Soil anchors in Israel - vision and reality

This article examines what is happening in the field of soil anchors about ten years after the issuance of the first Israeli standard in the field.

The soil anchors have been used for many years in the construction industry to stabilize buildings, slopes, retaining walls, piers, dams, etc., and are designed to transfer forces from the front of the building to a stable area underground. Stability is achieved by significantly increasing the normal forces acting on potential destruction planes. Driving the anchors in advance for required labor may reduce the future displacements of the anchored structure. A choice of using soil anchor alternatives should take into account the advanced technology used in it to recruit relatively high workforces, while immediately checking the endurance both in the short term and in the long term.

The article was published in the professional journal of the Association of Construction and Infrastructure Engineers "Construction and Infrastructure Engineering" Issue No. 90, March 2021.

1. The role of soil anchors and their importance in construction and infrastructure industries

A. Ground anchors have been accepted in the construction and infrastructure industry in the world and in Israel for many years. The anchors are used in a wide variety of applications, such as: walled excavations for basements, in places where open excavations are not possible, due to considerations of space and risk to buildings and infrastructures, a means of protection against slope sliding, to receive the troubles expected from the planned construction such as dynamic forces caused by wind and earthquake disturbances, as well as lifting forces caused due to underground construction, below the groundwater level.

Ha Lochem St. Bnei Brak. Sheathing walls on stilts are supported by anchors and soil nails

B. The above speaks for itself and emphasizes the importance of the anchor component as complex and unusual construction elements. A review of the Israeli standards and general and special specifications reveals that these elements constitute a complex category, which includes a constructive aspect on the one hand, and a geotechnical aspect on the other. This aspect requires the action of construction planners skilled in the field with geotechnical consultants in the field of soil and foundation.

C. Geotechnical structures are divided into three categories (as can be seen, in T.I., 940 part 1 and T.I. 940, part 3.1: geotechnical category No. 1, 2 and 3). According to this division, elements that include defined soil anchors, usually, as category 3. This means that failure of their function may cause serious damage to the nearby environment, disruption of the operation in the area, heavy economic damage, damage to people and property. Also, there is great difficulty in repairing and restoring the site and its surroundings, which requires a lot of time.

D. It is clear that structures and anchored elements require special attention, both in the planning aspect and in the performance aspect. Failures in this area also create a negative media resonance. Over the years, many failures in this field have been published, as can be found for example on the Internet, which include, for example, the collapse of drywall walls in Ra'anana, Beit Shemesh, in a large project in Bnei Brak and many others known to those in the field. On the other hand, you can find impressive projects that have been successfully carried out (attempting to attach images from these sites is difficult to impossible due to copyright issues. Readers interested in this can find them by searching the Internet).

E. Every year, about 20,000 anchors are installed in Israel, in many and varied projects. The majority of the anchors (about 95%) are temporary anchors, which are replaced during construction by permanent structural elements, such as ceilings. The rest (about 5%) are permanent anchors that are supposed to be used for the entire length of the structure, which in public projects reaches 120 sleep.

F. Anchors are a complex mechanical element in its structure, in the way it is installed and in the way it functions and in the existing length of the structure. Failures in anchors can be due to the following reasons:

Inadequate design of the support system, which includes the anchor + the constructive element it supports (conventional wall, reinforced concrete facade, bridge, etc.). An area under the responsibility of the planning team, mainly.

Failure in the structure of the anchor itself, on its various components, which includes mechanical components, protections against corrosion, drilling and installation of the anchor. This area, for the most part, is the responsibility of the anchor contractor.

Failure to test the anchor and guide it, including full monitoring of the anchor. The responsibility for this area is discussed in detail within this article.

2. The anchor, its components and standard requirements

A. In 2011, a detailed Israeli standard was issued for the first time, dealing with ground anchors, TI 940 part 4.2, called "Geotechnical design: strengthening and stabilization of structures for engineering purposes - ground anchors made of piles". This standard is defined as a recommended standard, which is not legally binding (unless it is determined as such in another legal framework, which adopts it as binding).

B. The anchors develop relatively high service tolerances for each anchor. This allows for a reduction in the number of anchors per square meter of wall façade and makes them more economical than other anchoring methods. This advantage is accompanied by a disadvantage, since the system has lower redundancy, due to which failure of a single anchor may create progressive failures in adjacent anchors, which may lead to a general failure.

C. In light of the above and the complexity of the anchor, it is necessary to ensure that the requirements regarding the anchors are clear and include, among other things:

Proven documentation of the various anchor components.
The components of the anchor will be compatible with each other, in a way that guarantees the use of spare parts, without improvisation and meeting all the requirements.
The properties of the materials will not change during the existing length of the anchors, (up to two years for temporary anchors and for the entire period of the structure's existence, for permanent anchors).
The use of new technologies and methods is allowed if there is documentation, tests and knowledge proving the anchor's resistance to all standard requirements.

Ha Lochem St. Bnei Brak. Sheathing walls with piles supported by anchors and soil nails Bnei Brak. Failure case: A wall with anchors that moves

D. Within the framework of the existing standard, there are two types of anchors: cable anchors and anchors from treading rods (in addition to this, there are currently polymer anchors on the market made of films and/or round polymeric rods, which are not yet included in the standard and with which experience is limited).

E. The anchors operate according to the principle of transferring the stresses acting on them from the structure, through the head of the anchor, to a stable area in the subsoil, outside the scope of mutual influence between them, which could cause a general failure (see attached anchor diagram).

Drawing 1 - Geometry of ground anchors

F. The front anchor part is connected to the structure using the anchor head system, adapted to the type of anchor (cable anchor, or rod anchor). The back part of the anchor is occupied, in a stable area of the ground and is anchored in it using cement mortar or resin mortar ("capture" area). The central part of the anchor passes through the ground which is in the "gliding circle", which is unstable. Therefore, the tread steel (cables or rods) is separated from the ground there, inside a flexible pipe. This area is defined as a "free section".

G. All anchor components are designed against corrosion and corrosion, according to their existing period. The cement mortar is tested for its resistance against corrosion and all the tread steel and normal steel components are protected against welding, in accordance with what is required of the anchors. The synthetic anchor parts are protected against UV radiation, before their installation. In temporary anchors, at least one layer of protection against corrosion is required, while in permanent anchors two such layers of protection are required.

H. The drilling of the anchors is done using a machine and equipment suitable for the type of soil, its stability and possible penetration to the groundwater level (more on soil drilling and their characteristics).

I. According to the requirements of the standard, the production of the anchors will be done by a manufacturer with knowledge, technological ability and experience, who will be approved by a competent authority.

J. Pouring an internal grout between the cables/treading rods and the threaded pipe allows, according to the standard, to consider this grout as a protective layer. Conditional on performing grouting under factory conditions, with high-quality cement grout, so that the amount and width of the cracks in the grout meet the requirements of the standard and comments on this topic, as detailed below).

K. There is a requirement in the standard to protect the anchor components at all stages of execution. That is, from the moment it leaves the factory until it is installed in the hole designated for it. To protect the anchor, the production conditions must be taken care of, it must be transported to the site from the factory, and then it must be installed in the borehole. It can be understood that in the case of casting the "root" of the anchor in the factory, lifting it onto the truck, transporting it on the road, taking it off the truck, putting it down, lifting it again and inserting it (sometimes by force) into the borehole, could, with great certainty, cause many cracks, so that his wages were lost. In order to avoid such a thing, each anchor must be led in a designated template, which will prevent damage to it, until the moment of insertion into the borehole. Obviously, all this involves additional costs. The alternative is to establish a factory on the site, which is economical only in large projects, or to find another creative solution.

L. After the anchors are installed, the standard requires that each anchor be stepped on to test the anchor's function on all its components. Only after this stage is completed can the anchor be locked in a planned work load.

M. The standard requires that the entire process of monitoring and analyzing the findings be done by an experienced and qualified person, under the supervision of the inspector or another qualified body, when all the aforementioned bodies will be approved by the planner, in accordance with the documents presented.

3. Vision and reality in the field

A. Issuance of general specifications between offices in 2005 and later, a detailed standard in 2011, were a milestone in the attempt to regulate the anchor industry, which had been operating for a long time before without regulated standardization and without clear procedures and rules, when the issue broke out and without a proper framework. The execution of the anchors then, was subject to special technical specifications issued by the planners, each according to his skills and understanding, without standardization.

B. In many projects in the past, the execution of the anchors, their inspection and guidance, were carried out by the anchor contractors themselves without proper quality control and assurance, when the contractor is the operator and inspects himself at the same time.

C. There is no doubt that the introduction of the specification and subsequently the standard contributed to raising awareness and the quality of the anchors produced, but not to the required extent and without proper supervision. At the same time, in recent years there has been a large increase in the number of anchor contractors entering the field. The intense competition led to a situation where the prices of the anchors dropped significantly (probably beyond the possibility of meeting all the requirements of the standard). One area that is particularly affected is the quality control over the installation of the anchors. There is currently a trend that contradicts the requirements of the standard, of going back to inspecting the anchors by the anchor contractors, despite what is stated in the standard and contrary to engineering logic.

D. In the construction and infrastructure sector in the public and governmental sector, it is acceptable to carry out projects according to a quality procedure in which the contractor performs quality control as part of the agreement, while the client performs quality assurance. This issue is problematic in the field of anchors, which requires expertise and knowledge from the inspector. This procedure does not usually exist in the private market .

E. In the prevailing situation in the field today, of the tough competition in the industry, the quality costs (quality control and assurance) and the additional time needed for high-quality execution of the anchors, there is a financial problem and a problem of schedules, both for the customer and the contractor of the anchors. The problems create repeated pressures on the planners and the management team. This could lower the quality of the product and endanger the project and the people.

F. The Israeli standard for anchors has existed for ten years. It has not been revised yet, as required therein. It seems to me that it is time to re-examine the standard and update it, among other things to prevent further deterioration in the field, as detailed above. Encouraging news can be found in updated general specifications issued by Israel Railways in 2018.

* Eng. Moti Yuger - union member, geotechnical cell, with a bachelor's degree and a degree in civil engineering, within the "Polytechnic Institute of New York". His specialization as part of his master's degree was in the field of foundation and geotechnical engineering. His professional activity as a freelancer, in the field of consulting for foundations and geotechnics, began in 1981, and in 2002 the firm moved to operate within the framework of Eng. The company specializes in solving difficult geotechnical problems and unique foundation methods in addition to its current activity.

- Assisted in the preparation of the article, Eng. Shlomo Lieberman, from G.A.S. - Construction and Bridge Engineering Ltd.