Ground Engineering Solutions

Effective Cone Penetration Testing Strategies

Introduction

The most common question we get asked is why use cone penetration testing (CPT) instead of intrusive investigations? It’s a fair question. The simple answer is that CPT works best as part of a broader geological understanding of the site. In the UK and Ireland, intrusive investigations are predominant – tactile examination of the soil is still preferred and that’s fine – a CPT contractor is a niche contributor. However, in and of itself, CPT testing is incredibly powerful owing to the quantum and quality of data returned per unit effort of investigation – the cost efficiency of cone penetration testing is unmatched. CPT is most effective where undertaken systematically and under dynamic management – more specifically:

  1. In a fashion that allows rational statistical methods of data analysis to be applied to the data in order to quantify and understand uncertainty. In other words, not in an ad-hoc fashion by undertaking probes at positions without forethought as to what data from this position will yield to the overall design concept
  2. Managed in real-time such that the fullness of the data is realised on a site-specific basis – predetermined depth, for example, is not something we would advocate. There’s nothing worse than a probe stopping up in soft soil or a sampling tube that’s not quite catching the target soil horizon

To achieve a goal, there must be a strategy comprised of a rational tactical basis – this is how investigations should be prepared and managed.

Cone penetration testing

Foundation Engineering Strategies

It is always better to correlate a design estimate such as bearing capacity, directly to a ground investigation result. This is much more preferable than interpreting an engineering parameter and then applying, via a theoretical basis, to the design estimate. Let’s take, for example, a large residential development – as a CPT contractor, we have recently undertaken a series of foundation engineering studies which provided a CPT trace at every plot position on site where ground conditions were difficult or unknown. Then, as a geotechnical designer, we were able to provide detailed bearing capacity and settlement estimates for shallow foundation for each plot – the net result of these studies has been to remove up to 50% of piling from the site and in some cases change a further 25% from piles to a more cost-effective foundation solution (such as stone columns). We’ve been able to do this because we’ve efficiently acquired abundant data at each plot rather than interpolating sparse data over a large area of the site.

Soft Ground Engineering Strategies

There is formally no better tool for investigating soft ground than cone penetration testing. The particular advantages are:

  • Complete profile delineation including the identification of thin soil horizons which may govern behaviour
  • Porewater pressure measurement in the undrained condition
  • Dissipation testing for consolidation behaviour
  • Facility to sample soft ground very accurately and easily
  • Very solid empirical and theoretical basis for the direct or indirect estimation of design parameters

Recently, we’ve undertaken a systematic study of an area of soft ground within a broader development in which we derived geotechnical solution categories based on the depth and strength of the soft ground and, using a systematic grid of CPT probes, were able to delineate the soft ground to the extent that over 10,000m2 was moved to a lower intensity category (resulting in substantial cost savings). In addition, we identified a small area of very deep soft ground that was previously unknown thereby removing ground-based risk from the site.

Value Engineering Strategies

Value engineering is the systematic refinement of the deliverable to produce value in the service. It’s a term that gets bandied about a lot. Our particular strength lies not just in the collection and collation of cone penetration testing data as a CPT contractor but also in the use of the data for the preparation and refinement of geotechnical designs. Our mission is to provide value and certainty to our customers and it is our firm belief that the meeting of contracting and design in this fashion is how this should be done – it’s what the construction / civil engineering industries are missing. Value engineering is a strategic venture, not an ad-hoc activity. GES has the ability as a CPT contractor and a provider of geotechnical design and temporary works design services to develop value engineering strategies and see them through to site.

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