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

5 Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing

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

12 Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing

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

14 Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing

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

17 Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing

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

18 Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing

??

Cement Dosage & Strength

19 Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing

• Some published data (Suzuki):

Mix Design

Typical Range:

Cement Dosage = 150 – 350 kg/m3

w/c ratio = 0.4 – 0.8

20 Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing

Plant & Equipment – Wet Mixing

some examples

22 Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing

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

29 Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing

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

34 Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing

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

35 Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing

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

39 Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing

Some Test Results

40 Dr K S Ho - Strengthening of Marine Deposit by Deep Cement Mixing