Hollow Core Soil Nail

!!!!!!!!!!This text is just a student project for "Soil Improvement" course!!!!!!!!!!!!

HOLLOW-CORE SOIL NAILS STATE-OF-THE-PRACTICE

Part 1) Introduction about typical soil nails:

There are many techniques for supporting ground excavations. These excavation support (also known as shoring) techniques may be broadly classified as external and internal (FHWA, 1999). External support techniques rely on resistance provided by elements outside the face of the excavation and include the use of elements such as berms, rakers, anchors, cross-lot bracing and cantilever walls. Internal support techniques rely on the installation of reinforcement into the existing ground and include the use of elements such as soil nails.

Soil nailing technique has been applied to civil engineering project at Mexico City back to 1960s and has gained popularity in Europe since 1970. During the development of soil nailing technique, cementitious grouted drilled nail, post-grouted driven nail, percussion driven nail, jet nail, and etc have been devised and improved.

The basic concept of soil nailing is to reinforce and strengthen the existing ground by installing closely-spaced steel bars, called ‘nails’, into a slope as construction proceeds from ‘top-down’. This process creates a reinforced section that is in itself stable and able to retain the ground behind it. The reinforcements are passive and develop their reinforcing action through nail-ground interactions as the ground deforms during and following construction.

Advantages and disadvantages of soil nailing:

Advantages:

1. Allow in-situ strengthening on existing slope surface with minimum excavation and backfilling, particularly very suitable for uphill widening, thus environmental friendly,

2. Allow excellent working space in front of the excavation face,

3. Sub-vertical cut surface reducing loss of space,

4. Avoid unnecessary temporary works,

5. Only requires light machinery and equipment,

6. Flexible at constraint site and excavation shape,

7. Can be used for strengthening of either natural slope, natural or man-made cut slopes,

8. Robust and higher system redundancy,

9. Thinner facing requirement.

Disadvantages:

1. Nail encroachment to retained ground rendering unusable underground space,

2. Generally larger lateral soil strain during removal of lateral support and ground surface cracking may appear,

3. Tendency of high ground loss due to drilling technique, particularly at course grained soil,

4. Less suitable for course grained soil and soft clayey soil, which have short self support time, and soils prone to creeping,

5. Lower mobilized nail strength at lower rows of nailing,

6. Suitable only for excavation above groundwater

SOME AVAILABLE DESIGN METHODS:

1-2-1) BS8006: 1995, Code of Practice for Strengthened/Reinforced Soils and Other Fills

1-2-2) HA 68/94, Design Methods for the Reinforcement of Highway Slopes by Reinforced Soil and Soil Nailing Techniques

1-2-3) FHWA, Manual for Design and Construction Monitoring of Soil Nail Walls

RECOMMENDED DESIGN PROCEDURES:

The recommended design procedures are predominantly based on the methods outlined in FHWA’s manual as it is comprehensive. The major steps involved in the design are summarized as follows:

Step 1: Set Up Critical Design Cross-Section(s) and Select a Trial Design

This step involves selecting a trial design for the design geometry and loading conditions. Table 1 provides some guidance on the required input such as the design geometry and relevant soil parameters.

Table 2: Suggested Ultimate Bond Stress (from Tables 3.2 and 3.3, FHWA, 1998)

In HA 68/94, the allowable bond stress, Q can be determined using the following equations:

Q = σ’n tan φ’des + c’des (kN/m2)

Where:

σ’n = average radial effective stress

φ’des, c’des = design values for the soil shearing resistance

The average radial effective stress, σ’n acting along the pull-out length of a soil nail may be derived from:

σ’n = ½ (1 + KL) σ’v

Where:

σ’v = average vertical effective stress, calculated mid-way along nail pull-out length

KL = coefficient of lateral earth pressure parallel to slope

If active conditions (i.e. σ’h = Kaσ’v) are assumed to develop perpendicularly to the slope, Burd, Yu & Houlsby, 1989 has shown that:

KL = ½ (1 + Ka)

Where the value of Ka may be taken as (1 - sinφ’des) / (1 + sinφ’des).

Table 3: Ultimate Bond Stress – Rock (from Table 3.4, FHWA, 1998)

Table 4: Recommended Value for Design – Facing Pressure Factors (from Table 4.2, FHWA, 1998)

Table 5: Nail Head Strength Factors - SLD (from Table 4.4, FHWA, 1998)

Table 6: Strength Factors and Factors of Safety (from Table 4.5, FHWA, 1998)

Note:

Group I: General loading conditions

Group IV: Rib shortening, shrinkage and temperature effects taken into consideration

Group VII: Earthquake (seismic) effects (Not applicable in Malaysia)

* Soil Factors of Safety for Critical Structures

† Refers to temporary condition existing following cut excavation but before nail installation. Does not refer to “temporary” versus “permanent” wall.

Figure 1:Definition of notation used in Table1.

Step 2: Compute the Allowable Nail Head Load

The allowable nail head load for the trial construction facing and connector design is evaluated based on the nominal nail head strength for each potential failure mode of the facing and connection system, i.e. flexural and punching shear failure.

Flexural strength of the facing:

Critical nominal nail head strength, TFN

TFN = CF (mV,NEG + mV,POS) (8 SH/SV) Eqn. 1

mV,NEG = vertical unit moment resistance at the nail head

mV,POS = vertical unit moment resistance at mid-span locations

SH = horizontal nail spacings

SV = vertical nail spacings

CF = pressure factor for facing flexure (Table 4)

Vertical nominal unit moment,

mv = (AsFy / b) [d – (AsFy/1.7f’cb)] Eqn. 1A

As = area of tension reinforcement in facing panel width ‘b’

b = width of unit facing panel (equal to SH)

d = distance from extreme compressive fiber to centroid of tension reinforcement

f’c = concrete compressive strength

Fy = tensile yield stress of reinforcement

Punching shear strength of the facing:

Nominal internal punching shear strength of the facing, VN

VN = 0.33 (f’c (MPa))1/2 (π) (D’c) (hc) Eqn. 2

D’c = bPL + hC

Nominal nail head strength, TFN

TFN = VN [1 / 1 – CS(AC-AGC) / (SVSH – AGC)] Eqn. 3

CS = pressure factor for puching shear (Table 4)

AC, AGC – refer Figure 2

The allowable nail head load is then the lowest calculated value for the two different failure modes.

Figure 2: Bearing plate connection details (from FHWA, 1998).

Step 3: Minimum Allowable Nail Head Service Load Check

This empirical check is performed to ensure that the computed allowable nail head load exceeds the estimated nail head service load that may actually be developed as a result of soil-structure interaction. The nail head service load actually developed can be estimated by using the following empirical equation:

tf = Ff* KA* γ* H* SH* SV Eqn. 4

Ff = empirical factor (= 0.5)

KA = coefficient of active earth pressure

γ = bulk density of soil

H = height of soil nail wall

SH = horizontal spacing of soil nails

SV = vertical spacing of soil nails

Step 4: Define the Allowable Nail Load Support Diagrams

This step involves the determination of the allowable nail load support diagrams. The allowable nail load support diagrams are governed by:

a) Allowable Pullout Resistance, Q (Ground-Grout Bond)

Q = αQ x Ultimate Pullout Resistance, Qu

b) Allowable Nail Tendon Tensile Load, TN

TN = αN x Tendon Yield Strength, TNN

c) Allowable Nail Head Load, TF

TF = αF x Nominal Nail Head Strength, TFN

where

αQ, αN, αF = strength factor (Table 6)

Figure 3: Allowable nail load support diagrams (from FHWA, 1998)

Step 5: Select Trial Nail Spacing and Lengths

The following empirical constraints on the design analysis nail pattern are therefore recommended for use when performing the limiting equilibrium analysis:

a) Nails with heads located in the upper half of the wall height should be of uniform length

b) Nails with heads located in the lower half of the wall height shall be considered to have a shorter length in design even though the actual soil nails installed are longer due to incompatibility of strain mobilized compared to the nails at the upper half. This precautionary measure is in accordance with the recommendations given by Figure 4.

The above provision ensures that adequate nail reinforcement (length and strength) is installed in the upper part of the wall. This is due to the fact that the top-down method of construction of soil nail walls generally results in the nails in the upper part of the wall being more significant than the nails in the lower part of the wall in developing resisting loads and controlling displacements as shown in Figure 5.

Step 6: Define the Ultimate Soil Strengths

The representative soil strengths shall be obtained using conventional laboratory tests, empirical correlations, etc.

Step 7: Calculate the Factor of Safety

The Factor of Safety (FOS) for the soil nail wall shall be determined using the “slip surface” method (e.g. Simplified Bishop method, Morgenstern-Price method, etc.). This can be carried out using commercially available software to perform the analysis. The required factor of safety (FOS) for the soil nail wall shall be based on recommended values for conventional retaining wall or slope stability analyses (e.g. 1.4 for slopes in the high risk-to- life and economic risk as recommended by GEO, 2000).

Figure 4: Nail length distribution assumed for design (from FHWA, 1998)

Figure 5: Concrete soil nail behavior (from FHWA, 1998)

Step 8: External Stability Check

The potential failure modes that require consideration with the slip surface method include:

a) Overall slope failure external to the nailed mass (both “circular” and “sliding block” analysis are to be carried out outside the nailed mass). For residual soil slopes, the analyses must consider both general and non-structurally controlled slip surfaces in association with the strength of the ground mass, together with specific structurally controlled slip surfaces in association with the strength characteristics of the relict joint surfaces themselves. The soil nail reinforcement must then be configured to support the most critical condition of these two conditions.

b) Foundation bearing capacity failure beneath the laterally loaded soil nail “gravity” wall. As bearing capacity seldom controls the design, therefore, a rough bearing capacity check is adequate to ensure global stability as shown in Figure 6.

Figure 6: Principal modes of failure of soil nail wall system (from FHWA)

Step 9: Check the Upper Cantilever

The upper cantilever section of a soil nail wall facing, above the top row of nails, will be subjected to earth pressures that arise from the self-weight of the adjacent soil and any surface loadings acting upon the adjacent soil. Because the upper cantilever is not able to redistribute load by soil arching to adjacent spans, as can the remainder of the wall facing below the top nail row, the strength limit state of the cantilever must be checked for moment and shear at its base, as described in Figure 7.

For the cantilever at the bottom of the wall, the method of construction (top-down) tends to result in minimal to zero loads on this cantilever section during construction. There is also the potential for any long-term loading at this location to arch across this portion of the facing to the base of the excavation. It is therefore recommended by FHWA, 1998 that no formal design of the facing be required for the bottom cantilever. It is also recommended, however, that the distance between the base of the wall and the bottom row of nails not exceed two-thirds of the average vertical nail spacing.

Figure 7: Upper cantilever design checks (from FHWA, 1998).

Step 10: Check the Facing Reinforcement Details

Check reinforcement requirements, minimum reinforcement ratios, minimum cover requirements, and reinforcement anchorage and lap length as per normal recommended procedures for structural concrete design. See Figures 8,9,10.

Figure 8: Reinforced shotcrete facing perspective (from FHWA).

Figure 9: Construct of reinforced shotcrete facing

Figure 10: Typical cross section of a soil nail wall (from FHWA).

Step 11: Serviceability Checks

Check the wall function as related to excess deformation and cracking (i.e. check the serviceability limit states). The following issues should be considered:

a) Service deflections and crack widths of the facing

b) Overall displacements associated with wall construction

c) Facing vertical expansion and contraction joints

Step 12: Construction Checks

For very high and steep slopes, the critical duration may be during the construction phase. Therefore, construction conditions shall be checked as per recommendations of HA 68/94 by missing out the lowest nail, but using short term soil strength parameters, (or using effective stress parameters with the value of ru relevant during construction).

In addition, it is also recommended that the critical stages of works for soil nailing to be highlighted to the contractor and be included as part of the construction drawings and work specifications to ensure satisfactory performance of the soil nailed slope in the long-term and also during construction.

Part 2) Introduction about Hallow-core soil nails:

Hollow core bars have been used in the United States as self-grouting soil nails for approximately 10 years.

FHWA is interested in the hollow core soil nail technology and would like to see hollow core nails used on transportation projects where they would add value. Jerry A.DiMaggio and et.al believe that the technology is different enough from the traditional drill-and-grout soil nail that study is needed to develop specific hollow core soil nail guidelines.

This method has advantages over solid bar nails primarily because fewer installation steps are involved, especially where the solid bar technique would require temporary casing of the hole. The hollow bar method differs most from conventional solid bar methods, and where further research, evaluation and testing would have the greatest impact in assuring quality of hollow bar soil nails and soil nail walls.

The soil nailing technique is described in two recent FHWA documents (FHWA, 1996, 2003). Both FHWA documents concentrate on the use of the _drill-and-grout_ soil nailing technique wherein a solid-bar nail is placed in a pre-drilled hole and then tremie-grouted. A typical nail installation sequence using the solid-bar nailing technique is shown in Figure 10. In the hollow-core soil nail construction technique, the drilling, installing and grouting of nails (Steps 2 and 3 in Figure 10) are combined into one step, which may be attractive to contractors. Initially this technique was used primarily for temporary shoring.

Figure 11. Typical drill-and-grout construction technique using solid-bar nails (FHWA 1994, 2003)

2-1) Research Objectives:

The purpose of this document is to present a SOP summary on the use of the hollow-core nail technology. The scope of this document is limited to the use of hollow-core nails in soils.

Where appropriate, guidance for use of this technology and suggestions for further studies are provided.

2-2) Research methodology:

- study type:

Experimental

- The hallow-core soil nails technology

The conventional solid-bar nail installation technique using the _drill-and-grout_ approach is most efficient in soils where the open-hole drilling method is possible. In caving ground conditions, such as in running or raveling sands and loose soils with cobbles and boulders, casing would be required to support the drill-hole excavation. Use of casing makes the solid-bar soil nailing process slower and therefore costlier.

Figure 12. (a) Schematic of hollow-core soil nail installation and grout paths, (b) Schematic of a cross-section of hollow-core soil nail and the grout body.

-Experience and research base

As part of the preparation of source document, the authors of the source report relied on their own experience and research (Nowatzki and Samtani, 2004; Samtani, 2003; NCS, 2005) as well as discussions with manufacturers of hollow-core soil nails and specialty contractors who install hollow-core soil nail.

-Wall heights: 3 m (~10-ft) to 20 m (~65-ft)

-Wall slopes: vertical to ½ H:1V (H: Horizontal, V: Vertical)

-Wall configuration: single height to multiple-bench (tiered) walls

-Nail spacing: 1.5 m x 1.5 m (~5-ft x 5-ft) to 1.83 m x 2.74 m (~6-ft x 9-ft)

-Required (design) minimum nail hole diameter: 76 mm to 102 mm (~3- to 4-inches)

-Nail Lengths: up to 15 m (~50-ft)

-Nail corrosion protection systems: grout only to fusion-bonded purple marine epoxy-coated (ASTM A 934) nails encapsulated in grout

-Nail configuration: rectangular to diamond (quadrilateral)

-Facilities behind top of wall: none, roadways, bridge abutments with deep foundations (piles and shafts), swimming pools, and buildings

-Type of soils: running sands, clayey sands, sandy clays, lean clays, silty sands, gravelly soils, cobbly soils, variably-cemented soils, compacted embankment fills, existing embankment fills, MSE wall backfill, and weak sandstone

-Nail testing: proof testing (on production and sacrificial nails), verification testing and ultimate (pullout) testing

-Measured nail adhesion values: 11 kN/m (~0.75 kip/ft) to 117 kN/m (~8 kips/ft).

-Steel and thread types

As with solid-bar nails, the steel for the hollow-core soil nails must meet the requirements of ASTM A615. Grade 60 is the minimum requirement for the soil nails, although higher grade steels are available and commonly used.

Figure 14. Various CTS-Titan hollow-core bars (Aschenbroich, 2005).

-Drilling equipment and drill crew experience

Hydraulic rotary or rotary-percussion drill rigs are commonly used.

The rigs can be excavator-mounted (Figure 15).

Figure 15. Excavator-mounted drill rig installing hollow-core soil nails to repair a rockery wall in a remote area.

-Grout equipment (Mixer and Pump)

In general, any plant suitable for the mixing and pumping of fluid cementitious grouts may be used for the grouting of hollow-core soil nails. Here the author used a typical paddle mixer with a holding tank as Figure 16.

Figure 16. A typical paddle mixer with a holding tank.

-Drill bits

Asacrificial drill bit is screwed on to the front of the threaded hollow core soil nail bar (Figure 17)

Figure 17. Mobile _star_ type steel centralizer on hollow-core soil nail.

-bearing plates

At the nail head: beveled washers, nuts and bearing plates (see Figure 18)

Figure 18. Headed stud bearing plate with a spherical hex nuts.

Figure 19. Preparing the unbounded length.

Figure 20. A hollow-core bar being installed with attached PVC tube sealed at both ends.

2-3) Conclusion:

A state-of-the-practice (SOP) document regarding hollow-core soil nails is presented herein.

Issues peculiar to the use of hollow-core soil nails are presented and, where appropriate, a commentary is provided based on the authors_ experience.

The hollow-core soil nail technology has been used successfully by the authors and others throughout the United States and elsewhere. Since the hollow-core soil nail technology is a relatively recent development, a sufficiently long track-record is not available. There are several issues that require attention before the hollow-core soil nail method can be accepted as a mainstream technology and regularly used for permanent applications. Some of these issues may be addressed by related literature review while others will require testing and analysis.

Comprehensive laboratory and field investigations are suggested to evaluate the above issues.

Careful exhumation and examination of nails should be part of the field investigations to verify issues such as the extent of corrosion protection offered by epoxy coatings and the amount of protection of unbonded length after nail tests.

The goal of further studies should be the development of a unified model that incorporates different nail types and construction methods and provides limiting values for design, e.g., limiting steel stress values for a given thread type from a crack width viewpoint.

References:

Chow Chee-Meng & Tan, Yean-Chin. Soil Nail Design: A Malaysian Perspective. Gue & Partners Sdn Bhd, Kuala Lumpur, Malaysia

FHWA(2006). HOLLOW-CORE SOIL NAILS STATE-OF-THE-PRACTICE. Naresh C. Samtani and Edward A. Nowatzki (Prepared for FHWA)

FHWA0-IF-03-017(2003). GEOTECHNICAL ENGINEERING CIRCULAR NO. 7 Soil Nail Walls. Carlos A. Lazarte, Ph.D., P.E., Victor Elias, P.E., R. David Espinoza, Ph.D., P.E., Paul J. Sabatini, Ph.D., P.E.

Liew Shaw-Shong(2005). Soil Nailing for Slope Strengthening. Gue & Partners Sdn Bhd, Kuala Lumpur, Malaysia