Design of Soil Nail Heads

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Geotechnical Engineering Office, Civil Engineering and Development Department The Government of the Hong Kong Special Administrative Region

GEO Technical Guidance Note No. 21 (TGN 21)Design of Soil Nail Heads

Issue No.: 1 Revision: - Date: 10.9.2004 Page: 1 of 8

[3707][D:\My Documents\TGN\TGN 21.doc] [10.9.2004] [YKS]

1. SCOPE

1.1 This Technical Guidance Note (TGN) provides technical guidance on the design of soilnail heads for soil cut slopes. Only soil nail heads in the form of isolated concretepads are covered by this TGN.

1.2 Any feedback on this TGN should be referred to Chief Geotechnical Engineer/Standardsand Testing of the GEO.

2. TECHNICAL POLICY

2.1 The technical guidance promulgated in this TGN was agreed by the GeotechnicalControl Conference (GCC) on 2.9.2004.

3. RELATED DOCUMENTS

3.1 Department of Transport (1994). Design Manual for Roads and Bridges: DesignMethods for the Reinforcement of Highway Slopes by Reinforced Soil and Soil Nailing

Techniques, HA68/94, Department of Transport, UK.

3.2 GEO (2000). Technical Guidelines on Landscape Treatment and Bio–engineering forMan-made Slopes and Retaining Walls (GEO Publication No. 1/2000), GeotechnicalEngineering Office, Hong Kong.

3.3 Shiu, Y.K. & Chang, G.W.K. (2004). Soil Nail Head Review. Special Project ReportNo. SPR 8/2004, Geotechnical Engineering Office, Hong Kong.

4. BACKGROUND

4.1 The guidelines contained in this TGN are based on the findings of a review on soil nailhead design (Shiu & Chang, 2004) completed by the GEO.

5. TECHNICAL RECOMMENDATIONS

5.1 DESIGN OF SOIL NAIL HEADS

5.1.1 Soil in a nailed slope consists of two distinct zones, namely the active zone and theresistant zone, as shown in Annex TGN 21 A1. Soil nails and nail heads act togetherto tie the active zone to the resistant zone.

5.1.2 A soil nail head serves the following functions: (a) working as a reaction pad for the

mobilization of tensile force in the soil nail, (b) confining the soil in the active zonebehind the nail head, and (c) preventing local failure between nails.







5.1.3 The sizes of nail heads should be determined by one of the following methods:

(i) the design table derived from numerical analysis as shown in Annex TGN 21A2,

(ii) the lower-bound nail head design method adopted from the one given by UK Department of Transport (1994) as shown in Annex TGN 21 A3, or

(iii) the prescriptive design approach as shown in Annex TGN 21 A4.

5.1.4 The structural design of the nail heads shall follow the recommendations stipulated inrelevant structural design codes. Nail force acting uniformly on the nail head may beassumed in the structural design. Alternatively, the reinforcement details shown inAnnex TGN 21 A5 may be used.

5.1.5 As alternatives to isolated concrete pads, other forms of surface support such asreinforced concrete grillage beams may be considered. Grillage beams can providebetter resistance to local failures. They also allow easier detailing for greeningmeasures in comparison with large soil nail heads.

5.2 LOCAL S TABIL ITY OF SOIL BETWEEN NAIL HEADS

5.2.1 Soil between nail heads is unsupported. It is susceptible to local failure or surfaceerosion, especially when the slope is steep and the nail head spacing is large.Adequate surface protection should be provided to prevent such failures.

5.2.2 The use of a long-term non-degradable erosion control mat in conjunction with steelwire mesh has been successful so far in preventing local failures and surface erosion of soils between nail heads. Typical construction details can be found in CEDD StandardDrawing Nos. C2511/1 and C2511/2A.

5.3 AESTHETICS

5.3.1 Exposed nail heads may have substantial adverse impact on the appearance of a slope.

Guidance on visual treatment of soil nail heads is given in GEO Publication No.1/2000(GEO, 2000). For example, recessed soil nail head with details similar to those shownin CEDD Sketch No. R1067A may be used to mitigate the visual prominence.

6. ANNEXES

6.1 TGN 21 A1 - Load Transfer Mechanism of Soil Nailed Structure

6.2 TGN 21 A2 - Soil Nail Head Design Table Derived from Numerical Analysis

6.3 TGN 21 A3 - Lower-bound Nail Head Design Method Adopted from Department of Transport (1994) with Modification







6.4 TGN 21 A4 - Prescriptive Design Approach

6.5 TGN 21 A5 - Reinforcement Details for Soil Nail Heads Bearing on Soil

(R K S Chan)Head, Geotechnical Engineering Office






[3707][D:\My Documents\TGN\TGN 21.doc] [10.9.2004] [YKS] ANNEX TGN 21 A1

Load Transfer Mechanism of Soil Nailed Structure







4 0

8 0 0

8 0 0

6 0 0

6 0 0

6 0 0

8 0 0

8 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

3 2

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

4 0 0

6 0 0

6 0 0

6 0 0

4 0 0

4 0 0

6 0 0

4 0 0

4 0 0

4 0 0

4 0 0

S l o p e A n g l e ≥ 6

5 °

S t e e l B a r D i a m e t e r ( m m )

2 5

6

0 0

6

0 0

4

0 0

4

0 0

4

0 0

6

0 0

4

0 0

4

0 0

4

0 0

4

0 0

6

0 0

4

0 0

4

0 0

4

0 0

4

0 0

6

0 0

4

0 0

4

0 0

4

0 0

4

0 0

4 0

8 0 0

8 0 0

8 0 0

8 0 0

6 0 0

8 0 0

8 0 0

8 0 0

6 0 0

6 0 0

8 0 0

8 0 0

6 0 0

6 0 0

6 0 0

8 0 0

6 0 0

6 0 0

6 0 0

6 0 0

3 2

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

4 0 0

6 0 0

6 0 0

6 0 0

4 0 0

4 0 0

5 5 ° ≤ S

l o p e A n g l e < 6 5 °

S t e e l B a r D i a m e t e r ( m m )

2 5 6 0 0

6 0 0

4 0 0

4 0 0

4 0 0

6 0 0

4 0 0

4 0 0

4 0 0

4 0 0

4 0 0

4 0 0

4 0 0

4 0 0

4 0 0

4 0 0

4 0 0

4 0 0

4 0 0

4 0 0

4 0

8 0 0

8 0 0

8 0 0

8 0 0

8 0 0

8 0 0

8 0 0

8 0 0

8 0 0

8 0 0

8 0 0

8 0 0

8 0 0

8 0 0

8 0 0

8 0 0

8 0 0

8 0 0

6 0 0

6 0 0

3 2

8 0 0

8 0 0

8 0 0

6 0 0

6 0 0

8 0 0

8 0 0

6 0 0

6 0 0

6 0 0

8 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

6 0 0

4 5 ° ≤ S

l o p e A n g l e < 5 5 °

S t

e e l B a r D i a m e t e r ( m m )

2 5 8 0 0

6 0 0

6 0 0

6 0 0

4 0 0

6 0 0

6 0 0

6 0 0

4 0 0

4 0 0

6 0 0

6 0 0

4 0 0

4 0 0

4 0 0

6 0 0

4 0 0

4 0 0

4 0 0

4 0 0

c ' ( k P a )

2 4 6 8 1 0 2 4 6 8 1

0 2 4 6 8 1 0 2 4 6 8 1

0

S o i l N a i l H e a d D e s i g n T a b l e d e r i v e d f r o m N u m e r i c a l A n a l y s i s ( S h i u & C h a n

g ,

2 0 0 4 )

φ '

3 4 °

3 6 °

3 8 °

4 0 °

N o t e s :

( 1 )

D i m e n s i o n s i n m m u n l e s s s t a t e d o t h e r w i s e .

( 2 )

O n l y t h e l e n g

t h o f o n e s i d e o f s q u a r e n a i l h e a d i s s

h o w n i n t h e T a b l e .

( 3 )

T h e m i n i m u m

t h i c k n e s s o f s o i l n a i l h e a d s h o u l d b e 2 5 0 m m .

( 4 )

T h e s h e a r s t r e n g t h p a r a m e t e r s ( c ’ a n d φ ' ) o f t h e s o i l n e a r t h e s l o p e s u r f a c e s h o u l d b e u s e

d .







Legend:

P Design load of soil nailσ'v Effective vertical stressγ Unit weight of soil (kN/m3)

β Slope angle

ru Pore pressure parameter (=u/γh)u Pore water pressure (kN/m2)h Depth of overburden directly above point in question

Lower-bound Nail Head Design Method Adopted from Department of Transport (1994) withModification







Prescriptive Soil Nail Head Design for Soil Cut Slopes or the Soil Portion of CR Features

Square Nail Head Size(mm x mm)Soil Nail Steel Bar

Diameter (1)

(mm)Geology

Slope Angle <55° 55°≤ Slope Angle≤ 65°

25 or 32Highly decomposedgranitic or volcanic rock.

600 x 600 600 x 600

25 or 32

Soils including colluvial,residual or completetly

decomposed materials of granitic and volcanicorigin, and weatheredsedimentary rocks.

800 x 800 600 x 600

Notes: (1) Refer to GEO Report No. 56 for prescriptive design of soil nails for cut slopes andSpecial Project Report No. SPR 2/2004 for prescriptive soil nail design forconcrete and masonry retaining walls.

(2) The minimum thickness of the nail head should be 250 mm.(3) For slope angles larger than 65°, reinforced concrete grillage beams instead of

isolated soil nail heads should be used.







Nail Head Size Reinforcement

400 x 400 x 250 3T16U-Bars both ways

600 x 600 x 250 3T16U-Bars both ways

800 x 800 x 250 4T16U- Bars both ways

Notes: (1) All dimensions are in millimetres.(2) The clearance between the steel bar and the hole of the GMS plate shall not be

more than 2.(3) Construction of nail head shall be referred to particular specifications.

Reinforcement Details for Soil Nail Heads Bearing on Soil