FOUNDATION REPAIRS

EMERGENCY DEFORMATIONS OF NINE-STORY RESIDENTIAL

BUILDINGS ON HYDRAULIC-FILL BASES WITH WEAK

UNDERLYING LAYERS

P. A. Konovalov, I. V. Finaev, and V. Yu. Prokhorov

UDC 624.044:624.131.21

The increasing construction scope in our nation has led to the need to develop areas which were formerly considered to be unsuitable for construction. For example, in Nizhni Novgorod, in the environs of the "Meshcherskoe Lake" housing development, a zone of the lower floodplain of the Volga River, near its confluence with th~ Oka River, was selected for construction. The floodplain surface, about 2 km wide, was watered and up to 40% of it was swampled. Over different periods of the development of the area, many swamps were unwatered by construction of drainage cuts, part of which were filled with sandy soils, con- struction refuse, ordinary garbage, etc.

The central part of the area is occupied by the Mesherskoe Lake, which is 2.2 km long. The weak lacustrine--swampy deposits consist of peaty soils and peat, whose total layer thick- ness reaches 6 m at some places, and of an 8-10 m thick silt layer. In addition to the swampy, lacustrine, and technogenic formations in the area, 10-20 m thick deposits of present-day alluvium have developed which consist of sand of different sizes and clayey soils. The physicd mechanical properties of the soils are presented in Table i.

All these soils are characterized by a low bearing capacity, a high compressibility, and significant settlements of the structures constructed on them.

In connection with the future housing development of the floodplain, which will be flooded as a result of rise of the Volga River level due to backwater from the Cheboksary hydroelectric plant, a project for hydromechanized hydraulic-fill placement of a 4-10 m sand layer on the flooded zone was worked out. The hydraulic-fill work was carried out layer-by- layer by the trestleless end method with concentrated discharge of the sludge. Since the peaty soils, the vegetable topsoil layer, and the construction and household debris were contaminated with industrial and chemical refuse, based on ecological considerations they were not removed, but they were washed off with sand from Volga quarries.

In the writers' opinion, a serious deficiency of the project lay in the limited quantity of data from contour surveys of the peaty masses, which would have made it possible to deter- mine the most unfavorable engineering-geologic portions and to arrange the buildings in the most rational manner. Moreover, measures connected with construction of drainage cuts for discharge of groundwater from the closed depressions of the natural relief and for drainage of the hydraulic-fill mass were not implemented.

The presence of good quarries made it possible to place by the hydraulic-fill method a mass consisting mainly of sands of medium size (78%) and medium density with lenses and interlayers (up to 1.5-2.0 m) of fine and silty sands (17%) and of coarse sands. As shown by results of static penetration tests in the lower part of the hydraulic-fill mass, in the undrained depressions of the natural relief 0.5-2.0 m thick interlayers and lenses of silty and loose sands were formed which preserved the loose constitution at the age of 3-5 yr. In the lower levels of the sand mass there was intense increase in the modulus of deformation of the hydraulic-fill soil. Half a year after the hydraulic fill placement, the mean modulus of deformations of the medium and coarse sands was 25 MPa; after one year it was 35 MPa; and after five years it was 45 MPa. The groundwater level was established at a depth of 0.3-6.4 m, which indicated slow water outflow and an associated slowing down of the process of stabilization of both the hydraulic-fill and underlying soil masses.

All-Union Scientific-Research Institute of Foundations and Underground Structures. Gorki- Civil Engineering Institute. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 6, pp. 7-11, November-December, 1990.

2134 0038-0741/90/2706-0223512.50 © 1991 Plenum Publishing Corporation

TABLE i TABLE i

S "_~I" L i'__ ~ Physicomechanical properties ox oesxgnatlon u p ~. p,g/em ~ ~ MPa c, MPa ,, i r

Clayey .~oils with mix of vegetablel o ~ , n I . o.o2 I 14 I O.0a remnants I I I I I I I I I

Weakly peaty I ~ ~ , ~ o°:.~ Medl,tm-peaty l ~ ~ ~ ' i ! Severely peaty ol Peat

TABLE 2

Engineering- geologic ele- ment No.

Soil designation

Hydraulic-fill sands of medium size, losses, and of low water content The same, wet, water-saturated (with loam inter- layers ) Hydraulic-fill sands of medium size and density, from low-moisture to water-saturated Peaty learns Loams with mix of vegetable remnants Peat Alluvial sands of fine size, silty, and of medium size

P, glcm a

1.56

1,72

1.63 1,44 1,94 1.01

Ip

0.17

Physicomechanical characteristics

0,77

0.65 1,81 0 7~ 4192

C o!!a

0.0¢ 0.024

O.OO? 26--3~;

E, MPa Ipr

t,

1,5-2.2 O.H. 4 0 O?

04~, 0.6'2

a 00

z5

?o

65

b

I

Fig. i. Geologic section of building site soil base. a) Buildings an drelative deformations of their external walls; b) Nos. 5, 6, 6a- building numbers. 1-6) Soils according to Table 2.

Under such complex engineering-geologic and hydrogeologic conditions, design unification of large-panel nine-story residenfial buildings of several microdistricts was implemented. In connection with the fact that four such buildings in the fourth microdistrict were in the zone of wide occurrence of weak soils, observations on their settlements were conducted during both the construction and operation periods. The constructional solution for build- ings Nos. 5, 6, 6a, and 7 was worked out based on standard projects of Series 1-464D developed by the TsNllEPzhilishcha Institute jointly with the KievZNIIEP Institute. For preparation of the working drawings for the buildings, the PI Division of the Gor'kovgrazhdanproekt Institute provided maximum unification of their elements.

Hydraulic-fill placement of material on the site of the future microdistrict was carried out with interruptions and it continued for 16 months. Construction of the buildings was started 5-7 months after completion of the hydraulic fill and was continued on the average

during six months for each.

The lithologic morphology of the soil bases of buildings Nos. 5, 6, 6a, and 7 (Fig. 1) determined from explorations of the Gor'kovTISIZ Institute is presented in Table 2.

235

70

6~

~0

nDSD DDDD DD DDDD DD

Fig. 2. Geologic section of construction site of building No. 7 and schematic of pile arrangement in its base (the geologic designations are presented with reference to Table 2).

P, Pa

. .i5

1oo "~ "

, ~ 2 3 4 5 6 T, yr

~, Cm

Fig. 3. Development of settlements of buildings Nos. 5, 6, and 6a with time. I) Soil-base load caused by hydraulic-fill sand layer for each building site, respectively; II) load on soil base of building during construction and opera- tion period.

Taking into account the engineering-geologic conditions of the site for the two-section building No. 5, measuring 45.2 x ~3.0 m in plan, prefabricated strip footings were designed for installation at a depth of 1.8 m, their width being 0.8 m under the external walls and 1.0 m under the internal. The pressure beneath the foundation underside was 200 kPa. At the place of contact between buildings Nos. 5 and 6 (axis 25), the foundation underside width was 1.2 m.

In the soil base of buildings Nos. 6 and 6a under a layer of hydraulic-fill sand of medium size and medium density 6.1-6.3 m thick there was a 1.5 m thick loose sand interlayer underlain by a layer of clay with a 0.5-1.9 m thick layer of vegetable remnants. In the direction toward building No. 6a, they were underlain by peaty clays interbedded with peat, their total thickness reaching 2.2-2.5 m, and the peat layer thickness being 1.2 m. Below this material there were alluvial sands of medium density, ranging from silty to medium-size.

For the buildings, the designed foundations consisted also of prefabricated strip foot- ings at a depth of 1.7 m with the same dimensions and foundation underside pressure as in

building No. 5.

236

TABLE 3

Building

No.

.% 6 6A

Measured maximum settle- ment, cm

iConstruction period Too , yr

0.5{I 0.67 1.0

Settlement increase with time st, cm/% During con- ~ing period operation ;struction 2 3 yr 4 yr period s c, i yr yr cml%

7/41 16,5/97 17/10(I 19/50 37/9? 38/I00 20/58 32/94 34/ O0

At the place of contact between buildings Nos. 6 and 6a (axis 13), there is a through passage. The foundation underside width along axis 13 is 2 m.

There are no settlement joints between the buildings, and the walls of adjacent build- ings rest on the same footing.

In the soil base of the two-section building No. 7, which measures 45.2 × 13.0 m in plan (Fig. 2), directly under an 8-m thick layer of hydraulic-fill sands of medium size, medium density, and dense, there were lacustrine--swampy deposits consisting of loams with a mix of vegetable remnants, peaty soils, and peats, the total thickness being 2.6-5.2 m. In these deposits, the peat layer thickness reached 2 m. This material was underlain by a mass of alluvial fine sands. For this building, the foundation project called for use of driven reinforced concrete piles 12 m long and 30 × 30 cm in section in a one-row ar- rangement and with a pile design load P = 300 kN. The caps are of reinforced cast-in-place concrete construction, 50 cm wide, 40 cm high, and with their underside at a depth of 1.3 m. The end walls of buildings Nos. 7 and 7a rest at the contact place on a 105-cm wide cap with a two-row pile arrangement.

BuildiDg settlement observations were conducted by the Gorki GlavAPU Institute jointly with the Gorki Civil Engineering Institute by means of an NV-I level immediately after in- stallation of the plinth panels and the floor slab over the equipment cellar.

Initially, it was assumed that the building settlements would be insignificant, since the hydraulic-fill sand in the soil base was characterized by low compressibility, In this connection, after completing the construction work the instrumental observations were dis- continued. However, after development of cracks in the superstructures of some buildings, the observations were resumed and the measurement cycle frequency was increased.

On summing up the results of work performed over many years to study the deformations of the buildings on hydraulic-fill heterogeneous soil bases, the following can be pointed

out.

The foundation settlements of building No. 5 reached 12-18 cm (Fig. 3), which exceeded the limit values of the SNiP 2.02.01-83 Norms by a factor of up to 1.8. As a result of the presence of the highly compressible soil mass under the end portion adjacent to building No. 6, the building suffered significant differential settlements which caused local dis- placements of floor slabs, as well as of stairway landings. Furthermore, rotation of the building was recorded, which was manifested by displacement of the plane of the ninth floor with respect to the plane of the first floor, reaching 88 mm in the longitudinal walls in

the zone of mark M-6 (see Fig. i).

The foundation settlements of building No. 6 reached 20-38 cm, which exceeded the allow- able limit values by a factor of 3.8. The significant nonuniformity of the simultaneous deformations of the soil base and the structure was determined by the heterogeneous stratifi- cation of the highly compressible varieties of soils in the weak underlying layer, as well as by the presence of loose interlayers in the hydraulic-fill sand mass. They caused crack- ing of the external and internal panels in different parts of the building, for example, at the place of contaot with the arch-type passage, as well as near the end of building

No. 5.

The settlements of building No. 6a during this period amounted to 26-33 cm, and they were comparatively uniform. Only insignificant cracks were observed in the external panels

of the front part of the building.

An analysis of the development of the settlements with the time showed that even when five years had elapsed after start of construction they had not been stabilized at different placed, and that they will continue for a long time. From preliminary estimates, over the

237

, I: M-6~ I 6@ M-b3M'6fH-ZJM'61 M'60

? a j, ? M-72 IM-?! M-TO M-16 M-65 M-f~

54

7'07

Fig. 4. Foundation settlement diagrams for external walls of residential building No. 7 during fourth year after start of construction.

G.O 1~5.0

I / / ~ / @ , Q

/

Fig. 5. Characteristics and deformations of superstruc- trues of building No. 7 (in mm). The dashed lines represent the building position after settlement of the foundations.

next I0 years the settlements will increase by about 5 cm in building No. 5, by up to 8 cm in No. 6, and by up to 7 cm in No. 6a.

If a settlement s e at the end of the observations is arbitrarily assumed to amount to 100%, then the settlement s c over the construction period amounts to about 50% of the above value. In this connection, over the frist two years of operation the settlements reach 90% on the average (Table 3).

The most dangerous deformations were suffered by building No. 7, constructed on pile foundations. Its settlements reached 75 cm. At different places of the building in the wall bearing panels, there were cracks, warping of the elevator shaft, displacement of the stairway landings, and significant tilts. The cause of the emergency deformations of the building was low-quality pile driving.

Driving of a 12-m long pile 2.7 tons in mass was carried out by means of a rod diesel hanuner with a 2.5 ton mass ram. As a rule, the constructors discontinued the pile driving operation when the zero refusal had been reached for all practical purposes, although signifi- cant elastic vertical displacements of the piles were recorded. Driving was discontinued also on cases of damage of the pile heads. In most cases it was wholly evident that the piles were underdriven by 5 m with respect to the design level. Previously [I], attention had been drawn to the characteristics of driving of piles into soil masses with buried layers of biogenic soils (peats, peaty soils, or sapropelic materials). Under such condi- tions, the driven pile tip does not reach the top of the highly compressible layers, and the driving operation is subsequently discontinued. After each successive blow, the pile is displaced togetron with some volume of the surrounding soil over a distance equivalent to the elastic deformation of the weak layer, and it subsequently returns to the initial position. The hammer blow energy is absorbed by the elastic deformation of the buried bio- genic soil and this is not sufficient for overcomzng the friction along the lateral surface between the pile and the surrounding sandy soil layer. Under similar engineering-geologic conditions, it is more advisable to sink piles through leader holes driven to the weak layer underside, or to apply pressing-in. Moreover, it is essential to use mechanical or steam-

238

TABLE 4

ss.

Cm

34,0

36.3 20.7

Q: sz, cm

0.2,F, 25.2 0,18 I 1,6 0.gf, 20 q 0,1~ 17,0

I 7,0 1.3 7,0 t ,3

I,%.8 1.3 9,7 1.3

'Hydraulic-fill loose sand set- itlements caused iby the building, icm

0.8 0,4 1.5 0,8

Total design I Actual settle- settlement of i ment during

i fifth year of soil base s, i observations, i cm ] Cm

........

44.2 36.b 20.2 I '22.5 454 /44__ .6 28.7

air hammers with increased ram masses, instead of diesel hammers. However, in building No. 7 none of these measures was implemented.

Subsequent analysis of the pile foundation quality made it possible to establish that only in 32% of the total number of driven piles the tip had been sunk into the bearing layer; in 20% it just reached the top of the bearing layer or had failed to reach it by 0.2-1.0 m; in 22% the tip had not reached the bearing layer by 1-2 m; and in 26% it had not reached the bearing layer by more than 2 m.

Thus, in 68% of the piles the tips were in peat or peaty soils. The remaining 32% of the piles, whose ends were sunk in conformity with the project into the bearing layer, were located at the end portions of the building.

The center of gravity of the pile field with underdriven piles was close to the free end and the front of the building, which led to its tilt in the above-mentioned directions. The load taken by the piles through the hydraulic-fill sand layer was transmitted directly to the peaty soils and peats. Compaction of the last-mentioned soils led to significant differential settlement of the building.

The building settlement observation results show that the settlement diagram character- istics are directly related to the peat layer thickness and the disposition of the portions with underdriven piles (Fig. 4).

Five years after completion of construction, the maximum settlement reached 75 cm, which exceeded the allowable value by a factor of 7.5. The relative difference of the foundation settlements along axis A amounted to 0.013 and exceeded the allowable value by a factor of 8.2. The building tilt in a portion of row 25 reached 0.01, thereby exceeding the allowable normative limit by a factor of 2.

One of the main causes of such a large tilt is also the support of the end of building No. 7 at the place of contact with the adjacent building (No. 7a) on a common cap whose piles cut through weak soils and were reliably sunk into the bearing layer. The deviation of the top of the building reached 135-283 mm away from the free end side and 181-75.5 mm toward the front, which led to twisting of the building (Fig. 5).

Obviously, such differential settlements of the pile foundation caused widening of the joint between buildings Nos. 7 and 7a by 2 cm at the first-floor level and to 22 cm at the roof level, as well as deflection of the structural components, which affected the strength of the welded connections and of the wall panels in which the maximum crack width are ob-

served.

Analysis of the development of the settlements of building No. 7 indicates that the pile foundation settlements in rows 10-23 continued to increase (M-60, 61, 62, 63, 69, 70), while in rows 1-3 (M-64, 71, 72) adjacent to building No. 7a they were relatively stabilized. For further operation of the building, it was decided to strengthen the foundations by re- driving the piles in accordance with the procedure described in [2]. However, it was not possible to redrive any of the piles under the building. Although the jack on the pile developed a force of i000 kN, the sand friction along the lateral surface of the pile could not be overcome. Further increase in the pressing-in force led to damage of the pile heads. Thus, the foundation strengthening work did not yield any results and, taking into account the emergency state of the buildings, it became necessary to dismantle it.

The significant difference of the actual measured settlements of buildings Nos. 5, 6, and 6a with respect to their design values was caued by absence, in the norms, of recom- mendations for prediction of the settlements of foundations on hydraulic-fill bases with weak underlying layers. In contrast with similar stratifications of natural soil bases,

239

account must be taken of the specific compression characteristics of deeply lying layers of weak soil under the action of the load from the structure building site, the incomplete consolidation of a weak soil under the action of a seepage surcharge from the hydraulic- fill sand, and the settlement due to self-compaction of the sand owing to variation of the usual pressure for natural descent of the groundwater level, which had temporarily risen during the hydraulic-fill sand sludge placement process.

The construction pressure is the quotient resulting from division of the mass of the building by its area. The construction width introduced into the calculation of the settle- ment s I contributed to formation of a compressible mass depth which drew into action the deeply lying weak soil layer, not taken into account in the analyses made in conformity with the SNiP 2.02.01-83 Norms.

Hence, the settlement s of a building or structure on a heterogeneous hydraulic-fill soil base with a weak underlying layer should be determined from the equation

s = s , + s ~ + s ~ + s , , (I)

in which s I is the weak underlying layer settlement caused by the load within the construc- tion site limits; s 2 is the settlement due to incomplete consolidation of the soils, caused by the seeping sand layer surcharge; s 3 is the settlement of the hydraulic-fill sands under the foundations within the limits of their compressible mass; and s, is the settlement of self-compaction of the hydraulic-fill sands under the action of the usual pressure increas- ing with time under natural descent of the groundwater level.

The settlements s I and s 3 are determined by the layer-by-layer summation method. The settlement s 2 after start of construction should be determined from the expression

~ = ~ - s , , ( 2 )

in which s s is the sum of the final settlements of the underlying layers surcharged by the hydraulic-fill mass; and s t is the sum of the settlements of the same soils under the action of the hydraulic-fill surcharge over the period preceding the start of construction of the building; it is determined in conformity with Section 5 of [3].

The correctness of the recommended method of predicting the settlements was verified from data for buildings Nos. 6 and 6a, where a 1-1.3 m thick peat layer lay in the soil base.

For the settlement analyses, the values of the modulus of deformation of the soils were taken from Tables 1 and 2. The coefficient of consolidation of the clayey soils, including those with different peat-content degrees for a pressure p = 0.2-0.25 MPa according to especially performed tests, was C v = 4 × I0 -s cmm/sec = 0.126 m2/yr; for peat it is C v = 4 × i0 -~ cm=/sec = 1.26 m~/yr.

For settlement--time analysis, dual consolidation was adopted. Both the minimum and the maximum settlements were calculated.

The settlement analysis results for building No. 6 are presented in Table 4.

On analyzing the design data, it should be noted that the construction of buildings Nos. 6 and 6a was started six and eight months, respectively, after completion of the hy- draulic-fill work. Over this period, the underlying weak woil settlement under the hy- draulic-fill surcharge action was completed at 18-26%. The remaining, highly significant part of the settlement was merged with the building settlements. The basic part is pro- vided by the settlements of uncompleted consolidation of the weak soils due to the hy- draulic fill and the construction site loads (92-95%). Thus, the proposed method of analysis of the settlements of hydraulic-fill bases with a weak underlying layer yields satisfactory

results.

1.

2.

3.

LITERATURE CITED

P. A. Konovalov, Foundation Construction on Peaty Soils [in Russian], Stroiizdat,

Moscow (1980). A. L. Gordon and L. Pil'des, "Effective method of strengthening pile foundations," Stroit. Arkhit. Moskvy, No. 9, 18-20 (1976). Handbook of Design of Soil Bases of Buildings and Structures (in Conformity with SNiP 2.02.01-83 Norms) [in Russian], Stroiizdat, Moscow (1986).

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