High capacity tieback installation method

The present invention is directed to a method of installing a high-capacity tieback in coarse-grained soil. The method includes connecting a unitary hollow casing to a drill and inserting a prestressing steel tendon within the casing. A lost bit is affixed to one end of the casing. The drill and casing is positioned at the desired location of the tieback and the casing is inserted into the ground by rotating the casing with the drill. The bit is released and grout is pumped into the casing under high-pressure. Once the grout has reached the desired pressure, the casing is extracted by applying a high-torque of at least 12,000 ft.-lbs. to the casing with the drill which overcomes the frictional engagement between the grout and the casing, and by applying a pulling force with the drill.

TECHNICAL FIELD 
The present invention relates to a tieback installation method in 
coarse-grained soils. In particular, to a method wherein a one-piece 
casing, containing the tieback tendon, prestressing steel, is installed 
into the ground. Upon installing the casing to the desired length, grout 
is injected at a high pressure in excess of 150 psi through the casing 
fixing the prestressing steel to the ground as the casing is extracted. 
During extraction of the casing, high torque is applied to the casing to 
overcome the friction developed between the casing and the grout, so that 
a relatively moderate pulling force can be used to extract the casing. 
BACKGROUND OF THE INVENTION 
Earth tiebacks, also generally referred to as ground anchors, are often 
used in the construction industry to support or anchor various structures 
in the ground. For example, they are used to support retaining walls 
bordering highways or to support excavation sheeting to prevent cave-ins 
which would otherwise endanger lives and property. Such tiebacks generally 
comprise a prestressing steel tendon, fabricated from multi-element 
strands or a single bar, installed in the ground and secured at its outer 
end by an anchorhead to an excavation sheeting system or other structure 
to be supported. A cement grouted anchor is formed around the opposite 
distal end of the tendon to distribute to the surrounding soil forces 
applied to the tendon. 
High load-carrying capacity tiebacks are made in coarse-grained soils by a 
variety of methods. Such high capacity tiebacks are installed by pumping 
cement grout under high pressure, for example, at a pressure above 150 
psi, which anchors the prestressing steel (tieback tendon) to the ground. 
At such high grout pressure, water in the cement grout is quickly driven 
from the grout into the soil as the grout is pumped out the bottom of the 
casing. The cement particles in the grout remain creating very stiff grout 
surrounding the anchor tendon and the lower portion of the casing. This 
stiff, solidifying grout makes it difficult to withdraw the casing so that 
special techniques or methods have been developed to install high capacity 
tiebacks. 
In one prior art method a drill is used to rotate the casing into the 
ground; and in another prior art method the casing is driven into the 
ground using a percussion hammer. The drilled casing uses a "lost" bit 
while the driven casing uses a "lost" point, attached to the front of the 
casing. After the casing has been installed to the desired depth, the 
prestressing steel is inserted into the casing and the bit or point is 
removed. Cement grout is then pumped down the casing while the casing is 
extracted with hydraulic jacks. The drills used to install tieback casings 
normally have about 4,000 to 6,000 ft.-lbs. of torque and 16,000 lbs. of 
pulling force. The casing-pulling jacks used to extract the casing must be 
capable of applying more than 50 tons of pulling force. These methods have 
the disadvantage of requiring two separate pieces of equipment, i.e., the 
dril or percussion hammer to install the casing and the jack and hydraulic 
power supply to withdraw the casing. Also, sectional casing must be used 
since the drills and percussion hammers are not capable of installing long 
continuous casing. Sectional casing is both more expensive and time 
consuming to use than a one piece casing. When sectional casing is used, 
bar tendons must be used because strand tendons have become grouted to the 
casing when high pressure is used to apply the grout. 
Another method of installing high capacity tiebacks in sand has been 
attempted, but to date it has been unsuccessful. In this method the casing 
is drilled into the ground, to a desired depth. Thereafter, the 
prestressing steel is inserted into the casing. The anchor is grouted by 
pumping grout down the casing at high pressure as the drill attempts to 
extract the casing. In these attempts the drill has applied at torque of 
only approximately 6,000 ft.-lbs. This torque is not sufficient to 
overcome the friction developed between the casing and the grout placed 
under high pressure resulting in failure to extract the casing and thus a 
considerable amount of lost casing. This attempted method is therefore 
incapable of attaining high load-carrying capacity tiebacks. 
A further prior method of installing a high-pressure-grouted anchor, 
disclosed in U.S. Pat. No. 3,494,134, consists of drilling a casing into 
the ground and inserting the prestressing steel along with a special grout 
pipe into the casing. In another prior technique, grout is placed around 
the prestressing steel under low pressure as the casing is extracted. The 
drill is capable of extracting the casing because the friction between the 
low pressure grout and the casing is low. After the initial low pressure 
applied grout has set (approximately 24 hours), grout is injected into the 
anchor zone portion of the tieback under high pressure through the special 
grout pipe. The high pressure grout fractures the low pressure grout 
surrounding the anchor. Then the tieback anchor is grouted under high 
pressure through the special grout pipe. This technique is time consuming 
since it requires two separate grout applications with an extended time 
interval between applications. An additional component, the special grout 
pipe, is also required, as well as a larger hole. 
SUMMARY OF THE INVENTION 
The present invention is directed to a method of installing a high capacity 
tieback in coarse-grained soils. A unitary hollow casing is connected to a 
drill. Prestressing steel is inserted within the casing and a lost bit is 
affixed to one end of the casing. The drill and casing is then positioned 
in the desired location and the casing is rotated into the ground by the 
drill and drilling fluid such as air, water, or the like, removes the soil 
as the casing is advanced. Once the casing has been drilled to the desired 
depth, the bit is released and grout is pumped down the casing. After the 
grout pressure has reached a desired high level (greater than 150 psi), 
the casing is extracted by applying a torque of at least 12,000 ft.-lbs. 
on the casing with the drill to overcome the frictional engagement between 
the grout and the casing and by applying a pulling force generally along 
the axis of the casing with the drill. 
The inability to extract the casing in prior art methods resulted because 
the high grout application pressure created semi-solid grout surrounding 
the end of the casing which caused significant friction between the casing 
and the grout. Once the grout seized the casing, it was not possible to 
free the casing with the drill. In the present invention, the application 
of high torque enables the casing to be extracted due to the combination 
of the high torque and a relatively moderate pulling force applied by a 
single drill. The high torque breaks the friction between the semi-solid 
cement grout and the casing and then the casing may be extracted with the 
drill. The method of the present invention allows tiebacks to be installed 
which have a high unit load-carrying capacity of at least 9,000 lbs/linear 
foot of anchor. 
The present invention overcomes the disadvantages of the prior art methods 
discussed above by providing a method in which the casing may be easily 
extracted using one piece of equipment. Moreover, the casing may be 
installed in a single piece, so that the prestressing steel may be 
inserted into the casing prior to drilling. This is a significant 
improvement over other methods which use short-length casing sections. 
Using short-length casing sections requires that all the casing be 
installed before the prestressing steel can be inserted and requires the 
use of a bar tendon as the prestressing steel when high grout pressures 
are used. Since the one piece casing can be extracted in a continuous 
operation, multi-strand tendons can be used without grouting them inside 
the casing. The capability of using multi-strand tendons in high pressure 
grouted tiebacks where the pressure is applied to the grout through the 
casing is a significant advantage over prior art methods because 
multi-strand tendons can have higher load-carrying compacities and they 
are available in longer lengths than bar tendons. In the United States, 
bar tendons have an ultimate load-carrying capacity of 234,000 pounds and 
a maximum uncoupled length of 60 feet. Additionally, there is a 
significant advantage in using a one-piece casing which does not have 
joints and may be fabricated from tubing which is heavier and larger than 
the tubing used in conventional sectional casing, since the casing joints 
used with existing methods are very expensive and the weight of the casing 
is limited to what two people can carry.

DETAILED DESCRIPTION OF THE INVENTION 
A preferred method of installing a high capacity tieback in accordance with 
the present invention includes connecting a unitary hollow casing 10 to a 
drill 12, which is shown diagrammatically as a block. Drill 12 is a 
high-torque drill capable of providing a torque of at least 12,000 
ft.-lbs. and preferably higher. Drill 12 also provides a pulling force of 
at least 16,000 lbs. A prestressing steel tendon 14 is inserted within 
casing 10 from its bottom end, and a lost bit 16 is then fixed to the 
bottom end of casing 10 which is to be drilled into ground 18. Drill 12 
and casing 10 are positioned at the desired location of the tieback. 
Casing 10 then is rotated by drill 12 into the ground while a drilling 
fluid such as air, water, or the like, is injected by a pump 13 to remove 
the soil as casing 10 is advanced. 
As shown in FIG. 2, once casing 10 has been drilled into the ground to the 
desired depth, lost bit 16 is released and grout 22 is pumped down casing 
10 under high-pressure by a pump 20, shown diagrammatically as a block. 
Once the pressure under which grout 22 is being pumped reaches the desired 
high level, wherein water in the grout is forced to bleed from the grout 
typically at a pressure of at least 150 psi, drill 12 extracts casing 10. 
As water bleeds from grout 22, solidified grout is formed adjacent the 
casing end. Grout 22 must be capable of allowing such water bleed at high 
pressure. A hydraulic cement type grout is suitable. Drill 12 applies a 
torque of at least 12,000 ft.-lbs. to casing 10 which overcomes the 
frictional engagement between solidifying grout 22 and casing 10. Drill 12 
simultaneously applies a moderate amount of pulling force, typically no 
more than 16,000 lbs. to the casing 10 generally along its axis. Grout 22, 
when it solidifies forms an anchor around the distal end of prestressing 
steel tendon 14. This grouted anchor, over length 25, is shown attached to 
a support structure 27 in FIG. 3. 
In a preferred method, the tendon 14 will be a multi-element tendon 
fabricated from seven-wire prestressing steel strand. The present method 
permits a multi-element strand tendon to be used and high pressure grouted 
with its inherent advantages of longer lengths and higher potential 
ultimate load-carrying capacity. Alternatively, the prestressing steel 
tendon can be formed of a bar. 
The preferred method is optimally utilized when installing a tieback in 
coarse-grained soil having a penetration resistance of at least ten (10) 
blows per foot, as defined by ASTM D-1586, "Standard Method for 
Penetration Test and Split-Barrel Sampling of Soils". The permeability of 
the coarse-grained soil is greater than 10.sup.-4 cm/second, as defined by 
ASTM D-2434, "Standard Test Method for Permeability of Granular Soils 
(Constant Head)". 
After casing 10 is extracted, prestressing steel tendon 14 is anchored to a 
supporting structure as shown in FIG. 3. The installed tieback in 
accordance with the present invention creates a tieback with an ultimate 
load-carrying capacity of at least 140,000 pounds and a high unit 
load-carrying capacity of at least 9,000 lbs/linear foot of grouted 
anchor. As used herein, high load-carrying capacity tiebacks refers to 
this high unit load-carrying capacity. The unit load-carrying capacity of 
tiebacks installed by the present invention typically reach the 20,000 to 
30,000 lbs/linear foot of grout anchor ranges. While low pressure grouted, 
tiebacks can be constructed with a high ultimate load-carrying capacity, 
the length of the tiebacks would have to be significantly greater than a 
high pressure grouted, tiebacks with the same ultimate load-carrying 
capacity. Moreover, the method is significantly faster than other methods 
used and the equipment used overcomes the difficulty associated with 
grouting under high pressure and attempting to pull casing with low-torque 
drills. It is believed that grouting under high pressure results in high 
capacity tiebacks because the state of stress surrounding the tiebacks is 
significantly changed, resulting in high frictional forces between the 
grout and the surrounding soil. 
The detailed description of the method of the invention is for illustrative 
purposes only and modifications may be obvious to one of ordinary skill in 
the art, within the scope of the invention as defined by the broad general 
meaning of the terms in which the appended claims are expressed.