Offshore Wind Power Systems
of Texas llc
Anchorage Systems
This discussion provides guidance for the design and evaluation of anchor systems used to prevent the sliding and/or
overturning of laterally loaded structures founded on rock masses.

Anchor systems may be divided into two general categories tensioned and untensioned. The primary emphasis in the
design, or selection of an anchorage system, should be placed on limiting probable modes of deformation that
may lead to failure or unsatisfactory performance. The underlying premise of anchorage is that rock masses are
generally quite strong if progressive failure along planes of low strength can be prevented. Both tensioned and
untensioned anchors are suitable for the reduction of sliding failures in, or on, rock foundations. Tensioned anchor
systems provide a means for prestressing all, or a portion, of a foundation, thus, minimizing undesirable deformations or
differential settlements. Preconsolidation of rock foundations results in joint closure and what appears as strain
hardening in some foundations.

Tensioned Anchor Systems
A typical prestressed anchorage system. The use of grouted anchorages is practically universal, particularly with high
capacity tendon systems.
Upon tensioning, load is transferred from the tensioning element, through the grout, to the surrounding rock mass. A
zone of compression is established (typically assumed as a cone) within the zone of influence. Tensioned
anchor systems include rock bolts and rock anchors, or tendons.

The following definitions:

a. Rock bolt. A tensioned reinforcement element consisting of a rod, a mechanical or grouted anchorage, and a plate
and nut for tensioning or for retaining tension
applied by direct pull or by torquing.

b. Prestressed rock anchor or tendon. A tensioned reinforcing element, generally of higher capacity than a rock bolt,
consisting of a high strength steel tendon (made up of one or more wires, strands, or bars) fitted with a stressing
anchorage at one end and a means permitting force transfer to the grout and rock at the other end.

Section II
Methods of Analysis
Typically, analyses of systems used to anchor mass concrete structures consist of one of two methods: procedures based
upon classical theory of elasticity or
procedures based upon empirical rules or trial and error methods. The gap between the methods has been narrowed by
research in recent years but has not significantly closed to allow purely theoretical analysis of anchor systems. The
following discussions on methods of analyses are divided into tensioned and untensioned anchor systems.

a. Anchor loads. Anchor loads for prestressed tensioned anchors are determined from evaluation of safety factor
requirements of structures. Anchors may be
designed for stability considerations other than sliding to include overturning and uplift. Other factors must also be
considered. However, anchor forces required for sliding stability assurance typically control design.

b. Anchor depths. Anchor depths depend upon the type of rock mass into which they are installed and the anchor
pattern (i.e., single anchor, single row of anchors, or multiple rows of anchors). The anchor depth is taken as the anchor
length necessary to develop the anchor force required for stability. The entire anchor depth lies below the critical
potential failure surface. (1) Single anchors in competent rock. The depth of anchorage required for a single anchor in
competent rock mass containing few joints may be computed by considering the shear strength of the rock mobilized
around the surface area of a right circular cone with an apex angle of 90 degrees If it is assumed that the in-situ stresses
as well as any stresses imposed on the foundation rock by the structure is zero, then the shear
strength can be conservatively estimated as equal to the rock mass cohesion.

c. Anchor bonding. The above, presented for analysis of anchor system, assume sufficient bond of the anchor to the
rock such that failures occur within the
rock mass. The use of grouted anchorages has become practically universal with most rock reinforcement systems. The
design of grouted anchorages must, therefore, insure against failure between the anchor and the grout, as well as,
between the grout and the rock. Experience and numerous pull-out tests have shown that the bond developed between
the anchor and the grout is typically twice that developed between the grout and the rock. Therefore, primary emphasis
in design and analysis is placed upon the grout/rock interface. For straight shafted,
grouted anchors, the anchor force which can be developed depends upon the bond stress Structures should in principle
be anchored, when required, to rock foundations with tensioned or prestressed anchorage. Since a displacement or
partial shear failure is required to activate any resisting anchorage force, analysis
of the contribution of dowels to stability is at best difficult. Dilation imparts a tensile force to dowels when displacements
occur over asperities but the phenomenon is rarely quantified for analytical purposes.

Design Considerations
a. Material properties. The majorities of material properties required for the design of anchor systems are also typically
required for the investigation of other
aspects of the foundation design. The selection of appropriate material properties. Design anchor force derived from
calculations not associated with sliding instability must consider the buoyant weight of rock where such rock is
submerged below the surface water. Tests not necessarily considered for typical foundation investigations but needed
for anchor evaluations include
rock anchor pull-out tests and chemical tests of the ground water. Rock anchor pull-out tests  provide valuable data for
determining anchorage depth and anchor bond strength. Hence, a prudent design dictates that pull-out tests be
performed in
the rock mass representative of the foundation conditions and anticipated anchor depths. Ground water chemical tests
establish sulphate and chloride contents to be used as a guide in designing the anchor grout mix. In addition, the
overall corrosion hazard for the anchor tendon steel should be established by chemical analysis. Such analyses are
used to determine the amount and type of corrosion
protection required for a particular foundation.

b. Factors of safety. The appropriate factor of safety to be used in the calculations of anchor force and anchorage depth
must reflect the uncertainties and built-in conservatism associated with the calculation process. In this respect, anchor
force calculations should be based on the factor of safety associated with sliding stability of gravity structures.
Anchorage depth calculations based on the unit weight of the rock mass should use a minimum factor of safety.

c. Total anchor length. In addition to the anchor depth and anchor bonding considerations, the total anchor length (L)
is controlled by the location at which
the rock mass is assumed to initiate failure should a general rock mass failure occur, the assumed location of failure
initiation commonly used in practice. As indicated, three locations are commonly assumed: potential failure initiates at
the base of the socket; potential failure initiates at the midpoint of the socket; or potential failure initiates at the top of
the socket. The implication with respect to the total anchor length imposed by each failure location assumption. For the
design of anchors in competent or fractured rock masses where the bond length is supported by pull-out tests, the
potential for rock mass failure is assumed to initiate at the base of the anchor. For preliminary design where pull-out tests
are not yet available or in highly fractured and very weak material, such as clay
shale, the potential for failure is assumed to initiate at the midpoint of the socket. However, in the case of highly
fractured and very weak material, pull-out tests must be performed to verify that the bond length is sufficient to develop
the ultimate design load.

d. Corrosion protection. The current industry standard for post-tensioned anchors in structures requires double corrosion
protection for all permanent anchors.

e. Design process. The rock anchor design process is conveniently divided into two phases; the initial design phases and
the final detailed phase.

(1) Initial phase. The design process is initiated by an evaluation which finds that a given structure is potentially
unstable without additional restraining forces. If the potential instability is due to potential for sliding, the magnitude of
restraining forces is calculated according to procedures. Restraining forces necessary to control other modes of
potential instability, such as overturning, uplift pressures, or excessive differential deformations are determined on a
basis. The magnitude of the required restraining force is evaluated with respect to the economics and practicality of
using rock anchors to develop the necessary force.

(2) Final phase. The final detailed design phase is a trial and error process which balances economic and safety
considerations with physical consideration of how
to distribute the required restraining force to the structure and still be compatible with structure geometry and
foundation conditions. While sequential design steps reflect the preference of the general design constraints usually
dictate that the total restraining force be divided among a number of anchors. Foundation conditions control the
anchorage depth as well as the amount and type of corrosion protection. Anchor depths between adjacent anchors
should be varied in order to minimize adverse stress concentrations.