5. TYPE 3 PRESCRIPTIVE MEASURES
5.4 PRESCRIPTIVE SOIL NAILS FOR CONCRETE OR MASONRY RETAINING WALLS
5.4.1 Qualifying Criteria
5.3.3 Soil Nail Head and Facing
Guidance on the design of soil nail head and/or wall facing is given in Sections 5.2.3 and 5.4.3 respectively.
5.3.4 Corrosion Protection
Guidance on the design of corrosion protection measures for steel reinforcement of prescriptive soil nails is given in Section 5.2.4.
5.4 PRESCRIPTIVE SOIL NAILS FOR CONCRETE OR MASONRY RETAINING WALLS
5.4.1 Qualifying Criteria
Soil nail layouts for retaining walls have been standardised based on a review of past designs. Retaining walls are deemed to satisfy the required safety standards with the prescriptive soil nails applied as upgrading works, provided that the retaining walls satisfy the qualifying criteria given in Table 5.9. In the case where not all the qualifying criteria are satisfied, the prescriptive measures may be used as preventive maintenance works.
Table 5.9 Qualifying Criteria for Application of Prescriptive Soil Nails to Concrete or Masonry Retaining Walls as Upgrading Works
Subjects Qualifying Criteria for Application
Geometry Slope feature height, H ≤ 8 m
Engineering and geology
1. Apply only to either concrete or masonry retaining walls judged to require improvement works.
2. Apply to sites comprising colluvial, residual or saprolitic soils of granitic or volcanic origin. Also apply to sites comprising other materials with similar shear strength properties, with the exception of alluvial and marine deposits and sedimentary rocks containing argillaceous layers.
3. Criteria 3 and 4 under the column of “Engineering and Geology” in Table 5.2 are also applicable.
Wall type
1. For concrete retaining wall, it should be a mass concrete wall, a reinforced concrete wall of L-shaped, inverted L-shaped or inverted T-shaped, as shown in Figure 5.2.
2. For masonry retaining wall, it should be of a condition no worse than wall condition Class B and no worse than observed state of wall deformation No. (2) as defined in Tables 5.7 and 5.8 respectively.
Terrain profile
1. Gradient of terrain in front of slope feature, α (averaged over a horizontal distance of four times the wall thickness) should not be greater than 10°.
2. Gradient of terrain above slope feature, β should not be greater than 15°.
Extent of fill material Maximum thickness of fill, fw as measured horizontally from the top of wall back face should not be more than the height of retained ground, Hr.
θθθθ
αααα
Legend:
α Gradient of terrain in front of slope feature β Gradient of terrain above slope feature q Design surcharge
θ Wall face angle
ft Vertical thickness of fill layer above retaining wall crest fw Depth of fill layer measured from top of wall back face H Slope feature height
He Maximum effective height of slope feature, where He = H (1+0.35tanβ) + q/20 Hr Height of retained ground
i Wall back angle
Tw Average thickness of retaining wall
Figure 5.6 Simplified Geometry of a Mass Concrete or Masonry Retaining Wall Feature
θθθθ
αααα
Legend:
α Gradient of terrain in front of slope feature β Gradient of terrain above slope feature q Design surcharge
θ Wall face angle
ft Vertical thickness of fill layer above retaining wall crest fw Depth of fill layer measured from top of wall back face H Slope feature height
He Maximum effective height of slope feature, where He = H (1+0.35tanβ) + q/20 Hr Height of retained ground
i Wall back angle
Tw Average thickness of retaining wall
Figure 5.7 Simplified Geometry of a Reinforced Concrete Retaining Wall Feature
accordance with the guidelines given in the Geotechnical Manual for Slope (GCO, 1984) and Works Bureau Technical Circular No. 13/99 : Geotechnical Manual for Slopes - Guidance on Interpretation and Updating (Works Bureau, 1999).
(c) Determine whether the feature is substandard. A way of doing it is to compare the measured wall thickness with the minimum wall thickness required to satisfy the current geotechnical standards given in Figure 5.8 according to the respective wall standard to be achieved.
If the measured retaining wall thickness is equal to or greater than the minimum required thickness, the wall can be considered as being up to the respective geotechnical standard and no upgrading works are necessary. The chart in Figure 5.8 is applicable to both concrete retaining walls effective to assume the wall to be substandard and proceed with prescriptive design of upgrading works, or whether it would be more appropriate to carry out a stability assessment with detailed ground investigation.
(d) Determine the total length of soil nails, Ltotal, as follows (see Figure 5.5):
Ltotal = L + Lfree
where L = Length of the portion of soil nail in material behind wall backfill (m)
Lfree = Length of the portion of soil nail within the retaining wall and wall backfill (m).
Determine from Table 5.10 the number of rows, spacings and lengths of the soil nails, L. Lfree can be taken as the height of retained ground, Hr, or alternatively it may be determined by designers based on detailed information on the wall thickness and extent of fill behind the wall.
A typical pattern of prescriptive soil nailing to a retaining wall is shown in Figure 5.9.
The soil nail layout, derived from prescriptive design based on the consideration of the maximum effective height of the slope feature, may be applied to the entire slope feature.
Alternatively, the feature may be split into different sections, with the soil nail layout for each section designed according to the maximum effective height for the respective sections.
6
5
m) s (nesickTh 4
3
Minimum Wall
2
1
0
2 3 4 5 6 7 8 9 10
Maximum Effective Height of Slope Feature, He (m)
Legend:
Concrete retaining wall (‘existing’ wall standard) Concrete retaining wall (‘new’ wall standard)
Masonry retaining wall (‘existing’ and ‘new’ wall standards)
Note: This chart is only applicable to concrete or masonry retaining walls that satisfy the qualifying criteria listed in Table 5.9, and situations where the surcharge behind the feature does not exceed 10 kPa.
Figure 5.8 Minimum Required Thickness of Concrete or Masonry Retaining Walls where No Type 3 Prescriptive Measures are Needed
Table 5.10 Standard Prescriptive Soil Nail Layouts for Concrete or Masonry Retaining Walls Standard
Soil Nail Layouts
He (m) φφφφr (mm) φφφφh (mm)
‘New’ Wall Standard(1) ‘Existing’ Wall Standard(1)
N L (m) Sh (m) N L (m) Sh (m)
retaining wall are stipulated in the Geotechnical Manual for Slopes (GCO, 1984) and Works Bureau Technical Circular No. 13/99 (Works Bureau, 1999).
(2) He is the maximum effective height of slope feature, φr the soil nail diameter, φh the drillhole diameter, Sh the horizontal spacing of soil nails, and L the length of the portion of soil nail in material behind wall backfill.
(3) Total length of a soil nail, Ltotal, should include the length of the portion of soil nail within the retaining wall and wall backfill, Lfree, and L.
(4) For He between any of the two consecutive values in the above table, the soil nail layout corresponding to the higher He value should be adopted.
(5) N is the number of soil nails per vertical column required at the critical section, i.e. the section with the maximum effective height, He. At other parts of the retaining wall, soil nails should be provided at vertical and horizontal spacing similar to that at the critical section.
Alternatively, different soil nail layouts according to the He of that part of the retaining wall may be adopted.
(6) The vertical spacing (Sv) of soil nails, as defined in Figure 5.1, should not be less than 1.5 m.
If necessary, designers may adjust N and Sh to achieve the required minimum Sv value. In so doing, the adjusted layout should maintain the same soil nail density as that of the layout given in this Table, with Sh ≥ 1.0 m. Where the designers opt for a specific Sv value (e.g. 2 m) to suit site constraints or other considerations, the soil nail layout may be adjusted by maintaining the same soil nail density as that given in this Table, with Sh ≥ 1.0 m.
(7) Soil nails should be evenly spaced over the face of the retaining wall.
(8) Steel reinforcement for soil nails shall be of Type 2 high yield deformed bars.
(9) If rock is encountered in the process of drilling such that part of the soil nails will be installed in rock (e.g. installation through a PW50/90 zone or better, see Geoguide 3 (GCO, 1988)), designers may exercise professional judgement to reduce the soil nail length.
(10) Designers should check the land status to establish whether the soil nails would encroach into the adjoining land and, if so, whether this is acceptable to the land owner.
(11) Sections of a retaining wall that are lower than 2 m in height do not usually require reinforcement by soil nails.
Elevation
Section A-A Notes: (1)
(2) (3)
L = Length of the portion of soil nail in material behind the wall backfill; Lfree = Length of the portion of the soil nail within the retaining wall and wall backfill; H = Slope feature height; Hr = Height of retained ground; Tw = Average retaining wall thickness;
β = Gradient of terrain above slope feature.
N, L, Sv, Sh and other dimensions for the soil nails are given in Table 5.11 based on the type of soil nail layout specified by designers.
If tie beams and/or tie columns are provided, the soil nails may not necessarily be staggered.
Figure 5.9 Prescriptive Soil Nails on a Retaining Wall
5.4.3 Soil Nail Head and Facing
Skin walls, tie beams or tie columns should be provided to connect soil nails, in particular those installed in masonry retaining walls with poor wall conditions, such as dry-packed walls or walls with signs of distress. Isolated concrete soil nail heads can be
used in concrete or well-constructed masonry retaining walls, because these walls generally have better structural integrity. For well-constructed masonry retaining walls, recessed soil nail heads are preferred in order to preserve the wall fabric and appearance. Due care needs to be exercised in the temporary removal of the masonry blocks to facilitate construction of recessed soil nail heads.
Typical details of a prescriptive reinforced concrete skin wall for connecting the soil nails in a concrete or masonry retaining wall together are shown in the latest version of CEDD Standard Drawing No. C2520 : Typical Details of Skin Wall with Soil Nails.
Typical details of the prescriptive exposed reinforced concrete tie beams and tie columns for connecting soil nails in a concrete or masonry retaining wall together are shown in the latest version of CEDD Standard Drawings No. C2524 : Details of Exposed Tie Beam for Retaining Wall and No. C2523 : Details of Embedded Tie Column for Masonry Retaining Wall.
Typical details of the exposed soil nail heads are shown in the latest version of CEDD Standard Drawing No. C2522 : Typical Details of Soil Nail Head on Rock or Concrete Wall Surface.
The following factors should be considered in the choice of the types of soil nail heads and facing for use on a wall face:
(a) Slenderness ratio of the wall, defined as He/Tw (see Figures 5.6 and 5.7) – Skin walls should be provided to reinforce existing retaining walls with high slenderness ratio.
As a general guidance, a skin wall should be constructed over the entire face of a retaining wall with a slenderness ratio equal to or greater than 5.
(b) Wall condition – Skin walls should be considered to reinforce existing masonry retaining walls with poor wall condition. As a general guidance, a skin wall should be constructed over the entire face of a dry-packed masonry retaining wall, or a masonry retaining wall with condition Class B or observed state of wall deformation No. (2), as defined in Tables 5.7 and 5.8 respectively.
(c) Availability of space in front of the wall – If exposed tie beams or exposed soil nail heads are to be provided to a retaining wall overlooking a footpath, the lowest row of beams or soil nail heads should be placed at an elevation high enough to avoid pedestrians being affected.
(d) Slope appearance – GEO Publication No. 1/2000 : Technical Guidelines on Landscape Treatment and Bio-engineering for Man-made Slopes and Retaining Walls (GEO, 2000a) gives guidelines on landscape treatment to man-made features. If no skin wall is constructed over an existing
wall face, care should be exercised to avoid staining of the surface of the existing wall due to grouting of soil nails.
5.4.4 Corrosion Protection
Guidance on the design of corrosion protection measures for steel reinforcement of prescriptive soil nails is given in Section 5.2.4.
5.5 PRESCRIPTIVE SKIN WALLS FOR MASONRY RETAINING WALLS