Grouting of Dispersive Dam Foundations

Grouting of Dispersive Dam Foundations

W. F. Heinz

Rodio South Africa (Pty) Ltd

Email: rodio1@infodoor.co.za

P. I. Segatto

Rodio Geotechnics (Pty) Ltd

Abstract: Dispersive soils, their occurrence, identification and their judicious utilization for embankment dams and other applications have become part of the geotechnical toolbox of contractors and consulting engineers alike.

Southern Africa has its fair share of dispersive soils and much research has been done during recent decades to improve the identification process and to develop adequate preventive and remedial techniques. Early identification and the introduction of appropriate elements at design stage have gone a long way to solve or prevent potential problems with dispersive soils. South African geotechnical engineers and contractors are sensitized to potential problems associated with these soils.

Most research has focused on potential embankment problems resulting from the use of dispersive materials. Practically no publications are available in the field of grouting of foundations, which are dispersive.

The authors endeavour to present the most important aspects of dispersivity as they relate to foundation engineering with special reference to dam foundations. Little or no experience is available where dams have been founded on dispersive soils; the precautions and controls with respect to dispersive foundations during construction and during operation is the subject of this paper.

An important and interesting case study is used to illustrate the typical problems with dispersive residual granites used as foundation for a dam.

1 INTRODUCTION

Dispersivity or dispersive soils first appeared as possible ground engineering hazard in mid-1960. Several dam failures, notably in Australia, led to a more intensive investigation into the reasons for these failures; the conventional soil mechanics tests such as Atterberg limits, etc. did not provide any answers. However, it was found that clays rich in sodium cations were often structurally unstable, easily dispersed and, therefore, highly erodible. Furthermore, it was found that these dispersive clays eroded rapidly in moving water; even in stagnant water these soils would slake rapidly. In the agricultural field, dispersive soils had been recognized for many decades already as dispersivity has important repercussions for agriculture.

Today it is generally accepted that earth embankment dams constructed using dispersive soils often for economic reasons, will operate satisfactorily and safely for many years if the design is correct and the appropriate precautions are taken during construction and operation.

Practically all publications and experience accumulated during previous decades relate to dam embankments: failures, identification, remedial construction methods, precautions, etc. No information exists in the literature on similar aspects relating to dispersible soils in dam foundations.

In this publication the author discusses the effect of dispersive foundation soils on the typical drilling and grouting activities required for dam construction and recommends certain new techniques under these conditions.

2 DISPERSIVITY, DISPERSIBILITY, DISPERSIVE SOILS

2.1 Definition

Dispersive Soil structurally unstable soil that readily deflocculates or disperses in water into its constituent particles such as sand, silt or clay; normally highly erodible.

Dispersiblity of a soil as it affects the engineering characteristics of construction materials or foundations is a complicated interaction between water AND soils with specific material properties such as clay content, clay type, type and concentration of certain ions and the concentration of free ions in the soil. In addition the type and concentration of certain ions in the water affect the dispersibility of the soil.

Although dispersivity and its effects as well as controls are better understood today, the complicated interaction between potentially dispersive soils and water seems to indicate the use of several different identification tests for positive and reliable identification.

2.2 Identification

Several tests have been developed and improved during recent years. Today field and laboratory tests can identify reliably the dispersivity of a sample. It is important to note that the conventional soil mechanics laboratory tests such as Atterberg limits, grain size analyses and others are not able to determine the dispersivity of a soil; simply because dispersivity is a physical manifestation of chemical and mineral dependent physico chemical properties of soils and the soils interaction with water.

Identification will commence with the first site visit of a potential construction site. Dispersive soils exhibit typical erosion patterns such as jagged formations, deep erosion gullies (dongas in South Africa) and tunnels and piping. Washed-out fans of light colour and turbidity of stored water are indications of the presence of dispersive soils.

The high salinity of the soil often results in a stunted growth of vegetation or the appearance of certain species, which are less affected by the high salinity such as certain acacia species as well as the mopane trees (Colophospermum mopane).

Although these surface signals may indicate the presence of dispersive soils, the absence of such indicators is no guarantee that the foundation soil will not be dispersive simply because the upper layers may not be dispersive at all, while lower horizons may be highly dispersive.

Originally, it was believed that dispersive soils occur mainly as alluvial clays, as slope wash, lake bed deposits, loess deposits and flood plain deposits (Sherard et al, 1972). However, particularly in Southern Africa, dispersive soils have been associated with residual granites, granodiorites, mudstones and sandstones as well as alluvial deposits. Dispersivity is found in clayey, silty and sandy soils.

Earlier studies had indicated that dispersive soils were mainly found in arid and semi-arid regions, however, formations in more humid climates with well-established vegetation can also be dispersive.

Hence, identification based on climatic conditions, geographic location and geological origin is not reliable.

In addition the high variability of the degree of dispersivity in any dispersive foundation soil makes an assessment of the seriousness of the problem difficult. Also where free salts are found in the pore water in equilibrium with the in situ soil, dispersivity may be minimal. If this equilibrium is disturbed the potentially dispersive soil may in fact disperse rapidly. Therefore, although identification tests are well developed and will detect dispersibility reliably, the overall geotechnical engineering is an important aspect of the design and construction of dams. Where dispersive material is used for the construction of an embankment dam, the appropriate precautions required are by now well known. In cases where these precautions have been taken, satisfactory results have been achieved. For the purpose of this publication dispersive soils also refer to highly weathered rocks.

2.3 Field and Laboratory Tests

It is beyond the scope of this paper to describe in detail the various tests that have been developed for the identification of dispersivity, nevertheless a short description of the more important tests will be given to highlight some useful characteristics of these tests:

The Crumb Test

The crumb test is a simple field test; it gives a good indication of the potential erodibility of clay soils. A soil specimen of about 3.5cm is placed in about 250ml distilled water. The degree of turbidity (colloidal cloud) in the water is an indication of the degree of dispersivity of the specimen.

The Double Hydrometer Test

The grain size distribution is determined using the standard hydrometer test where the sample is dispersed in distilled water by a chemical dispersant and by mechanical agitation. A second test is done without chemical dispersion or mechanical agitation. The difference between the two curves is a measure of the potential dispersivity of the sample.

To quantify the result the percentage finer than 5 microns without chemical dispersion and mechanical agitation is divided by the same percentage with dispersion and mechanical agitation. The resultant percentage is an indication of the potential dispersivity of the samples.

The percentage dispersion does not correlate to the amount of sodium present in the soil, but the test is reasonably reliable in indicating problem soils.

A dispersion percentage below 15% is an indication of non-dispersive soil, while over 30% is considered moderate; severe problems have been experienced where the percentage exceeds 50.

Pinhole test

The pinhole test simulates directly the dispersibility of clay soils as water flows through a small pinhole in the soil specimen. The dispersion of clay colloids is observed by cloudiness in the discharge effluent. For dispersive soils the measured flow rate increases as the pinhole diameter enlarges. The pinhole test as modified by the USBR is regarded as the most reliable test in the USA (Sherard et al, 1976).

Chemical tests

Chemical tests refer to the determination of cation concentration in the soil and pore water. As sodium seems to be the dominant cation related to dispersibility of soils the ESP test is the most important test to determine potential dispersibility.

ESP = exchangeable sodium percentage

=exchangeable Na x 100

CEC

CEC = cation exchange capacity

SAR = sodium absorption ratio =

Soils with ESP larger than 15% are regarded as dispersive (Harmse, 1990), however, soils with ESP as low as 5% have been found to be dispersive (Van der Merwe et al, 1985). Harmse, (1980, 1988) has proposed a procedure for identification of dispersive soils by chemical testing as follows:

(Harmse, 1980,1988)

2.4 Occurrence

As mentioned earlier dispersive soils occur not only in arid and semi-arid regions but also in more humid climates. In Southern Africa these soils seem to be more predominant in areas with rainfall less than 850mm per year. (Bell and Maud, 1994) Rainfall in these areas occurs as flash floods with considerable erosion.

Dispersive soils have been derived from the sedimentary rocks of the Molteno Formation, the Beaufort Group, the Ecca Group and the Dwyka Group, which are all part of the Karoo sequence; the Witteberg Group, the Bokkeveld Group and the Table Mountain Group which belong to the Cape Supergroup; and the Malmesbury Group; the Nama Group; and the Kirkwood formation and Sunday River Formation which are assigned to the Uitenhage Group of the Cretaceous system. Dispersive clays also occur in all granites and in soils found over the granodiorites in the Swaziland Basement Complex. In the Cape Province soils are largely derived from granites or Malmesbury mudrocks and can possess dispersive characteristics even where the ESP values are less than 5. (Elges, 1985; Bell and Maud, 1994)

Dispersive clays can develop under the following circumstances in Southern Africa: (Elges, 1985)

Low-lying areas where the rainfall is such that seepage water has high SAR (Sodium absorption ratio) values; especially in regions where the N-values are higher than two. Soils developed on granites are especially prone to the development of high ESP values in low-lying areas.

In areas where the original sediments contain large quantities of illite and other 2:1 clays (montmorillonite, vermiculite) with high ESP values, dispersive soils will generally occur. This in particular is the case with the mudstones and siltstones of the Beaufort Group and the Molteno Formation in regions where the N-values are higher than two. Soils in the low-lying areas of the above formations are, virtually without exception, dispersive.

In the more arid parts of the country where the N-values exceed 10 the development of dispersive soils is generally inhibited by the presence of free salts despite high SAR values. Highly dispersive soils can develop should the free salts with high SAR values be leached out.

The N-value was introduced and defined by Weinert (1980) and is equal to 12Ej/Pa where Ej is the evaporation during the hottest month and Pa is the annual precipitation.

3 EFFECT ON EMBANKMENT

3.1 Experience

Experience from dam embankment failures as a result of dispersivity, subsequent investigations and corrective actions or precautions may hold some lessons for foundation treatment.

Most dam failures have occurred in small earth dams, which typically are not as well engineered and designed as large important dams (Donaldson, 1975). Some dams may have failed due to dispersibility in the past but these failures were not recognized as such, as dispersive soils had not been recognized as a potential hazard in the dam construction field at the time. (Wagener et al, 1981)

Causes of dam failure and corrective action and/or relevant precautions are listed below and may have some relevance for dispersive foundation soils.

1. Piping Most failures of dam embankments seemed to have been initiated by piping although no failure as a result of piping in a foundation has been reported.

2. Dispersive erosion in embankments commences typically in zones of higher permeability. These zones often occur:a.around rigid (concrete or rock) structures in the embankment where embankment material was not compacted sufficiently.b. where differential settlement has opened fissures.c.where compaction is inadequated.where desiccation has caused cracks in the embankment.e. by hydrofracturing.

However, where designs have been done carefully e.g. adequate and effective filters have been installed and all normal precautions have been taken during construction such as proper compaction (2% wet of optimum of say 98% standard proctor) no failures of any significance have been reported.

4 EFFECT ON FOUNDATION

4.1 Experience

Typically the occurrence of piping through deep foundations is very infrequent; almost all reported piping problems in dispersive soils were embankment related.

Furthermore, the weight of the embankment may assist in closing fissures. Also after impounding when seepage is accelerated, the initial water moving through the foundation will probably not cause dispersibility, as it will be in equilibrium with the in situ formation.

Experience has shown that dispersive soils have many construction related problems: severe caving during drilling, rapidly progressing deterioration of the subsurface formation as drilling progresses, erratic and unreliable water test and grouting results, low permeability but anomalous high takes of grout, problems with the installation of casing etc. Some of these problems may cause construction delays and possibly quality related problems.(See Annexure: Case Bokaa Dam).

The main objective related to foundation improvements is seepage control and strength rehabilitation:

The main activities are drilling, water pressure testing and grouting.

4.2 Drilling

Drilling can be rotary such as diamond core drilling or rotary percussion, rarely rotary tricone is used, except for the installation of casing only in highly weathered dispersive formations. (air core drilling is only an option for very soft formations such as loose alluvia).

Present State-of-the-Art as specified most frequently by consulting engineers requires the following types of drilling:

1. Diamond core drilling with water flushing

2. Percussion (destructive) drilling with water flushing

3. Percussion (destructive) drilling with air flushinga. with top hammerb. with down-the-hole-hammer

Diamond drilling is specified to obtain cores for reliable strata identification and to obtain cores for various laboratory tests (frequently N-size is specified).

Water flushing is required as this is the best technique for drilling and water as drilling fluid has many functions in the drilling process (Heinz, 1994).

Percussion with water flushing is specified to avoid blocking fissures, which should be left open for subsequent grouting.

In general, percussion is specified as it is faster and more economical especially for blanket grouting but also for curtain grouting.

As contact with water causes dispersion the introduction of water into dispersive foundations is problematic. The dispersion of soils by water is a very rapid process as is shown by the fact that most embankments that failed, failed on first wetting or filling. Hence, it must be assumed that any type of drilling with water in dispersive foundations must be detrimental to the foundation. It also makes a reliable analysis of subsurface conditions difficult as an intrusive process such as drilling changes the results, which are being investigated. Particularly where drilling is accompanied by water loss, dispersion may take place without being detected.

Drilling with water in dispersive soils will result in caving as the surrounding foundation will react with water and disperse. Therefore, the typical specification which consulting engineers use i.e. that drilling will only be allowed with clean, potable water must be revised wherever dispersive soils may be encountered.

An enormous wealth of knowledge on drilling in difficult formations has been accumulated in the oil industry. Practically all drilling in the oil fields is done in sedimentary rocks. Where difficult swelling, slaking or dispersive formations are expected, the following tests are made:

1. Clay mineral analysis, cation exchange capacity and exchange cations.

2. Balancing salinity a test to determine the salinity required to balance the in situ activity of sub-surface formations e.g. shales.

3. Swelling measurement.

4. Dispersion tests.

The drilling muds are then selected and designed on the basis of these test results.

Hole stability is achieved by:

a) Protecting the formation against water: several polymer based drilling fluids can achieve this, hydrophilic shales are controlled in this manner.

b) Using a saline drilling fluid which is in balance with the in situ cation concentration (or slightly higher). The in situ ground water cation concentration will be in equilibrium with the surrounding formation and hence should not cause dispersion.

Therefore, drilling should be air percussion where possible. Where diamond core drilling is required, the most economical solution is probably drilling with a well designed saline solution.

4.3 Water Pressure Testing

Water pressure tests are an integral part of normal dam grouting techniques. Typically the Lugeon test is used which requires the isolation of a certain stage of a borehole by packers. Water is then pumped into this section at increasing and decreasing pressures. The result is expressed in Lugeon values (=litres per metre per minute). Where the maximum pressure of 10 bars cannot be reached (e.g. inadequate volume of water, hydrofracturing) the maximum test pressure is reduced; the result is then normalized to 10 bars.

Pumping water into dispersive foundation soils can be detrimental, especially at higher pressures and high velocities.

Also it is not logical to test at ten bars when the height of the dam is only 30m. The simple normalization calculation to 10 bar is misleading if the maximum test pressure that can be achieved is only say 0.5 bar.

The simplest precaution in dispersive foundation soils would be to use balanced saline water for the test. Either water from a borehole on the site is used or the in situ salinity is determined and salts including small concentrations of CaSO4 are added to the test water to obtain the required salinity. The latter method seems more feasible as dispersive formations are often rather impermeable. The addition of polymers of very low viscosity can also be considered particularly as the Lugeon test is really a before and after test.

The other most important aspect of the water test is the pressure to be used.

The water test pressure should be in some way related to the water pressure at operation of the dam. The water test pressure should also be related to the grouting pressure used for the foundation, despite the fact that for various reasons Lugeon values and grout takes normally do not correlate.

Between the two extreme philosophies of grouting exemplified by the European: rather fracture to achieve some penetration and the Australian (Houlsby, 1990): dont hurt the patient, the Engineer has to find a proper way on site to solve the problem within the context of the specific foundation condition. This is possibly more important for dispersive foundation soils than for any other type of dam foundation.

Therefore, for drilling and water pressure testing we would recommend the following procedure where dispersive foundation soils have been encountered:

1. Determine dispersibility of the foundation material including the degree and extent of the dispersivity.

2. Determine the in situ salinity of the ground water. The ground water should be stagnant or slow moving.

3. Use water with the in situ balancing salinity and some CaSO4 to do water tests as well as for drilling.

4. Determine hydrofracturing pressure.

5. Operate the water tests at approximately 20 30% below the determined hydrofracturing pressure.

4.4 Grouting

In essence the task of the grouting engineer and/or contractor is threefold:

1. To determine the pressure at which the grouting material is pressed into the foundation.

2. To design a grouting material that will match the subsurface requirements i.e. to reduce the permeability to the specified level, economically.

3. To design and implement, on the basis of the specific site conditions, an efficient, effective and economical grouting procedure.

On grouting pressure, the world is still divided, in simple terms, into the high pressure grouters, mainly European and South African practice and the low pressure grouters mainly American and Australian practice. For example European engineers often specify grouting pressures at 1kg/cm per m depth (Heinz, 1987) the Australian specification (Houlsby, 1990) usually states 1lb/sq.inch per ft depth; the difference is a factor of 4. More recent grouting techniques have been developed by Lombardi. Lombardi proposed the GIN principle (Grouting Intensity Number). The most important principles of this method are: (Lombardi and Deere, 1993)

1 A single stable grout mix for the entire grouting process (water:cement by weight of 0.67 to 0.8) with superplasticizer to increase penetrability.

2 A steady low to medium rate of grout pumping gradually increasing pressure as the grout penetrates further into the rock fractures.

3 The monitoring of pressure, flow rate, volume injected, and the penetrability in real-time by PC graphics; and

4 The termination of grouting when the grouting path on the displayed pressure versus total volume (per metre of grouted interval) diagram intersects one of the curves of limiting volume, limiting pressure, or limiting grouting intensity as given by the selected GIN hyperbolic curve ( a curve of constant pressure times volume, pV, a measure of energy expended.)

In general dispersive soils are quite impermeable with predominantly fine fissures. Nevertheless in some foundations isolated zones with large grout takes were found. (Bokaa Dam, Vaalkop Dam). In the case of the Bokaa Dam one control borehole absorbed 3435kg after primary and secondary grout curtain holes had been drilled and grouted. At the Vaalkop Dam 3 boreholes accepted 62000kg. The major part of the Bokaa Dam foundation absorbed 4 to 6 kg/m. (See Annexure)

It is possible that these isolated takes may be due to pseudokarstic phenomenon resulting from suffosion which is a process of undermining of transported or residual soils by mechanical and chemical action of underground water. The phenomenon is closely associated with residual granites. Suffosion can lead to piping and is the main process responsible for the development of collapsible grain structures in residual granite soils (Brink,1979). Collapsible and dispersive soils are both associated with residual granite in the Southern African region. While the former is found predominantly in areas of annual water surplus, the latter is more readily found in areas of less rainfall. However, overlaps of these areas exist. Therefore, dispersive and collapsible soils may be found on one site.

On the basis of experience to date on dispersive materials and the present State-of-the-Art of grouting dam foundations, the following recommendations are presented:

4.4.1 Grouting Pressure

Grouting pressures should be as high as possible to achieve reasonable penetration in as short a time as possible; the grouting pressure should be redefined after appropriate tests on site have been made.

For example shallower horizons may require very low pressures whereas fissured granites below the weathered granites may require proportionately higher pressures than the shallower horizons i.e. a simple formula for the pressure linearly related to depth may not be adequate.

Hydrofracturing should be avoided, as it is difficult to grout all fissures that have been opened by fracturing. In dispersive soils this is of particular importance.

4.4.2 Grouting materials

As a result of the high sensitivity towards water, grouting materials should have a low water content. First choices would be normal OPC (Blaine 3500cm/g), rapid hardening cement (RHC) (Blaine 4500cm/g) and microfine cement (Blaine 12000 to 15000cm/g). Typically OPC is specified under normal conditions but as RHC is reasonably inexpensive in South Africa, RHC blended with pozzolanic products (South African coal mines produce a PFA of excellent quality) is often used and is an excellent grouting material. However, PFA binds the excess calcium hydroxide which is set free by progressive hydration. In dispersive soils this free calcium hydroxide may reduce the dispersibility potential, hence it may be better not to use PFA under these circumstances.

With thicker grouts, superplasticizers should be used as is recommended by Lombardi (Lombardi et al, 1993). Gypsum is often added to cement to achieve a) flash setting to combat lost circulation b) gelling or thixotropic properties and c) expansion properties in the set cement (very low 0.3%). About 4 to 10% of gypsum may be added to reduce the effect of dispersibility of the subsoil. Cement grouts containing salts help to protect shale reactions from sloughing and heaving during cementing and hence may reduce dispersibility of foundation soils. Because of the Na-sensitivity in dispersive soils it is probably better to use KCl for salt cements (Smith, 1987). Long term stability and durability of the cement grout should also be considered.

The above mentioned procedures are proven techniques from oil field cementing technology, nevertheless, each case must be considered on its own merits, and based on the specific subsurface conditions.

Where cement products may be too coarse to achieve any notable penetration in fine fissures sodium silicate is often the most economical alternative. However, there is still some doubt as to the permanency of sodium silicate particularly under conditions of frequent wetting and drying. Shrinkage can result in an increased residual permeability. Sodium silicate has many uses including as deflocculants, detergent bleach etc. In addition sodium salts may be exuded from silicate gels. Also the quality control of the sodium silicate process is difficult especially at the interface between grout material and soil. (Karol, 1990). No experience is available of sodium silicate grouting of dispersive soils. With the above characteristics sodium silicates do not seem to have the right properties for grouting dispersive foundation soils.

If the use of cement products, OPC to microfine, has been exhausted chemical grouting using acrylamides should be applied. However, costs are usually high. A combination of cement products and chemical grouts, except sodium silicate, may provide the most economical solution. See also the attached drawing for an indication of grout penetrability.

GROUT PENETRABILITY A: Suspensions B: Solutions4.4.3 Grouting Technique

The grouting technique refers to the how to grout and place the grouting material where it is needed. In general this refers to the split spacing method, tube--manchette (TAM) (Heinz, 1983), Multiple Packer Sleeved Pipe system (MPSP) (Bruce, 1991), the rate of thickening if required and the variation of the W:C ratio as required etc.

The grouting technique is not only important from the perspective of the quality of the end product but it may have a significant impact on the construction programme. While grouting activities may only be 2 to 5% of the entire cost of a dam project, delays of main contractor activities due to grouting may be very costly. (See Annexure: Case Bokaa Dam). As dispersivity adds an additional hazardous dimension to the grouting process and hence to the entire project it is imperative that identification of these potential problems is made early on in the construction process so that the construction team including its experienced geotechnical subcontractor can address these potential problems speedily in order to avoid expensive delays and still achieve an acceptable end product.

For a given pressure the maximum distance of penetration is determined by the yield strength (as defined by the Bingham model) while the viscosity governs the rate of flow i.e. the time to get there. (Lombardi and Deere, 1993)

For a given material and type of fissures high pressures will increase penetration distance, increase the hole spacing which is desirable in dispersive foundations but may cause hydrofracturing. Low pressures will decrease the penetration distance, reduce the hole spacing which is not desirable in dispersive foundations and will probably not hydrofracture which is desirable in dispersive foundation soils. Therefore, in general, the Australian method of grouting with very low pressures typically leads to very close spacing of grout holes. Hence in dispersive foundation soils the judicious choice of grouting pressures for various horizons is extremely important.

The best all round method in difficult formations where severe dispersion and/or caving occurs is the tube--manchette (TAM) method. A further development of this system is the multiple packer sleeved pipe system (MPSP). These techniques are described in detail elsewhere (Heinz, 1983; Bruce, 1991). In short, the holes to be grouted are drilled to their final depth in one operation, a sleeved pipe (sleeves = manchette = non-return valves) is installed and the formation is grouted through these pipes. The system has many advantages, however, it is rather slow hence generally more costly than the standard grouting techniques. The pipe and sleeves are permanently installed hence contribute to the costs; the MPSP technique is similar to the TAM method.

The most significant advantage of these methods is that drilling is minimized in each hole, which is an important factor in dispersive subsoils and, of course, collapsing or caving soils can be treated effectively. Without the effective treatment of caving soils, drilling and redrilling may significantly impact on the entire construction programme. While these techniques may be more costly, invariably expensive delays are avoided.

The decision to grout upstage or downstage should be made in favour of the method that guarantees least contact with drilling water, assuming that no satisfactory drilling fluid can be found. A comparison of the two methods is given below (Bruce, 1991). It seems that upstage grouting with packers at the top may be the most effective method in dispersive soils.

DownstageUpstage

A

D

V

A

N

T

A

G

E

S1. Ground is consolidated from top down, aiding1. Drilling in one pass

hole stability and packer seating and 2. Grouting in one repetitive operation

allowing successfully higher pressures to be without significant delays

used with depth without fear of surface leakage.3. Less wasteful of materials

2. Depth of the hole need not be predetermined:4. Permits materials to be varied readily

grout take analyses may dictate changes from5. Easier to control and programme.

foreseen, and shortening or lengthening of the6. Stage length can be varied to treat special

hole can be easily accommodated. zones

3. Stage length can be adapted to conditions as7. Often cheaper since net drilling output rate

encountered to allow special treatment. is higher.

D

I

S

A

D

V

A

N

T

A

G

E

S1. Requires repeated moving of drilling rig and 1. Grouted depth predetermined

redrilling of set grout: therefore, process is2. Hole may collapse before packer introduced

discontinuous and may be more time-consuming or after grouting starts, leading to stuck

2. Relatively wasteful of materials and so generally packers and incomplete treatment

restricted to cement based grout3. Grout may escape upwards into (non-grouted)

3. May lead to significant hole deviation. upper layers or the overlying dam, either by

4. Collapsing strata will prevent effective grouting hydrofracture or bypassing packer. Smaller

of entire stage, unless circuit grouting method fissures may not then be treated effectively at

can be deployed depth

5. Weathered and/or highly variable strata4. Artesian conditions may pose problems.

problematic5. Weathered and/or highly variable strata

6. Packer may be difficult to seat in such conditions problematic

4.5 Special Techniques

Several techniques which have been used to prevent seepage in dam foundations also may have some distinct advantages when used in dispersive soils. Some of the more recent proven techniques are discussed

4.5.1 Diaphragm or slurry trench wall

Very effective though rather costly especially where only certain lower horizons require impermeabilization. Possible problems: The effect of a Na-montmorillorite slurry on highly dispersive soils has not been researched. Probably a high bentonite concentration would be required for the slurry which could then give problems when the slurry is displaced by the concrete i.e. inclusions of soil or slurry in the concrete.

4.5.2 Jet Grouting

Extremely effective and cost effective especially where only certain deeper horizons require impermeabilization. Dispersivity is irrelevant with this technique.

4.5.3 Crystallization Process

As this process is less known in dam foundation engineering, a more detailed description of the process will be presented here. Further detailed information can be found in Ziegenbalg and Crosby, 1997 and Ziegenbalg and Holldorf, 1998.

Many naturally occurring processes are known which lead to closure of seepage paths. These processes are the result of recrystallization processes of coupled dissolution and precipitation processes. Normally nature requires many years to produce satisfactory sealing. However, if the crystallization process can be controlled and accelerated a very exciting grouting technique is created.

For example gypsum precipitation occurs immediately when a solution containing Na2SO4 is mixed with a solution containing CaCl2. However, the addition of an inhibitor allows a mixing process without spontaneous crystallization.

Ziegenbalg and Crosby (1997) have shown that it is possible to prepare large volumes of CaSO4 oversaturated grout solution under field conditions. Depending on the degree of supersaturation and the relative inhibitor content, the solutions lead to the artificial precipitation of gypsum within two to eight hours. In this way micro fissures can be sealed.

The method is particularly interesting for the sealing of dispersive foundation soils because:

1. The grouting material is a solution capable of penetrating fine fissures which typically occur in dispersive soils at least in the Southern African Region and

2. The process precipitates gypsum, which is the favoured and proven material to inhibit dispersibility of soils.

5 CONCLUSION

Dispersivity is a complex interaction between relatively pure water and soils with specific chemical characteristics. Dispersivity in earth embankments can be identified and controlled by including certain design features, during construction and operation.

Practically no information or data exist with respect to the control of dispersivity in foundation soils particularly for dam foundations where grouting is required.

Based on the available knowledge and experience worldwide as well as experience from the grouting of several dams on dispersive soils in the South African Region, the authors come to the following conclusions and recommendations

1. On dispersive soils, which can be identified, all activities that introduce water into the foundation such as drilling and grouting must be limited to a minimum.

2. The most important activity to utilize water is drilling rotary and percussion. To minimize the effect on potentially dispersive soils, drilling water designed with a balancing salinity and the inclusion of some CaSO4 should be used for all activities such as drilling and water testing.

3. Similarly, grouting procedures should be designed on the basis of the effect of the grouting material on dispersivity. Grouting materials with relatively low water cement ratios, salt cements or chemical grouts should be used. Sodium silicate should only be used after careful evaluation.

4. Techniques such as the TAM or MPSP have distinct advantages when grouting dispersive foundation soils; especially in the face of collapsing or severely caving conditions.

5. Other alternative techniques to the standard drilling and grouting such as slurry trenches or jet grouting have been applied with great success. However, whereas slurry trenches are costly, jet grouting is an excellent and economical technique, especially where only selected horizons require impermeabilization.

6. A very exciting and promising new grouting technique to impermeabilize dispersive soils is the controlled crystallization of oversaturated grout solutions, especially CaSO4 in the foundation. As a solution, the technique holds great promise for sealing micro fissures and to reduce dispersivity by gypsum precipitation.

7. No simple test can be relied on to identify dispersive foundation soils. The ESP test is still the most reliable test to identify dispersive soils in the Southern African region. By early identification of dispersive soils the required precautions and controls can be introduced timeously. In this way, safe and economic structures can be built even if foundation soils should be found to be dispersive.

Finally it seems abundantly clear that intricate or complex foundation conditions such as the drilling and grouting of dispersive foundation soils requires the nomination of specialized and experienced geotechnical and grouting contractors if unnecessary delays are to be avoided and an economical solution of high quality is to be achieved.

6 REFERENCES

Bell, F.G. and Maud, R. R (1994) - Dispersive Soils: a review from a South African perspective, Quarterly Journal of Engineering Geology, Vol 27.

Brink, A.B.A (1979-1985) - Engineering Geology of Southern Africa, Pub. by Building Publications, 4 Vol

Bruce, D.A. (1991) - Grouting with the MPSP Method at Kidd Creek Mine, Ontario, Ground Engineering.

Bruce, D.A. and Shirland, J.N (1985) - Grouting of completely weathered granite with special reference to the construction of the Hong Kong Main Transit Railway, Proc. 4th Int. Sym., IMM, Brighton.

Clarke, M.R.E (1987) - Mechanics, Identification, Testing and Use of Dispersive soils in Zimbabwe.Darley, H.C.H. and Gray, G.R. (1988) - Composition and Properties of Drilling and Completion Fluids, 5th Ed, Gulf Publishing, Houston.

Donaldson, G.W (1975) - The occurrence of dispersive soil piping in central South Africa. Proceedings of the Sixth Regional Conference for Africa on Soil Mechanics and Foundation Engineering, Durban.

Elges, H.F.W.K (1985) - Dispersive Soils in South Africa. State-of-the-Art, The Civ. Eng. in S. A.

Gerber, F.A & von Maltitz Harmse, H.J (1987) - Proposed procedure for identification of dispersive soils by chemical testing, Die Siv. Ing. in Suid Afrika.

Heinz, W.F. (1995) - Grouting the Future, Proc. Int. Drill 95. Annual Conf. ADIA, Australia

Heinz, W.F. (1994) - Diamond Drilling Handbook, 3rd Ed, Published by SADA.

Heinz, W.F. (1993) - Extrem tiefe Injektionsschrzen,. Proc. Int. Conf. On Grouting in Rock & Concrete. Balkema.

Heinz, W.F. (1987) - The Art of Grouting in Tunnelling, SANCOT seminar, Nov.

Heinz, W.F. (1983) Tube--Manchette: Description and Applications. Proc. Grouting Symp, Jhb.

Houlsby, A.C. (1990) - Construction and Design of Cement Grouting, John Wiley & Sons Inc., NY.

Karol, R.H. (1990) - Chemical Grouting, Marcel Dekker, New York, 2nd Ed.

Lombardi, G. and Deere, D. (1993) - Grouting Design and Control using the GIN Principle, Water Power and Dam Construction.

Sherard, J.L., Decker, R.S., Ryker, N.L. (1972) - Hydraulic Fracturing in Low Dams of Dispersive Clay, Proc. Spec. Conf. On Performance of Earth and Earth-Supported Structures, ASCE.

Sherard, J.L, Dunnigan, L.P., Decker, R.S. (1976) - Identification and Nature of Dispersive Soils, Journal Geotechnical Division, ASCE.

Smith, D.K. (1987) - Cementing, Monograph, SPE.

Von M. Harmse, H.J., Gerber, I.A. (1988) - A Proposed Procedure for the Identification of Dispersive Soils, Proc. 2nd Int. Conf. On Case Histories in Geotechnical Engineering, St. Louis.

Von M. Harmse, H.J. (1990) - Report on Dispersive Soils from Bokaa Dam Botswana, Eurosond report by Prof. Von M Harmse of the Univ. of Potch.

Von M. Harmse, H.J. (1980) - Dispersiewe Grond, Identifikasie en Stabilisasie, Ground Profile, no.22.

Wagener, F., Von M. Harmse, H.J., Stone, P., Ellis, W. (1981) - Chemical Treatment of a Dispersive Clay Reservoir, 10th International Conference on Soil Mechanics and Foundation Engineering, Stockholm.

Weinert, H. H. (1980) The National Road Construction Materials of South Africa, Academica, Cape Town.

Ziegenbalg, G, Holldorf, H. (1998) - The directed and controlled crystallization of naturally occurring minerals a new process to seal porous rock formations and to immobilize contaminants, IAH Syposium, Quebec.

Ziegenbalg, G, Crosby, (1997) - An overview of a pilot test to reduce brine inflows with controlled crystallization of gypsum at the IMC Kalium K2 brine inflow, Mineral Resources Eng., Vol. 6, No. 4.

ANNEXURE - Case Bokaa Dam

The Bokaa Dam is situated approximately 35 km NNE of Gaborone, Botswana. The dam is part of the Metse motshlaba Scheme. Dam construction commenced on January 1989 and was completed during 1993; the dam is situated on the Metsemotshlaba River.

The dam is an earth embankment dam, it is 1700m long, its maximum height is 30m. The spillway is on the left embankment and is approx. 500m long. The pumphouse is centrally situated close to the old riverbed.

Dispersivity in the foundation soil was not investigated at design stage, although during construction the dispersive foundation soils caused significant problems and subsequent delays

According to a report by Prof. Von Maltitz Harmse (Harmse, 1990) some of the fissures filled with clay could be classified as highly dispersive. The soil samples investigated could be prone to piping. ESP values as high as 60.6% where encountered on the site. For example the average ESP for seven samples between 1m and 7m depth on Pattern E was 30%.

The dam foundation is a typical residual granite foundation. Residual granite foundations as they occur in the Southern African region, are possibly one of the most difficult formations for dam foundations.

Typically the formation has the following characteristics: (see drawing below) (Brink, 1979)

1. Troughs of decomposition adjacent to tors of unweathered rock.

2. The presence of small to large core-stones within the residual granite soil (see drawing).

3. Abrupt changes from highly weathered to unweathered rock.

4. Moderate to slightly weathered rock weathering is mainly along joints; joints may be void, partially or completely clay filled.

5. Irregularly dispersive in degree and extent.

For grouting engineering purposes, this soil profile translated into the following characteristics:

1. Low permeability in the weathered to highly weathered horizons.

2. Anomalous high grout takes in some stages.

3. Low grout absorption over the entire grout curtain.

4. Rapid deterioration of soils and hence caving in boreholes, flushing out of weathered soils.

5. Erratic water pressure test and grouting results. After grouting of primary and secondary and sometimes tertiary boreholes with decreasing absorption, sudden relatively high grout takes in the final test and control boreholes.

(Brink, 1979)

6. Difficulties in installing casing in the rapidly caving (dispersing) formation.

7. Difficulty in defining base rock due to the presence of core-stones; makes it difficult to define depth to which casing must be installed.

8. Due to variability of degree of dispersibility it was difficult to assess the extent and seriousness of dispersivity.

Due to the above-mentioned problems with the foundation, long delays were caused as the Engineer blamed the Geotechnical contractor for incorrect grouting procedures. The Engineer did not realize the extent and seriousness of the dispersivity in the foundation soil. The complete drilling and grouting programme of 22,000m percussion and 3000m diamond drilling was completed within 6-7 months. Approximately a similar amount of work was requested by the Engineer and completed in an additional 18 months with little or no improvement in the total grout absorption. A claim of the main contractor against the client resulted. The client recovered a large amount of this claim in a lengthy arbitration case against the Engineer.

Summary of Characteristics of

Non-dispersive vs. Dispersive Clays

Test CharacteristicsDispersive Quality

Noncohesive silt, rock flour, fine sandsDispersive in water; highly erosive; not a dispersive clay.

Clay stone and shales laid down as marine depositsMay be dispersive.

Red, brown, gray, yellow-clay soils or combinationsMay be dispersive clay.

Black, organic soils; fine grained soils derived from in situ weathering of igneous and metamorphic rocks; and soils derived from limestone (except for granites and granodiorites)Likely to be non-dispersive clay.

Unusual erosional patterns with tunnels, deep gullies, with excessive turbidity in storage water. Poor crop production and stunted growth from saline soils.Indications of possible presence of dispersive clays.

Field identification tests: crumb test, drop test, UV test, turbidity test.Show indications of possible presence of dispersive clays.

Common laboratory tests:

Laboratory crumb test:Good indication of potential erodibility

SCS double hydrometer test:Soils likely to be dispersive.

Pinhole test:Direct physical test of dispersive character.

Chemical tests:Indications of dispersive soils.

Suggested rating system for potentially dispersive soils

Crumb TestClass RatingStrong ReactionModerateSlightNo reaction

4210

Dispersion TestClass RatingHighly DispersiveModeratelySlightlyNon-dispersive

4210

*ESP/CECClass RatingHighly DispersiveDispersiveMarginalNon-dispersive

(meq/100g clay)

5310

SARClass RatingOver 102 10Less than 2

310

pHClass RatingOver 86 8

Less than 6

21

0

The table above is an attempt by Maud and Brennan (1994) to come to terms with the complexity of dispersibility of soils. While this can be helpful it seems that the salinity or the lack of it of the eroding agent which after all is water, is not adequately reflected in this table. Cases have been reported in the literature where dams suddenly failed after many years of satisfactory operation simply because the salinity of the stored water was lowered significantly. For example an interesting case in Australia where a reservoir was replenished with water from a newly constructed pipeline adding water of very low salinity to the reservoir. After operating for many years satisfactorily, the dam failed within three days after filling the reservoir with fresh, low saline water.

(Reference: Bruce, D.A., 1991)

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