GLOSARIO ABARCA

Soil nailing is a slope stabilization method that introduces a series of thin elements called nails to resist tension, bending and shear forces in the slope.

A B

D

C E

F

G

Figure 2.7 Modelling of ponded water.

The reinforcing elements are usually made of round cross-section steel bars.

Nails are installed sub-horizontally into the soil mass in a pre-bore hole, which is fully grouted. Occasionally, the initial portions of some nails are not grouted but this practice is not commonly adopted. Nails can also be driven into the slope, but this method of installation is uncommon in practice.

2.3.7.1 Advantages of soil nailing

Soil nailing presents the following advantages that have contributed to the widespread use of this technique:

• Economy: economical evaluation has led to the conclusion that soil nailing is a cost-effective technique as compared with a tieback wall.

Cost of soil nailing may be 50 per cent of a tieback wall.

• Rate of construction: fast rates of construction can be achieved if ade-quate drilling equipment is employed. Shotcrete is also a rapid tech-nique for placement of the facing.

• Facing inclination: there is virtually no limit to the inclination of the slope face.

• Deformation behaviour: observation of actual nailed structures demon-strated that horizontal deformation at the top of the wall ranges from 0.1 to 0.3 per cent of the wall height for well-designed walls (Clouterre, 1991; Elias and Juran, 1991).

• Design flexibility: soil anchors can be added to limit the deformation in the vicinity of existing structures or foundations.

• Design reliability in saprolitic soils: saprolitic soils frequently present relict weak surfaces which can be undetected during site investigation.

Such a situation has happened in Hong Kong, and slope failures in such weak planes have also occurred. Soil nailing across these surfaces may lead to an increased factor of safety and increased reliability, as com-pared with other stabilization solutions.

• Robustness: deep-seated stability would be maintained.

The fundamental principle of soil nailing is the development of tensile force in the soil mass and renders the soil mass stable. Although only tensile force is considered in the analysis and design, soil nail function by a combination of tensile force, shear force and bending action is difficult to be analysed.

The use of the finite element by Cheng has demonstrated that the bending and shear contribution to the factor of safety is generally not significant, and the current practice in soil nail design should be good enough for most cases. Nails are usually constructed at an angle of inclination from 10°to 20°. For an ordinary steel bar soil nail, a thickness of 2 mm is assumed as the corrosion zone so that the design bar diameter is totally 4 mm less than the actual diameter of the bar according to Hong Kong practice.The nail is usually protected by galvanization, paint, epoxy and cement grout. For the critical location, protection by expensive sleeving similar to that in rock

anchor may be adopted. Alternatively, fibre reinforced polymer (FRP) and carbon fibre reinforced polymer (CFRP) may be used for soil nails which are currently under consideration.

The practical limitations of soil nails include:

1 Lateral and vertical movement may be induced from excavation and the passive action of the soil nail is not as effective as the active action of the anchor.

2 Difficulty in installation under some groundwater conditions.

3 Suitability of the soil nail in loose fill is doubted by some engineers – the stress transfer between nail and soil is difficult to be established.

4 The collapse of the drill hole before the nail is installed can happen eas-ily in some ground conditions.

5 For a very long nail hole, it is not easy to maintain the alignment of the drill hole.

There are several practices in the design of soil nails. One of the precautions in the adoption of soil nails is that the factor of safety of a slope without a soil nail must be greater than 1.0 if a soil nail is going to be used. This is due to the fact that the soil nail is a passive element, and the strength of the soil nail cannot be mobilized until the soil tends to deform. The effective nail load is usually taken as the minimum of:

(a) the bond strength between cement grout and soil;

(b) the tensile strength of the nail, which is limited to 55 per cent of the yield stress in Hong Kong, and 2 mm sacrificial thickness of the bar surface is allowed for corrosion protection;

(c) the bond stress between the grout and the nail.

In general, only factors (a) and (b) are the controlling factors in design. The bond strength between cement grout and soil is usually based on one of the following criteria:

(a) The effective overburden stress between grout and soil controls the unit bond stress on the soil nail, and is estimated from the formula (πc′D + 2Dσ_v′tanφ′) for Hong Kong practice, while the Davis method allows an inclusion of the angle of inclination; D is the diameter of the grout hole.

A safety factor of 2.0 is commonly applied to this bond strength in Hong Kong. During the calculation of the bond stress, only the portion behind the failure surface is taken into the calculation.

(b) Some laboratory tests suggest that the effective bond stress between nail and soil is relatively independent of the vertical overburden stress. This is based on the stress-redistribution after the nail hole is drilled and the surface of the drill hole should be stress free. The effective bond load will then be controlled by the dilation angle of the soil. Some of the laboratory tests in Hong Kong have shown that the effective overburden stress is not

important for the bond strength. On the other hand, some field tests in Hong Kong have shown that the nail bond strength depends on the depth of embedment of the soil nail. It appears that the bond strength between cement grout and soil may be governed by the type of soil, method of installation and other factors, and the bond strength may be dependent on the overburden height in some cases, but this is not a universal behaviour.

(c) If the bond load is independent of the depth of embedment, the effective nail load will then be determined in a proportional approach shown in Figure 2.8.

For a soil nail of length L, bonded length L_band total bond load T_sw, L_efor each soil nail and T_mobfor each soil nail are determined from the formula below:

For slip 1: T_mob=T_sw

In this case, the slip passes in front of the bonded length and the full mag-nitude is mobilized to stabilize the slip.

For slip 2: T_mob=T_sw×(L_e/L_b)

In this case the slip intersects the bonded length and only a proportion of the full magnitude provided by the nail length behind the slip is mobilized to stabilize the slip.

The effective nail load is usually applied as a point load on the failure sur-face in the analysis. Some engineers however model the soil nail load as a point load at the nail head or as a distributed load applied on the ground surface. In general, there is no major difference in the factors of safety from these minor variations in treating the soil nail forces.

The effectiveness of the soil nail can be illustrated by adding two rows of 5 m length soil nails inclined at an angle of 15° to the problem shown in Figure 2.4 which is shown in Figure 2.9. The x-ordinates of the nail heads are 7.0 and 9.0. The total bond load is 40 kN for each nail which is taken to be independent of the depth of embedment, while the effective nail loads are obtained as 27.1 and 24.9 kN considered by a simple proportion as given in Figure 2.8. The results of analysis shown in Table 2.7 have illustrated that:

(1) the Swedish method is a conservative method in most cases; (2) the Janbu

1

2 L_b

L_e

Figure 2.8 Definition of effective nail length in the bond load determination.

rigorous method is more difficult to converge as compared with other methods. It is also noticed that when external load is present, there are greater differences between the results from different methods of analysis.

During the computation of the factor of safety, the factor of safety can be defined as

(2.28a) (2.28b) The results shown in Table 2.7 are based on eq. (2.28a) which is the more popular definition of the factor of safety with soil reinforcement. Some commercial software also offers an option for eq. (2.28b), and engineers must be clear about the definition of the factor of safety. In general, the factor of safety using eq. (2.28a) will be greater than that based on eq. (2.28b).