Indian Geotech J

https://doi.org/10.1007/s40098-020-00466-8

ORIGINAL ARTICLE

Rock Anchors for Static Pile Load Testing: Considerations

and Case Studies
Sujan Kulkarni1 • Ravikiran Vaidya1 • Dixit Nakum1

Received: 10 September 2020 / Accepted: 3 November 2020

Ó Indian Geotechnical Society 2020

Abstract It is common practice in India to install rock with installation of inclined anchors for crown girder
anchors around test pile in order to obtain reaction for static assembly.
load tests wherever competent rock is encountered at rea-
sonable depths. Rock anchor design and installation require Keywords Rock anchors
Pile load tests
Fixed length

adequate attention as it is dependent on several parameters Rock formation
and checks. Authors have performed several static load
tests across India with rock anchors up to test load of
around 2000tons and also in various rock formations such Introduction
as Basalt, Breccia, Tuff, Granite, Sandstone, etc. Compe-
tent rock levels also have been seen to be varied to great Vertical load tests are commonly performed on pile foun-
extent such as around 20 m from ground surface and such dations in order to assess pile capacities. The reaction can
long free lengths offer several challenges including be obtained by kentledge or reaction piles or rock anchors
extension of strands equal to or more than ram stroke. in case of rock socketed piles. Kentledge type reaction
Typically, for rock anchors in soft rocks, computed fixed requires huge setup, and the process is time consuming. It
length may be more than 10 m, and BS code 8081:1989 is suitable for piles with smaller test loads say up to 500-
limits the fixed length to 10 m. In order to assess the rock tons. As the test load increases, size of kentledge increases.
anchor behavior for such condition where fixed length was This also increases the risk factors and complexities asso-
more than 10 m, an effort was made to test the anchors to ciated with the setup and field operations.
its full capacity and corresponding experience will also be For moderate to high test loads (more than 500 tons)
summarized in the paper in the form of case history. This installation of reaction piles results in speedier completion
paper addresses rock anchor design philosophy including and relatively safer operation. The test setup is compact as
several checks required for appropriate rock anchor design, compared to kentledge, and it is very much suitable for
approaches for rock anchor design in soft rock, author’s sites with limited space such as Metro Rails. Testing with
experience of rock anchor design and installation with reaction piles is mainly applicable for friction piles, where
various rock types across India, along with case studies. contractor can install reaction piles along with test pile so
Paper will also highlight benefits and limitations associated that testing can be executed after completion of 28 days of
pile casting resulting in time saving.
Sites where rock is present at reasonable depths, instead
& Ravikiran Vaidya
Ravikiran@geodynamics.net
of installing reaction piles, testing can be performed by
installing smaller diameter rock anchors by connecting
Sujan Kulkarni
them to girder assembly using HT strands. Installation of
Sujan@geodynamics.net
rock anchors requires adequate knowledge of Geotechnical
Dixit Nakum
Engineering and understanding of rock properties. Key
Dixit@geodynamics.net
benefits of testing pile with rock anchors include speedier
1
Geo Dynamics, Vadodara 390023, India construction of reaction setup, suitable for heavy loads

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(around 2000 tons) and early testing as it can be started highly weathered rock and 900 for moderately to slightly
once grout strength is achieved. Another advantage is as weathered rock) and also in selecting apex of rock wedge
the rock anchor drilling rigs are quite compact, testing can which shall be typically at the midpoint of fixed length and
be performed where space is major constraint. Although not at the base unless base plate is being used at the bottom
this test method offers lot of benefits that over other con- [3]. Group action i.e., overlapping of rock masses of dif-
ventional methods, design and construction of rock anchor ferent anchors shall also be considered. Typically this
requires adequate attention so as to perform the vertical approach is conservative as shear strength of rock in
load test successfully. Following sections highlight the neglected, and the evaluation is purely based on weight of
design philosophy to be adopted to rock anchors and var- rock mass against applied anchor forces. It should be noted
ious case studies demonstrating successful tests performed that, careful geotechnical data review, proper calculations
with rock anchors as well as challenges faced. of required checks is must. If adequate precautions are
taken during designing of rock anchors, there are less
chances of their failure during the test.
Design Philosophy of Rock Anchors

Rock anchors are installed to designed depths by drilling Considerations for Installation of Rock Anchors
the holes of generally upto300mm diameter and inserting
HT strands (typically 12.7 mm or 15.2 mm in diameter) Once rock anchors are designed as per guidelines or
followed by grouting with non-shrink grout up to top of acceptable engineering practices, contractor shall exercise
fixed length. Rock anchors have free length (i.e., no grout utmost care during installation of rock anchor so that they
zone and which is above fixed length) and fixed length (i.e., are constructed as per the design. Anchors shall be free of
grouted zone contributing toward pullout resistance of the dirt or any other deleterious substance. Suitable spacers
rock anchors). Guidelines for installation of rock anchors shall be provided at specified distance to ensure that strands
have been outlined in various codes worldwide, and are equally spaced. Coats of epoxy shall be applied on the
important points are discussed below. fixed length portion followed by sprinkling of quartz sand
IS 2911 part IV [1] covers the load test on piles and to increase the bond. Anchors shall have arrangement into
allows use of rock anchors as reaction system in clause central portion so that grout tube is lowered all the way to
7.1.3 (c). Rock anchors shall be installed at a distance of at the bottom at the time of grouting. The grout tube shall be
least two times of pile diameter from the edge of the pile checked to ensure that it is free of any foreign particles and
subject to minimum 1.5 m. IS 10270 [2] outlines guideli- grout can flow freely up to bottom of the drilled hole.
nes for design and construction of rock anchors, and rock Anchor shall be grouted only for fixed length zone, and it
anchors installed for obtaining reaction for vertical load shall be ensured that grout is not present in free length zone
test are categorized as temporary anchors. The force on HT so strands are free to elongate.
strands shall be limited to 70% of ultimate tensile strength Once rock anchor installation is complete, girder erec-
of the strand and hence the quantity of the strands in each tion and stressing of anchors can be performed after 7 days
anchor shall be finalized accordingly. The fixed length of of grouting. It is authors’ experience that the anchors are
the anchor shall be decided after several checks such as generally ready for stressing after 7 days. Following pho-
bond strength between strand and grout, grout and rock as tographs presented as Fig. 1, illustrate the rock anchoring
well as shear strength of rock. The bond strength between process.
grout and rock shall be checked based on site specific rock
properties such as unconfined compression strength (UCS)
and also based on table provided in IS 10270. A relation is Case Studies and Experiences with Rock Anchors
also established between UCS and ultimate bond strength
in the code. Typically bond between grout and strand is Authors have successfully performed several vertical load
much stronger, and hence is not a governing factor. tests across India and gained invaluable experience with
However, a check on bond strength is desirable before pile load testing with rock anchors. Rock anchors have
finalizing the design. been installed in variety of rocks including Basalt, Granite,
In order to make sure that even if rock mass shears, the Shale, Limestone, Sandstone, Breccia, etc. covering entire
anchor system is still stable, pullout check shall be per- range of soundness of rocks. Following sections presents
formed. The weight of sheared rock mass shall be more some of the case studies which shall summarize some
than the uplift force with minimum safety factor of 1.25 for interesting learnings from unique tests.
temporary anchors. The designer shall be careful in
selecting apex angle of rock mass (typically 600 in soft or

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(a) Rock anchor drilling in progress (b) Anchors made of strands (c) Anchors being lowered in borehole

(d) Grouting in progress (e) Rock anchors ready for stressing

Fig. 1 Rock anchor installation procedure

Rock Anchor Installation in Highly Weathered Rock reaction. Just to make sure that design is conservative, a
methodology applicable to design of piles in weak rock
It has been seen that at several sites that core recovery of when core results are not available or material is highly
rock samples is very poor, and rock is highly weathered, fragmented was used to estimate fixed length as outlined in
and hence it is not possible to perform unconfined com- Appendix 5 of IRC 78 [4]. The analysis was performed
pression tests. In fact sometimes SPT is performed in such considering weak rock formation, and this analysis resulted
formations as rock is too weak. In such cases, it becomes in fixed length of 14 m which was higher than IS guideline
challenging to assess fixed length of rock anchor without check mentioned above, and hence 4 rock anchors with
UCS values. 14 m fixed length were installed at site, and test was per-
At a project site in Nagpur, borelog indicated weathered formed successfully. The design was also reviewed by third
basalt underlain by weathered sandstone, and sandstone party consultant as per requirement of client. Photograph of
was underlain by weathered Gneiss. In basalt and sand- pile load test is presented as Fig. 2.
stone formation, RQD was nil and core recovery was For another site in Nagpur, test load on the pile was
ranging from only 3–10%. SPTs were performed in Gneiss 1900tons. Subsurface conditions consisted of highly
formation and blow counts were greater than 100 indicat- weathered sandstone underlain by completely weathered
ing refusal. Since UCS were not performed, rock anchors schist underlain by completely weathered Gneiss. SPTs
were designed based on allowable rock grout bond values were performed and indicated refusal blow counts. Core
as mentioned in Table 1 of IS: 10270, and fixed length was recovery and RQDs were not attempted probably because
estimated to be 10 m. The test load was 750 tons, and of presence of very weak rock. Hence an approach as
hence 4 rock anchors were designed to provide required mentioned above was adopted to estimate fixed length. As

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Fig. 2 Vertical load test in

progress at a site in Nagpur

rock was very weak, and load was very high, 8 rock designed accordingly. Closest borelog to test pile encoun-
anchors were planned with fixed length of 24 m. Here too, tered completely weathered rock at 5 m and highly
fixed length assessment was governed by IRC guidelines weathered Tuffaceous Breccia from 8 m up to 19 m with
and hence more conservative. RQD ranging from Nil to maximum 28. Slightly weathered
For this load test, available plate girders were not suit- Breccia was encountered underlying highly weathered
able to carry such high load. Hence, crown girder was Breccia with RQDs typically around 70. UCS results were
proposed which is very compact, and it was capable to available on samples collected at a depth of 12 m. UCS
carry the required test load. Benefit of crown type of girder values were ranging from around 3.92 MPa (40 kg/cm2) to
includes its light weight compared to conventional girders 13.7 MPa (140 kg/cm2). Free length was considered to be
and ideal at locations where space is limited. However, it 12 m, and fixed length was 16.5 m which was quite long.
requires installation of anchors at an inclination so that The fixed length computation was governed by the criteria
rock anchors are away from pile for at least 2 times pile based on site specific UCS values. Lowest UCS was used
diameter at rock level. Inclined rock anchor installation is in calculations as the rock formation was highly variable in
challenging, and there is increased risk of hitting under- terms of consistency and hence the design was
ground utility while drilling. Employing skilled driller with conservative.
suitable rig is very critical. Maintaining designed angle and Eight rock anchors were installed at the site as per
exact rock anchor location is also a challenge. During the contractor’s direction. After installation, during a revised
drilling work of this test, driller had to install steel liner to review, the consultant raised concerns regarding fixed
deeper depths than he anticipated and faced difficulty length of anchors. As per British Code [5], fixed length
during drilling due to collapse probably because of inclined shall be limited to 10 m. IS code has not made such pro-
drilling in overburden. Refer to Fig. 3 for crown girder vision. Although we had enough past experience with
setup. Rock anchors were installed successfully after similar tests, and design was conservative, consultant
dealing with several challenges, and test was performed insisted for additional 4 anchors having similar configura-
successfully although the time frame for the entire opera- tion. This was done as required with increased time
tion was more as compared to conventional test setup and schedule and re-working of girder configuration to fit in 12
vertical rock anchors. Figure 4 presents photograph of pile anchors within the setup. Eventually test was successfully
load test with crown girder. completed and everyone was satisfied with the perfor-
mance of pile as well as rock anchor system.
Vertical Load Test with Rock Anchors in Mumbai Once the test was completed, 8 out of 12 anchors were
disconnected from the system and additional load was
For a project site in Mumbai, 23 m long test pile was applied to check performance of rock anchors. As per
installed having diameter of 1000 mm. Design load was original design, actual load on each anchor was 245 tons.
785 tons, and test load was 1962.5 tons (i.e., 2.5 times Due to additional 4 anchors the actual load applied to each
design load). Test pile was located in very compact area, anchor was around 163tons during the test. Since the factor
and installation of rock anchors was extremely challenging. of safety in the design was at least 2.5, it was mutually
Initially 8 rock anchors were planned, and they were decided to load the anchors up to 1.25 times the design load

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Fig. 3 Crown girder setup

satisfactorily with only elastic extension of strands con-

firming that earlier design was conservative and was indeed
a workable solution in the first place. A photo of load test
setup is presented as Fig. 5 below.

Other Challenges with Rock Anchors

The biggest and common issue associated with installation

of rock anchors is encountered, when subsurface conditions
are significantly different than depicted in borelog on
which rock anchor design is based. The issue is majorly
experienced in Mumbai region the reason could be attrib-
uted to sudden geological changes in the region.

Fig. 4 Vertical load test in progress with crown girder setup

of each anchor i.e., 300 tons to see if they get pulled out of
ground. However, all four anchors carried the 300tons Fig. 5 Vertical load test in progress with 12 rock anchors

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For a recent work, test pile was planned between two anticipated. The success of rock installation is highly
pier locations and contractor provided only one borelog dependent on team efforts between designer and site crew.
and relevant rock test results specifying it is the closest The material such as strands, grout, etc., being used for
borehole. The borelog indicated presence of hard basalt installation shall be as per specifications.
rock and hence rock anchors designed accordingly. How- Generally, rock anchors are designed based on site
ever, while actual field work, Breccia was encountered specific UCS data for rock samples. If rock is highly
instead of basalt. Field work was halted, and issue was weathered, UCS data may not be available and in such
escalated to contractor when he provided another borelog cases other methods such as specified in IRC 78 can be
from another pier which indicated presence of breccia and adopted in addition to codal guidelines so as to make sure
more relevant to actual field conditions. Rock anchors were that design is conservative. Crown type girder setup is
redesigned which resulted in significant increase in fixed useful where space is constrain but inclined rock anchor
length. Similar situations were faced for few other tests installation is challenging. It is better to avoid it wherever
(e.g., rock encountered 2 m deeper than that indicated in possible and conventional plate girders shall be used for
borelog) as well and rock anchor design was altered as effective rock anchor installation. Authors have demon-
appropriate so as to make sure that their performance is not strated that rock anchors perform satisfactorily even if
compromised during load test. fixed length is more than 10 m. More studies and case
Another challenge associated with rock anchors is studies will be helpful in increasing the fixed length and
extension of strands equal to or more than jack ram stroke. subsequently increased use of rock anchors for testing. In
This is common where free length is very long because such cases, adequate precautions shall be exercised so as to
rock is at deeper depths, and it allows extension of strands avoid failure. Higher free lengths results in excessive
due to their elastic properties. For a project in Bangalore, strand extension and adequate packing steel plates shall be
free length was as high as 41 m, and test load was 1875 arranged before the test itself. Even if rock anchor design
tons. The stroke of ram was 150 mm. The theoretical and installation are challenging at times, authors have
strand extension within free length zone was around demonstrated that it is workable solution even for higher
220 mm. It was anticipated in advance that this issue would loads up to 2000 T.
occur during the test. In such cases, sufficient packing shall
be made available so full load can be applied to the pile and
the above stated limitation is overcome appropriately.
References

1. IS 2911 (Part 4) (2013) Design and construction of pile founda-

Concluding Remarks tions—code of practice. Part 4 load test on piles (second revision)
2. IS 10270: 1982 (Reaffirmed 2003) Edition 1.2. Guidelines for
Rock anchor are commonly installed to obtain reaction for design and construction of prestressed rock anchors
static pile load test where rock is present. Due care shall be 3. Nayak NV (2012) Foundation design manual, 6th edn. Dhanpat
Rai Publication, New Delhi
taken during design and installation of rock anchors so that 4. IRC:78-2014 (2014) Standard specifications and code of practice
they do not fail during testing. Design of rock anchors shall for road bridges. Section VII, foundations and substructures.
be as per codal guidance and also based on engineering Indian Road Congress
judgment. The designer shall be experienced and shall 5. BS 8081:1989. British Standard Code of practice for ground
anchorages
properly communicate the design requirements to instal-
lation crew. The site crew shall also follow the guidelines Publisher’s Note Springer Nature remains neutral with regard to
provided by designer in order for satisfactory installation. jurisdictional claims in published maps and institutional affiliations.
Importantly, site person shall inform the designer in case
he encounters different subsurface conditions than

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