Direct-Push Environmental Investigations

As technologies used for installing monitoring wells advances significantly, MATECO Drilling continues to be a leader in this competitive field.  Direct-push wells installed using our Geoprobe rigs offer the potential to save money and time for environmental monitoring.  Our Geoprobe rigs, models 6620 Track and 6610 Track, are smaller and lighter than conventional drilling rigs, so less damage is done to surrounding property.  We are able to reduce the potential for exposure to contaminated soils by the minimal amount of soil cuttings that are produced during drilling.  Furthermore, soil sampling is done at a faster rate.

Regulatory agencies have relied upon established well drilling techniques when writing regulations that would be proactive of human health and the environment for the installation of environmental wells.  The drilling methods were developed to access the groundwater and typically were based on 2″ and 4″ diameter wells drilled using hollow stem auger or other conventional drilling techniques.  Since the writing of the regulations, advancement in technologies like the direct-push method has brought change.  Installation of wells by direct-push technology may require variances in permitting for long term monitoring  for many states.  The basis of the variance involves the direct-push well lacking the large volume of annular space that is required for the proper construction of conventional wells.

For most installations, the direct-push drive rod is constructed of flush threaded steel for easy connection.  The drive rods are hollow to allow for protection of the well materials during installation.  Steel expandable drive points are inserted into the end of the rod prior to advancement.  Typically, schedule 40 or schedule 80 flush threaded PVC is used and installed through the direct-push drive rod.   The internal diameter of the PVC well ranges from 1/2 ” to 2″.  Slotted PVC screen with slot widths of 0.010″ to 0.020″ is most common.  Pre-packed filters consisting of an outer stainless stell mesh screen can be placed around the well screen and filled with silica sand prior to installation.

Several methods can be used to seal a direct-push well.  The seal can be tremie grouted as the drive rod is removed using a bentonite grout.  Prefabricated bentonite seals are available and they consist of a sleeve of dry granular bentonite wrapped around the wall pipe.  The sleeve expands when hydrated and is mostly used in applications below the water table.

There are limitations to direct-push wells.  The depth of penetration is controlled by the weight of the equipment and the size of the percussion hammer.  The application is most suitable to unconsolidated formations, limited in semi-consolidated formations and not possible in consolidated formations.