Evolution of the Drilled Steel Displacement (DSD)
Pressure Grouted - Piling System
Genesis: This cast in place piling system finds its genesis in several well-known specialty foundation systems, such as, Auger Cast in Place Pile (ACIP), Mini Piles and Hollow Core Anchors, to name a few. These systems are advanced with continuous flight augers, casing and all-thread by a top drive hydraulic drill and grouted in one manner or another to complete the pile. These piling systems have gained favor among designers around the United States and other parts of the world due to their efficiency and economy per ton of load transfer. However, these systems do have several detractions, such as, soil and grout spoil, continuity issues, material waste, equipment size and reinforcement installation reliability.
Evolution of Displacement: The desire to eliminate spoils and decrease material waste has lead to the evolution of displacement ACIP piles. In this application the continuous flight auger is replaced with a mandrel and only the bottom 5 to 10 feet is a continuous flight bit. Downward crowd force and torque are used to advance to the required tip elevation or length. The mandrel is then withdrawn as grout is placed in a similar manner to a typical ACIP pile. Unfortunately this method still demands equipment on the same order of magnitude as ACIP piling, production is reduced and reinforcement installation remains an issue.
The Next Step: The next step in the system evolution was to take the advantages of the displacement ACIP pile and over come the disadvantages that still exist. Drilled Steel Displacement Piles (Pressure Grouted) were developed to eliminate the remaining disadvantages.
The Difference: The DSD piling system utilizes a disposable lead auger and mandrel (drill Steel) to advance to system. The lead flighted bit is advanced to a predetermined torque and the mandrel, which is placed in sections, becomes the reinforcement detail of the completed pile. This method insures the placement of the reinforcement section. By placing the pile in sections, the system can be installed with substantially smaller equipment and in low headroom or limited access conditions.
Pressure Grouting: The other advancement in the system revolves around the grouting process. The consistency problems with redi mix suppliers and paddle mixers are taken out of the equation. Bagged grouts are used and special colloidal mixers are utilized. This dramatically reduces the water cement ratio required for producing a pumpable mix, thus generating higher grout strengths in less time. In addition, these mix/pump plants have the capability of pumping up to 1,500 psi if required by the soil conditions or the application.
Drilled Steel Displacement (DSD) Pile System Comparisons:
Spoils and Site Disturbance: The displacing action of DSD Piles eliminates any spoil to stockpile or remove from the site, creating a logistical and economic advantage over other systems. This also provides a substantially safer work area while saving time and money required to regrade sites after similar cast in place systems have been installed.
Reinforcement: Since the drill steel utilized to install the pile is left in place, it ensures proper and complete placement of the required reinforcement section and eliminates a step in the construction process. This pipe section also facilitates the pressure grouting of both the interior and exterior portions of the shaft
Tiebacks & Batters: The articulating nature of the installation equipment allow for unlimited capabilities related to installation angle and make batter piles as efficient to install as vertical piles, making lateral load transfer through battered piles to be very cost effective.
Underpinning and Structure Lift: The unlimited connection details available for the system with direct positive connection to the steel pipe section makes DSD Piles ideal for underpinning and supplemental load transfer.
Low Headroom and Limited Access: The small size of system installation equipment allows for unprecedented capabilities to access installation sites within and around existing structures. Coupled with the elimination of spoils, this system provides an effective and clean means of adding load transfer capability. The light weight equipment required for system installation can also be lifted into areas that could not other wise be reached by equipment required by other cast in place systems
Capacity: Working loads of 40 tons or greater can be obtained in most soil conditions, however site specific Geotechnical information must be obtained to fully evaluate capacities.
Environmental Installation: Since DSD Piles do not produce spoils; they are ideal in contaminated soil conditions. This is a significant improvement over almost all other cast in place piling systems.
Noise & Vibration: The small nature of the equipment required to install DSD Piles, the lack of percussion drilling and the use of hydraulic drills eliminate any installation vibration and produces minimal noise. This is very advantageous when working around facilities like hospitals, historic structures and residential areas.
Inspection: Several factors make inspection of DSD piles simple. First, the use of torque to determine the adequacy of the soil stratum ensures that the pile tip is not terminated prematurely. In effect every pile gives positive feedback related to location specific Geotechnical conditions, similar to a driven systems reliance on hammer energy reflected in blows per inch. In addition, the pressure grouting procedure ensures that all portions of the pile shafts are properly filled and any week soil zones are filled. Similarly, continuity of the shaft is guaranteed by virtue of the full-length steel shaft.
Installation Description
Hydraulic Rotary installation:
Hydraulic rotary drive unit, sized for appropriate pile capacity, shall by mounted to base installation equipment.
Rotary drive unit shall be plumbed into auxiliary hydraulics on base equipment with pressure gauges mounted on both the pressure and return side of the hydraulic system. Gauges shall be mounted in such a way, as to provide a clear view for the inspector and equipment operator.
Lead drill steel shall be placed in hydraulic drive unit with both drive pins securely in place.
Displacement collar, if applicable shall be placed around pile drill steel.
Lead drill steel section shall be advanced until butt end of lead section is just above working grade, pins removed and hydraulic drive unit removed.
The inspector shall monitor installation torque via pressure gauges and record pressure at termination of each drill steel section along with length of section.
After lead section has been installed, subsequent sections of drill steel shall be added and advanced in the same manner as the lead section.
Once the appropriate depth or target torque has been attained, the hydraulic drive shall be removed from the drill steel and moved to next pile location.
Pressure Grouting:
Grouting head assembly shall be fitted to the top of the exposed drill steel.
The grout mixing and pumping plant (Colloidal Only) shall be positioned as to minimize the distance to the pile.
Bagged grout shall be added to the mixer and proportioned for the required grout strength.
The Colloidal mixer shall be outfitted with a continuous agitation device in the holding tank to prevent any segregation of the mix.
After the lines are primed, the grout hose shall be hooked up the grouting head assembly.
Grout will then be injected, filling the inner annulus until full.
Once the inside diameter is full of grout the system will be gradually pressurized forcing grout outside of the ports located on the lead drill steel.
Grout will continue to be injected until grout return is viewed outside the top of the pile or positive pressure or 150 PSI is achieved.
The grouting head assembly will then be removed from the pile top.
The pile will be cut to final grade by chop saw or portable band saw.
After grade is attained the connection detail will be mechanical bolted or welded to the pile.
