A dig into the past: the first tieback wall

Abstract This is an investigative work by the authors, all of them former students and employees at Tecnosolo Ltd, the company founded by our late Professor Costa Nunes, to find and photograph the first tieback wall designed and built in 1957 by Tecnoloso under Prof Costa Nunes’ guidance.


Introduction
This is a story of digging into the past, which the Authors embarked on and eventually had a successful outcome on 26 June 2023.
In the late 60's the first three Authors were undergraduate students at the Federal University of Rio de Janeiro, while the fourth Author, 10 years senior, was a practising geotechnical engineer working at Tecnosolo SA.This company was founded in the early 50s by the late Engineer and Professor Costa- Nunes (1917Nunes ( -1990) -researcher and pioneer in ground anchors.
In his classes, Costa Nunes (Figure 1) talked about the first tieback wall built in 1957.He mentioned it was built at the site of an old 19 th -century hospital named "Beneficiência Portuguesa" in the city centre of Rio de Janeiro, and 1958 in Germany.Weatherby (1982) and Hunt & Costa Nunes (1978) reported that permanently tied-back walls have been the most common method of slope retention in Brazil since 1957.After 1958 tiebacks found their way into Europe in Germany (Jelinek & Ostermayer, 1966;Jelinek & Ostermayer, 1967) and later in Switzerland, England, and France (Ranke & Ostermayer, 1968;Dupeuble & Brulois, 1969;Descoeudres, 1969;Littlejohn, 1980).
The first permanent soil tiebacks in the United States took place in 1961 (Jones & Kerkhoff, 1961).
The Authors were engineers and colleagues at Tecnosolo since the early 70s and recently realized that no photographs of this early wall survived.This is the story of a quest to find this wall.

A quest to find a wall
The first Author contacted several old colleagues from Tecnosolo, but only the second Author, aka PH, knew exactly the wall location, as he had visited the site long ago.The other Authors shared information about this first wall.
Today it is located between an old and the recent hospital building (Figure 2 and Figure 3) and it is difficult to access, hidden between two buildings (Figure 4).

Eureka
On 26 June 2023, the first (AO) and the second (PH) Authors visited the site.It did not take long for PH to find it hidden between two buildings (Figure 4).Eureka!After conversations with the hospital management, permission was granted for this visit (Figure 5).

Description
Figure 6 presents the 6 m high and some 50 m long wall sketches.Hunt & Costa Nunes (1978) described this innovative construction method taken from the top (Figure 7), replacing gravity walls which demanded a lot more excavation volume to be built.Professor Costa Nunes once told in his classes that he designed the upper part of the wall with a concrete grid, given the high cohesion of this residual soil.Nevertheless, he changed his design of later walls and used a concrete slab, replacing the grid."Concrete grid only for rock slopes", he said.
The Authors found that the ground anchors were 20 mm in diameter, certainly CA-50 steel-grade, which became available a few years before.All anchors were tested to 90% of the steel yield load (170 kN), then reduced to the working load of about 100 kN.

Design method
It took a few years for Prof Costa Nunes to publish a paper on the design method of anchored walls, which he did in 1963 (Costa Nunes & Velloso, 1963).They devised a wedge method (Figure 8) and equations which were used in Brazil for the design of thousands of anchored walls, until the advent of computers and limit equilibrium specialized software.

Corrosion
Corrosion protection was certainly not an issue in this first tieback wall.Nonetheless, the corrosion level seems quite small (Figure 9), given its 60 years of ageing.It was the first of its kind, followed by a series of developments which included corrosion protection, and higher load reduction factors, among others, thanks to Prof Costa-Nunes' ingenuity.
The Authors are very pleased to re-discover this geotechnical engineering milestone and to remember our mentor, the late Prof Costa-Nunes.

Figure 2 .
Figure 2. Site view, foreground: the old Beneficiência Portuguesa Hospital, at the back of the Gloria D'Or Hospital in Rio de Janeiro city centre.

Figure 4 .
Figure 4. Sketch of the tieback wall location, squeezed into the new hospital building.