Technology

 But even without the given grievous circumstances   the task of economizing on the basis of groundbreaking engineering solutions and not through  a tough “administrative course” has always seemed  a sensible necessity    – for countries that can think of the future ... Whether Russia enters their number is a question    left without an answer so far including the sphere of   engineering and road and transportation designing.

The same pessimistic “perspectives” are prompted by information on the progressive developments  on the credit side and volumes in which they are being implemented in fact. The unwillingness of government business structures  to raise corresponding demands to developers does not inspire hope either. Thus, developers have no spur to ascend to a new stage of development and comprehension of the transportation complex goals.  Hence low degree of developers’ readiness to   implement and study  latest engineering solutions together with scientific establishments. To keep you abreast of the fact: the latter still exist...

The previous material shed light on risks actually bordering on “lucrative impulses”,  the negligent choice of a standard design solution of the bridge configuration  on the basis of a construction of supports without grillages and with cone-shaped buried abutments in seismic zones

This awareness helps look in a different way – without admiration - at the mushrooming overbridges with conic buried abutments strenuously constructed both on federal and regional public roads in many RF subjects.  Over the recent years there have been constructed dozens of overbridges only on an actively financed   at the expense of   federal funds   Olympic highway    Ì 4 “Don”.   There is no doubt they have immediately damaged the state budget.     For over a five-year term     there have been alternative engineering solutions in Russia   to reduce the construction length – one span  at each side that prevent building two supports.   Instead of such    resource-demanding and expensive elements it is proposed to use reinforced earth and a contractor’s pedantic attitude       to jobs under the vigilant supervision of the      developer and with scientific guidance. As a result a  tight road budget saves  from 20 tî 40% of estimate cost on each construction!

On bridges much longer than a kilometer where the specific weight of abutment     cost is insignificant compared to  a summary estimate for construction of dozens of spans  and supports,   the proposed savings are certainly not so huge but still distinct.  Such bridges are built in Russia in small numbers whereas 100-150 m long 4-span  overpasses  are built in tens if not in hundreds. Annually! Small and medium bridges and overpasses account for  about 80% of financing volumes in bridge engineering.   Mind that considerable sums are taxpayers’ funds …      

Before considering   engineering means of overcoming the deadlock,   where  conservative planners lead us being   bereft of an incentive to seek low-cost solutions by legislation and the system, let us  turn to bridge engineering history to trace the evolution of construction and technology solutions of bridgework junctions with bank vaults and  approach embankments where abutments are key links.      

The functional purpose of an abutment is to react loads from a superstructure as well as bank vault soil and approach embankment pressure. The second function caused building  bulky, heavy and   labor-intensive constructions of  abutments   made in deep trenches or on high-strength pile foundations. Earthfill and bank vault soils turned into an “aggressive” medium making immense loads. They needed   adequate constructions providing abutment relative     immobility which was required by propping conditions and superstructure jobs. Bridge abutments of old construction were massive and bulky   constructions of stone,  rubblework, concrete and later -  reinforced concrete. 

Gradually engineering came to the construction of  buried abutments with cones   inside of which there are abutment bearing components. This variant has virtually replaced gravity-type massive structures and is the most wide-spread   bridge and overpass abutment type.              Originally it was supposed to release the  abutment bearing components inside the cone from     excessive earthfill soil pressure    (fig. 1).

Looking at various buried cone-shaped conjunctions you may think that at last we have managed to rid of bulky and expensive abutments.   You may get an impression that earthfill soil   rests by itself having a safe cut, and bridge   bearing elements of the abutment   inside the cone can be light and cheap.  In other words, all out-dated  is successfully replaced by new and progressive.   Unfortunately, at this stage an advance has halted in Russia ...

With the cones usually having a bevel with 1:1,5 horizontal equivalent and at foundation high earthfills and loose soils        an increase up to    1:2 or more takes place, berms are made along the slope elevations and the artificial structure has an increased length.       For  small and medium bridges it amounts to 18-24 m plus two  piers.  The abutment bearing elements in the embankment  and cone are not free from horizontal soil pressure of the embankment soil. It is permitted to release bearing elements of the buried abutment from the embankment soil pressure      only if the embankment is constructed with  hydraulicking, it ensures   a high degree of soil compaction; or when the embankment is  compacted      up to bearing elements of the embankment and its foundation.   The latter seldom occurs in bridge engineering.  Multiple times tested cone-shaped conjunctions and   buried abutments have turned into a cliché   which helps rapidly made drafts    as quickly and painlessly receive the approval of Glavgosexpertiza (RF General Expert Examination Body)  thus facilitating the developer as a contractor frequently put in  undertime.   Eventually the customer, developer and bridge builder are satisfied because the fine-tuned scheme  enables  keeping up with uneven financing of projects   adjusting to the state  customs and life reality.  The project is finished “today” but the object is to be    handed over “yesterday”. In a complex political situation     nobody is willing to complicate their life, especially on territories,     trying to prove the necessity of introducing alterations in the project – the main thing is to meet the deadline and avoid problems…    

Driven by the desire to  satisfy the customer’s  “whim” at the same time applying a more expensive project variant      that increases the developer’s “fee”, the community has forgotten   that along with cones two more spans and piers appeared. In any case, positive antiexamples  on landmark bridgeworks are a good ground to study the issue...

In this connection A.D.Sokolov, candidate of engineering sciences, leading researcher of the Scientific Research Center “Mosty” (“Bridges”)  of  Central research institute of construction (ÖÍÈÈÑ) OJSC writes: “Construction Norms and Rules 2.0503-84* («Bridges and pipes» – author) regulates  loading bearing elements of the abutment with active soil pressure on a double pier width if the total width does not exceed half of the abutment width.   It is not known why on a double width. What if the total pier width exceeds half the abutment width (for example, it equals   0,55Â), then the pressure   is considered  applied to the whole abutment width! Thus  – two extra spans, two extra piers,  cone fastening and at that the same embankment load.

A renowned Soviet bridge builder N.M.Glotov awarded with the Lenin Prize for   development and implementation of piers without a foundation framework  hardly supposed that according to Construction Norms and Rules  they should be loaded with  soil pressure on a double width of piers. Only one occurrence of a well-weighted and reasonable application of this approach is known to me when an experienced designer B.F.Byalik Á designed a bridge pier without a foundation framework  across the Chusovaya river. He calculated it for a single but not a double soil    pressure value.  

We should note that cone-shaped conjunctions  with buried abutments  had appeared before reinforced earth was invented by a French engineer    À. Vidal. Today, when    reinforced earth systems of various applications are globally widespread and filling its niche in Russian transportation construction, buried abutments must sink into oblivion from bridge engineering especially in seismic areas” (fig. 2).

Nevertheless they have become customary in bridge engineering for many decades and occupied a predominant position in bridge engineering.  

Turning back to  construction in seismic areas we must remind you: the bridge or overpass bridge length reduction decreases the span mass,  thus decreasing the seismic forces effecting it.   As a result loads on piers reduce. Besides, reinforced soil     obtains different properties in comparison with   soil without reinforcing elements. This in particular makes embankment ends more stable. We actually use a “different” material that received inner adhesion by means of geoplastics. Earthquake loads can  turn sand into quick sand as its internal friction sharply falls because of  tremors, so it    ensures discrete material density.    Science abounds in examples when buildings light-heartedly erected on sandy ground in   seismic areas sank deep under earth because of natural forces as if they stood on water.

The statement of the Mosty Scientific research center goes briefly: “Cone stability decreases not only   as a result of seismic forces  but reduced shear soil characteristics at seismic oscillations”.

The essence of the alternative idea is that the extreme support reacts a load of a span and subplates as it differs little from intermediate supports.     Meanwhile  the load of  the embankment end zone soil   is reacted by the reinforced soil system   fully unloading the extreme support from  the embankment  force. The briefly described (and illustrated in figure 3) construction has received a name   “an abutment with separate functions” according to the construction mechanics scheme.     

The reinforced soil system included in the bridge construction is distinguished by a vertical or slightly inclined towards the embankment front wall. It is located in a short distance from the extreme support so that “two extra spans” and “two extra piers” inside the cone of standard bridgeworks are unnecessary and it  lightens bearing and foundation elements of the abutment. The cone and its fastening is naturally excluded. It is rational to apply a precast and cast-in-situ structure of subplates in the construction. The lower row of subplates is built from precast elements with dowels and the upper layer – in a monolith variant.      Such a project solution  allows  working without formwork which is especially reasonable     for abutments with separate functions where the subplate overlaps the gap between a pier backwall  and a front wall of the reinforced soil system (fig. 3).

The support of subplates is recommended not on a corbel  as in standard projects but on its rib. Thus a number of advantages are gained: