Procedure

The PPT Profile Procedure
The side-sensing PPT cone is pushed into a landfill by hydraulic rams mounted on a 20-ton cone truck. As the PPT is advanced into the landfill the data is displayed in real time and is recorded on a computer. The tip resistance or end bearing indicates the density of the Municipal Solid Waste (MSW). The sleeve resistance or side friction can indicate an increase of moisture in the MSW as the friction values decrease. Typically, as a PPT sounding in MSW progresses, the side-sensing transducer has been indicating a slight vacuum due to dynamic pull as the cone passes the MSW. The dryer the MSW the greater the dynamic vacuum. When the sounding is stopped to add another push rod (standard length 1 meter) the vacuum reading typically returns to zero. It is during this pause that a vacuum zone will be registered as the vacuum influence from a nearby extraction well. The PPT can map the zone of influence of an existing collector. It is quite common to measure vacuums and gas pressures in the same sounding . Vacuums moves out from a collector like fingers, it is not like a balloon.

Figure 1: Is a colorized PPT log to assist in identifying the different measurements displayed on the log. The brown color indentifies the dense/daily cover layers that have an influence on the pore-pressure readings. The yellow color indentifies the gas pressures encountered. The blue color represents liquid layers. The green color reprsents gassy liquid. This particular landfill had excessive amounts of liquid in it. LFG was bound up in the liquid producing gassy liquid. The leachate actually foamed out of the PPT hole. Note how the gas is trapped under the dense layers and the liquid and gassy liquid is sitting on top of the dense layers.

If a zone of high pressure is encountered during the sounding, which is high enough to overcome the dynamic vacuum, the readings will increase, the cone should be stopped and a stabilized test performed to measure the static pressure. (Figure 2) To clear the porous-element and to verify the pressure reading the cone should be pulled up slightly. Note the drop in pressure when the cone is pulled up in figure 2. The pressure then rises to 2.3psi, which is higher than the initial pressure of 1.9psi. If this was liquid the pressure would have decrease due to less hydrostatic head pressure.

It is during this stabilization test that it is determined if the pressure being read is liquid or gas. Basically liquid will produce a hydrostatic pressure slope (Figure 3) on the log based on the depth of the liquid and landfill gas will produce the same pressure regardless of the depth. If the pressure readings are higher than the hydrostatic pressure could possibly be for the depth, then it is obviously gas. If the pressure reading is in the hydrostatic range the PPT cone should be pulled up one foot, and another stabilization completed. This serves two purposes, one it clears the porous element from blinding from plastic or other MSW and also if the pressure drops by .5 psi (equivalent to 12 inches of water column) it is most likely liquid. If it remains the same or increases, this is interpreted as gas. As the cone is advanced once again and the pressure increases under dynamic forces then it is probably a thin layer of liquid under the influence of gas pressure. Lower friction values from the CPT is also an indication of wet conditions and an area of gas production.

It has been observed that landfills have their own average compaction density. The average tip resistance in a landfill can give an indication of the density of the MSW and therby the permeability of the material. This information is useful when designing a LFG collector system.

As the next push rod is being connected the behavior of the PPT should be monitered. If a sounding does not indicate any measurable gas pressures, it will generally be found that the end bearing readings will be over the landfill end bearing average indicating very dense material, which means very little organic material is present. This would be a poor location to install a collector.

One of the most important factors in using the profiling technique is to prevent installing a collector through a perched liquid zone and providing a conduit for the liquid to migrate to the bottom of the landfill and leak to the groundwater or flood the collector. Please push the button labeled Push-in Collectors for more information on a better, faster, cheaper method for LFG extraction. The push-in well screen is placed only where the gas plume is known to be, which is usually deeper than 20 feet and not in liquid containing zones.

The PPT can be used to map the vacuum influence of existing collectors. This information can be used to increase the production of the collector or to determine why a collector is not producing as it once did. If the PPT indicates no vacuum and no gas pressure then the organic component of the refuse is either too dry or the gas potential of the organic material is used up.

Approximately 500 feet of PPT sounding can be performed in a 10 hour day.

During the field operations if it is decided that Push-in collectors should be installed immediately following the soundings, the PPT logs, which are plotted in the field, are used to determine where to install the screen sections of the collectors.

If a PPT 3-D Profile is to be created following the field operations, the PPT data is color coded as in figure 1 and is input into Auto CAD and Micro Station. Survey coordinates and precise depths are also inputted into the computer. To create a 3-D profile the PPT soundings performed in the field must be in a box formation to provide depth to the image. Plan views and cross sectional plots can also be created.