See typical load transfer curves in
Figure 10.
For another project Terracon placed
TIP wires inside hollow bar micropiles
to verify the required actual grout
cover and anticipated diameter of the
piles (Figures 11 and 12).
According to Terracon, while load
tests on micropiles are generally performed
on one to two per cent of the
production piles, the ease of TIP testing
allows for inspection coverage of
10 to 20 per cent. It can be performed
relatively easy and is another valuable
and innovative method to determine
proper micropile installation.
A new patent has been applied for, for
the use of hollow rebars, replacing pipes
for post-grouting the base or tip of drilled
Test Load Effective debonded
(kips) length elevation ( ft)
26 7.4 78.6
51 10.2 75.7
75 10.8 75.2
100 12.3 73.7
126 13.6 72.1
149 15.5 70.5
Seating Load
shafts, improving load carrying capacity
and also decreasing their length.
Post-grouting the base or tip of
drilled shafts refers to a variety of
practices related to the injection of
grout under pressure below the tip of
a drilled shaft foundation to improve
the resistance of the shaft to top-down
loading when subjected to compressive
axial load. The grout may be neat
cement grout (e.g. Portland cement
and water). Post-grouting is accomplished
using a grout delivery system
that is incorporated into a drilled shaft
during construction. The grout delivery
system generally includes tubes
or pipes that pass from the top of the
shaft to a grout distribution apparatus
located at the tip of the shaft. When
adequately instrumented and properly
monitored, the post-grouting process
has been proven to provide increased
load capacity compared to conventional
(non-grouted) drilled shafts,
since the process provides a measurable
indication of performance.
However, providing the additional
pipes is an additional expense that
adds to undesirable congestion of the
steel reinforcement cage used for the
drilled shaft foundation. Furthermore,
once post-grouting is complete, the
pipes used for post-grouting provide
no structural contribution to the
drilled shaft foundation and are a relatively
inefficient component.
Accordingly, it is desirable to provide
a more efficient mechanism for
using a post-grout delivery system of
a drilled shaft foundation and replace
the pipes or tubes with hollow rebars.
Summary
This new method using hollow rebars
for NDT of drilled shafts or micropiles
will eliminate all previously
experienced problems which caused
unnecessary expensive repair costs
and provides more accurate reliable
test results. As a bonus, the large diameter
hollow rebars (e.g. 73/53) replace
standard rebars and contribute to a
stiffer drilled shaft rebar cage for safer
handling and placing into the drilled
shaft. An additional bonus is using the
same hollow rebars for post-grouting
the base or tip of drilled shafts, which
increases the load capacity and can
shorten their length.
References
1. Armin Stuedlein, Professor,
School of Civil and Construction
Engineering, Oregon State of
University, Corvallis, OR., 2016
Deep Foundation Institute.,
Comparison of Non-Destructive
Integrity Tests on Experimental
Drilled Shafts.
2. Bernhardt H. Hertlein, FACI,
M.ASCE, Senior Consultant – NDE
& Geophysics, GEI Consultants,
Inc., Vernon Hills, Ill.
3. Bill Wright, P.E., Terracon
Consultants, USA. 2017 ISM
Vancouver. Micropile QA//QC Using
Thermal Profiling.
TECHNICAL
SOHO Lofts, Test #3 (12/30/03), Strain Gage Residual Load
SWC Las Vegas Boulevard and Hoover Avenue
TERRACON Project No. 64035159
Bar Load (kips) tension = positive values
Ground Surface
Test Load L1
gage #1
gage #3 gage #2
pre-test seating load (5 kips)
U-2 (51to 5 kips)
U-5 (126 to 5 kips)
L-1 (26 kips)
U-3 (75 to 5 kips)
U-6 (149 to 5 kips)
U-1 (26 to 5 kips)
U4 (100 to 5 kips)
pos. test, no load
Grouted Length 63.5 ft. to 86 ft.
Free Length 86 ft. to 100 ft.
Elevation (ground surface = 100 feet)
30
25
20
15
10
5
0
-5
-10
-15
-20
-25
-30
55 60 65 70 75 80 85 90 95 100 105
Figure 10
Radius vs Depth – P-7 – 05/23/16 19:39 Cage View – P-7 – 05/23/16 9:13
Radius (in)
0 2 3 4 5 6 7 8 9
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
42
44
h– 1
–• Avg Rad
–• Cage
–• Shaft
6.5 Depth (n) Concrete Cover (in)
0 2 3 4 5 6 7 8
0
5
10
15
20
25
30
35
40
Figure 11 Figure 12
1
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