent bearing pressures.

Cost Efficiencies: Although the overall package costs for Rigid Inclusions, taking into consideration the cost of constructing the load transfer platform, will be higher than the costs associated with vibro ground improvement, use of the technique will result in significant savings when compared to a fully suspended piled slab and substructure.

Key Considerations

There are several aspects of rigid inclusion design and implementation that need to be understood when assessing the suitability of the system, some of these considerations will be discussed in more detail below.

Load Transfer Platform

The Load Transfer Platform (LTP) is arguably the most critical aspect of the successful design and implementation of a rigid inclusion scheme. It must be dimensioned appropriately to the size and spacing of the rigid inclusions, the stiffness of the shallow soils, and the magnitude of the applied loads.

ASIRI 2012 recommends –

The load transfer platform must exhibit high shear strength and stiffness to ensure that internal arching effects develop, and may, in the case of an LTP formed after completion of column construction, be reinforced with a geotextile layer.

In UK practice it is commonplace to form the LTP at, or near to, underside of slab level and to use it to support the piling rig during the rigid inclusion construction phase. This requires that the rigid inclusion head levels are reduced by some means after construction is complete. In the case of augered rigid inclusions this is generally carried out while the concrete is still wet, through localised excavation. The LTP is then recompacted over the rigid inclusion heads to bring it back up to the correct level. This process is clearly sub-optimal if it is not strictly controlled, as it leads to an increased potential for localised areas of LTP with reduced strength and stiffness, directly over the RI locations. The optimal solution from a design perspective is to install the rigid inclusions from a working platform of suitable thickness, at some proportion of the required final LTP thickness, to reduce the levels as described above whilst the concrete is wet, and to then build the LTP up to its full design thickness once all RI’s are installed.

When preparing a load transfer platform, or finalising proposals for RI head reduction, the elevation tolerances present in ASIRI (2012) should be recognised –

In short, RI’s must have a depth of penetration into the LTP and should be within 50mm of design level at completion.

The important thing is that this is discussed and agreed before any work begins!

Maximum and Minimum RI spacings.

ASIRI (2012) is explicit in its recommendations for maximum and minimum rigid inclusion spacings, as a function of RI size, and the proposed testing regime.

Minimum spacings –

Maximum spacings –

In UK practice, where rigid inclusions are most often installed at diameters < 500mm using displacement augering techniques, a minimum centre to centre spacing of 4 x diameter would be expected, along with a maximum spacing of 3.0m or 9m² of foundation area per RI.

Maximum Load on a Single RI.

For small pad foundations supported on a single column, ASIRI (2012) stipulates that –

This, coupled with the minimum spacing criteria noted above, sets some limitations on the maximum bearing capacity that can be applied to small, isolated pad foundations.

Whilst not all the load transferred at top of LTP level will be transferred to the RI, it is clear that at the higher bearing pressure ranges, 250kPa – 300kPa, this upper limit of 150kN unfactored load on a single inclusion could be exceeded.

ASIRI (2012) also places limitations on the minimum required distance from edge of RI to edge of footing as –

Taking all these points into consideration, and as an example, for a typical RI diameter of 320mm formed using displacement augering methods, a single RI could be installed below a pile cap with dimensions of approx. 0.6m x 0.6m. To increase this to an appropriately sized cap over two RI’s, without contravening the minimum spacing constraint of 4 x diameter, would require a pile cap as large as 1.5m x 1.5m.

Foundation and LTP Overhang.

The reaction of the rigid inclusion at underside of LTP level is shown diagrammatically below, with conditions required for development of the full shear failure mechanism.

In most practical cases of isolated foundations, and at the edge of a slab, insufficient overhand may exist for this full mechanism to develop. The ASIRI (2012) document considers the range of conditions encountered in practice, with a procedure detailed for the reduction in the design value of N_q (bearing capacity factor) from a maximum at L_max, to a minimum in the zero-overhang case, again this is shown diagrammatically below –

This does not limit the use of RI’s but provides an additional factor for consideration and inclusion in the design validation report.

Reinforcement Content

Whilst most Rigid Inclusion projects comprise unreinforced displacement augered concrete inclusions, on occasion it might be necessary to reinforce the rigid inclusions against any shear forces or bending moments which might develop. This is a potential concern for RI’s at the perimeter of the loaded area, below an embankment slope, or for RI’s supporting foundations subject to inclined loading.

Want to know more? Contact us today to find out how we can help you with your rigid inclusion project.

Advantages and Disadvantages of Vibro Stone Columns

Vibro stone columns are a popular ground improvement solution used to increase available bearing capacity and reduce soil settlement below shallow foundations & ground bearing slabs.

Vibro stone columns can be formed using various methods, comprising either fixed mandrel or vibroflot tooling, with aggregate placed either via top feed or bottom feed methods. In all cases the completed ground improvement work consists of pillars of densely compacted aggregate formed through shallow compressible soils.

While we at GeMech offer this method of ground improvement within our range of solutions, it’s important to consider both the advantages and disadvantages to determine if it’s the right solution for a particular project.

Advantages of Vibro Stone Columns

Improves Soil Strength and Stability

One of the primary benefits of using vibro stone columns is the significant improvement in the strength and load-bearing capacity that can be achieved in compressible soils, particularly where a high area replacement ratio is adopted in design. This allows more heavily loaded and smaller foundation footprint areas to be adopted, whilst maintaining a satisfactory factor of safety against shear failure.

Reduces Settlement

Typically, although not in every case, stone columns help to transfer loads through the shallow compressible soils, to deeper, more stable layers of soil at depth. Whilst in the majority of cases this situation exists, it must be recognised that ‘floating columns’, which do not terminate in a more competent soil, can be used in some circumstances to good effect.

Vibro stone columns are effective at reducing total settlement, with settlement reduction factors of up to 2.5/ 3.0 commonplace for shallow foundations over vibro stone columns. The completion of ground improvement on a regular grid aids in producing greater uniformity of load/settlement response, helping to minimise differential settlement, which is critical for ensuring the long-term serviceability of structures.

Improves Drainage

Stone columns are formed using graded aggregates with a target grading of 40 – 80mm, resulting in the presence of high void ratio high permeable columns at regular centres in the improved soil mass. This can lead to greatly reduced consolidation periods when vibro stone columns are installed in fine grained (Clay & Silt) soils, making the technique very suitable for use in conjunction with static surcharging, or for earth embankment support where permissible rate of construction can be increased owing to the beneficial effects of the stone columns.

In addition to the improved consolidation performance, the stone columns will also provide a preferential pathway for downward migration of surface water. This can be particularly useful on sites where shallow impermeable Clay soils exist over more permeable soil layers at depth. Puncturing the Clay soils with stone columns will allow surface water to migrate downwards more rapidly into the permeable soils, helping with site drainage during the construction stage. Although, in these circumstances the potential for downward migration of contaminates must be considered.

Cost-Effective Solution

The use of Vibro stone columns in conjunction with lightly reinforced shallow foundations and ground bearing slabs will often result in a more economical substructure solution than traditional fully suspended piling alternatives, especially for medium-sized structures or larger structures with a reasonable tolerance for settlement – typically, vibro stone columns are designed to limit long term settlements to < 25mm.

Environmentally Friendly

Moving from traditional piling methods to vibro ground improvement can have several embodied carbon benefits –

Working platforms tend to be less onerous for vibro stone columns rigs than for comparable sized CFA or top driven piling rigs.

Stone column installation does not lead to the generation of spoil for offsite disposal.

Stone columns can be formed, in most circumstances, using recycled aggregates. This results in lower embodied carbon when compared to the use of quarried material, and a significant reduction when compared to traditional piling methods which require the use of carbon intensive concrete.

Total reinforcement weight in the foundation will be lower with vibro stone columns when compared to a traditional piling solution.

The thickness of slabs formed over vibro stone column improved soils will be less than for slabs suspended over traditional bearing piles.

Although potentially larger in width and footprint area than suspended foundations over bearing piles, shallow pad and strip foundations cast over improved soil tend to be very thin – in the order of 300mm – resulting in a net reduction in concrete requirements for the substructure foundations.

Flexible Usage

The method is highly adaptable and can be used for a wide range of structures and ground conditions. GeMech have successfully used vibro stone columns for a wide range of projects, including embankment construction, residential housing projects, commercial and student accommodation buildings – up to 5 storeys, tank bases, salt barns and industrial units.

Disadvantages of Vibro Stone Columns

Some Limitations on Suitable Soils

While vibro stone columns are great for improving compressible soils, they are not suitable for every soil condition.

In Clay soils a minimum shear strength is required to provide lateral support to the stone columns in bulging. For residential housing projects a minimum undrained shear strength of 30kPa must be proven to enable use of vibro stone columns – this is driven by building warranty guidance. On commercial projects, or projects not subject to building warrantor conditions, vibro stone columns can be successfully used in Clay soils with an undrained shear strength as low as 15kPa, provided bearing pressures are low and the structure has reasonable tolerance to settlement.

Stone columns should not be used on sites with extensive layers of Peat, or where organic soils are present which cannot be relied upon to provide lateral support to stone columns in bulging, and which might decompose over time.

Stone columns will provide no betterment in terms of required foundation depths in soils which are subject to volume change owing to either water demand of trees and planting, or seasonal variation in water contents.

The viability of Vibro stone columns, and ground improvement methods in general, should be carefully considered on sites where recent placed poorly compacted Clay fill exists. It will be difficult to control the ongoing self-weight consolidation settlement of a recently placed Clay fill with Stone Columns, particularly when this ongoing settlement is considered in conjunction with the settlement resulting from the loads imposed by the new structure. Collapse compression is also a very significant risk in poorly compacted recent Clay fill, this will not be controlled by stone columns, but may actually be accelerated owing to the increased permeability of the soil mass after completion of stone column construction.

Lastly, and in common with the point raised above with regards to Peat and organic soils. Stone Columns should not be used in Made Ground which contains a significant proportion of organic material which might be subject to longer term decomposition. Decomposition of the material supporting the stone columns will result in a reduction of lateral support, which in turn will manifest as increased long term foundation settlement.

Vibration-Induced Problems

Vibro stone columns are formed using vibratory processes, either at the vibroflot tip, or a fixed mandrel attached to a high frequency rig mounted vibrator.

By virtue of the high frequency levels associated with vibro treatment, the ground borne vibration levels are unlikely to be damaging for robust structures, but in many cases perception of vibration is as significant as the risk of damage.

Effective methods, such as vibration mitigation trenches, and pre-augering can often be used to minimise the transfer of ground borne vibration to neighbouring structures. However, on highly sensitive sites, low vibration non vibratory replacement piling methods might be preferable.

Not Suited for High-Rise Structures

Owing to the flexible nature of the improvement method GeMech limit the use of vibro stone columns to a maximum of 5 storeys, beyond this height angular distortion can become a concern.

This situation rarely develops, as the loading requirements for high rise structures tend to be in excess of what is achievable using flexible ground improvement methods.

Limited Depth of Improvement

Using our in-house fleet of top feed Vibro Stone column rigs GeMech can install stone columns to a maximum depth of 7.0m. Where compressible soils existing beyond this depth and the loading regime for the new structure results in the transfer of significant loads to this depth – typically, increases in effective stress are considered ‘significant’ when they exceed 20% of the pre-existing effective stress state – alternative ground improvement or piling methods may need to be considered.

GeMech can, and have successfully on several occasions in the past, undertake partial depth treatment ground improvement schemes, with stone columns installed to a predetermined target depth within a layer of Made Ground or compressible soil. This depth is calculated based on long term settlement potential, taking into consideration any settlement occurring in the unimproved soil below pile toe level.

Want to know more? Contact us today to find out how we can help you with your vibro stone column project.

How Do Piling Companies Choose the Right Piling Methods?

How Do Piling Companies Choose the Correct Piling or Ground Improvement Method?

It’s important for Ground Engineering specialists to select an appropriate method for each individual project, taking into consideration details such as, but by no means not limited to –

Access, headroom and working room constraints
Proximity to neighbouring structures
Ground conditions
Loading requirements
Required load/settlement performance
Long term structural durability

The vast range of methods available in the Ground Engineering marketplace provides contractors with a lot of potential options, and on many occasions, there will be more than one method suitable for a given project.

It is important that whatever system is adopted it is safe to construct and will result in a foundation which will remain fit for purpose throughout the design life of the completed structure.

In the highly subjective discipline of Geotechnics, the term ‘correct’ is itself subjective, and it must be recognized that even the most thorough review of ground conditions is limited by the scope and quality of site investigation information made available.

In this blog, we will provide an insight into how we at GeMech select what we consider to be the most appropriate piling or ground improvement method for a project under review.

Access, Headroom and Working Room Constraints

Before making any assessment of the technical aspects of pile structural capacity, load/settlement response or ground conditions etc., firstly it is important to understand which equipment can be safely delivered and used on site efficiently. This can often be confirmed via a desk study, but in some cases a site visit is necessary before a design can be meaningfully progressed.

Soil Type and Ground Conditions

To safely and efficiently design deep piled foundations or ground improvement, it is important to have a detailed understanding of the ground conditions across the entire building footprint area. Some key considerations when preparing a ground investigation scope and specification are –

Ensure levels are taken at each borehole and trial pit location, allowing the soil profile to be related to proposed building foundation levels.

If bedrock is anticipated, ensure that the scope of intrusive investigation works is sufficient to record the gradient of the rockhead and the direction of dip.

Accurately characterise strength and stiffness of all soil layers, using a combination of in-situ and laboratory techniques.

In soft or loose soils displacement methods, or vibro ground improvement might be preferable, provided location and structural intent are suitable.

In stiff clays, or weak rock formations CFA methods might be preferred.

Where competent rock formations are present, particularly at shallow depths, or where rock sockets are required to generate large lateral or tensile capacities, rotary bored or rotary percussive methods are often best.

Load Requirements

Each method of piling or ground improvement is associated with a range of safe working load capacities in the compressive, tensile and lateral directions. Large diameter rotary bored piles are associated with the highest capacities available for traditional piling projects, whilst rigid inclusions offer the largest capacities available for ground improvement projects.

Where access, working room, ground conditions and structural form are best suited to CFA piling for example, the most cost-efficient piling solution will be achieved with the smallest potential pile diameters appropriate to the specified loading conditions.

For ground improvement projects, where the shallow soil conditions permit, vibro stone columns will be the most cost-effective solution for supporting bearing pressures of up to 150kPa and 50kPa for isolated foundations and ground bearing slabs respectively.

Alongside considerations of ultimate load capacity, the likely settlement performance of the chosen solution under working load conditions must also be factored into the decision-making process. As a guide, a hierarchy of expected load/settlement performance, from highest to lowest anticipated settlements, can be listed as –

Vibro ground improvement
Rigid inclusion ground improvement
Bearing piles – all/a proportion of load transferred to pile toe level
Bearing piles – all load carried in shaft friction alone

Environmental Considerations

Environmental factors can play a role in determining the most appropriate piling or ground improvement method.

For example, factors such as ground water level, proximity of the site to bodies of water and rivers, or the presence of neighbouring structures can influence decision-making.

Vibration damage is a function of both the magnitude of vibration – measured in peak particle velocity – and the frequency of vibration. As a rule, high levels of low frequency vibration should be avoided in close proximity to existing structures, particularly sensitive structures, and first consideration should be given to replacement piling methods.

In close proximity to bodies of water, or where ground water is at shallow depth, pre-formed driven displacement pile types might be favourable, giving the combined benefit of not generating saturated spoil arisings at working platform level, in addition to avoiding the environmental concerns around placing concrete near watercourses.

If the near surface soil is contaminated, displacement piling or ground improvement techniques are typically preferred as they do not produce contaminated arisings for off site disposal, and they limit exposure of the site workforce to contaminants during the construction phase. However, it is important to consider the potential detrimental effects to shallow aquifers if displacement piling methods lead to either preferential pathways for downward migration of contaminated surface water, or to contaminated materials being displaced into the aquifer during the construction process.

A solution such as vibro stone columns would be good option where no concerns with downward migration of contaminants exist. However, if contamination to shallow aquifers is of concern, steel cased bottom driven piling might be preferred, as the pointed tip of steel cased bottom driven displacement piles tends to result in lateral displacement of material during driving, rather than downwards displacement of material into the underlying soil layers.

In a more unusual set of circumstances, GeMech have been involved in projects where the use of displacement driven piling or vibratory ground improvement methods have been specifically precluded due to the potential detrimental effects on migratory fish in a neighbouring river. Under these conditions only replacement or displacement augered methods could be safely used.

As an overarching environmental sustainability consideration, the chosen solution should minimise, as far as possible, the use of carbon intensive materials such as concrete. The carbon reduction associated with a conversion from traditional piling methods to vibro ground improvement must be recognised – particularly on projects where columns can be constructed using recycled aggregates – and every effort made to convert to ground improvement methods wherever the project requirements allow.

Material Availability

Whilst this is seldom an issue for projects in the UK, the local availability of suitable materials must be part of the decision making process. Some examples of potential constraints and concerns are provided below –

Remote sites, located a significant distance from the nearest concrete plant – can the project be serviced by the concrete supplier to enable the use of CFA piles, or will the material be out of specification before it can be placed?

Remote sites with limited access for articulated delivery lorries – can U/C sections, or pre-formed pile sections be safely and efficiently delivered to the working area?

For ground improvement projects, can a sufficient quantity of suitably graded aggregate be procured locally?

GeMech offer a broad range of piling and ground improvement techniques, allowing us to offer the most cost efficient and technically appropriate system, taking all of the various constraints into account.

Want to know more? Contact us today to find out how we can help you with your vibro stone column project.

What is Ground Improvement Testing?

The term ground improvement covers a range of techniques for improving the shear strength and settlement characteristics of the near surface soils, either through immediate compaction and densification, or through reinforcement with materials of high shear strength.

Whilst not providing an exhaustive list, this blog explores some of the different types of testing used to validate the range of ground improvement methods GeMech offer – vibro stone columns, dynamic compaction and rigid inclusion – their importance, and their applications.

We will also explore some of the common terms used when discussing the design and validation of ground improvement work for the purposes of ground bearing slab design.

Ground improvement testing can be differentiated between methods used for quality control checks of completed elements of the works, and those used for the estimation of long-term settlement performance.

Types of Testing Commonly Used on Ground Improvement Contracts

1. Plate Load Testing

Small diameter plate load tests – using test plate diameters ranging from 450mm up to 600mm – are used primarily for quality control checks, to validate the stiffness of vibro stone columns, or the load settlement response of a load transfer platform over rigid inclusions.

A short duration test method, the maximum applied load is typically 2.5 times working load, allowing an assessment to be made of the shallow soils’ immediate response to load application.

The depth of meaningful stress increase will be a function of test plate diameter, ranging from approximately 700mm to 900mm for 450mm and 600mm plates respectively (1.5B). Given this constraint on meaningful depth of stress increase it is clear that plate load tests can give no indication of the load/settlement performance of the improved soil mass, with the potential exception of very small shallow pad foundations. However, satisfactory test results can give confidence that the work has completed in accordance with the design requirements.

Whilst discussing plate load testing, it would be useful to consider two parameters often used in ground bearing slab design that are obtained from plate load tests, namely –

The procedure for carrying out an in-situ CBR test on site is described in BS1377-9:1990 section 4.3. It involves forcing an approximately 50mm diameter plate into the soil at a constant rate of 1 +/-0.2mm per minute, with time readings taken for each 0.25mm of penetration up to a maximum of 7.5mm. The small dimensional area of the CBR test plate is such that the stresses imposed during the test are limited to just the upper 100mm or so of soil. As a result, the recorded CBR value can give no meaningful indication of the settlement performance of the improved soil but can give useful information about the stiffness of the subgrade soils providing support to ground bearing slabs.

Modulus of Subgrade Reaction (ks)

TR34 Concrete Industrial Ground Floors, a guide to design and construction, provides the following guidance –

The measured modulus of sub-grade reaction is a function of the load applied to induce 1.25mm settlement of a 750mm diameter plate. A test plate of this diameter will induce meaningful stress increase over approximately the upper 1.15m of soil. In common with the CBR test, the recorded modulus of subgrade reaction value can give no indication of the settlement performance of the improved soil but can give useful information about the stiffness of the subgrade soils providing support to ground bearing slabs.

2. Zone Load Testing

Large scale zone load tests, or, for residential housing projects, dummy strip footing tests, are used to validate the load/settlement performance of the improved soil mass.

Owing to the complexities involved in providing sufficient reaction for larger scale tests, zone load tests carried out in the UK tend to use a plate size of up to 3.0m x 3.0m square, reducing to a1.5m long x 0.6m wide test plate for a dummy strip footing test.

For a typical vibro stone column design bearing pressure of 150kPa and allowing for advancing the zone load test to 1.5 x working load, a 3.0m x 3.0m square zone load test will require over 2,000kN of reaction, achieved either through the installation of reaction piles, or, more commonly, through the mobilisation of 200t + of kentledge. In cases such as this steel ballast weights would ordinarily be used as kentledge, although even then, taking unit weight of steel as 7.0t/m³, the height of the kentledge would be over 3.0m high, or a bespoke test frame would need to be constructed – no mean feat!

Given the complications and costs associated with the testing described above it is preferable to minimise the zone load test size as far as practical, which reduces the effectiveness of the testing method for validating the load/settlement performance of the improved soil, for the reasons illustrated below –

Figure 1. Idealised schematic of a 3.0m x 3.0m zone load test on a hypothetical site, which is underlain by an initial 4.5m of Made Ground over natural Clay soils.

Increases in effective stress are deemed ‘meaningful’ with respect to foundation settlements where they exceed 20% of the in-situ effective stress state, as estimated by the Boussinesq approximation at an approximate depth of 1.5 x foundation width – assuming that no significant plastic deformations occur.

Figure 2. Idealised schematic of a 4.0m x 4.0m x 1.0m deep working foundation on a hypothetical site, which is underlain by an initial 4.5m of Made Ground over natural Clay soils.

The contrast between the two conditions is clear. The 3.0m x 3.0m zone load test applied at surface level will measure the load settlement performance of the 4.5m depth of improved Made Ground. Whereas the meaningful increases in stress associated with the 4.0m x 4.0m working foundation placed at 1.0m below ground level will extend into the deeper natural Clays soils. Also, the performance of the working foundation will take some benefit from the positive effect of the overburden pressure above base of foundation level.

This consideration of the depth of meaningful effective stress increase is exacerbated when applied to wide loaded areas subject to uniformly distributed loads, where the depth of influence would be expected to be significantly deeper than that associated with a typical load test.

With all things considered it must be recognised that zone load testing is unlikely to provide a completely accurate indication of load/settlement performance of loaded slabs, or foundations which have a much larger footprint area than the test plate dimensions. Additionally, to obtain a true indication of the load/settlement performance of smaller foundations, it is advisable to progress the test from a similar depth to the proposed depth of the working foundations.

3. Geophysical Testing (CSW)

A novel method of testing and validating ground improvement work is the use of geophysical seismic methods, such as continuous surface wave testing.

For a more detailed description of this technique please visit https://www.soilsafe.co.uk/

A brief outline of the technique is described below –

Step 1 –

The equipment is brought to site in a 4×4 vehicle, and is set up as shown in the image above, with a vibration unit positioned at a known offset from an array of geophones positioned at surface level.

Step 2 –

The vibration unit is operated at a range of frequencies, and the response times of the surface waves are recorded at each geophone position. Using the geophone response time data, and the known distance between each geophone in the array, the velocity of the surface waves can be calculated – minimum strain shear stiffness can be calculated as a function of shear wave velocity. A typical profile of shear wave velocity recorded during CSW testing is given below.

The wavelength associated with each frequency can be attributed to soils at varying depth below survey level – both simple and advanced methods inversion methods are available for establishing a shear wave velocity profile with depth from the recorded data.

Step 3 –

A profile of minimum strain shear stiffness modulus is derived from shear wave velocity graph.

Step 4 –

A strain softening function is used to estimate an appropriate value of operational shear stiffness, based on estimates of shear strain occurring in the stressed layers of soil below base of foundation level. Typically, appropriate levels of operational stiffness are in the order of 10% – 30% of the minimum strain shear stiffness values estimated from shear wave velocity.

It is true to say that the inversion problem has no single unique solution, and that no specific strain softening function has been devised appropriate to all soil types, or, perhaps more importantly, to inhomogeneous layered soils. However, for the reasons provided above where zone load testing is discussed in greater detail, the results obtained are potentially no more or less accurate than the load/settlement relationship suggested by physical testing techniques.

The major benefit of using CSW testing in lieu of zone load testing is cost benefit. For the cost of a single zone load test, it is possible to arrange three, perhaps four days’ of CSW testing, yielding between thirty and sixty unique small strain shear stiffness profiles, allowing a lot more of the improved soil to be analysed. A reasonable compromise could be the use of CSW testing in combination with a single Zone Load test, allowing the results of the two tests methods to be compared.

For best results from CSW testing site ‘noise’ must be minimised. Best outcomes are achieved when no plant is used on site whilst the testing is in progress, ensuring that the recorded geophone data is not detrimentally affected by competing surface waves from other sources.

5. Integrity Testing

Specific to ground improvement using displacement or replacement augering rigid inclusion methods, integrity testing can be used to assesses the structural integrity and continuity of the unreinforced inclusions, identifying defects such as cracks, voids, and unwanted inclusions.

The ASIRI (2012) document – the leading design guidance for rigid inclusion design and implementation – recommends that displacement rigid inclusions are integrity tested wherever they are installed at spacings closer than four times rigid inclusion diameter.

In UK practice it is common to install rigid inclusions from top of load transfer platform level, and to reduce the rigid inclusions whilst the concrete is wet. The load transfer platform is then recompacted over the reduced inclusion head, leaving them hidden upon completion of the rigid inclusion construction phase, hindering the successful completion of integrity testing after the inclusions have cured.

6. Concrete Cube Testing

Again, specific to ground improvement using displacement or replacement augering rigid inclusion methods, concrete cube testing can be used to measure the strength of the concrete to firm the rigid inclusions, and to confirm it meets the design requirements.

It should be noted that when buying a designated mix from a QRMC contractor the design strength is effectively guaranteed by the supplier, who must take regular samples and keep a record of the achieved strengths for the designated mixes they supply. With this being the case, concrete cube testing becomes more an exercise in identity testing – ensuring the correct grade of concrete has been used – rather than strength validation

It should also be recognised that the entire process of concrete sampling, cube making, storage and curing needs to be strictly controlled. In our experience, low strength results are more often the result of poor procedure than defective concrete. In our experience, very few sites have either the facilities or necessary expertise to carry out the procedure correctly. As a result, GeMech use external accredited materials testing subcontractors to sample, make and test concrete cubes on our behalf.

Want to know more? Contact us today to find out how we can help you with your ground improvement project.

Bath project close up of piling machinery

Understanding Building Design Codes for Foundations: A Comprehensive Guide

Before starting any construction project, ensuring that the foundation is properly designed in accordance with the relevant building design code is essential. Foundations are a critical part of the construction process as they transfer the building loads safely into the ground.

Some of most important building design codes for piled foundations in the UK are:

BS 8004:2015 + A1:2020
BS 8002:2015 – TC
Eurocode 7 – Geotechnical Design
Eurocode 2 – Design of Concrete Structures

These are augmented by a large number of best practice ‘special execution documents’, industry position papers & supplementary guidance as well as requirements for designing foundations This is to support sensitive structures such as roads or railway lines.

Another important document to be considered when designing or installing piled foundations is the recognised specification document for the UK – ICE Specification for Piling and Embedded Retaining Walls (SPERWall)

Imperial park driven piling

The Importance of Foundation Design Codes

The design codes in common use in the UK adopt a system of partial (safety) factors, applied to actions – building loads etc. and resistances – soil strength & resistances.

The partial factors adopted for use in the UK range from 1.0 to 2.0, and have been selected to ensure that, through a use of a prescribed combination of factors, the completed foundation is suitable remote from risk of failure.

The application of the Eurocodes, and the partial factors adopted, vary by country and typically cover a range of design considerations, including –

Building Safety: Ensuring that the completed foundation is remote from the risk of failure, defined by –

Risk of collapse – the partial factors adopted within the design have been selected to ensure that, in so far as reasonably practical, the completed foundations will not suffer from a catastrophic failure.

Excessive settlement – the Eurocodes require consideration of foundation settlements under the applied loading conditions, to ensure that excessive movement doesn’t result in building defects.

Structural Durability: Ensuring that the completed foundations are suitably robust to ensure consistent performance throughout their intended design life – design life in the UK typically tends to range between 50 & 120 years.

Foundations machine ABI TM11

Types of Piling & Ground Improvement

Understanding the types of piling solutions and ground improvement solutions, and when they are used is the first step in understating and complying with the design code requirements – many of the commonly used piling solutions have their own special execution documents For example the special execution document for driven piles is –

BS EN 12699:2015 – TC, Execution of special geotechnical works. Displacement piles

Here are some of the piling and ground improvement types you will see in construction:

CFA Piles: CFA Piles have become one of the more popular methods of piling in the UK because of their versatility. Modern CFA rigs have the capacity to form piles in Very Soft Soils through to Weak Rock formations.
Driven Piles: Driven Piles are most commonly used in Soft Soil conditions, and are particularly advantageous on sites with contaminated soils as displacement piling methods avoid the generation of spoil arisings.
ODEX Piles: ODEX Piles are used when the ground situation is a lot more challenging, with shallow competent rock, existing unreinforced foundations, or the presence of boulders for example.
Vibro Stone Columns: using Vibro Stone Columns in lieu of traditional piling methods can offer significant cost, time and environmental benefits. The system can be used in a wide range of ground conditions, and across a wide range of structural types.

Key Requirements for Successful Foundation Construction

1. Site Investigation

Before construction begins, a thorough site investigation is required. This will determine the ground and ground water conditions to a suitable depth below site level, with a range of test methods available to characterise the soil strength and stiffness.

A specialist site investigation contractor will carry out this site investigation, typically comprising an initial desk study followed by a site based intrusive investigation including trial pits and boreholes.

The site investigation specialist will recommend a suitable scope of investigation, based on their prior knowledge of the ground conditions in the area. We would also recommend getting a piling and ground improvement contractor involved in specifying the scope of site investigation, to ensure the information gathered is sufficient to meet the design requirements.

A note of caution – opting for the cheapest possible site investigation, resulting in poor characterisation of the ground conditions can result in significant costs and delays during the foundation construction work. If you are unsure, please get in touch with us for free, no obligation advice.

2. Design Requirements

The design of the foundation must be in accordance with the Eurocode requirements, and any other relevant design codes and guidance documents.

The starting point is to employ a Structural Engineer who can accurately assess the loads transferred from the building to the foundations. The loads calculated by the Structural Engineer can then be used to develop the foundation design, resulting ultimately in a schedule of loads, or bearing pressure requirements that can be passed on to the Piling Contractor or Ground Improvement Contractor.

The design prepared by the contractor should ensure that the piles or improved soil can safely support the loads, without risk of collapse or excessive settlement.

3. Material Specifications

The various Eurocodes, special execution documents and SPERWall specify the types of materials that can be used for building foundations. Common materials used on GeMech Foundations Limited projects include:

Ready Mixed Concrete
Non-shrink Construction Grout
High Strength Reinforcement Steel
Structural Steel Sections
Graded Aggregates

The relevant documents for each material outline the minimum strength requirements and standards for mixing, curing, and testing of these materials to ensure they meet the necessary specifications for strength and durability.

4. Construction Practices

The construction of piled foundations, retaining walls and ground improvement work must be carried out in accordance with the requirements of the Eurocode special execution documents, SPERWall, and any site specific requirements, including the Structural Engineers Specification document.

The piling or ground improvement contractor should prepare a design document, a quality method statement and an inspection and test plan, all combining to identify how they propose to ensure the work is completed in accordance with the specification requirements.

6. Inspections and Testing

Inspection and testing methods for piling and ground improvement work vary significantly across methods, and the requirements often change on a site by site basis.

Guidance on appropriate regimes of testing are provided in the various Eurocode documents and SPERWall. Industry organisations such as the Federation of Piling Specialists (FPS) have issued a position paper on behalf of their members, entitled the ‘Handbook on Pile Load Testing’, detailing appropriate test methods and regimes of testing based on project size and piling method.

A critical consideration when determining the inspection and testing regime for piling or ground improvement works are the specific requirements of any building warranty provider with an active involvement in the scheme.

The testing and inspection requirements of building warranty providers are often extremely onerous with respect to piling and ground improvement work and can occasionally outweigh the building design code or Structural Engineers own specification requirements.

We would recommend that the foundation design package, containing details of the proposed testing regime, is presented to the building warranty provider at least two weeks prior to any work starting on site, to ensure that the proposals meet with their needs.

Oxford Vermeers foundation project

What Is Pile Testing?

Pile testing can be used to evaluate the integrity, ultimate capacity and settlement performance of piles. Piles are deep foundation elements that transfer the load of a structure to deeper, more stable soil layers or rock formations. This blog explores the different types of pile testing, their importance, methods, and applications.

Concrete foundations and piling

Benefits of Pile Testing

Pile testing serves several essential purposes:

Ensuring Safety: Where lower partial safety factors are adopted within the pile design calculations, to ensure economical design depths are scheduled, pile load testing will confirm that the piles can support the intended loads without excessive settlement, ensuring the safety of the completed structure and its occupants.
Quality Control: Certain pile testing techniques can be used to confirm that the piles have been installed correctly and meet the required specifications.
Design Verification: When load testing is required, the results are used to validate the design assumptions and help refine future designs by providing real-world load/settlement data.
Cost Efficiency: Taking positive benefit from previous pile load testing results, helps avoid overdesigning or under designing piles on future projects, leading to optimal use of materials and resources.
Regulatory Compliance: The scope of required pile testing will often be determined by the Project Engineer, or the Building Warrantor’s Engineer. Proving compliance with this schedule regime of testing ensures that the construction meets the required building codes and standards.

Bath project close up of piling machinery

Types of Pile Testing

There are several methods of pile testing, each serving a specific purpose and providing different types of data. The primary methods include:

1. Static Load Testing

Preliminary static load testing of sacrificial test piles is the most direct method of determining a pile’s load-bearing capacity, through measurement of the applied load at the achievement a predetermined ‘failure’ criterion. More commonly, static load testing is carried out at working pile locations, with the load/displacement response recorded in gradual load increments, up to a maximum of 150% of working load.

Procedure: A hydraulic jack applies a load to the pile, and the displacement (settlement) is measured using precise instruments. The load is applied incrementally until the pile reaches the desired load or fails.
Types: Static load tests can be axial (compression or tension) or lateral, depending on the direction of the applied load.
Applications: Preliminary static load testing is used for critical projects where precise load capacity data is required, such as high-rise buildings, bridges, and industrial structures. Maintained load testing to working piles is carried out where specified by the project team, or where the partial safety factors adopted in design require design validation.

2. Dynamic Load Testing

Dynamic load testing evaluates the pile’s performance by applying a dynamic load, usually via a hammer blow, and analysis of the pile’s settlement response.

Procedure: A pile-driving hammer strikes the pile, and sensors measure the resulting stress waves and displacement. The data is analyzed to determine the estimated pile head deflection at working loads.
Advantages: Faster and less expensive than static load testing. It can be performed during or after completion of the pile installation and allows a greater number of piles to be tested.
Applications: Commonly used for driven piles, especially in large-scale projects where many piles need testing.
Disadvantages: The application of a dynamic load does not accurately replicate the long term load/displacement performance of a deep foundation, particularly when piles are embedded in fine grained Clay soils.

3. Pile Integrity Testing (PIT)

Pile integrity testing assesses the structural integrity and continuity of augered piles, identifying defects such as cracks, voids, and inclusions.

Procedure: A small hammer is used to tap the pile at surface level, following pile cropping, and the response is measured using accelerometer also placed on the surface of the cropped pile. The resulting signal is analyzed to detect anomalies.
Types: Low strain (impact echo) and high strain methods are available, with the choice depending on the required detail and depth of analysis.
Applications: Used for quality control in new augered pile installations and to assess the condition of existing piles in older structures.

4. Crosshole Sonic Logging (CSL)

Crosshole sonic logging is a non-destructive testing method used to evaluate the integrity of cast-in-place concrete piles or drilled shafts.

Procedure: Tubes are installed in the pile before pouring the concrete. After curing, ultrasonic transducers are lowered into the tubes, sending and receiving signals through the concrete. The signal quality indicates the concrete’s integrity.
Advantages: Provides detailed information about the pile’s internal structure, including defects like voids or honeycombing.
Applications: Ideal for critical infrastructure projects requiring high confidence in pile integrity, such as bridges, dams, and high-rise buildings.

5. Bi-directional Load Testing

Bi-directional load testing, also known as Osterberg Cell testing, uses an embedded hydraulic jack to apply loads to the pile from within.

Procedure: A hydraulic jack (Osterberg Cell) is installed at a specific depth within the pile. The load cell expansion reacts against the concreted pile above & below the cell location, and the pile’s response is measured.
Advantages: Allows testing of very large piles and provides a detailed understanding of load distribution along the pile length.
Applications: Used for large-diameter bored piles and drilled shafts in heavy construction projects.

Drilling foundation tool

A Bright Start to 2024

The start of the new year has been frantic at GeMech, with new projects started and large projects continued through January and February, with March looking equally busy and a catalogue of work for April.

January and February has seen us continue CFA piling works for ISG at their prestigious Fairwater Campus development in Cardiff, with load testing completed in January, works are aiming to finish mid-March.

Our largest rig in the fleet, the Comacchio CH450 moved from Cardiff to Stratford to complete CFA piling work for repeat client Watkin Jones on a new student accommodation development.

Our driven pile crews have also been busy, utilising our newly adapted precast driven piling rig. GeMech installed 250mm square precast concrete piles to varying depths in Weston-Super-Mare, with further precast driven pile projects on the horizon. Thanks to the versatility of the Vermeer HL2500, the crews have also been busy installing steel cased bottom driven piles for clients LCB Group, ADRA Homes.

And finally our ground improvement projects have taken off, with our CAT 330 and ABI TM11 being put through their paces installing stone columns in Slough for Francis Construction and Stafford for Spellar Metcalfe. The CAT 330 installing 1,300 stone columns to provide improved ground bearing capacity for two new warehouse units, and the ABI TM11 installing 600 columns. Once completed, these rigs will be moving on to sites in Basingstoke and Bristol, keeping them busy until mid-April.

CFA Piling – Locking Parklands, Weston-Super-Mare

GeMech specialists and our fleet were down on site in Weston-Super-Mare for repeat client Curo Group, installing over 400nr. 300mm diameter CFA augured piles for housing plots on the former RAF Locking site. Using the versatile GEAX EK110, we are installing piles in to Mercia Mudstone from 4.5m depth and terminating at a maximum depth of 6.0m. Whilst the geology around Weston-Super-Mare generally is suited for a driven piled solution to be used, the ground conditions on this site called for an augured piling method.

We are pleased to be working again with Curo Group, providing a robust and cost effective solution.

Driven Precast Concrete Piling

Driven precast concrete piling is being added to the GeMech repertoire. We have an exciting update on our “drive” to offer more engineering solutions for our valued clients. Our colleagues at AGD Equipment LTD have been busy this last week transforming one of our steel cased bottom driven piling rigs in to a restricted access precast concrete piling rig. As well as being able to offer a solution, where vibration and noise restrictions may apply, we are still able to provide an efficient alternative in steel casings. Once the rig is adapted, we will be able to offer up to 250mm square precast concrete piles in as little as 7.0m headroom up to any length depending on ground conditions.

GeMech are looking forward to adding the adapted machine to our fleet, and will be putting it to work in Haywards Heath in two weeks time.