Integrated Ore Concentration Plant in the City of Stary Oscol

Settlements of foundations, underpinned by means of micropiles at the Integrated Ore Concentration Plant in the City of Stary Oscol.

Integrated ore concentration plant in the city of Stary Oscol is situated on the site, formed by the deposits of Quaternary, Nummulitic (palaeogene) and Upper Cretaceous rocks.

Three of four existing sections of concentrating mill, each 72m long and 84m wide (Fig.1), have been in operation since 1970. They were erected on 300x300mm precast piles going through the Quaternary soils and resting on palaeogene silty clays.

The 4th section, immediately adjacent to 3rd one, was constructed in to increase the production of ore concentrate. The construction of 4th section is not finished; in 1990 its frame was erected on the soil foundations with the bases resting on the threpel paleogene clays, EGE (engineering geological element)-7 (Table 1), characterized by relatively low modulus of deformation (17 MPa) and high porosity index(1.24). Besides, partially constructed reinforced concrete frame of 4th section on the soil foundation stood without roof and surface drainage from 1990 till 2005.

Because of the load increase due to equipment modernization, affecting the foundation bases of 4th section, up to 0.5MPa, it was decided to underpin them by means of micropiles. This decision was substantiated by the comparative calculation of settlements of 4th section soil foundations and the foundations underpinned by means of micropiles. Main characterization of the underpinned foundation and piles was given earlier [Popsuenko et.al, 2010]. Present article is dealing with the analysis of new data of the settlement observations, executed in the 2007-2011 years. Some results of the observations of the settlements of the full operationally loaded foundations of 4th section ( axes БB) are given in Figure 1.

It was rather unexpectedly to find, that predicted settlements of the underpinned foundations of 4th section (40-50 mm) with applied full operational load significantly exceeded observed those (7-9 mm).

Predicted settlement of the soil foundations (Popsuenko et.al, 2009) was calculated by finite elements method (FEM) with the aid of PLAXIS plain Version 8.2 software using Mohr-Colomb soil models for longitudinal section of the building of 4th section along line Б-В (Fig.2). Physical and mechanical soil characteristics, assumed in calculation, are given in Table 1.

It is necessary to note that the loads, assumed in the calculation, correspond to 75% of the total calculated load as it was supposed that the settlement due to the influence of 25% of the total calculated load occurred before the reconstruction of the section. The calculation results show that the expected settlement of the soil foundations under the effect of 75% of the total calculated load without their underpinning by means of micropiles can amount to 367mm (Fig.2). It largely goes over the limit preset by the building codes. Predicted settlement of the foundation underpinned by means of micropiles along axis Б-В reached 42 mm (Fig.3). Thus, it was decided to underpin the foundations by means of micropiles.

Control tests of micropiles 200mm in diameter and 12m in depth demonstrated that their bearing capacity equals 30-37 tons (Fig.4). Such bearing capacity required more than 800 piles for 4th section foundation underpinning. There were also tested different kinds of piles, applied earlier (Popsuenko et.al, 2009).

The foundations being underpinned (Fig.1) are massive cast-in-place reinforced concrete constructions 2.5-1.8m thick, reinforced with 3-5 vertically arranged reinforcement cage-meshes with 200x200 – 250x250mm cells. The fourth section of the concentration mill is 69x83.5m on the axes 59-70 within the rows A-E.

Foundation sizes and full operational loads differ according to the axes (Fig.1):

Fig.1.

Plan of the foundations of 4th section (numeral axes 59-70) and 5th section under construction (axes71-76) of integrated ore concentration plant, combined with the diagram of observed final (2011)settlements of the foundations along the letter axes БB

• on the axis “A” the foundation is a strip one, 5.2m wide, cut into 4.8—5.2m segments; the distance between foundations equals 0.5—0.9 m; total pressure under the foundation base is 436kPa;
• on the axes “БВ” - the construction of the foundations is similar to the foundations on the axis “A”, i.e. they are actually strip foundations 10.2m and 4.8m wide , cut into strips which are respectively 23m and 7.8m long, the distance between foundations being equal 0.5-0.9m and the total pressure under the foundation bases – 410kPa and 283kPa, respectively;
• on the axes ”A/1” and “ГД” the foundations are standing separately with the axial pitch being equal 6m and 12m along the letter axes, with the base size on the axis A/1 - 2.7x3.3m, on the axis “ГД” – 6.6x10.5 – 4.8-6m, and the total pressure under the foundation bases – 470kPa and 320kPa, respectively.

The necessary rigidity of the pile foundation of the section being constructed were selected on the basis of the results of the static load tests and by means of backfigure recomputation of the deformation moduli under the pile tips.

To derive the utmost reliable forecast of the deformations of the existing sections of the concentrating mill and those under construction there were made test piles and the field static load tests were carried out during which two piles of each type used during construction of the mill were tested, namely: precast piles with 300x300mm section, micropiles with 200mm diameter, the length of each pile being equal 12 m and others (Fig.4).

On the basis of test results deformation moduli of the soil under the pile tips were derived by means of inverse computation (backfigure) according to the formula for single piles, standing separately (Poulos H.G ,et.al,1980, СП, 2011):

S = P * Is / Esl / d (1)

where:

Р - load, applied to a pile, t,

Is - index of settlement effect which depends on l/d ratio and relative pile rigidity λ = Ер/Еsl,

Еsl- modulus of soil deformation on the level of pile tips, t/m2,

Ер- elasticity modulus of pile material, t/m2,

D - diameter or a side of a square pile, m,

l - pile length, m.

Index of settlement effect (settlement influence factor) for pile Is may be derived according to the requirements of code documents (CП ,2011) or using the aproximatly graphic (Poulos H.G., 1980).

The comparison of moduli of soil deformation, derived during the compression tests of soil specimen taken from the soil investigation holes, determined by the above mentioned method, demonstrates (Table 1), that the soil deformation moduli, specified on the basis of the results of the pile static load tests, are the highest and, probably, the most reliable when estimating settlements of pile foundations. However, the observed final settlements of foundations , underpinned with micropiles were founded less than predicted in 3 and more times. In the present paper prediction of the settlements of 4th section foundations, underpinned with micropiles, was carried out using methods of Russian national codes of practice:

• method of arbitrarily the accepted massive base (AAMB) for the pile groups (СП, 2011);
• method of combined raft-pile foundation (CRP) (СП, 2011).

According to the results of the method of arbitrarily the accepted massive base for the pile groups calculated settlement along axes “БВ” exceeds 51 mm. In fact AAMB and FEM (Fig.2) methods give similar computed results, 51mm and 41 mm respectively. Than, CRP method shows 16 mm - the most close to observed value of the settlement 7-9mm.

Sometimes the rapid practical estimation of group pile settlements is possible by use of settlement ratio method, presented by Poulos(1980). In the settlement ratio method, the group settlement, SG , is related to the single pile settlement as follows:

SG = Rs Siav , (2)

where Siav = settlement of single pile at the average load of a pile group, Rs = settlement ratio; Siav can be estimated from the results of a pile load test. In our case for a tested single pile U-2 (Fig.4) Siav =2,5mm under the load 25 t. The settlement ratio can be given according to Poulos (1980) by approximation :

Rs =nw, (3)

where n-number of piles in the group, w- exponent depending on pile spacing, pile proportions, relative pile stiffness, and the variation of soil modulus with depth. For the typical pile proportions and pile spacings Poulos suggested the following approximate rule w=0,5 for piles in clay, and w=0,3 for piles in sand. In our case for piles in clay w=0,5, n=124, than Rs =11, and SG =11x2,5=26 mm.

In our case all described methods gave rather different results from the observed displacements.

Considerable divergences between calculated and observable settlements can be explained by compaction and strengthening of the soils under the raft foundations under the weight of constructions in the period before their underpinning by means of micropiles.

The most reasonable predicted settlements show method of combined raft-pile foundation(CRP), accepted for application in Russian standards (СП , 2011). The next way for the rapid practical estimation of group pile settlement is using of the settlement ratio method, presented by Poulos(1980).

Carried out observations and analysis allowed successfully to place in operation 4th section of integrated ore concentration plant and itʹs settlement is much less than supposed.