Hybrid Drain Geosynthetics for fine- grain soil improvements Chandan Ghosh Prof. & Head [Geohazards] National Inst. of Disaster Management Ministry of Home Affairs, Govt. of India Kanto loam slurry Drain layer

Large strain slurry consolidation Do slurry behave similar to normal soil? Do PWP in slurry different from normal Terzaghi soil? Can internal PWP be measured? Do k, Cv, Cc, mv are constant with consolidation pressure? Permeability –direct and from Consolidation theory differ much?

3 Typical application/troubles!! 様々な工法や問題 Important!!! Drain flow capacity

Problem domain Local/problematic soils Load types Drain/reinforcement Time dep. Sett.? Clogging…flow capacity changes under confinement

Geosynthetics as drain Drain – clogging and efficiency in flow under pressure In-plane and x-plane flow be measured? Index properties of geosynthetics International codal practice Evaluation of clogging – from slurry stage Drain efficiency – hybrid drain system

6 AIM of present research To recommend suitable drain system for field application To assess nature of clogging and flow capacity of drains confined in fine-grained soil

7 Research needs Flow capacity of synthetic drains placed in-situ Assessment of clogging and its prevention Recommending design flow capacity of drains based on available hydraulic index test data Use of synthetic materials for improving fine-grained soils

8 Geosynthetics used Geocomposite-A GC-A Nonwoven –A NW-A Nonwoven –B NW-B Nonwoven –C NW-C Nonwoven cover Woven fabric

9 Grain size distribution In-plane flow X-plane flow

10 Clogging mechanism Geosynthetic clogged by Kanto loam

Consolidation of soil – ideal vs natural

AIM

Leh cloud burst Slurry formation Mud slides Prevention of mud slides Measuring in-plane flow and drain efficiency using a large dia consolidation apparatus Measuring internal PWP during test Validation of consolidation theory

NEW BUS STAND SONAM NORBOO MEMORIAL HOSPITAL BSNL OFFICE HIGHLY POPULATED COMMERCIAL AREA

Cloud burst at Leh, 4-5 Aug 2010 Boundary Wall of DIHAR Broken by slurry

16 Apparatus developed Basis of developing experimental methods

17 Test procedure

18 Flow tests in drains placed within Kanto loam and silty clay during consolidation Soil slurry Drain (120x50mm) Consolidation pressure Drain pipe Kanto loam slurry Drain layer

19 Various in-plane flow tests carried at in-situ Test situation – double layer drain system Double layer drain and in-situ state Geocomposite-A GC-A Nonwoven –A NW-A Nonwoven –B NW-B Nonwoven –C NW-C 2.7mm thick 4mm thick 5.5mm thick 6.2mm thick

20 Various drains tested in-situ during consolidation of slurry Toyoura sand

21 Status of drains after test

22 Permeability factor – in-situ Permeability factor = ‘k’-no clog drain/’k’- soil 1.Permeability factor >10 5, which is satisfactory 2.With increasing pressure this factor increases, which is also a good indication

23 Flow tests in drains placed within Kanto loam and silty clay during consolidation Soil slurry Drain (120x50mm) Consolidation pressure Drain pipe Kanto loam slurry Drain layer

24 Average in-plane flow capacity 平均水平流動量 1.Flow in HYBRID drain is the highest 2.Without sand mat flow is the low, NWC drain is the lowest 3.Confining pressure causes 70 to 80% reduction in GC drain

Clogging of drains How to evaluate clogging? How to remove clogging of drains? – ultrasonic removal Hybrid drains – combination of geotextile and sand mat

26 Clogging of geocomposite Inner woven part Kanto loam Silty clay Nonwoven cover

Large dia – 1D consolidation Can PWP be measured directly? Slurry and normal soil states – at 50 kPa Variation of internal excess PWP – with height ? Do excess PWP attains 100% immediately after applied pressure? Direct and indirect permeability – are they same? k, p, mv, Cc, Cv – are these constant with p? Can we measure Drained water during consolidation?

Consolidation from slurry stage Kanto loam – slurry (is there a relation between LL and slurry water content?) Why p=50 kPa?- transition from slurry to Terzaghi soil Kanto loam slurry Drain layer

At p=400kPa (dia =150mm, H=57mm)

Kanto loam – local soil and silty clay - commercial MaterialKanto loam (local soil) Silty clay (commercial) Plasticity LL % PL % PI Natural water content (%)1013 Particle density,  s (Mg/m 3 )  d max (kN/m 3 ) Optimum w%8217 Initial void ratio of slurry Grain size –Clay, Silt, Sand10%, 27%, 63%43%, 55%, 2% Triaxial (UU),  uu, c uu 4 0, 19.6 kN/m (CU),  cu, c cu 25 0, 5.9 kN/m

Excess pore water pressure ratio (u/del p) Vertical strain of slurry sample

t50 at p=400kPa

Is there a unique transitional stage at p=50kPa?

mv, Cv, k – are not constant

With more “p”, u/p reduces

Excess PWP (p=20 to 400kPa)

PWP-3 PWP-2 PWP-1 H50 Consolidation pressure (p=50 kPa) Drain face u 150mm diameter

PWP inside specimen

PWP-3 PWP-2 PWP-1 H50 Consolidation pressure (p=50 kPa) Drain face u 150mm diameter

Excess PWP is more than net increase in “p”

Permeability - direct

e – p curve

55 New developments New multi-purpose test apparatus developed – Flow capacity at in-situ state & increasing confinement – Excess pore water measurement inside specimen – Direct measure of ‘k”, vertical strain, drained water

Conclusions Transition between slurry and normal Terzaghi soil state occurred at consolidation pressure of 50 kPa. Water content at this transition state found fairly close to the respective liquid limits of the Kanto loam and silty clay. Peak excess PWP represents the state at which soil undergoes maximum rate of compression. ‘e-log p’ curves for slurry state and normal soil state are different and it has two separate C c values.

Thank you