During the Phase 1 of Clearwater Dam Major Rehab project in 2006, the project Specifications required that “Equipment for drilling and installing standpipes in the embankment, overburden or foundation materials must meet the requirements of ER 1110-1-1807. The rotary-sonic method of drilling, using minimal or no water, is considered to satisfy this requirement while providing increased productivity. The Contractor shall use this method, or may propose another method that meets the requirements of ER 1110-2-1807. It is preferred that water not be used during the drilling of the embankment material.”
The issue of injecting water, even in minimal amounts, during embankment drilling is a major consideration. The contract documents require that drilling through the embankment must meet the requirements of ER 1110-2-1807, with the stipulation that the rotary-sonic method, using minimal or no water, is considered to satisfy this requirement.
ACT proposed to utilize two different overburden drilling methods at Clearwater Dam. Rotary-Sonic drilling was conducted by our Sonic Drilling sub-contractor and was utilized where continuous sample recovery is required. Minimal or no water was introduced into the borehole during drilling and samples was collected, logged, photographed and stored in accordance with the specifications.
In addition to the Rotary-Sonic method, ACT proposed to utilize our Interoc AN 160 unit equipped with a rotary duplex augering system in holes where continuous samples were not required. The system can be used with or without the use of water. When water is utilized, which is desirable as an auger lubricant to facilitate production, it is our opinion that the method is superior to sonic drilling with respect to protection of the embankment. When water is not utilized, it explicitly meets the requirements of ER 1110-2-1807.
The rotary duplex auger system consists of an outer casing with an internal auger system. The outer drill string will be equipped with a button bit casing shoe. The inner drill string will consist of solid stem augers and a solid face button bit with a small hole for water feed, if required. If water is necessary to lubricate the drill sting it will be fed through a small hole in the bit and a larger hole in auger stem located several feet above the bit. The inner and outer strings will be counter-rotated and percussed. As the tooling advances, cuttings are forced into the outer casing and fed to the augers. The cuttings are fed to the surface by the counter-rotating action of the augers and casing. At the surface the cutting are removed from the augers using a diverter system. Embankment samples can be collected from the diverter if requested. This drilling system has been utilized on previous embankment dam projects with good results including the sinkhole remediation at WAC Bennett Dam for BC Hydro.
In order to appreciate the inherent technical superiority of the rotary duplex auger system, it is important to:
Recognize the ability of the system to perform dual percussion
The ability of the system to run the inner auger and exterior casing strings simultaneously, independently, or not at all
To overcome the most difficult cobble/ boulder conditions and variations through the dam
And finally to visualize the flow paths of water during the drilling process (See figure on next page).
The sonic drilling sequence involves drilling ahead with the inner string followed by advancing the outer override casing. If water is injected while advancing the inner string, the water and cuttings must flow up around the inner string and find the annulus between the inner and outer casing. If the inner string is advanced without water flush, the soil enters the inner string and requires the driller to trip out and extrude the soil on the surface prior to further advancement. This slows the drilling production rate. The most important time water is required to advance sonic borings economically is during the advancement of the outer override casing. This is also the most critical phase of the drilling cycle with respect to potentially damaging the embankment. During advancement of the override casing, the water is either injected through the inner string (Mississinewa) or the casings are filled with water prior to advancement (Clearwater Sinkhole). Since the outer casing is attached to the drill head the water cannot return to the surface in the annular space between the two drill strings. Since the water cannot enter the annular space, it is forced along the outer casing, exposing the embankment to the water pressure, and either returns to the surface or the water return is lost when it reaches the pervious shell. When the water is lost, it is difficult to interpret the cause. The question that cannot be answered is whether the water loss was due to embankment damage or whether it was lost in the shell zone.
At Mississinewa, where all sonic drilling was in the impervious section, a number of water losses were recorded within the embankment. The water losses were either due to existing embankment damage or due to damage caused by drilling. This drilling was performed using methods that some in the sonic industry loosely refers to as dry drilling techniques. Water consumption on this project averaged 70 gallons per 10-foot drilling, or approximately 1,000 gallons for a 140 foot boring. At Clearwater, the sonic drilling techniques utilized were improved as water was not injected under pressure. On this project, the drillers filled the casings with water prior to advancing the outer override casing. This resulted in the static water head plus up to 20 feet (the length of each drill steel) plus the pressure resulting from advancement of the casing full of water capped at the surface. Water return to the surface was not observed during the embankment drilling at Clearwater due to assumed water loss into the pervious shell.
These identified flow paths during sonic drilling along with the achieved production rates on past projects, have led us to evaluate alternative systems that provided equal or substantially better protection for the embankment while simultaneously increasing production. This search led us to the rotary duplex auger system. If water is required when drilling with this system to assist in returning the cuttings to the surface, the water is always contained between the inner and outer drill casing. The only exposure to the embankment is at the bit face where the small hole exists. However, since the two strings are advanced simultaneously, the flow path from the bit face to the inner annular space is only inches and therefore, it is the preferred flow path due to its short length. In addition, the diverter is always open to the atmosphere and no additional water pressure is induced by the down-pressure or advancement of the drill string. In contrast, during sonic drilling, water must travel up to 20 feet along the inner casing to reach the annular space when advancing the inner casing, and, even more importantly water must travel along the outside of the drill string for the entire length of the hole when advancing the outer casing. The flushing fluid flow paths for each system are illustrated in the figure below. The duplex auger system clearly provides greater protection to the embankment than the sonic drilling methods. Our research also indicates that higher productivity should also be achievable using this system.
While it is virtually self-evident from consideration of the flow paths that using the rotary duplex auger system with carefully controlled amounts of water should be a superior system, we also recognize that it is not a pre-approved method in the contract documents.
ACT proposed and demonstrated to the USACE that this alternative drilling method met the requirements of ER 1110-1-1807 provided that the exception for water use in the shell. With this drilling system, ACT drilled approximately 105,000 ft of embankment and overburden at the Clearwater Major Rehab project.
ACT’s rotary duplex auger system was also used to perform the overburden drilling at the Left Rim and Groin of the project.