Geotechnical Engineering Office
GEO Circular No. 5
Control of Compaction of Fill Slopes
Issue No.
1
Revision
B
Date
16.8.00
Page
1 of 12
1.
PURPOSE
1.1
This Circular details the policy and procedures for ensuring adequate compaction of soil
fill slopes. It supersedes GEO Circular No. 5A which is hereby cancelled. Any
feedback on this Circular should be referred to CGE/I.
2.
POLICY
2.1
Loose soil embankments are liable to liquefaction on collapse. Continuing geotechnical
control is required to ensure adequate compaction of fill slopes.
2.2
Italicised text is provided for guidance only. All other requirements are mandatory.
3.
RELATED DOCUMENTS
3.1
Buildings Ordinance (Chapter 123)
3.2
General Specification for Civil Engineering Works (1992)
3.3
GEO District Handbook. Practice Note No. 112 - Buildings Ordinance Section 23 and
24A Orders
3.4
GEO District Handbook. Practice Note No. 113 - Requirements for Qualified
Geotechnical Supervision of Private Works
3.5
Geotechnical Manual for Slopes (1984)
3.6
LWBTC No. 3/88 - Geotechnical Control Office Checking of Geotechnical Designs for
Government Works
3.7
PNAP 55 - Site Formation: Temporary or Permanent Filling Work
3.8
PWDTC No. 13/78 - Compaction of New Fill Slopes
3.9
Project Administration Handbook for Civil Engineering Works, Volume III
3.10
GEO LPM Branch Works Division Manual
3.11
GEO Technical Guidance Note (TGN) on Good Practice in Relation to the Investigation
of Old Fill Slopes and Use of the Recompaction Method for Loose Fill Slopes
4.
DEFINITIONS
4.1
A fill slope in the context of this Circular is one with height 5 m or more, or one that is
less than 5 m high but which poses a direct risk to life.
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Geotechnical Engineering Office
GEO Circular No. 5
Control of Compaction of Fill Slopes
Issue No.
1
Revision
B
Date
16.8.00
Page
2 of 12
5.
AUTHORITY AND RESPONSIBILITY
5.1
The APs of private developments and the Project Offices of public works and Housing
Authority projects are responsible for the supervision and site control of compaction, and
are required to submit monthly reports to BD and GEO respectively. For LPM works, the
CGEs managing the LPM works and the respective LPM Consultants are responsible for
the supervision and site control of compaction of in-house managed LPM contracts and
consultant managed LPM contracts respectively.
5.2
District Divisions would carry out independent site visits and field checks to determine
whether the compaction works are carried out in accordance with the approved design
and specifications.
5.3
Authority for carrying out field checks and obtaining assistance from the AP or Project
Office is given in Buildings Ordinance Section 22 and LWBTC No. 3/88 respectively.
6.
PROCEDURE
6.1
SPECIFICATION
6.1.1
For private projects, specifications for the compaction of fill slopes shall be clearly
written on plans (drawings) to be approved. The specifications shall comply with the
minimum requirements as stipulated in Appendix A to PNAP 55 Site Formation:
Temporary or Permanent Filling Work. Qualified supervision by a full-time Category
III site supervisory personnel shall be imposed under Buildings Ordinance Section
17(1)6(e) for the compaction of fill slopes.
6.1.2
For LPM works, public works and Housing Authority projects, specifications are
covered by the General Specification for Civil Engineering Works and, where necessary,
the Particular Specification. When commenting on the submission, the district CGE
shall draw the attention of the respective CGE managing the LPM works, the LPM
Consultant or the Project Office to the need for site control and requirements for
supervision of compaction.
6.2
RECORD OF WORK
6.2.1
The requirements to be imposed for a private project under Buildings Ordinance Section
17(1)6(b) & (d) at the time of granting approval and consent to building works with fill
slopes are as follows:
The AP is required to submit monthly reports to BD during
construction, giving records of tests on compaction of fill slopes
together with a covering assessment on the adequacy of
compaction. The reports are to be prepared by the Geotechnical
Consultant and signed by the AP and the Geotechnical Consultant.
The content of the covering assessment shall include:
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Geotechnical Engineering Office
GEO Circular No. 5
Control of Compaction of Fill Slopes
(i)
Issue No.
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confirmation that the ground preparation and underground
drainage works have been properly carried out,
(ii) a summary of the results of insitu density tests and laboratory
compaction tests of the fill, highlighting areas of
non-compliance with the specified compaction standards,
and
(iii) details of any corrective measures that have been taken to
rectify areas of inadequate compaction including the
re-testing results for areas of non-compliance.
The district GE shall request BD to refer the assessment to GEO for comments. The AP
or the Geotechnical Consultant shall keep records of compaction tests on site for
inspection by BD staff and GEO district GEs in accordance with PNAP 55.
6.2.2
For public works and Housing Authority projects, in the GEO reply memo commenting
on the site formation submissions, the district GE shall request the Project Office to
submit to GEO monthly records of tests on compaction of fill slopes during the slope
construction period, together with a covering assessment of the adequacy of compaction.
The content of the covering assessment shall include the items stated in para. 6.2.1 above.
The district GE shall check that such records/assessments are submitted by the Project
Office at the time of construction of the fill slopes in question. The district GE shall also
request the Project Office to keep records of compaction tests on site for GEO inspection.
6.2.3
Where LPM works designs have been submitted to the district CGE for comment, the
respective CGEs managing the LPM works or the LPM Consultants shall submit to the
district CGE monthly records of tests on compaction of fill slopes during the slope
construction period, together with a covering assessment of the adequacy of compaction.
The content of the covering assessment shall include the items stated in para. 6.2.1 above.
Such records/assessments shall be submitted by the CGEs or the LPM Consultants at the
time of construction of the fill slopes in question. The CGEs or the LPM Consultants
shall also keep records of compaction tests on site or at the contract depot for the
inspection by the district GE. Where LPM works designs have not been submitted to the
district CGE for comment, the district GE may request the above-mentioned documents
for information and/or for record purpose.
6.2.4
In para. 6.2.1, 6.2.2 and 6.2.3 above, if considered necessary, the district GE should
include a condition in the replies to the BD, the Project Office, the respective CGE
managing the LPM works or the LPM Consultant requiring that, prior to the completion
of the compaction works, GCO probes and trial pits with insitu testing be carried out in
order to verify the standard of compaction.
6.2.5
If compaction records/assessments are not submitted for private projects, the district GE
shall request the BD to apply sanctions such as the threat of withholding consent for the
next phase of work or the Occupation Permit. For public works and Housing Authority
projects, if written requests and reminders are not effective, the district GE shall refer the
matter to the district CGE for action at Directorate level in the department concerned.
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GEO Circular No. 5
Control of Compaction of Fill Slopes
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For LPM works, the district CGE shall refer the case to the respective CGEs managing
the LPM works or the LPM Consultants for follow-up actions. The CGEs or the LPM
Consultants shall inform the district CGE of the actions to be taken to rectify the
non-compliance.
6.3
FIELD CHECK
6.3.1
District Divisions may carry out field checks on the compaction of any fill slopes in
private projects and LPM works.
6.3.2
For private projects, the district GE shall request the BD to include in the
approval/consent letter to AP a requirement to the effect that prior notice be given to
GEO of the date of commencement of fill slope construction. For public works and
Housing Authority projects, the district GE shall request the Project Office to advise
GEO in advance of the date of commencement of fill slope construction. For LPM
works, the respective CGEs managing the LPM works or the LPM Consultants shall
advise the district CGE in advance of the date of commencement of fill slope
construction. This will allow the district GE to carry out site inspections and field checks
as appropriate.
6.3.3
As a general guide, the district GE should make at least one site visit during formation of
each slope. The number of site visits should be dependent on site constraints and the
scale of the project. During site visits, the district GE shall check that the following are
present:
(i)
Qualified supervision as required by BD/GEO.
(ii)
Adequate compaction plant at the time when compaction work is in progress.
(iii)
Records kept on site by the AP or the Geotechnical Consultant for private
projects (as required by PNAP 55) or by the Engineers Representative for LPM
works, public works and Housing Authority projects. These include:
(a)
compaction test records showing that the fill has been
compacted to acceptable standards,
(b)
records of appropriate compaction and testing equipment on site,
and
(c)
records showing correct compaction procedures are followed
with adequate qualified supervision.
The district GE shall make a note in the divisional file or in the relevant record sheet to
record the site visit, and the compaction plant and test records seen. A copy of the record
should be sent to the respective CGE managing the LPM works and the LPM Consultant.
6.3.4
If considered necessary, the district GE shall arrange for field checks to be carried out,
for example, initially using GCO probes (guidance on Correlation between GCO probe
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Geotechnical Engineering Office
GEO Circular No. 5
Control of Compaction of Fill Slopes
Issue No.
1
Revision
B
Date
16.8.00
Page
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value and fill density annexed), and supplemented by observations of fill material in
trial pits if any appreciable cobble and boulder content or other unsuitable materials in
the fill are suspected. Depth correction factor must be applied as per the guidance notes
to the General Specification for Civil Engineering Works to obtain the blow counts
corrected against depth effects. The district CGE shall request the AP, the respective
CGE managing the LPM works, the LPM Consultant or the Project Office to arrange for
the provision of skilled personnel and labourers to carry out the probing operation and
trial pit excavation. If loose fill is detected or suspected, the district CGE shall request
the AP, the CGE, the LPM Consultant or the Project Office to arrange for a laboratory,
which is accredited under the Hong Kong Laboratory Accreditation Scheme, to carry out
insitu density tests of the loose layer in the presence of the district GE and BD staff as
appropriate. For private projects where extensive loose fill is found which would pose a
significant threat if not rectified, the district GE shall recommend a Cease Works Order
under Buildings Ordinance Section 23 and 24A. If major difficulties are encountered in
securing co-operation from the AP or Project Office in carrying out field tests, e.g. in
gaining access, provision of labourers, etc., then the district GE shall bring the case to the
attention of district CGE who will seek resolution at a higher level.
6.3.5
For LPM works, when the district GE is dissatisfied with any items stated in para. 6.3.3
and 6.3.4 above, he shall notify the Engineers Representative immediately on site and
the district CGE shall refer the case to the CGE managing the LPM works and the LPM
Consultant for follow-up actions. The CGE or the LPM Consultant shall inform the
district CGE of the actions to be taken to rectify the non-compliance.
7.
ANNEX
7.1
GC5 A1
Correlation between GCO Probe Value and Fill Density
(R K S Chan)
Head, Geotechnical Engineering Office
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Geotechnical Engineering Office
GEO Circular No. 5
Control of Compaction of Fill Slopes
Issue No.
1
Revision
B
Date
16.8.00
Page
6 of 12
1.
CORRELATION BETWEEN GCO PROBE VALUE AND FILL DENSITY
1.1
Phillipson (1979) recorded the findings of Binnie & Partners (B&P) that a Probe
value (blows per 100 mm penetration) of less than 5 indicated fill of low density (or
wet), and that a Probe value of over 20 indicated probably well compacted fill.
Such findings are based on compaction studies carried out by B&P in 11 sites of CDV
fill and 13 sites of CDG fill. The best fit lines of correlation between Relative
Compaction (i.e. the ratio of the field dry density to the maximum dry density) and
Probe value, together with B&Ps data, are plotted in Figures 1 and 2. The test
results are scattered. However, in all the 24 sites, the Relative Compaction
corresponding to a Probe Value of 5 varies from 60% to 90% with a mean value of
about 80 to 85%. Hence, B&Ps conclusion that fill with a Probe value of less than 5
is loose is reasonable.
1.2
Phillipson (1979) also reported the results of a study carried out by the GCO in a site
comprising CDG fill (Figure 3). The scatter of the data is less than that of B&P s,
possibly because of the relatively more uniform material and test condition since the
GCOs test was carried out at one site. The GCOs test results also confirm that fill
with a Probe value of less than 5 is loose.
1.3
Lai (1987) described the correlations between Relative Compaction and Probe value
deduced by Scott Wilson Kirkpatrick & Partners (SWKP). The SWKPs data
presented in Lai (1987) was compiled by Dr. H. Y. Wong. According to Dr. H. Y.
Wong, the material was CDG fill. It can be inferred from SWKPs correlations that
fill with a Probe value of less than 3 would be loose (the average Relative Compaction
less than 88%). The range of SWKPs lines of correlation is shown in Figure 4, in
which the best fit lines obtained by B&P and GCO are also plotted. Lai (1987)
reported that high degree of scattering on the available data were noted in their (i.e.
SWKPs) correlations. SWKPs correlations seem to have overestimated the
Relative Compaction of fill which has a low Probe value (e.g. the Relative
Compaction is estimated to be between 80% to 90% when the Probe value approaches
zero).
1.4
According to the results described above, one could reasonably expect that a fill is
likely to be loose if its Probe value is less than 5 and that it is loose if the Probe value
is less than 3. The reliability of such an assessment may be improved if:
(a)
larger numbers of Probe tests are carried out to minimise any
bias arising from the scatter of data, and
(b)
the material is thoroughly inspected to confirm the soil type and
to note any characteristics which may influence the Probe value
(in this respect, sinking a hand auger hole or digging a trial pit
by the side of the Probe hole would be useful).
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ANNEX GC5 A1 (1/6)
Geotechnical Engineering Office
GEO Circular No. 5
Control of Compaction of Fill Slopes
2.
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LIST OF RELATED DOCUMENTS
(a)
Lai, S. W. (1987). Correlation of Penetration Tests Results with Other Soil
Parameters. Final Year Report, University of Hong Kong.
(b)
Phillipson, H. B. (1979). GCO Probe, Draft Standard and Notes. GCO Report
15/79, 21p.
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ANNEX GC5 A1 (2/6)
Geotechnical Engineering Office
GEO Circular No. 5
Control of Compaction of Fill Slopes
Issue No.
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Revision
B
Date
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Page
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Figure 1 - Correlation between Relative Compaction and Probe Value
(B&Ps Test Data on CDV Fill)
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ANNEX GC5 A1 (3/6)
Geotechnical Engineering Office
GEO Circular No. 5
Control of Compaction of Fill Slopes
Issue No.
1
Revision
B
Date
16.8.00
Page
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Figure 2 - Correlation between Relative Compaction and Probe Value
(B&Ps Test Data on CDG Fill)
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ANNEX GC5 A1 (4/6)
Geotechnical Engineering Office
GEO Circular No. 5
Control of Compaction of Fill Slopes
Issue No.
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Revision
B
Date
16.8.00
Page
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Figure 3 - Correlation between Relative Compaction and Probe Value
(GCOs Test Data on CDG Fill)
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ANNEX GC5 A1 (5/6)
Geotechnical Engineering Office
GEO Circular No. 5
Control of Compaction of Fill Slopes
Issue No.
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Page
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Figure 4 - Correlation between Relative Compaction and Probe Value
(SWKPs Test Data on Fill)
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ANNEX GC5 A1 (6/6)
Geotechnical Engineering Office
GEO Circular No. 5
Control of Compaction of Fill Slopes
Issue No.
1
Revision
B
Date
16.8.00
Page
12 of 12
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ANNEX GC5 A1 (6/6)