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Hollow Bar Anchor System
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The Williams / Atlas Copco MAI Hollow Bar Anchor System is today’s solution for a fast and efficient anchoring system into virtually any type of soil. The MAI system has historically been known as “self-drilling anchoring” because the hollow fully-threaded bar serves as both the drill string and the grouted anchor, thus installation is performed in a single operation. The sacrificial drill bit is threaded onto the end of the MAI Bar and left in place following drilling. The drilling fluid (air, water, or grout) is introduced through the hollow bar and allows the spoils to flush from the borehole.
The Williams / MAI System is particularly suitable for soils that do not allow for open-hole drilling (i.e. granular soils that are collapsible in nature). In such cases, drilling with a grout fluid serves the purpose of flushing spoils from the borehole and prevents looser, surrounding material from collapsing due to the higher relative density of the grout. The Williams / MAI Hollow Bar Anchor System should be considered on any project desiring fast production that would otherwise need to involve a casing system in order to maintain borehole stability.
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Advantages of the Williams / MAI Hollow Bar Anchor System |
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- Fast, single-step anchoring system with simple equipment.
- Eliminates the need for a cased borehole in collapsing soils.
- Efficient installation since drilling and grouting can be performed in a single operation, saving both time and money.
- Continuously drilling and grouting under high pressure causes the grout to permeate into looser soils and creates a bulb-effect for increased bond capacity.
- Suitable for working in limited space and areas of difficult access.
- Multiple ranges of drill bits suitable for most soil conditions.
- Installed with standard track drill (top hammer) or hand-held drilling equipment, eliminating the need for larger casing rigs.
- R25-R51 bars contain standard left-hand rope thread pattern to easily adapt to most drilling equipment.
- Continuously threaded bar pattern can be cut and coupled anywhere along its length.
- Full stock of 3 meter long anchors, accessories and drill bits available in two different warehouse locations.
- Corrosion protection systems available upon request.
- FHWA approved for use as a micro pile.
- Rope thread exceeds requirement of DIN 1045-1 for permissible grout crack width formation under tensile loading.
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Advantages of the Williams / MAI Hollow Bar Anchor System |
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In general, Williams / MAI recommends using the hollow hollow bar anchor system in difficult soils that do not allow for open-hole drilling. Ground conditions featuring large voids or the presence of underground water are generally not suitable for a drilled and grouted hollow bar. In hard rock, conventional DTH (Down the Hole Hammers) in open-hole drilling offer a more efficient alternative. In all such cases, Williams offers solutions in their complete line of solid bar anchor systems and multi-strand tendon systems. |
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Williams / MAI Hollow Bar Anchors |
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| Anchor |
Outside
Diameter |
Internal
Diameter
(Average) |
Effective Cross
Sectional Area
(Average) |
Ultimate
Load
Capacity |
Yield
Load
Capacity |
Weight
per
Foot |
| R 25 |
1"
(25 mm) |
0.55"
(14 mm) |
0.38 in2
(245 mm2) |
45 kips
(200 kN) |
33.8 kips
(150 kN) |
1.5 lbs
(0.68 kg) |
| R 32N |
1-1/4"
(32 mm) |
0.73"
(19 mm) |
0.61 in2
(394 mm2) |
63 kips
(280 kN) |
52 kips
(229 kN) |
2.3 lbs
(1.04 kg) |
| R 32S |
1-1/4"
(32 mm) |
0.59"
(15 mm) |
0.76 in2
(490 mm2) |
81 kips
(360 kN) |
63 kips
(280 lN) |
2.85 lbs
(1.29 kg) |
| R 38 |
1-1/2"
(38 mm) |
0.75"
(19 mm) |
1.11 in2
(716 mm2) |
112 kips
(498 kN) |
90 kips
(400 kN) |
4.0 lbs
(1.81 kg) |
| R 51L |
2"
(51 mm) |
1.42"
(36 mm) |
1.20 in2
(774 mm2) |
124 kips
(550 kN) |
101 kips
(450 kN) |
4.7 lbs.
(2.13 kg) |
| R 51N |
2"
(51 mm) |
1.30"
(33 mm) |
1.46 in2
(942 mm2) |
180 kips
(801 kN) |
142 kips
(630 kN) |
5.6 lbs
(2.54 kg) |
| T 76N |
3"
(76 mm) |
2"
(51 mm) |
2.84 in2
(1832 mm2) |
360 kips
(1601 kN) |
270 kips
(1201 kN) |
10.0 lbs
(4.54 kg) |
| T 76S |
3"
(76 mm) |
1.77"
(45 mm) |
3.72 in2
(2400 mm2) |
428 kips
(1904 kN) |
338 kips
(1503 kN) |
13.2 lbs
(5.99 kg) |
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Threaded Bar Profile |
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The Williams / MAI Hollow Bar is manufactured from heavy-wall steel tubing that’s cold threaded to form a standard left-hand rope thread (R-Series) profile for R25-R51 bar diameters. The standard rope thread of the bar produces an excellent bond between the grout and the bar as well as enabling connection to all conventional drill rigs and allows the use with a wide range of drill steel accessories and bits.
The T76 hollow bar bar features a trapezoidal thread profile. All diameters of the Williams / MAI Hollow Bar are readily available in standard three-meter lengths. Upon special request, lengths up to 12 meters are available with added lead-time. |
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Comments on Crack Width |
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The rope thread profile of the Williams / MAI Hollow Bar exceeds the requirements of German Standard DIN 1045-1 for permissible crack width in grout under incremental tensile loading, up to 1.5 times the working load of the bar. In all tests, the crack width at the steel surface was clearly below 0.1 mm in width. This testing was performed and documented by Brandenburg Technical University. Upon request, Williams can provide a complete copy of the test report. |
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Applications
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Soil Nails
Soil Nails are non-tensioned, in-situ reinforcement for the stability of excavations and embankments in top-down construction. The Williams / MAI Hollow Bar System is an ideal choice for soil nailing in difficult soils as it offers high installation rates. Hollow bar anchors have gained European approval for permanent soil nail use and have been used extensively for permanent use in the United States.
Prestressed Ground Anchors
Williams / MAI Hollow Bar Anchors can be used as a choice for pre-tensioned anchors in loose or collapsing soils without the need for a casing. A free length must be installed onto the anchor if the project specifications call for a pretension load to be applied from the bond length. Please consult with a Williams specialist for suggestions to properly attach a free length sleeve. Note to Designer: Consult with Williams for an appropriate level of corrosion protection if prestressed ground anchors are intended to be used for a permanent application.
Micro Piles
Williams / MAI Hollow Bar Anchors offer an excellent choice for micro piles in difficult ground conditions where open-hole drilling isn’t possible. The continuously threaded bar profile lends itself perfectly for restricted headroom applications because the bar can be cut and coupled at any length. The FHWA has approved hollow bar anchors for permanent use in micro pile applications.
Tunnel Spiles and Forepoling
The Williams / MAI Hollow Bar System are often used for spiling in NATM tunneling. Spiles are continuously drilled and grouted pre-support reinforcement to enable the heading of a tunnel to advance without the risk of falling debris. The anchors can also be used as face stabilization of portals.
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 Williams Form Engineering Corp. All rights reserved. |
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