PORT WORKS DESIGN MANUAL

CORRIGENDUM No. 1/2020

This corrigendum contains amendments to the Port Works Design Manual, 2002 Edition, and shall
be read in conjunction with Corrigenda 1/2006, 1/2014 and 1/2018.

PART 3 – Guide to Design of Reclamation

(a) Section 2.6.1 – Add the following paragraph after the 1st paragraph:
General In recent years, there is a tendency of not removing the sediment in
reclamations, mainly on environmental reasons. Apart from the drained
method, other types of soil improvement techniques can be considered.
These are described in the ensuing paragraphs.

(b) Section 2.6.7 – Replace Section 2.6.7 by the following sections:

Other Soil
Improvement 2.6.7 Deep Cement Mixing Method
Techniques
(1) Introduction of Deep Cement Mixing

The principle of deep cement mixing (DCM) is based on chemical reactions

between clay and chemical agents. Ordinary Portland Cement and Portland
Blast Furnace Cement, etc. are the most commonly used admixture
stabilizers. The purpose of mixing chemical agents with the soil is to improve
the stability, strength and stress-strain properties of the soil. The stabilization
mechanism generally involves the following chemical reaction processes :

 Cement reacts with the pore water of soft clay to form a series of
hydrates.
 Hydrates exchange ions with clay particles and form large
conglomerates.
 Clay particles react with the excess calcium ions from the
hydration process and are bonded together.

The design considerations and design approach of DCM method for the
foundation of seawalls and breakwaters are given in Section 4.5.2 and
Section 4.6.2 of Part 4 of the Manual respectively. For reclamation, designer
could consider, apart from conventional reclamation methods (including
drained method and dredged method), using DCM as one of the non-dredged
methods to form land by strengthening in-situ soft marine sediment.

(2) Advantages of Deep Cement Mixing

The advantages of DCM method as compared with conventional reclamation

methods are :

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  DCM method solidifies the soft marine sediment by mixing it with
cement slurry to form DCM columns in a shorter time frame to
support the seawall and filling materials above, thus reducing the
time required for reclamation. The reclaimed land could be
delivered for early development.
 DCM method is an environmental friendly method as it does not
require dredging of marine sediment and importing materials for
filling back the dredged area. DCM method can also significantly
reduce the consolidation settlement arising from conventional
drained method. Hence, the demand for importing fill materials
would be reduced and disposal of dredged marine sediment would
also be minimized.
 DCM is flexible in application because the amount of stabilizing
agent and form of treatment can be adjusted to suit different soil
properties and engineering requirements.

(3) Considerations of Deep Cement Mixing

The following considerations should be taken into account in the choice of

reclamation method :

 The cost of DCM method may be higher than that of a

conventional reclamation method.
 Stringent quality control and monitoring is required during the
mixing process to ensure that the required strength is developed
in the soil. It may be necessary to carry out field trials to obtain or
verify design parameters and construction method statement for
practical application, such as an optimal site-specific soil to
cement ratio, water cement ratio, blade rotation number, etc.
 The rotating blades of the DCM machine may not work properly
if obstructions of size larger than 250 mm are encountered during
the mixing process.
 Investigations should be carried out to assess the possible
environmental impacts associated with marine application of
DCM and to determine if mitigation measures are necessary for a
particular site. Site trial demonstration may be required. The
time implication should be considered.
 It does not work well in certain soils, notably those which have a
high organic content and acidic soils (Suzuki, 1982).
 It may not be applicable for very stiff or very dense ground or for
treatment depth more than 40 m for land construction (Bruce,
2000) and 70 m for marine construction (Kitazume and Terashi,
2013).
 The change of unit weight of deep mixed zone is not significant.
Therefore, no noticeable additional surcharge will be induced on
the underlying soil strata due to the DCM stratum.

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  The design of DCM works shall take into consideration the future
land use and control the residual settlement to the acceptable
level. The design consideration includes the geological
conditions, treatment depth, design strength of DCM columns,
spacing of DCM columns, filling depth and overburden pressure
etc.
 The layout of DCM works shall be designed to cater for any
horizontal stress arisen during construction and/or in the
permanent state.
 The future developers of the land should be alerted to the modified
ground conditions as the variability of sub-soil properties shall be
taken into account during their foundation design and
construction works.
 For pile foundation design, designer shall take into consideration
the properties of DCM-treated soil in determining piling design
layout and construction method, with a view to penetrating
through DCM-treated soil to a deeper stratum for assuring pile
capacity and reducing settlement of the superstructure.
 If DCM stops penetration in marine sediment without fixing on
hard stratum (i.e. floating type improvement), some amounts of
ground settlement will take place in the marine sediment beneath
the DCM.
 The operation of DCM may cause heave and horizontal
displacement of ground. The amount of ground deformation
depends on the improvement area ratio and construction
sequence.
 Spoils from DCM operation require treatment and/or disposal.

2.6.8 Other Soil Improvement Techniques

Soft deposits may also be strengthened in-situ by vibro-replacement (stone

columns) or by soil mixing techniques including lime columns or any other
soil improvement techniques. Other form of seawall construction methods or
combination of methods may also be considered (e.g. using cofferdam steel
cells sinking to hard stratum). Designers should consider the cost
implications and possible effects on piling and excavation in future land
development. These specialist techniques may require detailed assessment to
demonstrate their viability.

(c) REFERENCES Add the following references:

Bruce, D. A. (2000). An Introduction to the Deep Soil Mixing Methods as
Used in Geotechnical Applications, Publication No. FHWA-RD-99-138.
U.S. Department of Transportation, Federal Highway Administration,
Virginia.

Kitazume, M. and Terashi, M. (2013). The Deep Mixing Method, CRC Press,

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Taylor & Francis Group, 410p.

Suzuki, Y. (1982). Deep Chemical Mixing Method using Cement as

Hardening Agent. Proceedings of the International Symposium on Recent
Developments in Ground Improvement Techniques, Bangkok, pp 299-340.

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