a first aid course in the design of lateral support and deep foundations

Pietermaritzburg, May 2006 Malcolm Jaros MOORE SPENCE JONES

A First-Aid Course in the Design ofLateral Support & Deep Foundations

z Lateral Support

z Deep Foundations

Procedures used to make a steep or vertical bank self supporting

Foundations that transmit structural loads down through unsuitable shallow soils to stronger underlying materials.


A First-Aid Course in the Design ofLateral Support & Deep FoundationsFirst-aid is what you use when you have to make

decisions with inadequate or limited resources.

The resources that may be lacking include:z Experience or Training Few of us have the opportunity to become as

familiar with design methods for Lateral Support and Deep Foundations as we are with design of Columns and Beams, or Roads and Services.

z Information Geotechnical Investigations seldom provide all of the information that one would like, or that could usefully be obtained, given sufficient time and money.

First-Aid training is not only about avoiding bad decisions and limiting damage. It should also indicate how best to benefit most from the specialist help available.


Important First-Aid Manuals

1.) The Franki Handbook: A guide to Practical Geotechnical Engineering in South Africa

2.) The SAICE Code of Practice on Lateral Support in Surface Excavations

These books were compiled by people who worked with, or were taught by, the late J.E.B. Jennings.

Professor Jennings had an enormous impact on the development of Geotechnical Engineering in South Africa. He was highly regarded by people - both contractors and engineers - who needed to make sound practical decisions, sometimes in life-threatening situations.


Important First-Aid Manuals1.) The Franki Handbook


Important First-Aid Manuals1.) The Franki Handbook

The Franki handbook, written initially by Ian Braatvedt in 1976, revised by John Everett in 1986 and extended and up-dated by a team of writers lead by Gavin Byrne and Ken Schwartz in 1995, is now available on CD at a nominal cost.

If it cost hundreds of Rands, which it does not, it would still be found on the desks of most Geotechnical Engineers in South Africa, and in several other countries, alongside Tony Brinks four volumes on the Engineering Geology of South Africa, and the SAICE Code of Practice on Lateral Support in Surface Excavations, soon to be joined with SANS 207.

The Franki Handbook covers a wide range of subjects and there will be differences of opinion on some of these subjects.

An issue on which many geotechnical engineers would not agree with the Handbook is the classification of cohesive soils in terms of SPT N values, as described in one of the following slides.


Important First-Aid Manuals2.) The SAICE Code of Practice


Limit State or Ultimate Strength Design

Limit State Design is the outcome of growing maturity in the understanding of design principles and the properties of materials. Only when we truly understand a process can we accurately separate its component parameters and quantify their relative importance in relation to overall performance.

Structural Engineers took a step towards this happy state in the 1970s (CP-110), another in the 1980s (BS-8110) and several more in the 1990s (SABS 0100 & 0162 and Eurocodes 2 & 3).

Geotechnical engineers have been working on parallel lines (Piling Panorama 1980 at the Wanderers Club, Johannesburg) that are now converging (Eurocode 7).


Limit State Design for Lateral Support

British Standard BS-8006 : 1995 contains guidelines and recommendations for the application of reinforcement techniques to fills or in-situ soils. The Standard is written in a limit state format and guidelines are provided of safety margins in terms of partial material factors and load factors for various applications and design lives.

SANS 207 : based on BS-8006 but modified to suit South African conditions, is currently circulating in draft form for comment


Lateral Support to Steep Earth Faces

When would you want:-

z A Dry-Stacked Masonry Wall ?

z A Reinforced Earth Fill ?

z A Soil Nailed Cut ?

z Ground Anchors ?


A site in dire need of lateral support

Unbelievably, this developer would not accept that he already had a dangerous site.

He wanted to know if he could furtherunder-cut the bank without causing it to collapse.


Do these Banks need Support ?

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When is Lateral Support Necessary ?

Both the banks in the preceding slide are slowly failing. Pieces, up to a tonne or more, have fallen out of the vertical bank and the 30 slope is experiencing gulley erosion.

The banks over-look a public sports ground, but are unlikely to receive any attention because no persons, or structures, are immediately threatened.


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One tonne block fallen from face


Slope FailureSlope Failure can be prevented by tying back the Active Zone through the Failure Surface to the Passive Zone

The bank can be stabilized with ties, nails or anchors embedded into the Passive Zone, behind the Failure Surface.

Passive

Active


Base Failure of Cuts, or Fills

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Whether in cut or fill, if the height H multiplied by the bulk density approaches the ultimate bearing capacity Qult below the bank, large settlements will occur. Adding tie-back forces will not prevent base failure.


The diagram opposite is from

Theoretical Soil Mechanics

by Karl Terzaghi (1943)

The principles hammered out by the intellectual giants who developed Geotechnical Engineering have been condensed into user friendlycomputer programs that produce seductive charts and diagrams.Should the colourful print-outs help us sleep peacefully at night? Only if we truly understand the significance of the data that we put in.


Down-load from www.damonclark.co.za


Computer Aided DesignDamon Clark has provided sterling service to the precast concrete

industry, and his program for designing dry-stack masonry walls (see previous slide) is used in various forms by many people.

The program comes with an excellent manual that describes the theory and methodology in detail, but like most manuals it is seldom read.

It seems that the least competent people are the first ones to run out of patience and start pressing buttons to print out some kind of answer.

It is not unusual for geotechnical consultants like myself, or Damon, who pay good money for our Professional Indemnity insurance, to receive calls from people who have used the program but want someone to explain to them the significance of what they have done.

Sometimes the caller has no site investigation data of any kind, and has simply used the program default values for the soil strength parameters. Such people are, quite literally, a danger to the community.


Dry-Stacked Masonry Wall

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The blocks are stacked progressively on a concrete foundation as the backfill is placed and compacted in layers


Dry-Stacked Masonry WallS

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Common failure modes are:-

z Buckling of the strut

z Bearing capacity failure of the foundation supporting the blocks.

The blocks protect the bank against erosion, but they act principally as a compression strut.


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If the wall needs to be more than 3m high, the simplest solution is to create a sequence of terraces, each one no more than 3m high.

This flattens the slope as a whole, improving overall stability which must be separately checked.


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The blocks can also be used as the non-structural facing to a stabilized earthmass gravity wall.


Reinforced Earth

is built from the bottom up:


A Soil Nailed Face

is built from the top down:


Soil Nailed Face

is built from the top down:


Ground Anchors

Is a Ground Anchor a Soil Nail ?


Ground Anchors

A Ground Anchor is a SoilNail with a stressing head

and ?


Ground Anchors

A Ground Anchor is a Soil Nail with a stressing head and a de-bonded free length

The anchor is de-bonded through the Active Zone, so that a pull-out test measures the

anchor force in the Passive Zone


Ground AnchorsGround Anchors are post-tensioned and are made of pre-stressing strand instead of reinforcing rods, or bars, because:

z Strand has a higher strength / weight ratio than normal reinforcing bars and can be made up to any length without couplings

z Strand is a familiar product in the pre-stressed concrete industry.

However, the highly stressed slender strands do present corrosion protection problems.


Corrosion of Steel Buried in FillBS 8006, the British Code of Practice for Reinforced Soils,

recommends (Table 7) that the thickness of steel buried in selected (non-corrosive) fill material be increased by a sacrificial thickness on each exposed face.

For Land-based structures (out of water) some of the recommended thicknesses are:-

0Stainless steel0Galvanized steel

0.05Stainless steel0.3Galvanized steel1.15Black steel50 years

0.35

Thickness (mm)Black steel10 yearsMaterialDesign Life


Corrosion of Steel Buried in FillSANS 207 (now in draft) has a similar table for Black

Steel and Galvanized Steel, in which the thickness values are up to twice those shown above. The table in SANS 207 also has a footnote:

Black steel should not be used as a reinforcement material for a design service life greater than 30 years, unless based on a specific study.

This means that steel buried in the ground should be considered structurally ineffective after 30 years, unless it was galvanized or stainless, or otherwise protected.


Corrosion Protection of Ground Anchors

From SAICE Code of Practice, 1989


Anchors or Nails ?

z The corrosion protection measures required for stressed strand ground anchors have become relatively expensive, which has promoted the use of less expensive soil nails.

z The downside of soil nails is that their load capacity in the passive zone cannot be determined by ordinary pull-out tests. Furthermore, soil nails are not pre-tensioned and the lateral deflections of nail-supported walls are greater than walls supported by tensioned anchors from 5mm per meter height in stiff clays and dense sands, to twice this value or more in less-stiff clays and loose sands.


Deep FoundationsDeep foundations transmit loads down

through soft or compressible strata onto harder materials that can sustain high bearing pressures with reduced settlement

Deep foundations are usually more expensive than shallow foundations, so the first step is to decide whether deep foundations are needed.


Why not Shallow Foundations ?Shallow foundations may be unsuitable if

they bear on a Problem Soil.

To understand Problem Soils read the classic State of the Art Papers published in The Civil Engineer in South Africa of July 1985 and presented at the SAICE Symposium on Problem Soils in South Africa in September of the same year.


Problem SoilsCollapsible Soils:

Apparently strong granular soils into which water easily penetrates can settle rapidly around leaking services or after rain.Fill embankments, even well-compacted, often experience collapse settlement. Some natural soils are particularly susceptible.

Heaving Soils:Soils that swell, or rise, as moisture builds up and then settle, or shrink, in drier periods. Damage is recurring, or cyclical.

Cavernous Soils:Soils that include voids, or very weak zones, between much harder materials. Dramatic collapse can be caused by minor disturbance, especially groundwater movment.


Collapse of Grain StructureA problem on cut-to-fill sites without adequate site preparation is that the underlying sand settles under the weight of the fill.

Deep Foundations

Sand with co

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Fill


Heaving Clays

Piles support the structure when the ground shrinks, but heave may rupture water pipes or crack surface beds supported by the ground

Heaving ground lifts (some of) the piles, unless they are anchored at sufficient depth below the active zone and adequately reinforced in tension


Weak GroundEven if not a Problem Soil the shallow ground may be

too soft or compressible to carry the foundation without excessive settlement.

In general, a foundation is suspect if there is a stratum below the foundation with an Unconfined Compressive Strength (UCS) that is less than the imposed vertical stress, including surcharge.

This rough approximation follows from TerzaghisBearing Capacity Equation for cohesive soils: -Allowable Bearing Pressure, Qall (Cu x Nc) / (F of S)

where Cu = UCS / 2, Nc 6, F of S 3


The Allowable Bearing PressureFor simple structures, not unduly sensitive to settlement, a

crude assessment can be made of the allowable bearing pressure from the perceived strength, the consistency, of the soil.

If not on a Problem Soil, shallow foundations are unlikely to settle more than 1% of the foundation width if the imposed vertical stress (including surcharge), in kiloNewtons per square metre, is less than ten times the SPT N value of the weakest underlying stratum.

For cohesive soils, this is an extension of the rough guideof the previous slide, assuming that Cu 5 x SPT(N), from Stroud (1974) see Franki Handbook, Figure 3.3.8.

For sandy soils the relationship still holds, provided that there is no risk of collapse settlement.


How Strong is the Soil

The Allowable Bearing Pressure of the previous slide is expressed in terms of SPT N values.

Often we do not have such values, and must rely on qualitative strength assessments of the soil profile.

This requires good profiling skills and reliable correlation factors.

It is therefore most important that the descriptive terms used in profiling have a standard meaning.


Field Assessment of Shear Strength

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The N values In Table 3.3.4 in the Franki Handbook, here shown shaded, are for very sensitive or high plasticity clays.For most clays, use the values in the right-hand column.


Working Strength and Limit Strength

Currently, structural engineers design buildings using Limit State Design codes and pass characteristic loads to geotechnical engineers, who happily design their foundations using Working .

This process has led to some unfortunate accidents, and much inefficiency of design.


These three icons of Durbans Golden Mile were fast tracked in the 1980s.

One was tracked so fast that only when the piling contractor was on site was it discovered that the pile design loads had been badly underestimated.

Piling costs caused a serious dent in the budget and the structural engineer suffered a significant career setback.


Communicate with the Structural Engineer

Make sure that you understand how the loads were derived.

For silos and tanks, check the range of bulk densities that can apply to the product. What happens if the product gets wet? Is there any chance that the silo may be used for a different product in the future?

For buildings, get copies of representative layout drawings and check the loads on at least some of the piles.


Settlement Criteria - be realisticSome of these silos move up and down by as much as 50mm in a

full/empty cycle. This is a nuisance, but not a serious problem.


Keep Settlement Criteria RealisticThe specified movement tolerances for the foundations

of the main turbines at Lethabo Power Station near Vereeniging were less than one millimetre.

The subsequent heave of the entire site, by hundreds of millimetres, provided the theme for Professor Geoff Blights Rankine lecture at Imperial College in 1997.

As far as I know, after the initial panic and some remedial work, the turbines are working well.

The unthinking application of mechanical tolerances to the support structure made a mockery of the specification.


Pile Design Methods are EmpiricalIn the 1970s and 1980s pile load tests to failure

(or near failure) were frequent, often forming part of a pre-construction investigation.

Such tests were done for sound commercial reasons as new, more economical, pile construction techniques were being fine-tunedto suit local conditions.

As the industry matures such tests are seldom done, and some design methods are being used without verification in conditions where they may be dangerously inaccurate.


Durban Pile Symposium, 1984Frankipile, as reported by John Everett, Clive Wilson

and Professor Ken Knight (Symposium on Piling Along the Natal Coast, 1984), conducted four load tests to failure at a site in Prince Edward Street in Durban in 1983.

The results of these tests encouraged a new method of forming the pile shaft for the iconic Driven Cast In-Situ Franki pile. The faster slump castmethod has in recent years largely supplanted the original hammer compacted shaft.

The tests also indicated that Continuous Flight Auger CFA piles formed with pumped concrete had lower shaft friction capacity than similar piles with pressure-grouted shafts.


Durban Pile Symposium, 1984Esor, as reported by Graham Plant and Malcolm Jaros

(1984), loaded a pressure grouted CFA pile constructed in sand on the Marine Parade, to an equivalent axial shaft stress of 23 MPa. The average shaft friction mobilised when the deflection at the head of the pile reached 7,6mm was estimated using the method of Chin & Vail (1983) to be more than 140 kPa.

The results of this test confirmed previous reports (Roscoe, G.H 1983) that, for pressure grouted shafts in sand, the value of the design parameter Ks.Tan could exceed unity to a depth of at least twenty times the pile diameter. Subsequent events have shown that this very high level of performance can only be attained in ideal conditions using an auger rig that has sufficient torque to reach the founding depth without pulling soil out of the ground, and pressure-injecting the grout.


Pile Design in South Africa Text-book methods for assessing pile capacity are usually

conservative. In South Africa the frequency of failure of pilesunder load has been comfortingly low, and most failures have been found to be the result of construction defects.

However, while construction defects usually only occur in a few piles, a design defect may affect all of the piles installed on a site.

In sandy soils, most design failures have been caused by the use of Total Stress design methods which under-estimate, or neglect, the effects of groundwater.

In cohesive soils, the use of dynamic cone penetrometers has been the principle cause of design failures.


Total Stress DesignPrediction of the bearing capacity of piles based exclusively on

N values of the SPT was a paper presented by LucianoDcourt at the 2nd ESOPT conference in Amsterdam in 1982.

It gave a simple method for calculating the shaft friction capacity of most pile types from SPT test values alone, a design method so simple to apply that it was widely adopted, until failures started to occur.

Nowhere does the paper mention a water table, but it recommends that end bearing capacity be ignored completely unless several centimetres of settlement can be tolerated.

In other words, it depends on two compensating errors to arrive at, hopefully, a reasonably safe design that wont result in too much settlement.

Such methods are useful for cost estimating purposes but do not constitute an engineering design. They are useless for the purpose of Limit State Design.


The Behaviour of Continuous Flight Auger Piles in the Quaternary

Sediments Underlying Durban, (1994) by Francis T.E. &

Aucamp C.A.

The valuable historical data presented in this paper has been misinterpreted as a design method, which it clearly is not. The paper pointedly concludes that No correlation was noted between pile length, diameter or settlement. It further suggests that the results are so intractable to analysis that a Total Stress rule of thumb is probably as good as any classical design method. These conclusions are at odds with the findings of experienced pile designers.


The Walnut Grove pilesFrancis & Aucamp (1994) has a particular difficulty with the

Walnut Grove pile load tests to failure, and suggests that construction techniques for once-off special piles differ(ing) from those applied to normal piling contracts.

The harbour beds are succinctly described as an approximately 25m thick succession of fine and medium grained silty sand, interbedded with clayey sand and clay layers. Particular attention is given to the depositional environment that produced a highly complex stratigraphy.

The paper correctly infers that, because the CFA piles derive most of their load capacity from shaft friction, they are less sensitive to stratigraphic variations than end-bearing piles. It takes this inference too far, however, and ignores local variations in stratigraphy that have a critical influence on many of the test results presented.


The Walnut Grove pilesThe Walnut Grove piles were designed using a detailed

stratigraphic model constructed from borehole and Dutch Probe tests done at the pile test site. The ultimate capacitieswere deliberately limited to be within the capacity of the test equipment, so that the piles would fail under maximum load.

The purpose was to verify design parameters for the many hundreds of piles to be installed for the International Convention Centre.

A reliable design strategy was developed from the Walnut Grove tests that reduced the sensitivity of Pressure Grouted CFA piles to minor variations in stratigraphy. The strategy provided an advantage for these piles over pile types that derive their load capacity predominantly in end-bearing.

This advantage has since proved to be most successful in the interbedded clays and sands of the Harbour Beds, where Total Stress design methods have caused expensive failures.


The Walnut Grove pilesThree piles constructed in very poor ground were loaded to failure.

The working load on each pile may be defined as the load at which the deflection was 4mm.

2,02,332,14F of S

20001750750Max. Load kN

1000750350Load kN at 4mm

161412Length m

600400300Diam. mm

Load decreasing in 16m deep pile as clay below toe yields


The Walnut Grove pilesAll three of the test piles were within 7m of borehole BH8, depicted on the left hand side of the geological section. The water table was about 3,5m below ground level

The SPT N values show that the shaft of the 12m deep pile was in loose sand which correlates with the moderate performance of this pile (F of S = 2,14).

The bottom 2m of the 14m deep pile was in medium dense sand, with dense sand below the toe, which is consistent with the improved performance of this pile (F of S =2,33).

The soft clay immediately beneath the toe of the 16m deep pile accounts for the decrease in load capacity after 13mm deflection.

If constructed near BH3, instead of BH8, these piles would all have carried higher loads.

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CLAY


The Walnut Grove piles

The design strategy, reduced to a spreadsheet for the ICC contract, has been validated by load tests in the Harbour Beds at Cato Creek (2004), Bayhead (2006) and in the Berea Sands of Umhlanga Ridge (2001)


The Walnut Grove piles


A Warning !In the previous slides, effective stress

design principles for the sandy horizons were mixed with total stress analysis for the clay layers. Experience has shown that the results of this hybrid analysis are reasonably accurate for the interlayeredstrata of the Harbour Beds. Much greater errors can be introduced by the use of dynamic cone penetrometer (DCP, DPL or DPSH) tests in plastic clay soils.


Dynamic Penetrometers in Plastic ClayThe use of Dynamic Penetrometer Test results for pile

design in plastic soils has caused some expensive design failures.

This is because cohesion of the plastic soil on the driving rods can greatly increase resistance to penetration, even when the cone tip is weak material.

On a site in Fordsburg, Johannesburg, several DCIS piles driven into residual Diabase failed to meet the specified load/settlement criteria and hundreds of piles already installed had to be down-rated.

On a site in Roodepoort a large building started settling while under construction. The CFA pile foundations installed in residual Greenstones had to be augmented before the building could be completed.


This dynamic cone penetration test, recently conducted in Prospectonindicated that the clays below 4,5m metres depth were very stiff.

However, an adjacent borehole showed conclusively that all of the material below 4m depth was a soft or very soft plastic clay.

Dynamic Penetrometers in Plastic Clay


The Problem of Rod Friction

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Increase in blow-count with depth was due solely to the squeeze of the ground on the rods, not point resistance.


The Problem of Rod Friction

Piles installed are too short

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Re- test in basement excavation shows no increase in strength of

ground.

a first aid course in the design of lateral support and deep foundations

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