GB2363153A - Testing the load bearing capacity of concrete foundations - Google Patents

Abstract

A concrete structural foundation element, such as a pile, includes a load bearing capacity testing means comprising two sets of reinforcing columns (20, 20') embedded respectively in upper and lower portions (11, 12) of the element and a jacking means (25), having a small transverse cross section, forming a coupling between the two sets of reinforcing columns. The jacking means may include a number of small diameter hydraulic jacks or cylinders. Also a means for promoting a break between the upper and lower portions of the pile may be included such as a blade (31) connected to a twisted bar (30). In an embodiment hydraulic fluid is fed to the hydraulic cylinders of the jack via a channel (21) in the reinforcing column and the pile includes cage sections (23, 24) connected to the reinforcing columns. The movement of the cylinders can be measured by extensometers fixed to the cage. The concrete element can be split by applying a load via the jacks once it has gained sufficient strength.

Description

2363153 Testing Foundation Load Bearing Capacity 5 The present invention

relates to the measurement of the load-bearing capacity of deep foundations by the 'maintained load' method. This invention is applicable to a variety of foundation elements, such as (but not limited to) rotary bored piles (also called drilled shafts), continuous flight auger (auger cast) piles, screw displacement piles, driven cast-in-situ piles, and diaphragm 10 wall (slurry wall) elements. It is of particular advantage for use with continuous flight auger (CFA) piles.

Many deep foundation elements are tested by applying a load and measuring the vertical displacement at varying loads. The traditional method 15 is often called a maintained load test.

The majority of maintained load tests are carried out on piles, by applying a vertical force at the head of the pile. In its simplest form, the vertical force may be provided by means of a static mass (kentledge) placed on 20 an arrangement of beams which are placed on top of the pile head.

Alternatively a similar arrangement of beams may be connected to a system of tension piles (drilled shafts) or ground anchors (tie-backs). These methods of providing a vertical force at the head of a pile are expensive, in terms of both installation time and direct costs (transport, reaction piles, etc).

In the 1980s, a different method (of carrying out a load-bearing test on piles) was introduced by Osterberg (Osterberg, J. "New device for load testing driven piles and drilled shafts separates friction and end bearing", Proc. of the Int. Conf. on Piling and Deep Foundations, London. May 1988, pp 421-427.

30 This was a development of work carried out by Frischman & Fleming in the 1960s (Frischman, W W and Fleming, W G K (1962), "The use and behaviour of Large Diameter piles in London Clay", J. Instn. Struct. Egns. Vol. XL, No 4, April 1962, pp 123-13 1). Frischman & Fleming introduced a flat jack in an under-reamed pile, at the head of the under-ream. They used this jack to measure the load at that level, during a maintained load test, with an external 5 load being applied at the head of the pile. Subsequently they used the flat jack to apply a load to the upper and lower sections of a pile.

The technique of applying a load to the upper and lower sections of a pile requires a specially designed jacking arrangement, one form of which is 10 often referred to as an Osterberg Cell, which is cast into the pile at some depth below ground. The arrangement comprises one or more hydraulic jacks sandwiched between two plates (usually made of steel) which are aligned horizontally. The diameter of the plates is usually less than the diameter of the pile bore. Methods of installation vary, depending on circumstances. In one 15 method, concrete is placed from the base of the pile to a pre- determined level; the jacking arrangement is then lowered on to the (unset) concrete surface, and the remainder of the shaft is then filled with concrete. Reinforcing steel may be used as required, both above and below the jacking arrangement. After the concrete has gained adequate strength, and during the test, the pressure applied 20 to the jacking arrangement generates an equal force to the lengths of pile above and below the cell. It is necessary for the designer of the pile to determine the appropriate level of the jacking arrangement. Maximum load may be applied if the skin friction of the shaft above the jacking arrangement is approximately equal to the combination of skin friction and end bearing below the cell.

All of the methods described above apply load to the pile by means of horizontal bearing plates, which essentially cover the majority of the section of the pile.

30 The general objective of the invention is to provide an improved form of pile load bearing capacity testing means.

According to one aspect of the invention there is provided a concrete structural foundation element including load bearing capacity testing means located at an intermediate position in the element and forming a coupling 5 between the upper and lower portions of the element, characterized in that the load bearing capacity testing means comprise two sets of reinforcing column means embedded respectively in the upper and lower portions of the element and jacking means forming a coupling between the upper and lower reinforcing column means, the reinforcing column means and the jacking means all being 10 of relatively small transverse section.

Thus the loading test can be carried out without the need for external reaction systems, which reduces the costs and assembly time. This arrangement also allows the jack pressures (which are applied on the upper and 15 lower columns) to be in excess of the concrete strength; in that situation, the jack loads are transferred to the concrete via the embedded column sections, so avoiding the jacks crushing the concrete.

The present invention thus provides a radically different method of 20 transferring load from one or more jacks into sections of a deep foundation element. In the following description the application to foundation elements consisting of piles is discussed, but the principle is equally suitable for diaphragm wall elements.

25 The invention is particularly applicable to CFA piles: the Osterbergtype Cell, which offers economies compared with traditional pile head tests, cannot be installed satisfactorily in a CFA pile because there is no open bore.

The reinforcing column means preferably comprise steel column means 30 in the form of two steel sections (a column or any other suitable section).

These are installed on either side of the embedded jacking arrangement. These steel sections transfer the load from the jacking arrangement to the upper and lower parts of the pile, by means of bond with the concrete.

The present system can readily be used in foundation elements which 5 are not vertical.

A pile including load bearing capacity testing means embodying the invention, and two variants thereof, will now be described by way of example and with reference to the drawings, in which:

10 Fig. I is a longitudinal section through the pile; Figs. IA and 1B are sections through the pile; Figs. 2, 2A, and 2B are corresponding views of the first variant; and Figs. 3, 3A, and 3B are corresponding views of the second variant.

15 Fig. 1 shows the central regions of a pile 10, which is split into upper and lower portions I I and 12. Upper portion 11 has a steel section 20 embedded in it, as shown in section in Fig. IA; lower section 12 has a similar section 20' embedded in it. As shown, these steel sections have relatively low cross-sections. The steel sections 20 and 20' are connected together by a 20 number of small diameter jacks or cylinders 25 operating at high pressure, say 10,000-20,000 psi or even higher. As shown in Fig. IB, the jacks 25 have a relatively small cross-section.

The complete unit which is installed into the pile consists of an upper 25 section of cage 23 connected to the upper column section 20 which in turn is connected to the lower column section 20' by means of the jacks 25. If required, a cage 24 can also be connected to the lower column section 20'. After the concrete has gained sufficient strength, the concrete at the jacking arrangement can be split by applying a load via the jacks 25.

The steel section 20 has a channel 21 which is preferably central, through which the feed lines to the hydraulic cylinders are fed. The movement at the head and base of the hydraulic cylinders can be monitored by mechanical or electrical extensometers (not shown) fixed to the main steel of the cage 23.

5 If a lower cage 24 is installed, this allows movement at the toe of the pile to be monitored (as above, by extensometers fixed to the rebar of both cages 23 and 24). If required, strain gauges or any other measuring devices can also be fitted, at any position over the entire length of the pile.

10 In Fig I the shaded regions 20 and 20' of the pile show the regions over which the concrete of the two sections of the pile is mechanically coupled, i.e.

by bond to the steel sections. Beyond those regions, more conventional reinforcing means can be used in the pile.

15 Fig. 2 shows a variant including a cutting blade 31 connected to a twister bar 30. This can be used to physically cut the concrete soon after it has achieved initial set. A weakened horizontal zone can thus be created in the concrete at the jacking arrangement.

20 Fig. 3 shows a further variant, in which the steel sections are Hshaped instead of star-shaped, and the jacks have a corresponding layout.

The combination of column sections, small diameter jacks, cage, etc.

provide little obstruction across the section, and allow relatively easy insertion 25 into a CFA pile. The reinforcing cage, together with the steel sections and hydraulic cylinders, can be installed by means of the service line fitted to typical CFA rigs. Alternatively, depending on the weight and length of the composite cage, a separate crane may be used. As with standard reinforcing cages, it may be advantageous to assist installation by means of a vibrator fixed 30 to the head of the cage.

Furthermore, with this system, if trouble is experienced in installing the composite cage into the pile, it can be removed, cleaned off and installed in another pile.

5 This type of test pile closely resembles a working CFA pile: i.e. it is bored, concreted and the cage installed as conventional.

Referring to other types of foundation elements, it can be seen that the method of load transfer can be used generally. For pre-cast piles it would be 10 necessary to install the new load-transfer elements into the pile during fabrication.

This method of applying load to test a pile can readily be applied to piles which are inclined; because the load-applying assembly is composite with 15 the reinforcing cage, the load applied is axial. (In contrast, because an embedded jacking arrangement such as an Osterberg Cell essentially covers the section of the pile, it would be difficult to ensure that its reaction is axial).

Claims (1)

1. Claims

   5 1 A concrete structural foundation element including load bearing capacity testing means located at an intermediate position in the element and forming a coupling between the upper and lower portions of the element, characterized in that the load bearing capacity testing means comprise two sets of reinforcing column means embedded respectively in the upper and 10 lower portions of the element and jacking means forming a coupling between the upper and lower reinforcing column means, the reinforcing column means and the jacking means all being of relatively small transverse section.

   2 A concrete structural foundation element according to claim 1 15 characterized in that the reinforcing column means comprise steel column means in the form of at least two steel sections (a column or any other suitable section), one above the jacking means and one below.

   3 A concrete structural foundation element according to either previous 20 claim characterized in that the jacking means comprise a plurality of small diameter hydraulic jacks or cylinders.

   4 A concrete structural foundation element according to any previous claim characterized by means for promoting a break between the upper and 25 lower portions of the element.

   Load bearing capacity testing means as defined in any previous claim.

   6 Any novel and inventive feature or combination of features specifically 30 disclosed herein within the meaning of Article 4H of the International Convention (Paris Convention).