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Strengthening of marine deposit by the state-of-the-art Deep Cement Mixing (DCM) method for construction of seawall foundations, airport runways, containment of contaminated mud. One of the best environmental friendly marine construction methods to minimize pollution and cross contamination. A feasibility and conceptual design presented.
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Dr K S Ho
October 2012
Introduction
The Issue
Proposed Ground Improvement (DCM)
Design Approach
Plant & Equipment
Verification
Q & A
2 Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing
CWB Dwall Portion 2
Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing 4
The Project
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Some Considerations
6 Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing
Deep Cement Mixing
(DCM)
Deep Cement Mixing DCM is based on chemical reactions between clay and chemical agents,
which are usually quick lime or Ordinary Portland Cement (OPC). This is done in the field by machines with rotating blades for supplying the chemical agent into the soil and for insitu mechanical mixing of the soil with the agent. The chemical agent absorbs the pore water and reacts with clay particles to form pozzolanic reaction products.
The objective of DCM is to improve the soil shear strength and reduce the deformation characteristics by mixing the soil with chemical additives that react with the soil.
In the Wet Mixing Method, the medium of transportation of the chemical agent is water.
Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing 8
DCM – Wet Mixing
Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing 9
Penetration
Inject Cement Slurry
Soil Mixing
The execution of wet mixing is usually carried out following the procedures: 1. Correctly position the mixing tool 2. Penetrate the mixing shaft to the designed depth of treatment with simultaneous
disaggregation of the soil by the mixing blades and injection of the cement slurry 3. Withdraw the mixing shaft after reaching the designed depth of treatment. In some cases
cement slurry is injected again and mixed with the soil during lifting of the mixing shaft
Design Approach
To Design…………..
Required strength of the treated soil
Pattern of installation (e.g. block, wall, grid, column, etc)
Cement Dosage (Mix Design)
Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing 11
Lateral Squeezing of Soft Clay
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Transportation Research Record 1192 Required minimum undrained shear strength (Cmin ) to avoid lateral squeezing of the soft clay: Cmin ≥ Pa/(BL2) Cmin ≥ one-third of applied stress FOS ≥ 1.3
soft clay
Required Soil Improvement Example:
Cmin = g’hfLB(hmd/2)/BL2
Assume:
g’ = submerged unit weight of fill = 10 kN/m3 hf = thickness of fill = 10 m L = 15 m B = length of embankment = 1 m hmd = thickness of MD = 10 m Cmin = 33.3,………….. say 35 kPa
Apply factor of safety of 2 (instead of 1.3),
13 Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing
Cu of soft clay = 70 kPa
Laboratory & In-Situ Strength Suzuki (1982): the unconfined compressive strength of laboratory improved soil
(Ucl) is represented by the following formula, Ucl = a’ b’ g' l' sc where: a'= configuration factor = 1/0.9 = 1.11 b'= lap factor = 1/0.8 = 1.25 g' = deviation factor = 1/0.7 = 1.43 l'= laboratory strength/site strength ratio = 1 sc= required in-situ compressive strength = 2Cu of soft clay Therefore, Ucl = 1.11 * 1.25 * 1.43 * 1 * 2 * Cu = 4Cu
Example: Ucl = 280 kPa
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Improvement Ratio
Improvement Area Ratio:
R = A/(d1d2)
Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing 15
A
Design Strength of Treated Soil
Uq = Ucl / R
where Uq is the design unconfined compressive
strength of the soil after improvement by DCM.
Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing 16
Pattern of Installation
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Note overlapping / interlocking of DCM columns
Approx improvement area ratio, R, of this example is about 0.6 to 0.7
Dw
all
Laboratory Soil-Cement Mix Trials
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??
Cement Dosage & Strength
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• Some published data (Suzuki):
Mix Design
Typical Range:
Cement Dosage = 150 – 350 kg/m3
w/c ratio = 0.4 – 0.8
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Plant & Equipment – Wet Mixing
some examples
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Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing 23
Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing 24
Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing 25
Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing 26
Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing 27
Verification
Verification
SPT
CPT
Mechanical Coring
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References Larsson, S. (2005) State of Practice Report – Execution, monitoring and
quality control. Deep Mixing 2005. (SGI)
CDIT (2002) The Deep Mixing Method – Principle, Design and Construction, 123p. (Japan)
Bruce, D.A. (2000) An Introduction to the Deep Soil Mixing Methods as Used in Geotechnical Applications. FHWA Report No. RD-99-138. (US)
Tse, K.H. (2001) Study on Foundation Improvement Methods for Marine Structures. GEO TN6/2001. (HK)
BS EN 14679:2005 (2005) Execution of Special Geotechnical Works – Deep Mixing (UK)
Ho, K.S., Kitazume, M., Terashi, M. & Tsuboi, H. (1997) Development of Deep Mixing Method and Current Practices. CPD Short Course, HKU.
Suzuki, Y. (1982) Deep Chemical Mixing Method using Cement as Hardening Agent. Symposium on Recent Developments in Ground Improvement Techniques.
30 Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing
Q & A
Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing 32
Specifications
Technical Specifications
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Materials Water-cement ratio: 0.4 to 0.8
Cement dosage: 100 to 350 kg /m3
Drilling Soil-cement column: 600 to 800 mm diameter
Mixing auger at the end of continuous flight
Hollow stem auger
Rotation speed: 25 to 50 rpm
Penetration depth: min 20 m
Penetration speed: 0.5 to 1.5 m/min
Retrieval speed: 1 to 3 m/min
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Injection Rate 50 to 100 litre per min
The rate must be limited to avoid increasing the in-situ volume of the soil-cement columns
Injection pressure: 2 to 5 bars
Monitoring Location and depth of soil-cement column
Penetration and retrieval speed
Rotation speed
Injection pressure
Feed rate of cement slurry
Regular testing of soil-cement samples (strength, flow, uniformity, etc)
Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing 36
Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing 37
HKU Short Course (1997)
Hong Kong Marine Mud
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Some Test Results
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