Ground water sampling with direct-push systems (DPS) has gained widespread acceptance in the environmental industry over the past decade because of the versatility, relatively low cost, and the mobility of these systems. The two major classes of direct-push platforms are cone penetrometer testing (CPT) and percussion hammer systems.
Cone PenetrometerWhen geotechnical instruments are advanced, the push rods typically are advanced at a controlled rate of 1 centimeter to 2 centimeters per second. CPT systems are capable of pushing CPT systems; ground water, soil gas and soil samplers; and piezometers to depths in excess of 100 feet in unconsolidated material. This methodology provides detailed hydrogeologic profiling at an average rate of 400 linear feet to 500 linear feet per day, depending on the subsurface materials and conditions. Collection times for samples vary depending on many factors, including the media being sampled, sampling depth, presence and abundance of ground water, and types of soils, as well as whether the soils are conducive to soil gas sampling. For example, collecting shallow ground water samples through the push rods may be quick, whereas collecting deep soil samples take longer. Clay can be more difficult and slower to penetrate than most sandy soils.
Percussion HammerWith a percussion hammer system, the throughput associated with sampling downhole geotechnical or analytical sensors is highly dependent on the material present in the subsurface, and the objective of the sampling or sensing event. Under typical pushing conditions, a percussion hammer system can push about eight to 10 30-foot holes per 8-hour day. Actual throughput will vary by the tools being used and the objectives of the investigation.
Advantages and LimitationsFollowing are advantages and limitations associated with direct-push technologies in general, and should be considered when determining whether they are appropriate for use on a site or project. Advantages and limitations specific to either the CPT or the percussion hammer systems will be discussed later in this article.
Unless using dual-tube continuous coring, direct-push technologies do not generate cuttings or excess soil, so there is no potentially contaminated soil to dispose of. Costs of investigation are reduced, and the process is simplified.
Direct-push systems are quicker and more mobile than traditional drill rigs. Sampling and data collection are faster, reducing the time needed to complete an investigation and increasing the number of sample points that can be collected during the investigation. In-situ emplacement of geophysical and analytical instruments allows a great deal of information about subsurface soils and contaminants to be collected in near real time. Closed sampling systems and on-board analytical instruments allow samples to be analyzed in the field, avoiding laboratory turnaround time, remobilization time and associated expenses.
Direct-push systems do not have high masts and, hence, can better operate where there is conventional overhead electrical wiring.
Direct-push systems can be used to install prepacked monitoring wells. While prepacked wells generally are smaller in diameter than the conventional 2-inch outside diameter well, the largest diameter dual-tube has been used to place a 2-inch monitoring well. The costs associated with installing a prepacked well are substantially less than the costs of installing a monitoring well with a traditional drill rig.
Direct-push technologies are limited to unconsolidated materials, and are limited in their penetration depths. They cannot be used to penetrate bedrock layers, concrete footings or foundations, and sometimes caliche layers and very fine-grained, saturated sands can cause refusal.
Changes in geological density can limit the use of these technologies. The presence of soft layers overlying hard layers can alter the alignment of the probe and can bend, break or refuse the rod.
There are advantages and limitations associated with CPT systems in particular that should be considered when determining the technologies used on a site or project. Because of the complexity of the analytical methods and the specialized requirements for operating CPTs, their operation takes considerable experience. Most CPT systems are limited by their size and mass. They cannot be used in tight quarters as readily as many of the percussion hammer configurations.
Because percussion hammer systems can be installed on numerous size platforms, with varying mobility, they are more likely (than a CPT or conventional drill rig) to access areas within buildings or off-road.