strengthening of marine deposit by deep cement mixing

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


Some Considerations

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.


DCM – Wet Mixing


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)


Lateral Squeezing of Soft Clay


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),


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


Improvement Ratio

Improvement Area Ratio:

R = A/(d1d2)


A

Design Strength of Treated Soil

Uq = Ucl / R

where Uq is the design unconfined compressive

strength of the soil after improvement by DCM.


Pattern of Installation


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


??

Cement Dosage & Strength


• Some published data (Suzuki):

Mix Design

Typical Range:

Cement Dosage = 150 – 350 kg/m3

w/c ratio = 0.4 – 0.8


Plant & Equipment – Wet Mixing

some examples

Verification

Verification

SPT

CPT

Mechanical Coring


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.


Specifications

Technical Specifications


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


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)

HKU Short Course (1997)

Hong Kong Marine Mud


Some Test Results


strengthening of marine deposit by deep cement mixing

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