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ABSTRACT: Knowledge of pile capacity over a long period of time after the end of initial driving is important for proper design, construction and estimation of the cost of pile foundations. In this paper, assessment of pile capacity as a function of time has been performed for cohesive soils. On the basis of an existing formula, a new relationship between pile capacity and time after pile installation has been derived. This relationship takes into account pile capacity at the end of driving and an actual time after pile installation. Derived results can be used as a guide for evaluation of long term capacity of piles in cohesive soils.

1 INTRODUCTION

Piles have to withstand design loads for a long period of time. Therefore the consequences of soil modification around the pile are essential with respect to changes of pile capacity. During pile installation, the soil around the pile experiences plastic deformations, remolding, and pore pressure changes. Excess pore water pressure developed during driving reduces the effective soil shear strength and ultimate pile capacity. After the completion of pile driving, soil reconsolidation in cohesive soils, manifested by the dissipation of excess pore pressure at the soil-pile interface zone, is usually accompanied by an increase in pile capacity (soil set-up). The amount of increase in pile capacity and the time required for return of equilibrium conditions depend on soil properties and pile characteristics. For example, the disturbed zone around a pile is more or less proportional to the soil volume displaced during driving and dissipation of excess pore water pressure occurs faster in friction soils.

The phenomenon of time-dependent strength gain in cohesive soils related to pile driving has been studied and published, for example, Fellenius et al. (1989), Randolph et al. (1979), Skov & Denver (1988), Seed & Reese (1955), Svinkin et al. (1994), Thorburn & Rigden (1980), Tomlinson (1971), Wardle et al. (1992), and others.

Assessment of pile capacity as a function of time is, of course, important in the design and construction of pile foundations. Having knowledge of general tendencies of pile capacity with time after driving would certainly be beneficial in economical standpoint. Such information may be used during construction to reduce the design penetration and pile capacity at the end of initial driving and also to choose relevant time for dynamic testing at restrikes or static loading test. In this paper, an attempt is made to present the relationships between pile capacity and elapsed time after the end of initial driving (EOID) for cohesive soils and to show some benefits for estimation of pile capacity from this approach.

2 EXISTING FORMULA

Skov and Denver (1988) found the following formula for ultimate pile capacity, R_u(t), as a function of relative time between different tests

(1)

Some designations in this formula are different from those given in the original expression. The pile capacity at the first restrike, R_RSTR-1, is the lower limit for appreciable increasing in pile capacity when some time elapsed after initial driving results in developing soil set-up. Quantity, t, is a time elapsed from the end of initial driving and capacity R_RSTR-1 is determined for t=t₁. A factor, A, is dependent on soil conditions.

Consolidation of cohesive soils around the pile after pile installation requires much more time in comparison with other soil types like sand and gravel to regain in soil strength and pile-soil adhesion after EOID. For this reason the existing formula is pertinent for clay and cohesive soils.

Case histories presented by Svinkin et al. (1994) confirmed that formula (1) is a good indicator of the pile capacity versus relative time relationship after pile installation. Besides, it was shown that the factor, A, depends not only on the soil but also on the pile type and size.

The application of formula (1) usually requires to make the first restrike in 1-2 days after pile driving. It might be especially convenient for dynamic testing at construction sites where many piles should be tested by restrike in a short period of time.

Nevertheless, the existing formula yields relative set-up versus relative time for assessment of the pile capacity after the first restrike. This is a contradiction to the set-up approach commonly used in geotechnical practice and inconvenient for certain construction sites to restrike piles on 1-2 days after pile installation.

3 PROPOSED FORMULA

The main goal to derive a new formula for evaluation of the set-up effect of cohesive soils in pile capacity is to take into consideration the pile capacity at EOID and the actual time elapsed after pile installation.

For a soil set-up straight line passing through two points corresponding pile capacity at EOID, R_EOID, and pile capacity at any time after pile installation, R_u(t), a formula with a logarithmic time scale can be written

(2)

Time is calculated in days after pile installation. The time for EOID is taken 0.1 (2.4 hours) that negligibly affects increasing in the pile capacity at EOID but gives an opportunity to use the logarithmic time scale.

After simplification formula (2) becomes

(3)

Formula (3) is similar to formula (1). However, proposed formula (3) has certain advantages. This formula (a) uses the traditional set-up formulation, (b) takes into account the actual time in days passed after pile installation, (c) provides determination of the soil set-up independently of the time of the first restrike.

4 CASE HISTORIES

For verification of formula (3), in the following case histories the existing and proposed expressions were used to calculate the pile capacity as a function of time after EOID.

4.1 Case 1

Three piles were considered in this study from total number of seven prestressed concrete piles tested for a bridge approach (GRL Report 1987, Svinkin et al. 1994). A pile description is presented in Table 1. The depth of penetration of each pile was approximately 24.4 m. The soil consisted of about 25.6 m of mainly gray clays followed by a bearing layer of silty sand. Water table was at the ground surface. A Delmag D 46-13 hammer was employed for initial driving and restrikes. For each pile, 3 to 4 dynamic testings were performed after pile installation. For piles TP3 and TP4 static loading tests (SLT) were made as well. The elapsed time after EOID, the penetration resistance and the time dependent ultimate capacity of piles tested are shown in Table 1. Pile capacities from dynamic testings were determined by CAPWAP analysis and the Davisson criterion of failure load was used for static loading tests (GRL Manual 1993).

Pile capacity calculation according to formula (1) was made by Svinkin et al. (1994). Quantity, t₁, was equal to 2 days for piles TP1 and 1 day for piles TP3 and TP4. Measured data and results calculated in accordance with the existing formula have been plotted in ordinates R_u(t)/R_RSTR-1 and log₁₀(t/t₁) as broken and straight lines, respectively, in Figure 1. Note that the set-up coefficients from the field tests generally match well to those obtained from the existing formula.

For piles under consideration, the factor, B, has been found on the basis of back calculations using formula (3). This factor ranges from 1.6 to 3.5 (Table 1). Scattering of the factor, B, is the same for the existing and proposed formulae. The set-up coefficients derived from both the field tests and from formula (3) are given in Table 1 and depicted in ordinates R_u(t)/R_EOID and Time after EOID in days (logarithmic scale) as broken and straight lines, respectively, in Figure 1. Good agreement is found between calculated and measured values of pile capacity as a function of time after EOID.

Figure 1. Pile capacity-time relationship for prestressed concrete and steel piles in clayey soils

4.2 Case 2