Session 68: Other Geosynthetic Applications Geosynthetic's 87 Conference New Orleans, USA

PARTOS, A.M. and KAZANIWSKY, P.M.
SITE Engineers, USA

Geoboard Reduces Lateral Earth Pressures


ABSTRACT

A two level below grade parking garage under a forty story office building adjacent to a low rise retail plaza is nearing completion in Philadelphia, Pennsylvania. A subway is located adjacent to the south property line. The exterior grades at the north are approximately 2.2m (7 ft) higher than those at the south. The difference in elevations and the potential for a future excavation along the subway would result in unbalanced lateral earth pressures at the location of the plaza. It was not feasible to brace the north wall of the garage below the plaza or employ permanent tiebacks. A design was completed to reduce lateral at-rest earth pressures against the north basement wall be mobilizing active lateral earth pressures. Prefabricated, expanded polystyrene bead drainage geoboards were installed against the exterior face of the north basement wall. Compacted backfill was constructed against the 25 cm (10 in) thick drainage geoboards. Load cells and extensometers were installed to monitor the magnitudes of lateral earth pressures and deflections of the drainage boards. This paper presents the design and monitoring system data together with conclusions regarding the performance of the system.

INTRODUCTION

Figure 1 shows the project site which occupies a full city block in the business center of Philadelphia, Pennsylvania. The completed project will consist of a fourth story west office tower, a future forty story East office tower, and a centrally located plaza with one and two story retail areas. Two basement levels will provide parking below the entire site. The nearly completed West office tower is supported on drilled piers socketed in mica schist rock. The garage below the West tower and plaza is completed and supported on drilled piers and spread footings, respectively.

Figure 2 presents a cross-section through the plaza area. The lowest garage floor and the Market Street subway platform are approximately at the same elevation. Market Street grade is 2.2m (7 ft) below JFK boulevard grade. A temporary retention system consisting of soldier beams, wooden lagging, and soil tiebacks protected the north west, and south sides of the approximately 10 m (33ft) deep excavation. A 1.9 m (6 ft) diameter storm sewer was installed along the north wall of the excavation.

SITE engineers, inc. was retained by the owner to provide geotechnical engineering services in connection with design and construction. Figure 3 presents a summary of subsurface conditions based on borings drilled along the north retaining wall.

PRESENTATION OF PROBLEM

All below grade walls were designed to be non-yielding, braced by floors, and designed for the at-rest earth pressures computed on the basis of the existing soil conditions behind the temporary retention system. The design at-rest earth pressures are shown in Figure 3.

Due to differences in elevations between Market Street and JFK Boulevard, unbalanced lateral earth pressures exist for the length of the plaza as shown in Figure 4. The unbalanced design load computed by the Structural Engineer was on the order of 233 kN/lin m (16 kips/lin ft) of wall.

Further excavation along the Market Street subway for repair or reconstruction could further increase the unbalanced loading conditions, but such additional loading was not considered in the design since maintaining stability would be the responsibility of others.

The Structural Engineer's review disclosed that modification to the light plaza steel frame by providing bracing to the lateral loads was not feasible. Permanent tiebacks anchored under JFK Boulevard could not be considered due to easement problems beyond the JFK Boulevard property line. Therefore, it was necessary to search for methods of reducing the lateral earth pressures against the North wall of the plaza.

METHODS OF REDUCING LATERAL EARTH PRESSURES

Several schemes were considered as listed in Table 1. The relevant functions of each method are also listed.

Table 1: Descriptions of Methods of Reducing Lateral Earth Pressures

Description of Method Function

1. Light Weight Cellular - Reduction in density
Concrete BAckfill - Mobilization of high value of
Cohesion

2. Stabilized Backfill - Increase of angle of internal
friction
- Increase of cohesion

3. Elastic Drainage - Inducing quasi-active stress
Board on Face of Wall - conditions by providing yielding
media

After taking into consideration costs, scheduling, etc., the Construction Manager selected the drainage board scheme.

BACKFILL SOIL PROPERTIES AND DESIGN OF DRAINAGE BOARD

The excavated on-site granular materials were proposed as backfill for use the full height of the retaining walls. The physical and engineering properties of the on-site granular soils based on laboratory testing and the anticipated field properties are presented in Table 2.

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FIGURE 10 AND 11

Very small deformations were observed for the extensometers as shown in Figure 10. Maximum deformations of the drainage board at full fill heights are 0.25 cm (0.1 in.), 0.50 cm (0.2 in.), and 1.5 cm (0.6 in.) for extensometers E-3, E-2, E-1, respectively.

Figure 11 shows the computed strain-stress relationships for the field observations together with the laboratory data. Essentially fairly good correlation between field measurements and laboratory data is observed. Bothe curves appear to be nearly parallel, but there appears to be an apparent shift between the curves indicating that, under the same stress, the computed strain for the field condition is less than the laboratory curve. There may be several explanations:

- The laboratory curve is based on unconfined conditions.
Field conditions are not unconfined.
Therefore, deformations in a confined condition generally
would be less than those in an unconfined state.

- The accuracy of the field readings is limited to
0.25 cm (0.1 in.). This is well below the accuracy of
laboratory instruments with an accuracy of 0.0025mm
(0.0001 in.).

- The extensometers required some seating load as backfill was
being placed before the gauges were zeroed out. Thus it
could be possible that measured total deformation,
and subsequently the strain, is less than the true value.

FIGURE 12

Curve 1 in Figure 12 shows the at-rest earth pressure against the wall to the stone layer, based on theoretical relationships and not considering the influence of soil materials beyond the temporary retention structure.(5)

Curve 2 presents the theoretical relationship (Coulomb) of an active earth pressure against a wall due to the sand and gravel and stone backfills.(5)

Curve 3 presents the actual observed pressures against the wall at the three meter elevations and interpolation between the points. It is recognized that the actual pressure distribution may not be necessarily linear. (4)

Comparing Curves 1, 2, and 3 it is obvious that essentially a condition of very low lateral pressure against the retaining wall has been attained by application of a yielding material which in this case was the “Geotech” drainage board.

It is acknowledged that the temporary retention structure may not have permitted full development of the theoretical at-rest pressures; however, it should be noted that continuous monitoring of the temporary retention system indicated a steady creeping of the retention system toward the south direction during backfill construction. Thus we conclude that lateral pressures against the temporary retention system were transferred onto the backfill and subsequently onto the permanent basement wall.