Method for constructing groundwater monitoring well inside prb trench

A method for installing a groundwater monitoring well inside a PRB trench, specifically comprising following steps:

• Step 1:
• excavating a PRB trench:
• excavating with trenching equipment for construction of the PRB trench, to a design depth, width, and length, and cleaning soil from the excavated trench;
• Step 2:
• installing a trench side-wall support:
• when a rotary drilling machine is used to excavate the PRB trench, a large-diameter steel casing serves as the trench side-wall support, wherein the large-diameter steel casing has a diameter larger than 1 meter;
• when a long-reach excavator or a double-wheel milling machine is used to excavate the PRB trench, steel sheet piles serve as the trench side-wall support;
• Step 3:
• positioning a monitoring well borehole:
• determining an installation location of the groundwater monitoring well inside the PRB trench at a center of the large-diameter steel casing or at a mid-point between the steel sheet piles on both sides of the PRB trench, and drilling 0.3 to 0.5 meters into an aquiclude layer with a drilling machine to form the monitoring well borehole, and using the monitoring well borehole as a positioning and fixing support point for a bottom of a monitoring well pipe;
• Step 4:
• installing an outer installation casing for pre-burying the groundwater monitoring well:
• Placing, with a crane or vibratory hammer, the outer installation casing for the groundwater monitoring well at the bottom of the excavated PRB trench, wherein the outer installation casing is coaxial with the monitoring well borehole;
• Step 5:
• installing a monitoring well positioning bracket:
• the monitoring well positioning bracket including a hoop mounted outside the outer installation casing for the groundwater monitoring well and a retaining structure attached to the hoop, wherein the hoop is formed into a ring shape by rolling and pressing a steel bar;
• the retaining structure is a set of straight rebars with one end welded to the hoop and the other end in contact with the trench side-wall support;
• Step 6:
• installing the monitoring well pipe:
• the monitoring well pipe consists of a PVC well casing, a well screen, and a sand-settling casing, the well casing, the well screen, and the sand-settling casing being assembled to form the monitoring well pipe in a manner of the well casing connected to a top of the well screen and the sand-settling casing connected to a bottom of the well screen;
• following completion of assembly, lowering the monitoring well pipe into the outer installation casing for the groundwater monitoring well using a crane and placing the bottom of the monitoring well pipe into the fixing support point that is formed by previously drilling at the bottom of the monitoring well pipe;
• Step 7:
• installing monitoring well filter packs and seal materials:
• filling a space between the monitoring well pipe and the outer installation casing for the groundwater monitoring well with the filter packs;
• then filling the space with buffering sands on a top of the filter packs to a thickness of 0.5 to 1 meter;
• and then filling the space with bentonite from a top of the buffering sands to a ground surface to seal the outer installation casing for the groundwater monitoring well;
• Step 8:
• backfilling the PRB trench with PRB trench backfill materials:
• backfilling the PRB trench with the PRB trench backfill materials between the trench side-wall support and the outer installation casing for pre-burying the groundwater monitoring well, in layers of a thickness of 2 meters, wherein the PRB trench backfill materials include a trench media and a covering soil layer;
• Step 9:
• removing the outer installation casing for pre-burying the groundwater monitoring well:
• suspending the backfilling of the backfill material every time the thickness of the backfill material reaches 2 meters, pulling up the outer installation casing for pre-burying the groundwater monitoring well with a crane or a vibratory hammer by 2 meters, and resuming the backfilling afterwards;
• repeating the alternated operations of backfilling of the backfill material and pulling up the outer installation casing for pre-burying the groundwater monitoring well until the trench is completely backfilled and the outer installation casing for pre-burying the groundwater monitoring well is completely pulled out of the ground;
• wherein, in this way, installation of the groundwater monitoring well and backfilling the PRB trench are completed concurrently;
• Step 10:
• removing the trench side-wall support:
• following removal of the outer installation casing for pre-burying the groundwater monitoring well, removing the trench side-wall support with a vibratory hammer or a hoisting device;
• and,
• Step 11:
• conducting well completion and well development:
• lastly, installing a well head assembly for the groundwater monitoring well installed inside the PRB trench, and conducting the well development for the installed groundwater monitoring well to complete preparation for groundwater sampling.