Rarely does Geoinvestigate carry out intrusive phase 2 site investigations these days using large machine dug trial pits. I dare say the same is true for many other geotechnical and geo-environmental site investigation specialists. The drawbacks of big machine dug pit are many, including the large volume of disturbed soil they create which is liable to settlement and subsidence, mixing and cross contamination of previously distinct geological horizons, the creation of pathways between different horizons and the contamination they may hold, releasing contamination previously trapped in a permeable soil pocket or lens, bringing large amounts of potentially contaminated soil to the surface and of course the unsightly disturbance and damage left at surface which can look like a freshly dug graveyard. Having experienced first hand the chaos caused by big pitting on a small site which had been excavated on 4 separate occasions by different site investigation companies it changed my mind on how much disturbance was acceptable on a site and that there must be a better way of routinely obtaining good quality geoenvironmental data which triggered my interest in window sampling. Today one can’t imagine carrying out a trial pit investigation in many parts of London and its suburbs let alone the difficulties of getting a JCB through traffic to the site.
There are also safety issues with large machine trial pits. I’m sure many of us can recall that sinking feeling when the JCB driver tells you they’ve just completed their certificate and this is their first site investigation job just as you’re dodging the unpredictable swing of the bucket across the back end of the pit while the drivers shouting “sorry I wasn’t looking and I’m still getting the hang of it”.
On many sites today fencing is a pre-requisite for public safety where large pitting is to be carried out whereas in earlier times it wasn’t. I can remember one particular Manchester site investigation on derelict open space used by the occasional walker. Having just finished a deep pit I was logging, when I noticed a lady and her cocker spaniel approaching my direction. Inexplicably as she got closer she suddenly unleashed the dog which enthusiastically bounded and leapt across the field towards me. One bounce to many and it was over the spoil heap hiding the pit thereafter disappearing into the abyss. Silence and dread was followed by the hysterical cries of the dogs master brandishing a limp empty lead. “Where’s he gone she screamed. What have you done !”. “Down there” I replied peering into the pit where the dog was now sitting at a depth of 4m. Quite contented, perfectly unharmed and wagging his tail the dog looked inquisitively up at me and his master. Luckily, a ladder was to hand and he was out in a jiff upon which the villain became the much hugged hero.
The forerunners of mini-drilling in the UK were Pennines dynamic probe (it must be said a reasonably sophisticated bit kit for its time ) and other rather cruder contraptions including one which had a man assisted, car battery powered pulley drop weight system. At the same time mini-drilling was much more advanced in Germany, dynamic probing having become extensively used there soon after WW2 as a rapid means of profiling bomb cratered and infilled ground and the rapid appraisal of ground conditions for the reconstruction Germanys shattered infrastructure and cities.
The introduction in the UK some 20 to 25 years ago of hand held percussive window sampling followed by tracked mini hydraulic percussion and drop weight rigs such as Carl Hamm’s MRZB and The Dando Terrier offered a cost effective alternative to the traditional machine dug trial pit. The poor quality of the hand auger samples and the slow and physically demanding nature of the work with its attendant health and safety concerns, means that it never really offered credible competition to the machine dug trial pit or the advent of window sampling.
The mini borehole has subsequently more or less rendered hand augering obsolete in engineering geological investigations though occasionally it has advantages and applications which it is better suited – and of course its still used by a small number of diehards though some might say this more to do with cost than looking after the well being of their employees.
Enthusiastically supported in the early days by Carl Hamm’s geotechnics development director Axel Wieczorek, Geoinvestigate was one of the first companies in the UK to introduce and widely advertise window sampling as a new site investigation tool in this country. I remember one of the first site investigation jobs in Leeds we had was to drill down through a deep pipeline trench which had collapsed badly injuring a worker. Soon afterwards the trench which had been unsupported at the time was infilled. The insurer had wanted to establish very quickly at what depth the worker had been working at when the collapse occurred. I remember being told about of the look of astonishment on the face of the insurer’s agent when he saw for himself the string and labels (which the worker had been using at the time of the collapse) in the bottom of the window sampler after it had just been recovered from 5m.
Essentially for the first time in site investigation with the advent of the window sampling techniques cheap machine trial pit could be replaced by comparatively similarly cheap short mini-holes with the advantage that they created very much less damage to the body of the ground and its surface while recovering high quality core samples in open-sided window samplers or reasonably undisturbed samples in windowless samplers with removable transparent plastic core liners.
The development of the Duplex casing systems in recent years has enabled borehole to be advanced through unstable ground while preventing collapse in loose fill or sand in some cases allowing hole to reach up to 15m though 3m to 7m is more common. Water and gas monitoring pipes can also be installed in mini holes.
The miniaturisation of boreholes follows other similar trends in mini-tracked excavators, skid steers, rotary drilling and piling rigs. Small hand held rotary drill rigs have been used in the coal mining industry for many years and are capable of probing up to 15m while others can routinely achieve 20m to 30m.
Mini tracked rigs are small enough to be driven though doorways, through gates and into garden, alleys and other tight spots. They are also capable of tracking up slopes of up to 35 degrees and slope drilling. Some rig operators have developed systems for drilling on very steep slopes or long reach capability to drill over voids. Many of these rigs have the capability of undertaking heavy and super heavy dynamic probing and in-situ SPT tests. Concrete coring can be performed either from a changeable coring head fitted to the mast or using power take-off from the rig to drive a stand alone hand held coring tool. Interchangeable rock core drilling attachments are also available which allow rotary follow-on.
Given the advantages of mini boreholes one can understand why they have replaced the large machine dug trial pit as the standard investigation method for sample recovery and logging for shallow geotechnical and geo environmental investigations in the UK today. However a major concern, many in the industry have about mini- boreholes are the number of companies and especially one man outfits who are currently working these tracked rigs single handed. Though this practice is not illegal a responsible employer should seriously question whether this practice is acceptable either in terms of health and safety or the quality of the output of the ground data obtained which must suffer. Many drilling jobs require early morning starts and long distance driving as well as the physically demanding work of operating these rigs in all weather. This is hard enough for 2 operators which Geoinvestigate always insists upon let alone one person doing everything by themselves including loading and offloading this manoeuvrable though still heavy plant which is capable of the unexpected piggish behaviour.
Trial pits still have their place in site investigation. Today they are used as starter pits to check for the presence of services such as gas and electricity before type of borehole is drilled. They are still the only way of exposing foundations in subsidence investigations and for creating starter pits for mini holes again to check for services. Trial pits and/or trenching are still frequently used for mine shaft location, locating edges of quarries or building basements, buried foundations and investigating the backs of retaining walls or soak-away tests. They are also used in combination with mini boreholes where it is critical to conclusively prove rockhead or to confirm that boreholes didn’t refuse on boulders or other obstruction above rock.
With regard to the later I remember hearing a story about a ground investigation which had been carried out by one of the UKs leading SI companies and had gone badly wrong. Apparently window sampling boreholes sunk on a steep wooded 40 degrees slope had refused at shallow depth and this had been interpreted as rockhead at 1m but, which, on subsequent trial pitting had turned out to be thick colluvium including boulders extending to 9m necessitating piling. The company had repaid the 12K or so SI costs under the threat that the client would report the incident in the NCE as a case study in inadequate SI. The double moral being – “Without appropriate site investigation the ground is a hazard” and expect the unexpected.
So to come back to the original question – yes in many instances trial pits have been superseded by short mini boreholes which offer a rapid and cost effective ground investigation tool for geotechnical and geo-environmental sample recovery, logging and ground profiling with minimal disturbance to the body and the surface of the ground. However, trial pits still have their place in current day site investigation practice though perhaps we have to think more carefully about how, when and where they are used.
From our offices across England, Geoinvestigate provides a good, reliable, cost effective, nationwide site investigation service. We provide expertise in the fields of site investigation, ground investigation, soil investigation, foundation investigation, engineering geology, geotechnical engineering, Phase 1 desk study and preliminary risk assessment, Phase 2 site investigation reports, geotechnical consultancy, environmental consultancy, geo-environmental consultancy, landfill gas and mine gas surveys and monitoring, contaminated land assessment and contaminated land remediation, building subsidence, slope stability, mineworkings, contaminated land validation surveys as well as data acquisition and soil surveys using trial pits, boreholes and soil and rock drilling.
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