Modelo Entidad - Relación

A technique for installing replacement piles, which is gaining increasing pop ularity, is that of con tinu ous flight auger or CFA piles, which in USA are also called auger- cast piles. They are installed by inserting into the soil a con tinu ous flight auger by the combined action of a torque and an axial thrust (Figure 2.15). The auger has a

hollow stem with an inner dia meter of 60 to 100 mm provided with a seal at the lower end. Once the desired depth is attained, a fairly fluid concrete is pumped down the stem releasing the bottom seal whose function is to prevent the soil from enter- ing. As concrete is pumped, the auger is pulled up without rotation removing the soil within the spiral flights; the rate of raising the auger must be com pat ible with the volume of concrete pumped. With this technique there is no need of temporary support, since the wall of the borehole is con tinu ally supported by the spiral flights and the soil within them, or by the concrete as it is pumped. Piles with a dia meter up to 1200 mm have been installed, but the typical range of dia meters is between 400 mm and 800 mm.

CFA piles offer con sider able ad vant ages: vibration is minimal and noise is the lowest for any pile of com par able size; productivity is high. The insertion of the auger involves a slight displacement of the soil; for this reason, some authors classify CFA piles among the partial displacement piles. Since the effects of the displacement are substantially cancelled by the removal of the soil- filled auger, it is believed that this pile type is included more prop erly among the replacement piles.

The drilling rigs are usually crawler mounted and quite tall, so that rel at ively long piles can be installed (Figure 2.16). A concrete pump is required; modern rigs are equipped with sensors to monitor and control the insertion para meters and the con- crete placement.

If reinforcement is needed, it is introduced into the fresh concrete either pushing the cage down manually, or vibrating or driving it. This is easily achieved for cages up to 12–15 m in length; with a proper concrete mix and a suitably assembled cage, depths up to 25 m have been reached.

Figure 2.15 Continuous flight auger (CFA) piles: (a) insertion of the auger; (b) auger withdrawal and concrete casting; (c) insertion of the reinforcement cage into the fresh concrete.

Review of pile types 33

In the cased CFA piles a counter- rotating casing tube is inserted and extracted together with the auger (Figure 2.17); this requires a double rotating head to operate simultaneously the casing and the auger. This kind of pile is frequently employed for the installation of secant piles diaphragms.

2.2.5 Micropiles

Micropiles are small dia meter (150 to 250 mm) replacement piles that can be installed in almost any type of ground where piles are required.

Drilled micropiles were invented in Italy in the 1950s as a means to underpin sen- sitive historic buildings. At present, they compete for use with conventional larger dia meter piles systems under many circumstances, and are especially suited in diffi- cult ground con ditions or with limited or difficult access, like inside buildings to be underpinned. A wide variety of drilling techniques can be employed: light auger, tri- cone, down- the-hole- hammer, casing with auger, percussion rod etc.

The ori ginal Palo Radice or root pile (Figure 2.18), is a small dia meter, cast in place reinforced drilled pile, gen erally designed to achieve a capa city in the range of 100 to 200 kN. Casing fitted with a tungsten bit is drilled into the ground to the full depth of the pile. The drilling fluid (either water or bentonite mud) is injected inside the casing and flows in the annular space between the casing and the soil carrying away soil cuttings. After drilling and placing the reinforcement (either a single bar or a small cage) sand- cement mortar is pumped via a tremie pipe from the bottom of the hole, displacing the drilling fluid. A pressure is applied to the grout during with- drawal of the casing. Pressure is typically in the range of 0.3 to 1 MPa, and is limited by the ability of the soil to maintain a grout- tight seal around the casing during its withdrawal.

In another micropile type presently avail able (Figure 2.19), a steel tube with no return valves (tube à manchettes or TAM), is placed in the hole and the annular space between the tube and the hole wall is filled from the bottom with a sand- cement mortar or neat cement grout. Fifteen to thirty minutes later, i.e. after setting but before hardening of the pri mary grout, a sim ilar grout is injected at a pressure of 2 to 8 MPa using a double packer inside the TAM, so that specific horizons can be

Figure 2.17 Installing cased CFA piles (TREVI) (a) insertion of the auger and of the case; (b) auger and case withdrawal and concrete casting; (c) insertion of the rein- forcement cage into the fresh concrete.

Reinforced cage Tremie pipe Compressed air

Drilling tube

Crown

Concrete

Figure 2.18 “Root” micropile (Palo Radice): (a) drilling; (b) reinforcement placement; (c) grouting with a tremie pipe; (d) extraction of the casing with pressure on the grout; (e) pile complete.

Tube à manchettes

Figure 2.19 Micropile with TAM: (a) drilling; (b) insertion of TAM; (c) insertion of injection pile with packers; (d) injections; (e) micropile’s shaft formed.

treated, if neces sary, several times. To sim plify the installation, sometimes all the valves are grouted simultaneously with a single packer placed just above the upper valve. At the end of the injection stage, the steel tube is filled with grout. The load is mainly resisted by the steel tube; capa city in the range of many hundreds to over 1000 kN may be achieved. The most usual dimensions and the max imum capacities are listed in Table 2.2.