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.