Geomembranes, or liners, are a brilliant way to stop liquid loss or contamination. Picking the right sort of geomembrane for an application could be a confusing process.

When scanning the market you’ll come across a good spread of geomembrane liners. That is because of the unique wishes of numerous industries they’re used in. The sort of geomembrane that’s appropriate for a particular application relies upon 1 or 2 different factors. The scale of the liner required ; the sort of liquid which is to be stored, whether the area will contains dangerous or non-hazardous liquids ; the longevity needed, what sort of environment the liner will be exposed to ; what guarantees are required ; if the liner needs defence against UV rays ; and so on. These are only some of the major elements one has to consider before choosing a geomembrane liner for their requirements.

These geomembranes can be found in different materials like buttressed polyethylene, braced polypropylene, PVC, XR-5, and assorted geotextiles for protection of the liners or for filtration. The reason there are that many adaptations is due to the wide selection of application Geomembrane liners are utilised for. A pair broad examples of where liners are being used today are for applications like pool liners, canal liners, unsafe containment liners and non dangerous containment liners.

Relying on the use of the geomembrane liners, material with different traits is utilized. This has an effect on the installation procedures, lifespan and performance of the liner. There are numerous liner producing firms which offer geomembranes and other liner products on the web. These liner makers can supply an enormous spread of products and regularly resolve any questions you will have about which sort of geomembrane liner would suit your requirements best. The liners supplied by these makers are built in the plant to fit your necessities.

They may specialise in the production of custom built geomembrane linings for many sorts of applications like huge pools in all sizes and styles, canals, brief construction pools, waste pits, golfing course pool liners, oilfield fuels & waste, vapor barriers, correction linings, and many more. A couple of these liner producing corporations take orders from around the globe and supply liners that are simply folded for convenient and cost efficient shipping.

They use leading edge kit like radio frequency welders, advanced heat welders, stitching machines, and grommet & d-line machines to make liners, to meet the varied requirements of their clients and make sure the liners are leak proof. Article Source = “http://www.articlesbase.com/industrial-articles/selecting-the-right-geomembranes-for-your-needs-4997332.html”.

A secure or sanitary landfill is a carefully built and lined depression in the ground, or a landfill built on top of the ground, into which wastes are put. Whichever it is, the aim is to avoid any hydraulic connection between the wastes and the encircling environment, particularly groundwater. Essentially, a landfill is a bathtub in the ground , a double-lined landfill is one bath inside another. Bathtubs can leak in two ways : out the bottom or ott. HDPE landfill liners are the most well liked choice for lining landfills internationally and the best HDPE lining systems are generally “composite” systems with a mix of HDPE surface above a clay type material for extra protection.

There are 4 urgent elements in a secure landfill : a bottom liner, a leachate collection system, a cover, and the natural hydrogeologic setting. The natural setting can be selected to reduce the likelihood of wastes escaping to groundwater underneath a landfill. The 3 other elements must be engineered. Each one of these elements is important to success.

GUNDLE geomembranes were one of the first manufactured and are produced using high density polyethylene ( HDPE ). GUNDLE is produced in diverse thicknesses and with either smooth or structured surfaces.

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The acclaim for High Density Polyethylene ( HDPE ) is essentially due to its low initial material cost and excellent chemical resistance. This permits thicker sections to be used compared to other geomembrane materials. A thick, durable, HDPE liner can be placed in exposed applications where the price of other materials would be prohibitory. HDPE has fantastic chemical resistance which is frequently the driving force behind the choice of HDPE. HDPE is a field assembled lining material that cannot be practically built in the shop .

Legislations in several nations around the planet have driven the market for landfill liners making waste containment the single biggest use for HDPE liners. HDPE landfill liners are in most cases cost-effective, have glorious chemical resistance to most chemicals, and have predicted maunfacturer quoted lifetimes ( in a properly designed and built landfill ) of hundreds of years. Dependent on the level of containment required HDPE can be mixed with other lining materials ( or other HDPE layers ) to form multi-layer landfill liners for dangerous waste applications. HDPE is infrequently utilized in landfill caps however a low friction angle boundaries its application on steep slopes.

All HDPE projects, irrespective of size, must be installed by trained installers. HDPE is a versatile material which is used widely across all applications. One of the key uses of HDPE is for landfill base liners where its chemical resistance is used to good effect. HDPE can also be utilized in a mess of secondary containments, pond linings, and water containment projects. HDPE is best used as an exposed lining material, and has the UV resistance needed for years of outstanding service. The primary use of HDPE for the previous couple of years has been in waste containment.

HDPE materials practically do not change their properties inside temperature go from -60 degrees C to +60 degrees C. They are resistant to the most chemical substances : acid, salts, alkaline, fats and for example. HDPE membranes are not spoilt by fungus, microbes, are resistant against plant root growing and ultraviolet radiation and do not affect the standard of drinkable water.

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The makers claim that world experience of HDPE application confirms sturdy life-span of HDPE geomembranes, not less than fifty years.

The high density polyethylene surfaces are usually made of 97.5% polyethylene, 2.5% carbon black and other stabilizers for UV protection, aging protection, anti oxidants. The product does not contain softening agents that would migrate in time.

This material offers the highest tensile strength, impact, tear and puncture resistance. It was very good ESCR ( Environmental Stress Cracking Resistance ) and the highest resistance to chemicals. Its downside is its rigidity which makes handling and installation more difficult. HDPE is the in most situations the number one choice in landfill lining, insulation of chemical plants, roads, gas stations etc . As well as in mining, due to its chemical resistance.

There are nonetheless many other geomembranes like Puraflex. Puraflex is a real example of a new hydrocarbon and chemical resistant barrier membrane designed for containment, separation, environmental and groundwater protection projects. Its proved resistance to hydrocarbons and toxic commercial chemicals provides a inexpensive answer for brownfield developments.

The US Fabricated Geomembrane Institute (FGI) offers its popular short course, “Constructing with Fabricated Geomembranes“, on 23 October 2009 in Lakewood, Colorado at the Sheraton Denver West Hotel. This course will be presented by Timothy D. Stark, Stan Slifer, John Heap, Daren L. Laine, Bill Shehane, Stuart Lange, Andrew Mills, Gary Kolbasuk and other speakers. Course participants are eligible for 6 PDHs.

Those involved with the design, construction, operation and closure of potable water and irrigation ponds, floating covers, canals, landfills, waste water lagoons, secondary containment, golf course ponds, decorative applications, corrective action activities at closed sites, etc. are encouraged to attend this course. Participants will gain a broad knowledge of what is required to properly design, specify and construct with fabricated geomembranes and advantages of fabricated products over rolled geomembranes.

The fabricated geomembrane information will cover manufacturing, formulation, fabrication, shipping, installation, long-term performance, wedge welding, testing of field geomembrane seams, updated ASTM testing of geomembranes, and design and installation of various applications, such as floating covers, canals, decorative and irrigation ponds, and secondary containment.

AGENDA

The 23 October short course will unfold as follows:

7:30 – 8:00 am Registration / Continental Breakfast
8:00 – 8:20 am FGI Introduction, Activities, and Research
8:20 – 8:40 am Fabricated Geomembranes vs. “Rolled Goods”
8:40 – 9:30 am Manufacturing Fabricated Geomembranes
9:30 – 9:45 am Break
9:45 – 11:00 am Fabrication and Installation
11:00 – 11:30 am Leak Location with Fabricated Geomembranes
11:30 am – 12:30 pm Lunch on Your Own
12:30 – 1:30 pm Floating Covers and Potable Water
1:30 – 2:00 pm Canals, Decorative and Irrigation Ponds
2:00 – 2:30 pm Wastewater Ponds
2:30 – 3:00 pm Break
3:00 – 3:30 pm Secondary Containment
3:30 – 4:30 pm Case Histories
4:30 – 5:00 pm Summary and Questions
5:00 pm Geomembrane Welding Demonstration

REGISTRATION

The registration fee for industry professionals is $100 and the course is free for government employees and students. All receive one day of instruction, short course notes, refreshments and lunch. Industry professionals should register by 25 September 2009 to get the best rate. See the online event registration page to start the process: http://fgi.eventbrite.com/

Inferior geocomposite drainage layers threaten landfill slips

Whether or not due to recessionary pressures on profits for contractors, or inexperienced contractors bidding outside their normal expertise and winning landfill/geo-engineering work, environmental experts ABG are reporting that inappropriate separation layers are increasingly being offered in drainage layer geotextiles.

These inferior materials crush, or simply bend under the normal soil loading and the drainage path between the underside of landfill capping sub-soils, and the low permeability capping layer which these drainage geotextile composites are intended to provide becomes non-existent.

The very real concern is that if these defective materials are accepted for use in the works, slip failures on the restored landfill surfaces will be inevitable during wet weather conditions. Water will build up on the layer between the top of the capping layer and the sub-soil creating a slip plane, and eventual failure.

The remediation costs after such slips, and disruption to use of the land, caused are to be avoided at all cost. Contractors and Designers and Site Engineers accepting geotextile drainage materials which subsequently block when the drainage path void becomes flattened and filled with soil, could also quite possibly be sued for negligence after such slip failures.

And yet, use of such materials is easily avoided by carrying out a simple test which can be carried out in less than 60 seconds on a small sample of any drainage geotextile composite offered. It is done by squeezing in the hand a sample (geomembrane, protection layer and the drainage stone (equivalent) layer) of the material between two resilient rubber pads to imitate the soft pressure exerted by the soil.

Inspection of the extent to which compression of the separation layer can be seen to occur is a good indication of their capability. Low performance of geocomposite drainage layers is due to combinations of drainage core compression and textile intrusion into the drainage core. Some products on offer will compress visibly to the point that the drainage void space can be seen to have been greatly reduced, and some very inferior samples show almost complete loss of open drainage voids.

Other more rigorous tests should also be considered appropriate to the application of these materials, but by use of this simple action alone the worst performing products would be discounted.

Goran Erak, Business Development Director for ABG, Environmental Geosynthetics and producers of the original Pozidrain product is very concerned about the loss of reputation of drainage geo-composites posed to the landfill remediation and restoration industry by the use of inferior products. He gave my company a set of rubber pads to use when we are offered these materials, plus a sample of their Pozidrain product, which shows no such problems.

Goran was also keen to point out that reliance on the supplier’s data on plate compression testing could also bring problems unless the supplier/manufacturer’s test protocol was checked in detail. Test results offered by some suppliers had been found to show compliance for stiff steel plate tests, whereas soft pads would give an entirely different and more accurate reflection of soil conditions in-situ. It is the requirement that standard flow capacity test must be carried out with soft platens, so any use of hard platens is a non standard test.

One of the most common locations for landfills are worked out quarries and quarries suitable for landfill are an increasingly valuable resource for this reason.

In a growing number of cases suitable sites include rare geological exposures of mineral bearing rock, or strata of regional importance which need to be kept exposed after landfilling for educational and also often for historical reasons.  In the United Kingdom these features are identified at planning permission stage and usually allocated Special Scientific Interest (SSI) status.

These SSI’s can result in conflict between conservation and waste disposal interests. The geological feature is usually below the intended restoration level and results in a low point being left in the restoration profile where the SSI is present.

Where quarries used for waste disposal contain Sites of Special Scientific Interest, it is necessary to maintain safe long term access to the geological exposure. The landfill operators will wish to minimise sterilisation of void space for the waste. These objectives can be met by the construction of a structure which limits land take and which maintains a safe barrier to the waste material.

However, it is possible to minimise the conflict and to provide for these geological SSI’s without undue difficulty, as we will describe.

The following list of considerations is broadly based on research described funded by the Nature Conservancy Council in the early 1990s, and has led to the identification of engineering measures designed to optimise landfill void in quarries whilst protecting, in the long term, geological Sites of Special Scientific Interest.

1.    To provide long term, safe, unhindered access to the geological exposure with minimal sterilisation of landfill void space for waste, it is necessary to provide an engineered structure which limits land-take and which maintains a safe and  secure perimeter barrier to the waste material. Long term slope stability must be checked by geotechnical analysis, but it is not the sole design consideration since the access to the exposure must remain drained, be free of leachate and free of significant concentrations of landfill gas.

2.   The presence of a geological exposure in a quarry used as a landfill may have a significant effect on the design and operation of the landfill particularly with respect to leachate management. In some cases in nations where leachate levels are not controlled by landfill site licenses it will be necessary to maintain leachate in the landfill at a much lower level than if a geological Site of Special Scientific Interest was not present.

3.   Natural drainage should be provided where possible to prevent the accumulation of surface water adjacent to the geological exposure. Where this is not possible or the base of the geological exposure is below the water table, pumping may be necessary to facilitate access to the exposure. In such cases it will be necessary to maintain the level of accumulated water below that at which it  will flow into the landfill to prevent the generation of unacceptable volumes of leachate. In addition, it will be necessary to minimise the volume of water which may become contaminated by leachate so rendering it unsuitable for discharge to the surface water system.

4.    It may be necessary to take measures to prevent the movement of leachate from the landfill site through or beneath the waste retaining structure towards the Site of Special Scientific Interest where it  may contaminate accumulating surface and groundwater. The measures may include excavation of the base of the landfill site to a lower level and maintaining leachate below the level of the  base of the geological exposure, reducing the leachate level by pumping and the construction of a low permeability leachate retaining structure keyed into the low permeability materials forming the base of the site.

5.     Landfill gas is flammable, is explosive if ignited in an enclosed space, and can also create an asphyxiating atmosphere. In Europe gas hazard sites (such as landfills) are controlled by the ATEX Directive and national regulations, such as the UK’s Dangerous Substances and Explosive Atmospheres Regulations. Landfill sites in most nations now have gas control systems, and with adequate control it is considered unlikely that landfill gas will accumulate in significant concentrations adjacent to a Site of Special Scientific Interest. However, this may not always be the case and especially if no landfill gas extraction is provided on the site, the area should be monitored for the presence of methane and carbon dioxide prior to access, and ATEX Rules applied as appropriate.

6.   Where the landfill perimeter slopes adjacent to the geological exposure are engineered and graded to a profile of less than 1:3 access by visitors on foot across mown ground should present no significant problems if all visitors wear suitable footwear. Where steeper landfill perimeter slopes are designed an engineered access route may be necessary in the form of a graded path across the landfill or the geological exposure or a purpose made staircase from original ground level to the floor of the exposure.

However, if the above criteria are met, there is no reason why a geological SSI and a landfill cannot co-exist without a significant conflict of interest.

Leachate drainage layers are necessary in most waste landfill sites to minimise the accumulation of leachate within the site and they reduce the risk of contamination of surrounding ground and groundwater. A cheaper and environmentally preferable option is be the use of scrap vehicle tyres, but is their use permissible and what happens to them under pressure? A paper in the proceedings of Waste 2004 by A.P. Hudson, R.P. Beavan, and W. Powrie helps us to understand this.

Normally layers of whole or shredded tyres exhibit excellent drainage properties, but if tyres are used as the main drainage layer at the base of a landfill the concern exists that they may compress under the overburden stress from the weight of the waste above and cease to act as an effective drainage layer.

The results of a series of tests undertaken by the University of Southampton are reported by the above researchers as presented in their paper examining the compressibility and changes in hydrogeological properties of shredded and whole tyres subjected to a range of stresses typical of landfill conditions.

In the UK over 400,000 tonnes of used vehicle tyres are produced each year (Hird et al. 2002). The problem of disposing of used tyres has been made worse by the EU Landfill Directive which prohibited the disposal of whole used vehicle tyres to new landfills from 16 July 2003. The disposal of shredded tyres to landfill will be banned on 16 July 2006. There is, therefore, a need to establish alternative methods of re-use, materials/energy recovery and disposal of tyres.

The Landfill Directive permits used tyres to be utilised as engineering material in landfills. Use of whole or shredded tyres are often a cheaper and environmentally beneficial alternative to aggregates for the construction of landfill drainage layers or trenches. However drainage layers at the base of landfills will be subjected to high overburden stresses from waste subsequently placed above.

There is little published research indicating i) the extent to which tyre drainage layers will compress under such stresses, ii) the reduction in hydraulic conductivity due to compression and iii) the effect of tyre shred size on the compressibility and hydraulic conductivity of tyre layers. However, these atters have been addressed in their paper in a large scale compression cell in order to investigate the above.

The data demonstrated that tyre layers will compress under stress and this will result in a reduction of drainable porosity and hydraulic conductivity. The construction of any leachate drainage layer using whole or shredded tyres within a landfill would need to take into account the compressive behaviour of the material under load.

Countries that have specified a minimum hydraulic conductivity for landfill drainage layers generally give values of between 1 x 10^-3 and 1 x 10^-4 m/s.

However, this group found that shredded tyres would easily comply with requirements as low as 1 x 10^-3 m/s at stresses up to 600 kPa, but would only meet the most stringent requirements of some nations at stresses below 400 kPa.

The data presented in this paper demonstrate that the hydrogeological properties of whole and shredded tyres change according to the applied stress. In general the data indicates that shredded tyres are suitable for use as a drainage medium in landfill applications.

In the design of municipal landfill leachate collection systems, some state regulatory agencies require carbonate content of leachate collection system aggregate not to exceed 15 percent by weight. This requirement comes from a legitimate concern about the possibility of aggregate degradation, or loss of mass due to contact with leachate.

Most involved in landfill design and development will have experienced as a result, the fact that in some areas it is difficult to find carbonate free stone within an reasonably economic distance from the site. Many potential aggregate sources have been eliminated for supplying drainage material, due to this stipulation in the specification, but is it really warranted?

While leachate in MSW landfills is capable of dropping to pHs of 6.5, and sometimes 6, it rarely falls below this other than for short periods. This does not seem to be so low that problems would necessarily be serious, and if any of the carbonate dissolved from the stone, the amount would presumably be low as the reaction would be self limiting due to the dissolved carbonate caused by the reaction being bound to raise the pH. High pH will not erode the carbonate so the problem is corrected.

There is not a huge amount of research work on this that we have been able to find. We would be very interested to receive comments if our readers have sources to research on this matter which are more authoritative than the paper I am about to refer to.

The best paper we have found which sets out to by experimentation over a reasonably extended time period (in this case just under 6 months) to investigate whether carbonate drainage stone, when submerged in leachate, will suffer damage, is the following paper:

Suitability of Carbonate Aggregate in Land fill Leachate Collection Systems; Christopher G. Rubak, PE John,O. Starke, PE William D. Upman, PG M. Merrill Stevens, PhD: Presented to the Nineteenth International Madison Waste Conference, September 25-26 1996, Dept of Engineering Professional Development, University of Wisconsin – Madison.

This paper summarizes a research project which evaluated the suitability of a carbonate aggregate with a municipal solid waste leachate. The tests were conducted over a 20 week period using site specific landfill leachate and collection aggregate. Laboratory bench reactors were constructed to simulate landfill conditions with leachate flowing through carbonate aggregate.

The reactors consisted of 12-inch diameter plexiglass cylinders each charged with 80 pounds of carbonate aggregate. Leachate was then circulated through the reactors. An anaerobic environment was maintained in the reactors by applying 0.5 Atmosphere of CO2.

Fresh leachate was added to the reactors on a regular basis to maintain a constant concentration level during the test. Leachate samples were analyzed to determine the change in dissolved solids throughout the test period. Aggregate material was measured before and after the test to determine net mass change. Chemical equilibrium speciation modelling was also performed and compared to the bench test results.

On the face of it this experiment showed that there was no need for concern about carbonate deterioration even down to the exceptional pH 3.0 (exceptional for an MSW landfill under good regulatory control, built to good current standards).

However, the strange thing about the experiment to the writer is that the leachate used was changed on only, I think, 3 occasions; other than on these occasions the leachate was simply recirculated.

I would have preferred to see results which would ensure that the natural circumstances of a landfill were replicated more closely, and that would have meant allowing fresh leachate to pass through the system all the time.

The views of our readers are encouraged. There is a commenting facility available on the Blog Site to enable you to very easily let us know your views on this.

Retaining Wall System Combining Sheet Piling and Geogrid Reinforcements Now Available from Northstar Vinyl Products, LLC and Tensar International

The new AquaTerraâ„¢ system combines sheet piling and geogrid reinforcements, offering considerable benefits to slope stabilization, marine and levee repair projects.

Atlanta, GA (PRWEB) December 2, 2007 – Northstar Vinyl Products, LLC (http://www.northstarvinyl.com/) and Tensar International recently unveiled the AquaTerra Retaining Wall System, a proprietary new concept that allows sheet piling and geogrid materials to be combined for the first time.

“AquaTerra is a solution for many complicated situations,” says Jeff Moreau, President of Northstar Vinyl Products, LLC. “AquaTerra is an excellent option when rock and hard soils present challenges, because in most cases, the sheet piles only need to be placed in a two foot deep trench. The system is also an excellent choice when piping and global stability issues are present because the sheet pile can be driven to a specified depth and then attached to the reinforcement system.”

Until now, there was not a connection for a ‘sheet pile-to-grid’ system.  
As communities continue to expand, land once considered unusable because of steep grades is now being improved with the use of retaining wall systems. AquaTerra is already having a dramatic effect on how retaining walls are designed and installed. Retaining walls once reserved for steel sheet piling are now being designed with light gauge vinyl or composite sheet piling supplied by Northstar.

Upland retaining walls are currently comprised of two components: a fascia system and a soil reinforcement system. Fascia systems are typically masonry block or pre-cast concrete panel systems. Soil reinforcements are either a geogrid or geotextile material. Using a variety of methods, fascia systems are connected to the soil reinforcement. The purpose of the geogrid is to diffuse the soil load that would otherwise act on the fascia by causing the soil to “stack vertically” as opposed to stacking at its angle of repose. Once the retaining wall is built, there is little soil load against the fascia.

“For years, engineers have known that if they had a method to connect geogrid or geotextile to a sheet pile, the same benefits would apply, and shorter, lighter gauge sheet pile could be used in most retaining wall applications,” Moreau says. “Until now, there was not a connection for a ‘sheet pile-to-grid’ system.”
AquaTerra also provides significant benefits to engineers designing tied back sheet pile walls. With AquaTerra, walls once reserved for steel, composite or heavy-gauge vinyl sheet piling can now be built with shorter sections of lightweight vinyl sheet piling. In addition, AquaTerra has no metal components, which is a huge plus in saltwater applications.

The versatility of the AquaTerra system lends itself particularly well to flood control and levee structures. The Modular Levee System is composed of two parallel sheet pile walls supported by layers of geogrid in the core. The geogrid mitigates the soil loading in the sheet piling and in many cases will allow the use of native soil in the core. The Modular Levee System by AquaTerra provides several key benefits:

• The footprint of the levee is dramatically reduced, solving many environmental and easement concerns.
• Lighter and shorter sheet pile sections reduce the cost of getting sheet pile material to remote sites;
• Lighter and shorter sheet pile sections don’t require the use of heavy machinery on an existing levee;
• Lighter and shorter sheet pile sections are less expensive that optional materials;
  The geogrid, in many cases, will allow the use of native fill in the core, saving the expense of trucking in non-native fill;
• Whatever the choice of fill, less of it is required; and,
• The AquaTerra Modular Levee System can be installed in less time than conventional designs.

BlogMaster: System looks useful: but is it available outside the US?

Leading drilling specialist Magpie has announced the deepest boreholes/wells for leachate extraction drilled to date.

SITA UK approached Magpie with a need for one of the most challenging landfill drilling projects undertaken to date.

Enderby Warren a closed landfill site in Leicestershire has existing leachate extraction chambers 74m deep. SITA wanted to retro drill some chambers adjacent to the existing chambers as a back-up to the wells. Magpie were contracted to drill two wells to an estimated target depth of 75m.

To Magpie’s knowledge this had not been achieved before in landfill in the UK. They completed the first well in just under two weeks, and on the second visit completed the second well in a week. This included drilling of the wells in 450/350mm diameters, with a double permanent steel installation. 406mm steel was installed to 50m and 273mm steel installed to final depth.

More on Magpie’s web site here.

The widely used definition of geophysics is that provided by Sheriff in the Encyclopedia of Exploration Geophysics.

For the purposes of this site, we refer more specifically to the following definitions – these focus on Environmental and Engineering Geophysics:

1. Geophysics is: The subsurface site characterization of the geology, geological structure, groundwater, contamination, and human artifacts beneath the Earth’s surface, based on the lateral and vertical mapping of physical property variations that are remotely sensed using non-invasive technologies. Many of these technologies are traditionally used for exploration of economic materials such as groundwater, metals, and hydrocarbons.
2. Geophysics is: The non-invasive investigation of subsurface conditions in the Earth through measuring, analyzing and interpreting physical fields at the surface. Some studies are used to determine what is directly below the surface (the upper meter or so); other investigations extend to depths of 10′s of meters or more.

Both of these definitions have a common component, namely that geophysics represents a class of subsurface investigations that are non-invasive (i.e. that do not require excavation or direct access to the sub-surface). The exceptions are borehole geophysical methods that expand the use of holes already drilled to access the subsurface on a very localized basis.