FROM THE EXPERIENCE OF CONSTRUCTION ORGANIZATIONS

TAMPED-TRENCH FOUNDATION CONSTRUCTION IN LENINGRAD V. N. Svarovskii

UDC 624.38.22(470.23)

The Leningrad districts and regions are characterized by extremely complex engineeringgeologic and hydrogeologic conditions. Middle Quaternary, Upper Quaternary, and Recent deposits occur widely, which cover the native rock, the upper mass consisting in most cases of heavily compressible soils characterized as weak, according to their construction properties (varved clays and loams, silty sands, sandy loams, and in many cases blanket and buried peats and peaty soils). Most of the Recent deposits lying in the near-surface layers are prone to frost heave. The groundwater level occurs near the surface (from i to i0 m) and its regime is of a complex nature determined by excess precipitation storage and the effect of the developed hydrographical network. Constructionwater lowering or unwatering is frequently made difficult by the great abundance of water in the deposits. As seen from a brief description of the engineering-geologic and hydrogeologic conditions of the region, typification of the solutions adopted for foundation construction is very complex. Frequently, deep foundations are designed, the soil is replaced, and different construction measures are applied -- settlement joints, reinforced bands, etc. For buildings subject to large loads, the most widely applied foundation construction method is the use of 20-30 m long piles resting on firm soils (moraine loams and fluvioglacial pebbles and sands). However, implementation of the above solutions involves considerable labor and materials consumption in the substructure construction, as well as increase in its time of completion, which reaches 30% of the total construction project period. For this reason, the Glavzapstroi, which has an annual volume of over 500 million rubles worth of industrial, agricultural, and housing construction, jointly with the Scientific-Research Institute of Bases, the Fundamentproekt, and the Leningrad Civil-Engineering Institute, has implemented during the last few years measures for reducing the labor and materials consumption of the substructure work and for introducing more economical solutions. The most effective of them have heen surface compaction of soils by heavy rammers and foundation construction in tamped trenches. Surface compaction of fill and weak soils up to 6 m thick by means of rammers 5 and i0 tons in mass, carried out at four projects during 1979-1983, reduced the labor and cost by one half (by i million rubles in comparison with the deep pile and column foundations adopted in the projects). In 1979, experimental work was started in order to introduce the method of foundation construction in tamped trenches, which resulted in adoption of this progressive method for 45 construction projects by 1983. During this period, construction techniques and technicalorganization schemes were developed which included static test methods, and new technical documents were drawn up. Significant efforts were made by constructors to re-equip the excavators and to fabricateg largely in accordance with their own projects, trench-tamping equipment including several new types of rammers (Fig. i). Massive introduction of this method has been promoted by its intensified advertising in the construction trusts, in particular by demonstration of the tamping process, timely examination of the requests of constructors for improved technology, joint carrying out of full-scale tests with them, and assistance rendered by the Scientific-Research Institute of Foundations. Some of the first projects which by 1983 had four years of service with foundations in tamped trenches were the materials conveyance and hydrotransportation trestles of the "Phosphorite" Industrial Association in the city of Kingisepp. The structure foundation bases were stripping-material dumps consisting of sandstone and limestone rubble and gravel with lumps up to i m in diameter and with a heterogeneous clay filler in the proportion of 15% Glavzapstroi, Ministry of Construction of the USSR. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 6, pp. 12-14, November-December, 1984.

0038-0741/84/2106-0243508.50

O 1985 Plenum Publishing Corporation

243

1"-1

>3 --

Tf --

--

if50

800

i: 4

T~

T-E

150

/FO0

100

I!1

Fig. i. New types of rammers for equipping excavators for trench tamping. T-I) Q = 3 tons, v = 0.84 mS; T-2) Q = 5 tons, V = 2.52 mS; T-3) Q = 3 tons, v = 1.66 mS; T-4) Q = 3 tons, V = 0.72 mS; T-5) Q = 4 to s, V = 1.91 m3; T-6) Q = 1 ton, v = 0.23 mS.. b

II II I'--:-0 1 r il

iooo.o,

I I

•

C"

//7" ~// //,

I

i

f250

Section --------

A--A E

1250

"t" Fig. 2o Trestle support foundations, a) At "Phosphorite" Industrial Association; b) at "Kirov Factory" Industrial Association. to 50%. The 6-12 m thick dumps had been placed 5-8 years before, which permitted classifying them as deteriorating (with the exception of the 1-3 m thick upper layer placed during the last few years). The loads on an individual column of the high trestle reached N = 1600 kN, and the most unfavorable combination consisted of a load N = 200 kN and a bending moment M = 400 kN-m. Tests on several foundations using 200- and 50-ton capacity jacks showed that their bearing capacity exceeded by a factor of two the values a d o p t e d i n the projects. The foundations, constructed during the winter--spring period of 1979-1980, had exhibited full stability over

244

a four-year period of operation and their settlements did not exceed i0 mm, that is, a value lower than the allowable by a factor of 5. The successful construction experience led to wide introduction of this method in the above-mentioned region under different geologic conditions -- during the last few years over 1500 foundations in tamped trenches have been constructed. Considerable design work was required by the changeover to tamped support-foundation systems of trestles of heating networks and cable galleries. Large moments (ii00 kN-m) combined withrelatively small vertical loads (300 kN) were transmitted by framed caps resting on tamped foundations for which account was taken of the balancing lateral earth pressure on them (Fig. 2). The construction site was covered by a layer of silty/sandy loams and silty loams 3.5 m thick, underlain by silty sands. The groundwater level at different places along the axis lay at a depth of as much as 1.5 m, and in some spots it rose to the surface. The ordinary and sandy foams were characterized by the following physicomechanical indices (respectively): (void ratio) = 0.9 and 0.6; c (cohesion) = 0.02 and 0.007 MPa; ~ = 17 and 28 ° . To prevent the possible effect of frost heave on the framed caps, a lO cm gap was left between them and the soil. After tamping a trench with a widened base obtained by tamping 1 m 3 of rubble, the rammer was left in the hole until the reinforcement cage was placed and the foundation was immediately concreted. When foundations have been constructed in tamped trenches at several industrial buildings, in order to transmit loads of 2000-3000 kN wide use has been made of double foundations joined by a cap, as well as of single foundations with "collars," that is, with a widening at the top, made after tamping the hole, by cutting the soil With the excavator in the upper part of the foundation, throwing it into the hole, and subsequently tamping it to the design depth. In particular, this solution was adopted for construction of a new shop at a mica factory in the city of Kolpino. The skeleton two-bay shop building (24 m bays), which measures 52.5 x 130.5 m in plan and is 12 m high, adjoins an existing shop at the factory, with brick and prefabricated protective walls. The distance between the axes of the new foundations and the existing shop foundations was 4.5 m. Many foundations of the single-story portion of the building had loads of 1400-1900 kN, and in the multistory part the loads reached 2100 kN. The base soils consisted of varved silty foams and clays 0.4-0.8 m thick and of layered grey silty foams with some gravel and sand interlayers lying to a depth of 2.6-9.1 m, which were characterized by the following physicomechanical properties: Smn (mean void ratio) = 0.26; Ydry = 1.39-1.41 g/cm 3. The soils had a high water content of 0.27 on the average and a degree of saturation of 0.75. From the test results on the experimental foundations carried out using T-I and T-2 rammers (see Fig. i), including a widened rubble base, during August-September 1981, a clear-cut relation between the design loads and the tamped rubble volume was found. The bearing capacity of the foundations on enlarged bases increased by a factor of 1.6-2.8. Taking into account the large loads and the Kolpino site soil characteristics, the design bearing capacity of the tested types of foundations was assumed to be lower by 15-20% than the obtained values. For single foundatians with increasedloads (2100 kN) use was made of a "collar" - a cantilevered widening in the foundation top measuring 0.7 m in plan at each side and 0.6 m thick. In connection with the possibility of soil frost heave, observations on the foundation displacements were carried out during the construction period (from September 1982 to May 1983) at a depth of 0.6 m below the cantilevers. However, neither horizontal nor vertical displacements of the foundations were found from these observations. Tamping of the foundation trench along an axis located near the existing shop (at 4.5 m), with metal-working machines in it, was carried out without stopping the factory. It should be noted that the tamping, performed with a T-2 rammer dropped from a height of 7-8 m, was carried out from a formerly used road at the factory with asphalt pavement, that is, on a very rigid base. During the construction process, a vibrograph was used for measuring the parameters of the vertical and horizontal vibrations of the ground surface and the building walls at a distance of 4-12 m from the tamped trench. The maximum vibration amplitudes reached magnitude 6 on S. V. Medvedev's scale of maximLrm magnitude 12, which for all practical purposes

245

is safe for buildings having satisfactory conditions. However, taking into account that the vibration velocities were very high, the height of rise of the rammer during the final tamping stage was reduced to 3-4 m. Visual observations on the components of the existing building did not reveal any deformations. Cast-in-place foundations in tamped trenches for farm buildings with relatively small loads (up to 300 kN) proved to be uneconomical because of large quantities of concrete required for filling the tamped trench, as well as of difficult supply of concrete to construction sites located at considerable distances from the main centers. Hence, such foundations have found application for larger buildings with designed cast-in-place or multiple-type prefabricated foundations -- mechanical maintenance shops, garages, technical service stations, etc. Cast-in-place foundations in tamped trenches for the "Raduga" greenhouse complex (city of Kingisepp) over an area of 2 ha, and for greenhouses of the Kirov plant in Leningrad over an area of 3 ha were constructed with considerable reduction in labor and materials. Filling, with concrete, of only the lower part of the tamped trench, which was constructionally coordinated with the original project for connecting the greenhouse columns to the foundations, and fabrication, for this purpose, of the T-6 rammer (see Fig. i), whose volume is 0.23 m s, made it possible to reduce the substructure cost by 60% in comparison with the project alternative. It should be noted that the foundations in tamped trenches of the "Raduga" greenhouse complex, built on a 1.5-m-thick sand fill layer, did not exhibit any settlements over a 3-yr operation period, whereas the foundations constructed in accordance with the original project (with digging of trenches) exhibited settlements which caused deformations of the components and damage of the greenhouse glazing. In connection with the above-mentioned characteristics of cast-in-place foundations, for farm buildings with small loads (cow-sheds, pigsties, mineral fertilizer warehouses, hay barns, etc.) prefabricated foundations were adopted. To transmit the acting loads to the different soils, the design organizations specify concrete beds under the standard prefabricated foundations, over an area of up to 4-5 m 2, which significantly increases the earthwork volumes of trench digging and backfilling. For insufficient supply of excavating and compacting equipment for small, remote projects, such solutions involve the need for labor-consuming excavation of continuous trenches for construction of the beds and foundations, and the weak backfill compaction results in development of differential settlements of the floors and other components. Alternative designs and tests made by the Glavzapstroi permitted introducing two improved types of prefabricated foundations in tamped trenches. In the first type, the tamping is performed by a T-3 rammer having a large bottom area (i m2), which compacts the trench base and makes it possible to eliminate the distributing bed and to reduce the earthwork by 90%. In the second type of foundation, which has been applied more widely (18 projects constructed over the last 1.5 years), use is made of driven reinforced concrete blocks (Fig. 3), whose dimensions

Fig. 3. Farm building foundations, a) For project; b) constructed by the Glavzapstroi. i) Foundation bed; 2) sand layer; 3) driven block; 4) compacted soil zone.

246

are identical to those of the T-4 rammer, with sockets measuring 0.4 x0.4 m, in plan, at the block top for installing the columns. In this method, trench tamping is carried out first to a depth equal to 60-75% of the rammer height (the lower values being for sands and the higher for clays), and in the trench a driven block is installed which, by addition of the same rammer, is carried down to the design level. As a rule, the deviations, in plan, for redriving the driven blocks under different soil conditions did not exceed 50-60 mm, whereas the vertical deviations did not exceed 10-15 mm. When b~ocks were driven into the fine silty sands of the Luga District, to fix their tops during driving use was made of a leader on a carriage installed on a guide. For remote projects, where transportation of heavy machines in trailers is difficult, the Glavzapstroi developed and introduced a truck-mounted rammer. CONCLUSIONS i. The experience with tamped-trench foundation construction has shown that this method can be successfully used not only in water-saturated clays, ordinary loams, sandy loams, and fills, but also in sands and gravel-sand soils. The investigations showed that in water-saturated fine silty sands, the addition of a small quantity of clay and rubble during the tamping process forms a smooth trench capable of standing during 3-6 h without wall collapse or penetration of water into it under the full hydrostatic pressure. Construction of a widened base by tamping rubble increases the possible load on the foundation by a factor of 1.6-3. 2. The use of foundations in tamped trenches instead of individual column footings, or pile foundations makes it possible to reduce the labor by a factor of 3-5, the earthwork by 90-100%, the concrete and steel by 15-50%, and the construction cost by 50-60%. Also, formwork and dewatering are eliminated, the transportation costs are lowered, the quality of the floor bases as well as of the equipment foundations is improved, and it is not necessary to compact backfills.

PILE FOUNDATION DEFORMATION DURING RESIDENTIAL BUILDING CONSTRUCTION UNDER FROST HEAVE ACTION E. V. Kosterin

UDC 624.139.22

During the construction of a Series 90 large-panel nine-story residential building in an Omsk microdistrict in the summer of 1979, deformations occurred in the building foundations, which consisted of cast-in-place caps and Class STs 6-30 piles wihout transverse reinforcement, an equipment basement having been incorporated in the building. The piles had been driven during the fall of 1978 and had been left over the entire 19781979 winter without heating the soil base. The pile caps were concreted at the end of April and beginning of May 1979. At the end of August 1979, when erection of the fifth floor was under way, the work was suspended because of foundation deformations. Erection was resumed after the foundations were strengthened during thespring of 1981. The buildingwas commissioned at the end of 1981. The foundation deformations over a length of 70.5 m (between axes i-i and 31-31) were evidenced by the fact that the internal pile caps settled and were detached from the building panel box, which turned out to be supported on the external wall foundations. Appreciable deformations were not noted in the wall panels. The maximum settlement occurred in the cap under the internal longitudinal wall of the building axis C (Fig. i). Between the wall panels and the edge of this cap a gap i to 7 cm wide was formed. The caps of the transverse w a l l s u n d e r w e n t differential settlements with respect to the bottom of the panels of these walls, with triangular gaps being formed which had the maximum opening in axis C at the places of union with the cap of the internal longitudinal wall, and the minimum opening at the places of union with the caps of the longituSibADI. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 6, pp. iS-17, November-December, 1984.

0038-0741/84/2106-0247508.50

© 1985 Plenum Publishing Corporation

247