Construction Material Survey

8/11/2019 Construction Material Survey

1/97

1Methods and Procedures for Prospecting for Road Construction Materials

oa s epar men

Methods and Proceduresor Prospecting or Road

Construction Materials


2/97

2 Methods and Procedures for Prospecting for Road Construction Materials

oads Department

Ministry of Works, Transport & Communications,Roads DepartmentPrivate Bag 0026Gaborone, BotswanaPhone + 267 - 313511

Fax + 267 - 314278

DECEMBER 2000

ISBN 99912 - 0 - 358 - 3

Reproduction of extracts from this Guideline may be made subject to due acknowledgement of the source.Although this Guideline is believed to be correct at the time of printing, Roads Department does not accept any

contractua , tort ous or ot er orm o a ty or ts contents or or any consequences ar s ng rom ts use. Anyoneus ng t e n ormat on conta ne n t e Gu e ne s ou app y t e r own s an u gement to t e part cu ar ssue

un er cons erat on.


3/97


oa s epar men

ROADS DEPARTMENT

Under the policy direction of the Ministry of Works, Transport & Communications, Roads Department is

respons e or prov ng an a equate, sa e, cost-e ect ve an e c ent roa n rastructure w t n t e or ers o

Botswana as we as or ac tat ng cross- or er roa commun cat ons w t ne g our ng countr es. Imp e n

these far-ranging responsibilities is the obligation to:

1. ensure t at ex st ng roa s are a equate y ma nta ne n or er to prov e appropr ate eve o serv ce or roa

users;

2. mprove ex st ng roa s to requ re stan ar s to ena e t em to carry preva ng eve s o tra c w t t e

required degree of safety;

3. prov e new roa s to t e requ re geometr c, pavement es gn an sa ety stan ar s.

The Department has been vested with the strategic responsibility for overall management of the Public HighwayNetwor PHN o some 18, 300 m o roa s. T s con ers aut or ty or sett ng o nat ona spec cat ons an

stan ar s an s eare respons ty w t t e D str ct Counc s an Department o W e an Nat ona Par s or

the co-ordinated planning of the PHN.

Roa s Department s a so respons e or a m n ster ng t e re evant sect ons o t e Pu c Roa s Act, ass st ng

local road authorities on technical matters and providing assistance in the national effort to promote citizen

contractors in the road construction industry by giving technical advice wherever possible. This task is facilitated

y t e pu cat on o a ser es o Tec n ca Gu e nes ea ng w t stan ar s, genera proce ures an est pract ce

on a variety of aspects of the planning, design, construction and maintenance of roads in Botswana that take full

account of local conditions.

Guideline No. 1 The Design, Construction and Maintenance of Otta Seals (1999)

Wor s op Procee ngs, Septem er 2000, A en um w t re erence to

Guideline No. 1 The Design, Construction and Maintenance of Otta Seals (1999)

Gu e ne No. 2 Pavement Test ng, Ana ys s an Interpretat on o Test Data 2000

Guideline No. 3 Methods and Procedures for Prospecting for Road Construction Materials (2000)

Gu e ne No. 4 Ax e Loa Surveys 2000


4/97


oads Department

FOREWORD

Construct on mater a s an re ate ssues typ ca y account or over 75% o roa construct on cost. T e genera

scarcity of conventional road building materials in Botswana has led to substantial research, over the past 25 years,

by Roads Department to identify, test and develop specications for the use of the available local materials. Thisresearc wor as een carr e out n- ouse an n co a orat on w t t e o ow ng organ zat ons:

Transport Research Laboratory (TRL) funded by DFID,

Norweg an Pu c Roa s A m n strat on NPRA , un e y NORAD,

Center for Scientic and Industrial Research (CSIR) funded by the Government of Botswana,

Dr. Frank Netterberg, funded by the Government of Botswana.

T e mater a s prospect ng tec n ques t at are em o e n t s gu e ne are t e resu ts o eca es o wor

and should provide an invaluable document for both engineers and technicians and trainers working in the road

construction industry in the country. Ultimately, it is myfervent hope that the Guideline will assist in insuring

t at t e est appropr ate mater a s are ent e or eac roa pro ect at econom ca au stances. T ere y,minimizing the construction costs and reducing materials related claims.

T e Gu e ne s anot er n a ser es o ocuments nten e to nsure t at s gn cant tec no og ca eve opments

within the Roads Department over a number of decades are put on sound sustainable footing for future efcient

development and maintenance of our road network.

Gaborone

December 2000

An rew N aro

Director of Roads

Roa s Department

M n stry o Wor s, Transport an Commun cat ons


5/97


oa s epar men

ACKNOWLEDGEMENTS

This Guideline is one of a series that is being produced under the Institutional Co-operation Agreement that existsbetween the Roads Department and the Norwegian Public Roads Administration (NPRA). This Agreement fallsunder a NORAD Technical Assistance Programme to Roads Department, which is co-funded by the Kingdom of

Norway and the Government of the Republic of Botswana.

The production of the Guideline has been a joint effort between the Roads Department, the NPRA, the TransportResearch Laboratory, TRL (UK), the CSIR (South Africa) and Civil and Planning Partnership, CPP (Botswana)who was the consultant for the Working Group. The staff members involved were:

C. J. Lawrance RLD. Jones CSIRP. Paige-Green CSIR

K. J. Motswagole CPP

The Working Group that guided the project and reviewed the Guideline, comprised of the following people:

Mr. C. Over y, NPRA

Dr. B. O a, DFIDMr. B. Kemsley, Roads DepartmentMr. M. Kowa, Roads DepartmentMr. E. Maswikiti, Roads DepartmentMr. M Sego go, Roa s DepartmentMr. M. Gaeeme we, Roa s Department

Mr. B.M S arma, Roa s Departmen

Mr. L. Von Straaten, ACE, commented the rst draft Guideline and his contribution is highly appreciated.

The following persons and their organisations are also acknowledged for their input during the interviews on past

exper ence an or t e r contr ut on n t e wor s op ur ng t e preparat on o t s gu e ne.

Mr P Monamets oa s DepartmentMrs A. Ga aa e oa s DepartmentMr N.Ra we a oa s DepartmentMr P. Selema oads DepartmentMr S. Seloka oads DepartmentMr E. Phang oads DepartmentMr D. C mtas u C v an P ann ng an Partners pMr S. Mosepe e C v an P ann ng an Partners pMr M. Ntsimanyana epartment of Geological SurveysMr A. Nanyaro Geotechnics InternationalMr J. Tidi Geotechnics InternationalMr M. Pu e Geotec n cs Internat onaMr B. McGee Gr na er-W y e

Mr M. Green AAS Consu tMr G. Lebitsa waneng Town CouncilMr. P. A. R. Oosthuizen Material Testing ServicesMr S. Jackson aterial Testing ServicesDr F. Netter erg Fran Netter ergMr J. San s San s C v sMr D. Brown Sladden InternationalMr L. van der Poll Stewart Scott InternationalMr K. Spackman Stewart Scott InternationalMr L. Ma ura n tecMr I. Le wape otswana Co ege o Agr cu tureMr W. Mojeremane otswana College of Agriculture

Mr L. Malela otswana College of Agriculture

otograp s were prov e y:

C. J. Lawrance RL. Jones CSIR

. Pa ge-Green CSIR. J. Motswagole CPP

C. Overby NPRA. Segokgo Roads Dept.

.T. Keganne D v Consu t


6/97


oads Department

TABLE OF CONTENTS

Roads Department ...........................................................................................................................................3

Foreward...........................................................................................................................................................4

Aknowledgement ..............................................................................................................................................5

1 INTRODUCTION............................................................................................................................................9

1.1 Background ............................................................................................................................. ...............9

1.2 Purpose an scope..................................................................................................................................10

1.3 Structure of the guideline............................................................... .......................................................10

1.4 Summary of procedure...........................................................................................................................11

2 ENVIRONMENTAL REGIONS OF BOTSWANA....................................................................................13

2.1 Introduction ............................................................................................................................. .............13

2.2 C mate...................................................................................................................................................13

2.3 Geomorp o ogy .....................................................................................................................................14

2.4 Geology ............................................................................................................................. ...................14 2.4.1 . Eng neer ng c ass cat on o roc types .....................................................................................15

2.5 Agr cu tura so s....................................................................................................................................16

2.6 Vegetation ..............................................................................................................................................17

2.7 Prospecting regions ............................................................................................................................. .17

3 GRAVELS IN BOTSWANA.......................................................................................................................... 0

3.1 Introduction ............................................................................................................................. .............20

3.2 Res ua grave s .....................................................................................................................................20

3.2.1 . Acid crystalline rock gravels.......................................................................................................20

3.2.2 . Basic crystalline rock gravels......................................................................................................21

3.2.3 . Arenaceous roc grave s .............................................................................................................21

3.2.4 . High silica gravels ............................................................... .......................................................213.3 Pedogenic gravels ..................................................................................................................................22

3.4 ransporte mater a s............................................................... .............................................................23

4 GRAVEL INDICATORS ............................................................................................................................... 5

4.1 Intro uct on ............................................................................................................................. .............25

4.2 Lan orm................................................................................................................................................25

4.2.1 . Landforms of rock regions ..........................................................................................................25

4.2.2 . Landforms of sand regions ..........................................................................................................25

4.3 Botan ca n cators ...............................................................................................................................25

4.3.1 . Factors inuencing the use of botanical indicators .....................................................................29

4.4 Animal indicators............................................................... ...................................................................30

5 INTERPRETATION OF MAPS AND PHOTOGRAPHS.......................................................................... 15.1 Introduction ............................................................................................................................. .............31

5.2 opograp c maps..................................................................................................................................31

5.3 Geological maps.....................................................................................................................................31

5.4 Agricultural soil maps............................................................................................................................32

5.5 Remote sens ng tec n ques ...................................................................................................................32

5.5.1 . Aer a p otograp s an nterpretat on............................................................... .........................32

5.5.2 . Satellite images and interpretation ..............................................................................................34

6 SURVEY METHODOLOGY........................................................................................................................ 6

6.1 Introduction ............................................................................................................................. .............36

6.2 Desk study..............................................................................................................................................36

6.2.1 . Pro ect r e ng ............................................................................................................................36 6.2.2 . Interpretation of background information ............................................................... ...................37

6.2.3 . Collation of information .............................................................................................................37

Page


7/97


oa s epar men

6.2.4 . Planning the eld survey............................................................................................................. 0

6.3 he eld survey ..................................................................................................................................... 1

6.3.1 . Reconna ssance survey................................................................................................................ 1

6.3.2 . Detailed survey............................................................................................................................ 4

7 EVALUATION OF A GRAVEL SOURCE .................................................................................................. 57.1 ntroduction ........................................................................................................................................... 5

7.2 Detailed eld study ............................................................................................................................... 5

7.3 r a p t ogg ng .....................................................................................................................................50

7.4 Samp ng ............................................................... ................................................................................52

7.5 Estimating material quantity............................................................... ..................................................54

.5.1 . La oratory test ng............................................................... ........................................................57

8 REPORTING.................................................................................................................................................. 8

8.1 ntro uct on ...........................................................................................................................................58

8.2 Report structure .....................................................................................................................................58

8.3 Materials inventory ...............................................................................................................................59

9 REFERENCES............................................................................................................................................... 0

10 ABREVIATIONS ........................................................................................................................................... 1

11 APPENDICES

APPENDIX 1 - OTHER RELEVANT ORGANISATIONS............................................................................63

APPENDIX 2 - WEATHERING AND WEATHERED MATERIALS...........................................................64

eathering ...........................................................................................................................................................

Residual materials ...............................................................................................................................................

ransporte grave s ............................................................... ..............................................................................

Pe ogen c grave s ...............................................................................................................................................

Origin of weathering products .............................................................................................................................APPENDIX 3 - BOTANICAL INDICATOR SPECIES..................................................................................67

APPENDIX 4 - VARIOUS LOGGING FORMS ............................................................................................95

LIST OF TABLES

Ta e 2.1 Prospect ng reg ons o Botswana...........................................................................................................19

Table 4.1 Landforms of rock regions, associated with gravel deposits .................................................................26

Table 4.2 ypes of gravel associated with rock regions ........................................................................................27

Ta e 4.3 Lan orms o san reg ons, assoc ate w t grave epos ts.................................................................28

Ta e 4.4 Grave types assoc ate w t san reg ons ............................................................................................28

Table 4.5 ndicator plants for pedogenic gravels ..................................................................................................29

Table 6.1 Appropriate methods for eld survey ....................................................................................................38

Ta e 6.2 nterpretat on o ca crete pro e resu ts mo e rom Netter erg 1996 ............................................ 1

Table 6.3 Differences between reconnaissance surveys and detailed surveys....................................................... 2

LIST OF FIGURES

Figure 1.1 Flow diagram stages, for material prospecting......................................................................................12

F gure 2.1 Annua mean ra n a n t e country an N-va ues ................................................................................13

Figure 2.2 Simplied geological map of Botswana ............................................................... ................................15

Figure 2.3 Prospecting regions of Botswana...........................................................................................................18

F gure 3.1 Re at ons p etween soa e CBR an proport ons o Ka a ar san m xe n w t ca crete............. 24

F gure 3.2 yp ca gra ng enve ope o Ka a ar san ...........................................................................................24

Figure 3.3 ypical density curve of Kalahari sand..................................................................................................24

Figure 5.1 Landsat. extract. Sekoma Pan plus pixel extract in top right.................................................................35

F gure 5.2 Aer a p oto - Se oma Pan an part o Se oma - Kang roa e ore t was tum n se . ....................35

Figure 6.1 Flow chart summary of desk study activities leading to early assessment of gravel types expected to be found 39


8/97


oads Department

igure 6.2 Flow chart-reconnaissance survey............................................................... .........................................43

igure 6.3 Flow diagram for detailed investigations...............................................................................................44

gure 7.1 I ustrat on o var ous samp ng pattern ..................................................................................................46

igure 7.2 Typical layout of trial pits ......................................................................................................................47

igure 7.3 Flow diagram, Evaluation of a gravel source ........................................................................................49

gure 7.4 Examp e o tr a p t ogs.........................................................................................................................51

gure 7.5 Met o o samp ng rom tr a p t..........................................................................................................53

igure 7.6 Correct method for reducing the size og sample ...................................................................................53

igure 7.7 Subdividing a borrow area in order to calculate gravel quantities.........................................................54

gure 7.8 Typ ca orrow p t ocat on p an ...........................................................................................................56


9/97

Methods and Procedures for Prospecting for Road Construction Materials Chapter 1Introduction

9

oa s epartment

1 NTRODUCTION

. ac groun

In order to maintain, construct, rehabilitate or upgrade any road network,large quantities of gravel are required. This Guideline gives advice on themethods and procedures that are appropriate in Botswana for the locationand proving of material sources for road construction. The Guideline

presents t e now e ge o exper ence , pract s ng prospectors n t ecountry so t at everyone can e aware o t e est pract ce ava a e. It as

been compiled by combining the technological approach of scientists withthe hard won experience of long-time prospectors.

T ere s no mater a s map, s m ar to a geo og ca map, cover ng Botswana.Grave s occur as re at ve y sma oca se epos ts, scattere aroun t elandscape and usually buried. The art of prospecting involves looking forclues to the occurrence of useful materials and then digging to see what

may be there. Learning to identify features that indicate the presence ofgravel from the interpretation of maps and other information is a centralact v ty n prospect ng. However, t e most mportant parts are t e esstu y o owe y t e e survey an p t eva uat on, t e atter e ng amechanical process that depends on the prospectors experience and thequality and thoroughness of preparatory work.

In ormat on a out grave s n t e an scape comes rom t ree ma n sources.T s s ana yse a toget er to assess t e e oo t at grave may occurat a particular place:

n Geo og ca n ormat on, com ng rom geo og ca maps an reports.

n So s n ormat on, com ng rom agr cu tura so s maps an reports.n andscape information, coming from topographic maps, aerial photographs and satellite images.

T e tec n ques g ven n t e Gu e ne may e t oug t o as a too t,each being available independently for a range of purposes. They are

presented within a framework of the procedure for materials studies, sothat the prospector can know when and how to use them. But it remains fort e prospector to ec e w c tec n ques are appropr ate at any stage or nany part cu ar ocat on.

Construction materials are a non-renewable resource and in view of the

increasing importance attached to protecting the natural environment, its mportant to ma e t e est use o a source, once oun . T s mp es

mapp ng t an escr ng t accurate y so t at t can e correct y c ass ean t ere ore correct y app e to a spec cat on. I ea y, a recor oextraction should also be kept so that reserves can be utilised economically.These comments are especially true of the Kalahari and Okavango regions,where materials occur only in small amounts, are widely scattered and can

e cu t to n .

The Guideline covers natural gravels that occur in the country and dealswith weathered, transported and pedogenic materials. Hard rocks areexcluded because the methods of nding and extracting these sources

are n many respects erent rom t ose appropr ate or weat ere ,transporte an pe ogen c mater a s.

View from a light aircraft. Ancient wateranne s an t e pans coneasity e seen.

The calcrete probe has been a very useful toolor materia s prospecting in t e a a ari region.


10/97

Methods and Procedures for Prospecting for Road Construction MaterialsChapter 1Introduction

10

oa s epartment

. urpose an scopeT e Gu e ne s nten e pr mar y or tec n c ans respons e or carry-ng out surveys to ent y new sources o mater a . It conta ns n ormat on

on how to plan, organise and carry out surveys. It also summarises the

types of gravel that occur in various parts of the country and describes thegravel indicators to look for during prospecting.

T e Gu e ne s a so nten e or g way eng neers an pro ect manag-ers who make design decisions based upon survey results. The Guidelinecontains information on the interpretation of maps and the application ofremote sensing survey methods, not normally used locally because of thenee or spec a se now e ge. T ere w e occas ons w en consu tancyserv ces s ou e use n or er to opt m se t ese tec n ques. T e a vant-ages o t ese met o s an ow to o ta n appropr ate ass stance areexplained.

Textbooks and other standard documents that relate to the Guidelineare ste n t e Re erences. T ese wor s are a ava a e as sources ore erence. T e r most mportant proce ura po nts are summar se n t eGuideline. Organisations that are able to offer other assistance, such assources of mapping and consultancy services, are listed in Appendix 1.

1.3 Structure of the guidelineThe Guideline is arranged into three sections covering the background,theory and the procedure by which this knowledge is put into practice, asfollows:

Natural gravels are dened in this Guideline asoc pro uc s a ave een par a y ro en

own c em c a y an p ys c a y y wea er n g.Some of these products may have undergoneransport and re-deposition, or reworking of their

constituents by water movement and chemicalprocesses within the soil prole.

Calcrete borrow pit. Tsabong area.

The end product, a new road Molepolole-amaga roa .

n ro uct on

2 Environmental Regionsof Botswana

3 Gravels in Botswana

Gravel Indicators

5 Interpretation of Mapsand Photographs

6 Survey Methodology

Evaluation of Gravel source

8 Reporting

9 References

10 rev at ons

11 AppencicesAppendix 1 - Other relevant

Organisations

ppen x - ea er ng anweathered materials

Appendix 3 - Botanical indicatorSpecies

ppen x - ar ous ogg ngorms

Dumped base course material. Molepoloe-ep epe roa .


11/97

Methods and Procedures for Prospecting for Road Construction Materials Chapter 1Introduction

11

oa s epartment

n e rst part covers t e geograp ca sett ng, t e types o grave s t atare found in different parts of the country, and the important landformsand other indicators that are a key to locating gravel deposits. Chapter2 prov es ac groun n ormat on on t e pr nc pa env ronmenta cotrols that have led to the development of gravels. This is followed by a

escr pt on o t e ma n types o grave t at occur, an nc u es a genera -ised map of prospecting regions, reecting a combination of materialstype, geograp ca actors an prospect ng met o o ogy. C apter 4describes the types of landforms that are usually associated with gravels.

n he second part (Chapter 5) introduces the interpretation of maps, aerialphotographs and satellite images. The chapter describes the principaltypes o maps use n t e etect on o grave sources, an ow to nter-pret them.

n he third part covers the methods and procedures to be followed in orderto pro uce a success u survey. T s nc u es summar es o t e es stu yand eld survey procedures in the form of checklists of the activitiesescr e . T e proce ure or eva uat ng a grave source s a so out ne .

Chapter 8 summarises the information that should be included in a reporton the materials survey.

. Summary of procedureThe overall procedure is reviewed here in order to show how informationa out t e route corr or s u t up n a og ca way. Re er to t e ow a-gram in Figure 1.1.

The foundation of all prospecting work is the desk study. During this stagethe main decisions are made concerning the types of materials that occur,w ere t ey are most e y to e oun an t e manner n w c t e esurvey is to be conducted.

The phases of the desk study are:n Identify the project requirement including road location and length;

spec cat ons or mater a s n t e ormat on an pavement ayers; vo -umes of material required. This information is not within the scope oft e Gu e ne.

n Pro uce a prov s ona mater a s map.

n P an t e e survey, e.g. route, s tes to e v s te an equ pment anresources required.

Indicators of the occurrence of gravel in the project area may come frommany sources including reports, maps, aerial photographs, and past experi-ence. T e pr nc pa outcome o t e es stu y s a prov s ona mater a s

map showing existing and possible sources of material within the explora-t on corr or. T s n ormat on s use as t e as s o r p ann ng a o ow-ing work. Planning of the eld survey forms the nal stage of the deskstu y an ass sts t e prospector w t assem ng a equate resources or t eeld survey and carrying it out efciently.

T e e survey cons sts o two p ases:

n Locat ng potent a sources o mater a

n Eva uat on o eac source.

T e e survey, w c nc u es a reconna ssance traverse o ows t edesk study. Having obtained an overview of materials within the routecorr or, t e most avoura e s tes are nvest gate u y w t tr a p tt ng

and sampling.At present, reporting the results of materials surveys is limited to the

production of a printed report.

The extent of the desk study should be linked tothe size of the eld survey in order to balance theefforts of each exercise.

Data should be recorded in a computerised mate-rials database in order to facilitate easy accessto information and help to plan future prospectingactivities.


12/97

Methods and Procedures for Prospecting for Road Construction MaterialsChapter 1Introduction

12

oa s epartment

PROJECT PREPARATION

DESK STUDYn Specication and quantity of materials

equired

n Interpretation of background information

Provisional materials map

PLAN SURVEYnDecide survey methodology and

rocedure

n Carry out in-house or use consultant

n Environmental considerations

FIND & MAP SOURCES

n Reconnaissance eld survey

nDetailed eld survey

SOURCE EVALUATION (FIELD)

nAreal extent

n Prole description

n Visual classication

n Sample

DATA CAPTURE & FEEDBACK

n Reporting

n aterials inventory (database)

SOURCE EVALUATION

(LABORATORY)

n Laboratory testing

Figure 1.1 Flow diagram stages, for material prospecting.


13/97

Methods and Procedures for Prospecting for Road Construction Materials 13Chapter 2Environmental Regions of Botswana

oa s epartment

2 ENVIRONMENTALREGIONS OF BOTSWANA

2.1 IntroductionBotswana can be divided into natural regions based primarily on the geol-ogy and climate. Different landscapes and geomorphological processeshave characteristic soil and weathering proles and these can be used formaterial location purposes.

2.2 ClimateC mate a ects roc weat er ng an ence t e ormat on o natura grave s.Hot, wet conditions cause the rapid chemical breakdown of rocks. In dry con-ditions, chemical reactions are restricted even though temperatures may be high.

Large a y temperature uctuat ons contr ute to rap p ys ca s ntegrat on

T e c mate over most o t e country s sem -ar , w t a narrow analong the southwestern border with South Africa classied as desert. Thus,c em ca weat er ng or t e most part s s ow. T e We nert N-va ue rangesbetween 2 at Kazungula to more than 30 in the extreme southwestern areas.Typically, where N < 5 chemical decomposition of rock material prevails,w e p ys ca s ntegrat on o t e roc pre om nates w ere N > 5. T smeans that chemical decomposition is restricted to the area north of about20S latitude under present-day climatic conditions.

The annual rainfall follows the same trend as the Weinert N-values withabout 650 mm of rain falling at Kazungula and less than 175 mm falling int e extreme sout -west areas.

Botswanas landscapes are the result of continentaluplifts, changes of climate, sea level and cycles oferoson an epos on occurrng over geo ogcatime. These geomorphological processes havehad a profound inuence on the weathering ofthe rocks and the development of surface andpedocrete deposits

or a ona n o rma on, re er o e n er s o o :The natural road construction materials of

southern Africa.

The standard deviation of the annual rainfall isvery high and extreme wet and dry periods are notuncommon.

The minimum rainfall exceeds the 100 mm thresholdvalue for dune activity and thus active sandmovemen s res r c e o une cres s n oca se

.

igure . Annual mean rainfall in the country and N-values.

an unes can e seen in t e extream sout -western part o t e country ( o spits .


14/97

Methods and Procedures for Prospecting for Road Construction Materials14 Chapter 2Environmental Regions of Botswana

oa s epartment

he climate has varied signicantly in recent geological time. Evidencesuggests t at c em ca weat er ng assoc ate w t a ternat ng wet andry cycles during the last two million years affected the weathering ofroc outcrops an ormat on o pe ocretes. T e presence o err crete, orexamp e, n eastern Botswana s ev ence o trop ca con t ons n t e past

. eomorphologyGeomorp o ogy s an mportant actor n eterm n ng t e c aracter st csan eng neer ng propert es o a weat ere mater a .

ree aspects o geomorp o ogy are o concern to t e prospector:

n eathering, past and present (see Appendix 2).

n andscape formation - erosion and deposition.

n e ocrete ormat on com nat on o t e a ove two .

Botswana can be divided into three principal geomorphological regions:

Eastern regionRoc an scapes, compr s ng s an va eys an conta n ng most yresidual and transported gravels.

ii) Central regionMixed landscapes, intermediate between Regions 1 and 3.

iii) Western regionAn ancient basin now lled with deep sand (Kalahari). Subduedlandforms with pans, ancient dune elds with inter-dune hollows and

old river valleys, now dry (e.g. Okwa River, Mmone River, NalediVa ey . Pe ocretes ca crete an s crete are t e pr nc pa construc-t on mater a s.

2.4 eologyWhere gravel is derived from weathered rock, the parent material is the mostimportant factor governing the properties of the gravels. The bulk of theexposed hard-rock geology occurs in the eastern third of the country, therema n er e ng covere w t young san s. In t ese areas, t ere ore, pro-spect ng or roa grave s s ou ta e t e oca geo ogy nto cons erat on.

Botswana consists of ve geological regions: four solid rock regions and one

sand region. In each of the rock regions there is a mixture of rocks but one ortwo predominate. The regions are:

n aroo se ments an avas.

n ixed igneous, sedimentary and metamorphic rocks.

n Non-metamorp c roc s.

n asement rocks, into which various igneous rocks (mostly granites)ave been intruded.

n Geologically recent sands (Kalahari).

A s mp e map o t e geo ogy s nc u e as F gure 2.2.

The Kalahari also contains ancient drainagenes, which are now dry. These ancient drainageystems are now the site of pedogenic calcretesnd silcretes within the Kalahari sand regions.

ere s ev ence a a arge r ver sys em oncean sou roug o swana n o e range

River. Similarly, the Makgadikgadi Pan area waspparently once fed from the Okavango River as

well as other smaller rivers to the east.

Geomorphology concerns the interaction of rocksparent materials) with the processes of weathering,

erosion and deposition, over geological time.Water is the primary agent of erosion, but wind andgrav y a so pay a cons era e par

Aerial photograph, showing Sekoma Pan.


15/97


oa s epartment

2.4.1 Engineering classication of rock types

T e actua roc s ma ng up t e our so roc geo og ca reg ons avebeen grouped by Weinert (1980) into 10 classes for road building purposes,of which seven are relevant in this Guideline. The residual component ofthe rst ve comprise natural gravels for use in roads.

n Bas c crysta ne

n Acid crystalline

n High silica

n

Arenaceousn Arg aceous

n Pe ocretes

n ransported soils

T e basic crystalline groupcompr ses t ose roc s w t p ag oc ase e -spar an ma c m nera s pyroxene, amp o e, o v ne as t e r ma n con-stituents and only small quantities (if any) of quartz present. Typical rocksin this group are gabbro, basalt, diabase, basic and ultrabasic metamorphicsand metavolcanics. When the Weinert N-value is < 5, these materialsweat er to montmor on t c c ays an t us ave g p ast c t es an c ay

contents. W ere N > 5, t e roc s s ntegrate, any c ays t at o orm arehydromicas, and their properties are less problematic in road construction.The basalts north of Nata (N < 5) are renowned for their high smectitecontents and poor performance in roads.

asa t outcrop in a roa cutting, asanegoma roa .

igure . Simplied Geology MAP - Stratigraphy and major rock types.


16/97


oa s epartment

Acid crystalline materialscons st pr mar y o gran tes, gne sses anfelsites with a signicant quartz component. The quartz is stable and thefeldspar components generally weather to low activity clays (illite andkaolinite). Although N > 5 in the area south of Nata, certain materials showev ence o c em ca ecompos t on n cat ng t at t e oca N-va ue as

pro a y een ess t an 5 n t e recent past. Some o t ese mater a s e.g.e s tes aroun Moc u ave marg na p ast c ty n ex va ues PI a out

10) in terms of base course specications.

The high silica materialsconsist mostly of quartzites and quartzporp yr es. T ese mater a s on y s ntegrate eav ng a quartz-r c san yto ne grave mater a . W ere present n su c ent quant t es t ese grave smake good base and subbase materials.

Arenaceous rockscomprise mostly sandstones and arkoses and are apotent a y mportant construct on mater a . T e aggregate strengt s aregenera y re ate to t e cement ng mater a an t ose t at are cemente y

silica are usually more durable and stronger than those cemented by calcite orclay.

The argillaceous rockgroup is represented mostly by mudstones and shales.T ese mater a s, y e n t on, conta n c ay m nera s an are se om su t-a e as construct on mater a s or roa s. Weat er ng o t e mater a s sgenerally in the form of disintegration resulting in clayey gravels to clays.At best, argillaceous rocks can be used as subbase although when slightlymetamorphosed they can be used as a base material in light pavementstructures. Arg aceous roc s ten to e mo sture sens t ve an strengttest ng o soa e mater a s s essent a .

Pedocretes areprobably the most useful natural gravel materials for road

construction in the country, particularly in areas covered by Kalahari sand.They include primarily calcrete and silcrete with lesser volumes of ferricrete.

Pe ocretes are var a e n t e r compos t on an t c ness, vary ng romisolated pockets of gravels (nodules or glaebules) to thick deposits ofhardpan that can be ripped or crushed. Their formation is related mostlyto drainage structures, with the best materials being associated with fossil

pans an stream courses.

Transported soils cover a large proportion of the country and have beenused in road construction. They include unconsolidated material suchas alluvial gravels and sands (deposited by rivers), wind-blown sands,

an mater a accumu ate y grav ty on t e ower parts o s opes ca eco uv um or ta us .

2.5 Agricultural soilsAgricultural soils are of concern to the engineer because some soil groupshave distinctive characteristics of drainage, texture or structure that affectaspects of engineering design, for example, subgrade bearing capacity or

ept to so e roc . From t e prospectors po nt o v ew, some groupseve op no u ar or zons t at can e use as grave or roa construct on.

The 1:1 000 000 soils map of Botswana (and the 1:250 000 series of theeastern half) can provide useful information on the types of soils occurring

in certain areas. Indications of ferricrete and calcrete are particularlyrelevant in this regard.

Prehistoric wetter conditions produced the for-ma on o err cre es on roc s n eas ern areaso e coun ry.

ic seam o ecompose granite grave at aoot i in os ong i age.

r ose grave urrie in a ossi river va eynear sa ong.

A calcrete borrow pit along Molepolole-Lephepe road.


17/97


oa s epartment

Botswana conta ns s x ma n groups o agr cu tura so s. T ey are escr ebelow:

n Arenosols.Sandy soils occupying the whole western two thirds ofthe country (the Kalahari). A large area around Metsematlulu (north

west o Mo epo o e s mappe as possess ng ur pan n t e so pro e,aar t oug not neceessar y t c or zon cemente y s ca.

n Fluvisols. Soils developed on recent alluvium, wet but open-textured.They are not of interest to prospectors.

n Solonchaks. Structureless, open-textured soils of high salinity, oftencontaining calcic or gypsic horizons. These are found in the Makgadik-ga Pans.

n Luvisols.Non-expans ve c ayey so s. T e uv so reg on occup es t erock landscapes east of the Kalahari sand region. Those luvisols devel

oped on Basement Complex rocks are mapped as stony, which suggestsan increased frequency of gravel occurrence, probably quartzose. Someo t e uv so s w conta n err crete.

n Vertisols. Expans ve c ay so s. T ese occur n poc ets, espec a y onbasalts. Not of interest to the prospector.

n Lithosols. and regosols. Soils less than 0.5 m thick over bedrock.They can constitute good engineering gravel.

. ege a on

T e vegetat on cons sts pr mar y o savanna tree, s ru , an grass w taquat c grass an surroun ng t e arge pans an swamps an m te orestn t e nort . Vegetat on can n cate t e occurrence o grave eneat t e

soil. The type and health of vegetation is partly a function of the propertiesof the soil. Certain plants have adapted to specic soil conditions and can

be used as indicators of the presence of particular materials. Of particularmportance are t e ca c um to erant spec es t at n cate ca crete, scussen C apter 4.

2.7 Prospecting regionsTa ng cons erat on o t e env ronmenta actors scusse prev ous y,

Botswana can be divided into three regions based primarily on geomor-

phology. These are subsequently sub-divided into specic prospecting

reg ons ase on geo ogy an c mate. T ese are s own n F gure 2.3 an

Table 2.1.

n e ma n types o mater a t at occur w t n reg on.

nBy implication, the techniques that are appropriate for prospecting ineach region.

n e reg ons w ere prospect ng may e part cu ar y cu t e.g. t eO avango De ta . T e map an Ta e n cate to t e prospector:

Calcrete vegetation indicator - P. Lcubnitziae.

The Okavango Delta has proved to beparticu ary i cu t to ocate roa constructionmateria s.


18/97


oa s epartment

rospecting regions o otswana.

Sand region.

ransistiona region.

Rock region.


19/97


oa s epartment

a e . . rospecting regions o otswana.

-tcepsorPnoigergni

cihpargitartSforedroni,spuorg

ecnanimoderp

lairetamtneraP gnireenignEtrenieW(slairetam)scilatinispuorg

rehtOfoscitsiretcarahc

noigereht

SNOIGERKCOR.1 .snoigerkcorfosmrofdnallevarG.kcormorfdeviredslairetaM

a1

seireSgrebmrotScisabxelpmoCtnemesaB

sevisurtni

metsySprodsretneV

savaltlasaB&scinaclovateM

stnemidesatem

scinacloV

enillatsyrccisaB.slairetamroopyllaitnetoPhgihevahslevarglaudiseR

-icossafidoogeratub,IP.eterclachtiwdeta

detropsnartdnalaudiseR-erebnactahtslevarg

rosuoeraclacotnidekrow.slevargsuonigurref

htiwdetaicossasetisleFsllihfosepolsydnas

b1

gnubeLakywD&accE

grebretaWenopuaT

tnemesaBnaeahcrA.xelpmoC

senotstlis&senotsdnaSsenotsdnas&selahS

snoisurtnicisabemoS

skcorcihpromateM

suoecalligra&suoecanerA,slairetamdexiM yltsom fo

.ytilauqroopotetaredom;slevarglaudiseremoS

htiwseterclac(setercodep)slairetamdetaicossa

tlasablaudisertnenimorP.aeraeworeSnislevarg.nedrubrevodnaskcihT

,secalpniliosnottockcalBdetropsnartybnialrevo

slioscitinarg

c1

evisurtnInairbmacerP

tnemesabcitinarg

tnemesaBnaeahcrAxelpmoC

setinarg,sessienG

skcorcitinarG

enillatsyrcdicA

slevargydnasytilauqdooGsetisleflaudiseR.IPwolfo

noitcurtsnocdoogmrofslairetam

KCORREVODNASIRAHALAKNIHT-SNOIGERLANOITISNART.2 sulp,kcormorfdeviredslairetaM.

.snoigerdnasdnakcorhtobmorfdeviredsmrofdnallevarG.dnasnihtiwdepolevedsetercodep

a2oorraKrevosdebirahalaK &eterclachtiwdnaS

eterclissavaltlasaB

setercodePcisaB

fotnelaviuqelanoitisnarTa1noigeR

b2

gnubeLrevosdebirahalaKakywD&accE

&eterclachtiwdnaSeterclis

senotstlis&senotsdnaSsenotsdnas&selahS

setercodeP&suoecanerA

suoecalligra

fotnelaviuqelanoitisnarTb1noigeR

c2

revosdebirahalaK

citinargnairbmacerP,awgawgeS,tnemesab

naeahcrA,enopuaTxelpmoCtnemesaB

&eterclachtiwdnaS

,setiztrauq,selahSeterclis,setimolod,saval

citinarg,scihpromatemskcor

,suoecanerAsetercodeP

,suoecanobrac,acilishgihcisab;dica

fotnelaviuqelanoitisnarT

c1noigeR

d2iznahGegdiR

accE)iznahG(grebretaWakywD&

&senotstlis,senotsdnaSesokra

senotsdnas&selahS

suoecanera;suoecanerAdetaicossa,suoecalligra&

etercodephtiw

.dnashtiwsporctuokcoRytilauqdoogsmrofesokrA

lairetamnoitcurtsnoc

SNOIGERDNAS.3 .sporctuokcorhtiwdnasyltsomerasnoigeremoS.dnasnihtiwdepolevedsetercodepylniaM.snoigerdnasmorfdeviredsmrofdnallevarG

a3agkidagkaM

snaPid

sdebirahalaK eterclis&eterclaC etercodeP sliosenilasyreV

b3

dnanrehtuoSlartnec

irahalaK

sdebirahalaK

,keohstnahpilOfosporctuOgnubeL,akywD&accE

dnuoraSniaramaDdnaobogogoBdnagnobahsT

seterclachtiwdnaspeeD.seterclis&

stnemidesdexiM

setercodep&slioS

;suoecalligra;suoecanerAsuoecanobrac;acilishgih

'N'citamilcsiyradnuobN5=eulav

kcorhtiw,dnasyltsoMsporctuo

c3 sdebirahalaK

)sdeBelteltoB(seterclachtiwdnaspeeD

seterclis&setercodep&slioS ;ecafrusnoisoreelteltoB

snapwef

d3 sdebirahalaK seterclachtiwdnaspeeD

seterclis&setercodep&slioS pmawscidoirep/lanosaeS

e3ognavakO

atleD

sdebirahalaK seterclachtiwdnaspeeDseterclis&

setercodep&slioS pmawstnenamreP

secruosslairetamweF

f3nrehtroNirahalaK

sdebirahalaK &seterclac,dnaspeeDseterclis

setercodep&slioS citamilctub,b3otralimiS5


20/97

Methods and Procedures for Prospecting for Road Construction MaterialsChapter 3Gravels in Botswana

20

oa s epartment

3 GRAVELS IN BOTSWANA

. n ro uc onT e grave prospect ng reg ons ave een e ne n t e prev ous c apter.

Typical gravels likely to be encountered in these regions are discussed in

t s c apter.

3.2 es ua grave sResidual materials are the in situ, weathered remains of any parent rock.

Res ua mater a s are argua y t e east var a e o potent a construct on

mater a s an w ere rea y ava a e wou o ten e t e rst c o ce con-

struction material. The chemical and physical nature of the material is

primarily a function of the parent material (composition and structure) but

s a so strong y a ecte y t e mo e o weat er ng. T e qua ty o res uasoils in terms of road building generally improves with depth. Characteris-

tics such as the plasticity decreases, the grading improves and the strength

o ot t e u mater a an ts aggregate component ncreases. At some

point, however, the material strength reaches a value that ripping becomes

impracticable and blasting is necessary.

3.2.1 Acid crystalline rock gravelsThe most common and most used residual gravels are derived from the

weathering of acid crystalline rocks, particularly granitic rocks, gneisses

an e s tes.

Granite gravels(and soils) are typically found in the southeastern parts

o t e country w ere t e Ga orone gran te comp ex outcrops extens ve y,

and in parts of the central region where the Mahalapye Granite outcrops

(Region 1c).

These residual granite gravels are typied by the medium to coarse sub-

roun e quartz gra ns an arge, pa e w te, su -roun e e spar gra ns o

the Gaborone granite. Granitic gravels usually exhibit very good compac-

tion characteristics, low to moderate PI (0 to 10) and moderate to high GM

1.5 - 2 , a o w c mprove w t ept .

Gneiss gravels(soils) are fairly widespread, being derived from metamor-

p c roc s. T ey are a un ant nort o t e res ua gran tes Reg on 1c

around Francistown. The gneiss gravels are similar in character to the

residual granite gravels. Their engineering properties depend on the extent

o t e weat er ng an t e ept o t e weat er ng zone, ot o w c are

con-tro e y c mat c N-va ue an geomorp o ogy.

Felsite gravelsare oun n t e sout ern parts o t e country, nort o

Ga orone Reg on 1c aroun Moc u . Fe s te res ua so s ten to ave

a higher clay content and therefore if completely decomposed may not be

su ta e or most pavement ayers. W ere s ntegrat on was t e pr marymo e o weat er ng, t e grave ex ts goo eng neer ng propert es.

n orma on a ou e or gn o res ua gravescomes mainly from geological maps and reports.

ecompose e site, a so s owing unweat erecore stones. rea near i wane.

ran te outcrops n t e amaga area.

granite outcrop in t e amaga area.

orrowpit in t e os ong area.


21/97

Methods and Procedures for Prospecting for Road Construction Materials Chapter 3Gravels in Botswana

21

oa s epartment

3.2.2 Basic crystalline rock gravelsBasic crystalline residual gravels are not very common. Where they occur,they seldom have good engineering properties. This is a result of basic cry-sta ne roc s weat er ng rap y to act ve c ay m nera s w ere t e c mat c

N-va ue s ess t an 5.

Basalt gravels occur as isolated pockets spread throughout the country butthe most prominent locations are within the Stormberg basalt lavas. Theyare oun most y n Reg ons 1a, 2a an 3g.

The basal res ua grave s are typ ca y assoc ate w t ac cottonclay, which are derived from the decomposition of mac minerals. Thegravel may have high plasticity, especially where the rock is completelydecomposed. Where the rock is disintegrated the gravels tend to haveangu ar roc ragments, w c mproves t e eng neer ng propert es o t emater a . W en mpregnate w t ca careous mater a t e res ua asa tgrave usua y ex ts goo eng neer ng c aracter st cs t e PI s ow.

Dolerite gravelsare not very common as they tend to be in the form oflinear or localised intrusions. They typically form kopjies or dykes inRegion 1b.

T e res ua o er te grave s are s m ar to t e asa t grave s. T e o er tehowever tends to weather into sub-rounded core stones through the onionskin peeling mechanism. This results in an abundance of oversized corestones, which are usually difcult to break during construction

3.2.3 Arenaceous rock gravels

Arenaceous residual gravels are widespread as the Transvaal, Waterbergand Karoo Supergroups, and the Ghanzi Group occupy a substantialoutcrop area (Regions 1b, 2b, 2d and 3b).

Most o t e san stones o not weat er nto goo qua ty grave s un essimpregnated with pedogenic materials. However, those cemented withsilica tend to be more durable.

T e ar ose o t e G anz group Reg on 2 weat ers nto goo qua tyres ua grave s. T e res ua ar ose grave s o ten cemente y ca crete,silcrete or ferricrete material to form pedogenic gravel.

3.2.4 High silica gravelsH g s ca grave s cons st pr mar y o quartz te an quartz porp yr es.

Quartzitesare assoc ate w t areanceous roc s o t e Water erg anTransvaa Supergroups Reg on 1c . T ey are usua y goo construct onmaterials as they are composed almost entirely of quartz. Their character-istic hardness can result in difculty during processing to an acceptablegrading.

A pebble marker s a stone or grave ayer etween res ua weat ereroc an over y ng transporte so , cons st ng o quartz t c pe es n asoil matrix. Pebble markers are not very common and are usually too thinand too localised to be considered as sources of road building material ontheir own. n outcrop o quartzite on a ri ge neari e pits.

Unweathered dolerite core stones can be usefulfor stone pitching and gabion basket lling.Mahalapye - Shoshong road, Bonwapitse rivercross ng.


22/97


22

oa s epartment

. e ogen c grave sPedogenic gravels are probably the most useful, and often the only suitableconstruct on mater a s n t e san y areas o Botswana. T e r un que mo eo ormat on a so ma es t em t e most var a e mater a s an t e most

difcult to locate, particularly as they tend to occur in relatively small andlocalised occurrences.

Calcrete is the most common pedogenic gravel in Botswana. It is founda most everyw ere assoc ate w t ot res ua an transporte mater a s.W ere ca crete s assoc ate w t res ua grave t s usua y no u ar n nature,whereby the nodules are derived from parent rock fragments (Regions 1, 2and 3g predominantly). Such calcretes are common in the eastern corridor,where borrow pits are often overlain by clayey soils, especially where the

parent roc s a as c crysta ne roc . Ca cretes orme y t e cementat ono ne-gra ne so s are t e most common an mportant construct onmaterial in Regions 2 and 3. These calcretes occur typically in pans of

various sizes in inter-dunal depressions and in areas of grey sands.

e ogen c graves or pe ocre es are ormew en roc or so par c es ecome mpregna ewith, or cemented or replaced by, an additionalomponent. The most common impregnating

omponents are calcium carbonate, to formalcrete, iron hydroxide to form ferricrete andmorphous silica to form silcrete.

Pedogenic gravels are identied mainly by inter-retation of landscape features (geomorphology),upplemented by information from soil maps andccasionally geological maps. (Except in the casef the Kalahari region, which is shown on theational geological map).(Exept in the case of thea a ar reg o n, w c s s own on e na onaeo og ca map .

Ferricrete borrow pit along theahalapye - Machaneng road.

Four types of Calcrete:

Upper right NodularUpper left Hard plan

ottom rig t a si e sanBottom left Powder

Powder calcrete, Sekoma area.

a ci e san , sau area. Nodular calcrete, Tsabong area.

Hardpan calcrete, Sekoma area.

or e a s on e orma on an eve opmen cacre e re er o e er erg . or

he location, selection and use of calcrete inBotswana, see TRRL Report No. 1122 (1984).

Silcretes are increasingly being quarried forrusher run base course and chippings forurfacing in areas lacking conventional sourcesf aggregate, particularly in the Kgalagadi andgam an s r c s.


23/97

Methods and Procedures for Prospecting for Road Construction Materials Chapter 3Gravels in Botswana

23

oa s epartment

Silcrete is the second most widespread pedogenic material. Silcretes areoften found in association with nodular or hard-pan calcrete. Most of thelarge calcrete pans have silcrete in the outer margin of the pan (Region 3).T ese are usua y cu t to rea w t a p c axe an may requ re r ngto exp ore an ast ng to exp o t.

Ferricreteis found along the eastern corridor and is probably associatedwith historically wet conditions, as it is un-likely that they would formunder the prevailing climatic conditions (Region 1).

3.4 Transported materialsTransporte mater a s are w esprea an var e . T e type an qua ty o t ematerial is highly dependent on the geology of the source, the type and extentof weathering, the mode and energy of transportation, the age of the transportedsediments and whether any pedogenic modication has occurred

Transported materials can be classied as:

n Co uv a - assoc ate w t s opes.

n A uv a - assoc ate w t r ver courses.

n Aeolian - windblown sands.

Colluvial gravels are typically coarse grained i.e. gravel, cobble andou ers an t ere ore ten to ave a g proport on o overs ze mater a .

Alluvial gravel can e oun a ong ot act ve an anc ent r ver courses.In active channels, gravel is usually found on the inner side of river mean-dersand on terraces. Rivers produce large quantities of coarse and nesand that can be used for concrete and for sand seals on surfaced roads.

Buried alluvial gravels associated with ancient basins and braided riversare potentially useful sources of materials in the Kgalagadi Region. Thesegrave s are part cu ar y use u n t e Mo opo R ver, La e Ngam an Gwetaareas. A uv a grave can e mpregnate w t ca crete, s crete or err creteespec a y w ere t s epos te w t n a ra nage as n suc as a pan. A u-vial soils range from clay, sands and gravel to boulders.

Aeolian soils are ne grained. The most abundant aeolian soils are thesingle sized sands of the Kalahari beds. These sands are found everywheren t e reg on west o 26E at tu e Reg on 3 . T e san s:

n Form dune features, which are easily distinguishable on aerial photo-grap s an sate te magery.

n Have varying colour and characteristics reecting variation in engineer-ng c aracter st cs. Co our var es rom pure w te common n poc ets

un er grey san to grey s , an rom g t rown to ar rown anre s .

n Have c aracter st cs t at c ange w t ept . T ey are norma y non-p ast c on t e 0.425mm s eve, ut may ex t p ast c ty PI > 10 onthe minus 0.075mm sieve.

It s mportant t at t e mater a prospector a so ta es note or even samp essources of Kalahari sand, as they could be used for road construction.

Kalahari sand blended with calcrete has been successfully used in pave-ments both as selected subgrade and subbase. Laboratory testing should becarr e out to eterm ne t e opt mum m x proport ons

Information on transported soils comes frominterpretation of features in the landscape, plusanalysis of soil maps. Sometimes geologicalmaps also give clues.

Typical quartz pebbly gravel found in burriedrai e river c anne s near i epits.

Kalahari sand has been successfully used as ll,se ec e su gra e an su ase on ow vo umeroads. The grey sands, which often have goodcompaction characteristics, occur as overburden tocalcrete gravels and on inter-dune depressions.

urrie a uvia grave west o weta.


24/97


24

oa s epartment

e two pre ominant types o materia s ine country. a a ari san an ca crete cover

about 80% of the country`s area.

Kalahari sands varies in thickness from a fewmetres to approximate y metres.

igure . . e ations ip etween soa e an proportion o a a ari san mixein wit ca crete.

e s etc map s owing avai a e roaconstruction materia s in t e country.

igure . . ypica gra ing enve opeo a a ari san .

Figure 3.3. Typical density curve ofa a ari san .


25/97

Chapter 4Gravel Indicators


oa s epartment

4 GRAVEL INDICATORS

. n ro uc onT ree types o grave n cator are recogn se n Botswana:

n Landform.

n Botanical indicators (specialised plants).

n An ma n cators traces e t y an ma act v ty .

4.2 LandformLandform refers to the conguration of the ground surface in a distinctive

shape. Landform is important in gravel location, as different types ofgravel are associated with particular landforms. This association is due to:

n e presence o mater a near t e sur ace g v ng r se rect y to aparticular type of landform. (e.g. a band of harder rock gives rise to abump on a slope or a at hill top, depending upon the rocks positionand orientation).

n e eve opment o a part cu ar type o grave n re at on to a geomor-p o og ca eature e.g. a r ver terrace typ ca y s ma e up o a uv agravel).

Identication of landforms should be done during the desk study usingaer a p otograp s an maps.

4.2.1 Landforms of rock regionsRock regions have the typical range of gravel landforms illustrated in Table4.1. This table is the key to the landform names given in Table 4.2. Table 4.2ta es t e ma n parent mater a s escr e n C apters 2 an 3 an n -cates t e an orms t at are common y assoc ate w t eac .

4.2.2 Landforms of sand regionsThe landforms in the sand regions of the country are given in Table 4.3,an t e grave s assoc ate w t t em are g ven n Ta e 4.4.

I grave part c es are present n t e so , t e processes o weat er ng anerosion will tend to bring some of these to the surface. The accumulationof a layer of stones and gravel on the surface can give the false impressionthat the soil is composed of gravel, or that a gravel layer lies at depth. Beforemar ng t e area as a potent a source o grave t s mportant to c ec t ata grave ayer s actua y present, y n ng an expose pro e. A gooindication as to whether there is a possibility of nding good gravel is tolook around the base of trees to see if growth has brought pebbles to thesurface. Check by digging a prole near the tree or by use of the calcrete

pro e

4.3 Botanical IndicatorsT e presence o certa n p ant spec es an somet mes t e nature o t e rgrowth can depend upon the mineralogical and physical properties of thesoil in which they are growing. Botanical indicators can thus be a useful

o a occurrences o grave an orms are nec-essarily associated with gravel suitable for roadconstruction, nor do all occurrences of gravelnecessarily give rise to an identiable landform.

num er o sma an arger pans occur in t ega aga i region. ese o ten contain goo

qua ity ca crete. is pan is t e e oma an.


26/97



oa s epartment

mrofdnaL )noitces-ssorc(margaiD

potllihtalFsegdeprahshtiw)uaetalp(potllihlevel,talF

snigramta

potllihgnipolSneewtebegdeprahshtiwpotllihtalf,denilcnI

pid)eltnegerom(dnaepolspracs)peets(epols

llihlacinoCniralucricsselroerom,potdetniophtiwlliH

htoomsroralugerriebyamsediS.nalp

dnuoMro,sepolsxevnochtiwpotllihdednuoR

potllihrouaetalpano'pmub'xevnoc

egdiRssorcagninnuregdirworran,thgiarts,gnoL

suoenginaybdemrofyllausU.yrtnuoc

nievztrauqaro)llisroekyd(noisurtni

hcnerTgninnurnoisserpedworran,thgiarts,gnoL

mrofdnalegdiRehtekiL.yrtnuocssorca-urtnisuoenginaybdemrofyllausu,)evoba(

gnidnuorrusehtesacsihtni,revewoH.nois.noisurtniehtnahttnatsisereromeraskcor

foriapaybdemrofsihcnertehtsemitemoSehybdemrof,segdirlellarapdecaps-ylesolc

ehtybdrah'dekab'gniebkcorgnidnuorrusnoisurtniehtfotaeh

epolstooF,epolsrepeetsafotoofehttaepolseltneG

fonoitalumuccaehtyb)esacsihtni(demrofehtni)etercirrefyllausu(levargcinegodep

eliforplios

ecarreTafoegdeehttadetautismroftalpdesiaR

reviraybdetisoped,yellav

nialpdoolF.revirgnidniwhtiwroolfyellavtalf,daorB

foedisniehtnoetalumuccalevargdnadnaSsdnebrevir

aid to materials location. However, plants are adaptable and the absence ofan indicator species does not necessarily mean that the material is absent,

w st t e presence o t e n cator spec es oes not a ways s gn y t at t eun er y ng mater a s su ta e or eng neer ng purposes.

A list of species used for the location of pedogenic materials is providedin Table 4.5. A guide to the identication of the listed species, togetherwith illustrations, is provided in Appendix 3. Plants are usually used to

e p ent y pe ogen c mater a s n san reg ons. In some nstances, ana un ance o certa n spec es n cates t e a sence o roa construct onmaterials near the surface. In rock regions landform features are usually amore direct indicator.

A st nct c ange n t e spec es type, a ense t c et o a part cu ar

spec es or a c ange n t e orm o t e p ants may e su c ent to ent ya potential source. The ability to identify the exact species is not alwaysnecessary in these instances.

Various publications have excellent leaf keys,p o ograp s, s r u on maps an escr p ons

a are su a e or e en ca on o ebotanical indicator species. Assistance can alsobe obtained from the Botany Department at theUniversity of Botswana.

oca n a an s are ques one or nvo v e nhe study, knowing the local names of plants is

often useful. Note, however, that identicationof plants using local names can be misleadingdue to the wide variety of names used for theame plant.

Common land form in the eastern part of thecountry. i ge a onge t e i a eya - gagodi ct. e op e - artins ri t.

Table 4.1 Landforms of rock regions,associated with gravel deposits.

Large borrowpit on the Lephphe-Shoshongoa . ase qua ity materia (granite, quartz-

ic grave .

Dumped gravel base course, Lephephe-

os ong roa .


27/97



oa s epartment

Table 4.2 Types of gravel associated with rock regions.

Granite, quartz-rich gneiss. Mmadinare area.

epytlevarG lairetamtneraP mrofdnaL smelborpgnireenigneelbissoP

SLEVARGLAUDISER

kcorderehtaeW avaL llihgnipols,potllihtalFpot

niatnocyamnoitisopmoccisabfoavaLslarenimelbarehtaew

ekydsuoengI hcnertroegdiR niatnocyamnoitisopmoccisabfosavaLslarenimelbarehtaew

llissuoengI llihfoedisnoegdiR niatnocyamnoitisopmoccisabfosavaLebyamlliS.slarenimelbarehtaew

nonoitisopotgniwo,tiolpxeottluciffid)nedrubrevo-(llihfoedis

ssienghcir-ztrauq,etinarG dnuoMenoN

elbarehtaewniatnocyamsessienGslarenim

enotsdnaS potllihtalf,dnuoMenoN

tfosrehtarebyamselcitraP

aiccerb,etaremolgnoC llihgnipols,potllihtalFpot

dnuoMenoN

.dednuoreraselcitrapetaremolgnoCyamselcitrapesraocehtfoseitreporP

xirtamehtfoesohtmorftnereffideb

etimolod,elbram,enotsemiL llihgnipols,potllihtalF

pot dnuoMenoN

ztrauqnieV sepytllafosessieng,etinarG egdiR selcitrapfokcolretnilacinahcemrooP

SLEVARGDETROPSNART

enilenotsztrauQ epolstooFenoN

epolstooF

.selcitrapfokcolretnilacinahcemrooP.senifynamootniatnocotdnetyaM

muivulloC epolstooF senifynamootniatnocyaM

muivullA ecarreTnialpdoolF

dnaydnasnetfo,selcitrapdednuoRsenifnignikcal


28/97



oa s epartment

mrofdnaL )noitces-ssorc(margaiD

'mroftalp'htiwnaPlaminimroonhtiwnapderoolf-talFhcnebwolasi'mroftalP'.noitategev

tubnapehtfoegdeehtnodetautis

.dnuoryawehtllagnidnetxetonyllausu sselro,ssorcam005nahteromebyaM

roevitcnitsidsayllausutonsimroftalP:etoNerehnwohssasuoivbo

'mroftalp'tuohtiwnaPfotnempolevedonhtiwnapderoolf-talF

ebyaM.egdeehtdnuorahcnebwolaebyaM.sselro,ssorcam005nahterom

sessargniatnocro,noitategevtuohtiw

noisserpeD,ecafrusdnasehtniwollohevacnoC

ssargehT.sessarggniniatnocnidegnarranetfoeraseitinummoc

noisserpedehtdnuorasenozcirtnecnoc

wollohenud-retnInilennahcevacnocthgiarts,gnolyreV

gnimrofynamfoenO.ecafrusdnas

swollohhtiwsesirraenillellarap fohtuosaeraehtnideifipyT.neewtebdepolevederaseterclaC.enaawstakaT.wollohehtfoenilehtgnolaslavretnita

)lennahcrevirdlo(yellaV.dnashtiwnidellif,yellavreviryrdA

eltneg,daorbafomrofehtsekaTrofsdnetxetahtnoisserpeddetagnole

neewtebneessa,sertemolikynamylisae,secalpnI.adreWdnaaekahKos,srehtonitubdnuorgehtnoelbisiv

yldrahebotsawollahsdnaediwelbatceted

dnasyerGdnasyergylno,feilercihpargopotoN

rohsiddergnidnuorrushtiwgnitsartnoc

sdnasnworb

mrofdnaL lairetaM dnascitsiretcarahCstnemmoc

mroftalphtiwnaPmirdnuora

ylbissop,eterclaCraludonronapdrah

etercliS

sieterclacytilauqtsebehTmroftalpnapehtnidnuof

tuohtiwnaPmroftalp

eterclaC

etercliS

tubruccoyameterclacdooG-erptonsiytilauQ.lausutonsi-nocyamsnapegraL.elbatcid

eterclacredluobrodrahniat

egralehtnismrofetercliSidagkalagKehtfosnap

noisserpeD .raludonebnaceterclaCro,ecnerruccoonnetfO

dnassuoeraclac

.eterclacytilauqroopyllausUsepolsedisehtnoruccoyaM

wollohenud-retnI -draheterclisdnaeterclaCrobmocyenohronap

raludon

slairetamytilauqdoog,yllacoLtsomrevoenonyllarenegtub

htgnels'wollohehtfo

revirdlo(yellaV)lennahc

ylbissop,eterclaCraludonronapdrah

slairetamytilauqdoog,yllacoLtsomrevoenonyllarenegtub

emoS.htgnels'yellavehtfoevisnetxeniatnocsyellav

sdnasdeificlac

yerg-mrofdnaloN-artnoc,ylnodnas

-dnuorrushtiwgnitsdnasdergni

.dnassuoeraclaCeterclacemosylbissoP

eterclacytilauqroopyllausUsidnasfirettebebyamtubsdnashsikcalB.citsalp-non

ytilauqrettebsdleiyyllausulairetam

Table 4.3 Landforms of sand regions,associated with gravel deposits.

Table 4.4 Gravel types accociated with sand regions.

river c anne . ( t e t a orrow pit area .Sehitwa - Tsau road.


29/97



oa s epartment

4.3.1 Factors inuencing the use of botanicalindicatorsAlthough botanical indicators are a valuable source of information for

material location, certain human and natural inuences can limit theire ect veness.

Species identicationmay e cu t or nexper ence persons.e erence to a su ta e ea ey an oo s or ent cat on w e e p u .

Human occupationin rural areas, where shifting cultivation and theregular cutting of trees changes the natural species mix, may suppresshe normal spread of indicator plants. Certain species have cultural

s gn cance or are use or me c na purposes. T ese are e t uncut ancan ecome sproport onate y requent.

n areas where livestock overgrazingoccurs, signicant changes in thevegetation become evident and indicator species can be trampled or eatenback to ground level. Selective browsing can give the false impression that

e rema n ng unpa ata e spec es are om nant. In areas w ere t e naturapasture grass s unaccepta e or graz ng, ot er more su ta e spec es areo ten p ante , t us g v ng a a se n cat on o t e eco ogy.

Natural bush resor regular burning of grassland for grazing purposesmay eliminate species that could be suitable for indicator purposes.

Riverine vegetationmay give false indications of the underlying mate-r a , ecause some o t e spec es may ave een transporte n rom stant

oca t es

e env ronmen a cons ran s assoc a ew wor ng a acen o r v ers mus a so econs ere e ore excava ng n ese areas.

a e . n icator p ants or pe ogenic grave s.

lairetaMepyt

emanlacinatoB nommoCeman

emanlacoL

eterclaC iikcelfaicacAarefillemaicacAiinworbenaicacA

sneiciferaicacAirdnaxelasetcarhpotaC

ebrebmimuterbmoCarenicsyhcatsorhciDsediociresulahpecoirE

ruolocibaiwerGavalfaiwerG

snecsevalfaiwerGsisnelagenessunetyaM

eaiztinbuelaehcseol-leuhcePsetarohpmacsuhtnanohcraT

sedionurpailanimreT

nrohtedalBnrohtkooH

aicacaretaW

allerbmueslaFnroht

nrohttepmurTdoowdaeL

hsubelkciShsubwonS

hsubydnarbeslaFhsubydnarB

yrrebyeknoDeertittefnoC

hsubrettiBhsubrohpmaC

uhahoM/alakoManagnoM

ayugnupurO

ayugnupurOeleselesoM

isiweLanawgoM

awltroMahtahpmagkoM

onohtoMidukoM/istemelaltoM

ahltahoMaraistoM

etercirreF sumocuenogopordnAmutalucipamuterbmoC

siragluvebeotS

ssargekalfwonSwolliwhsubdeR

hsubtorknaB

iriduhoM

dnaspeeD( setacidnI

foecnesba)slevarg

nolyxotameahaicacAsisneiassamaihpaB

anacirfaaekruBaeciresailanimreT

nrohtlemacyerGdoowmacdnaS

agnirysdeRailanimretrevliS

olohkoM

otanoMononogoM

Plants may be good indicators for pedogenicgrave . a opong area.


30/97



oa s epartment

. n ma n ca orsThe activity of certain animals results in gravel particles, soil or stones

e ng roug t to t e sur ace. Examp es are term tes, porcup nes, sur cates,groun squ rre s Se ate-Mos ma , Ant eater ta a u an serunya.

Examination of the material in termite mounds and anthills, and materialexcavated from burrows should be carried out for indications of anygravelly material brought to the surface.

nt- i s may give va ua e in ormation or agrave prospector. rapa area.


31/97

Chapter 5Interpretation of Maps and Photographs


oa s epartment

5 NTERPRETATION OFMAPS AND

PHOTOGRAPHS. n ro uc on

T e use o ex st ng maps, aer a p otograp s an n certa n nstances,sate te mages are essent a or a success u e survey. T ese are estut se ur ng t e es stu y w en a poss e sources o mater a can eidentied.

Topographic and geological maps are most useful for prospectors as theyprov e n ormat on a out an orm an parent mater a s. To a esserextent, agr cu tura so maps can a so e use .

Aerial photographs and satellite images are supplementary sources ofinformation as they show information additional to that contained in maps.Maps an p otograp s, nterprete toget er, orm t e est com nat on or

ent y ng poss e mater a sources.

. opograp c mapsMe um- or arge-sca e topograp c maps can n cate t e an ormsassoc ate w t grave s. T ey a so conta n ot er use u n ormat on sucas roa s, access, an -use, m nes an quarr es, eve opment an water,

which could provide indications of potential sources of material andassistance with appraisal of the area.

5.3 Geological mapsGeological maps are probably the most important source of preliminaryinformation available to a prospector. They indicate the parent materialsin the area and details such as quarries, faults, rock outcrops and igneousntrus ons. T e types o weat er ng pro ucts can e n erre rom t sn ormat on. A geo og ca maps are accompan e y a narrat ve memo r.

Reference to these is recommended. A generalised geological map is givenin Figure 2.1.

The features of geological maps that prospectors need to be aware of are:

n he map shows the rocks as they would appear if all the supercialdeposits such as soil or unconsolidated materials (e.g. thin windblownsand and alluvium) were removed. However, very thick surface depositssuc as t e san s o t e Ka a ar are s own ecause t ey are t e

om nant mater a or a pract ca purposes.

n Geological maps rarely indicate the presence of gravel deposits. How-ever, geological memoirs often contain information on the degree ofweat er ng or ractur ng o t e roc s an t ere ore n cate t e poss -

ty o a su ta e grave source. Memo rs a so escr e super c adeposits such as pedogenic and coarse alluvial materials.

n erpre e ea ures s ou e mar e on abase map to bring all information together andto guide eldwork.

Topographic maps, aerial photographs, air photomosaics and information about other sources ofremote sensing aid is available from the Departmentof Surveying and Mapping, Gaborone.

or a ona n orma on on erran evaua onand the interpretation of geological maps andaerial photographs for materials location, refer tothe Terrain Evaluation Manual and Road buildingmaterials in Botswana.

The 1:1 000 000 geological map of Botswana issufcient for an initial estimate of road constructionma era s or e purposes o prospec ng, anmore detail can be obtained from larger scalegeological maps. Regional geological maps areusually published in conjunction with a report (ormemoir), describing the geology of the area indetail. The Geological Survey in Lobatse may beconsulted for information on the availability ofgeo ogca maps.

Geological Map.


32/97



oa s epartment

n The map legend (the key to the rock types on the map) may identifyeatures t at are re ate to grave epos ts, suc as quartz ve ns, y es

or a uv a mater a s.

Geological maps are used as follows:

i) Draw the road corridor on the geological map or a copy, in order toeterm ne t e approx mate c a nages at w c geo og ca un ts cross

the road alignment. The 1:250 000 geological maps, usually appendedto the District Memoirs, are recommended for this exercise.

ii) Read the map legend and the accompanying geological report for anyment on o roc s t at potent a y conta n grave -s ze weat er ng

products, such as gneiss with quartz veins. Note the geologicalun ts w t n w c t ese are oun .

I ent y t ese geo og ca un ts on t e map, w t n t e exp orat oncorridor. Pay particular attention to oddities on the map such asm nor gneous ntrus ons an unusua y prom nent s o ten

named). These features often contain gravels when the surroundingcountry has none.

iv) Identify possible sources of material as detailed in Chapters 3 and 4an rom past exper ence.

v Stu y t e ent e areas o geo og ca nterest n aer a p otograp sto pinpoint the locations most favourable for eld investigation.

. gr cu ura so mapsAgr cu tura so maps an reports g ve n cat ons o t e presence o

pedogenic gravels if these occur in the area. Agricultural soils maps arenot concerned with the deeper parts of the weathering prole or withgravels specically, and therefore rarely contain information relating toin situ gravels or transported gravels. However, as with a geological map,t s wort rea ng t e so egen an reg ona so report or ment on o

features such as stony phase of a soil.

5.5 Remote sensing techniquesAerial photographs and satellite images (collectively called remote sensingsystems) are valuable sources of information, which can be used for mate-r a ocat on. T s n ormat on s o ta ne y ter ng out aspects o t elandscape that are relevant to prospecting and using this to supplement theinformation taken from maps.

5.5.1 Aerial photographs and interpretationT e reso ut on o aer a p otograp s s o g qua ty an every eta othe ground surface down to one or a few metres in size is shown. Featuressuc as m nor roc outcrops can e seen c ear y. Aer a p otograp s are notconsistent in terms of scale. Change in scale within a photograph does nota ect nterpretat on, ut t s s tuat on nee s to e apprec ate w enevermeasurement or calculation of distance is involved.

Aerial photographs can be viewed in three dimensions, that is, slopes andrelief can be seen. The vertical relief in aerial photographs is exaggerated,ma ng s opes appear steeper t an t ey rea y are. T s c aracter st c san advantage because it enables very small changes of elevation, thaton t e groun are are y scern e, to e seen. For examp e, n aer a

photographs one can readily recognise low relief landforms such as thecalcrete features in the Kalahari.

Black and white aerial photographs are the mostcommonly used remote sensing technique forgrave prospec ng n o swana.

Transferring information from aerial photographso maps is most easily done freehand to

overcome scale inconsistency. When usingaerial photographs for navigation in the eld,istances measured from a photograph may onlye approxma e.


33/97



oa s epartment

Aer a p otograp s can e a out an re-p otograp e as a s ng e s eet,called an air photo mosaic. Air photo mosaics are useful as compilations eets upon w c to recor t e resu ts o an a r p oto nterpretat on ormap interpretation exercise, especially in areas such as the Kalahari whererelief is very low and landmarks are few.

The air photo interpretation commences during the desk study.P otograp s cover ng t e corr or to e nvest gate s ou e or ere

prior to commencement of the desk study. These should be interpreted inpara e w t t e stu y o t e maps.

W en carry ng out an nterpretat on, eg n y ma ng a note o t efollowing general characteristics of the terrain. Afterwards, look in detailfor the landforms described in Chapter 4.

i) Relief and landform.Hill areas, rolling or undulating land, terraces, oodplains and low-lying land. Look especially for relief features that are too small to bes own on maps.

ii) Drainage.n Note areas w ere ra nage pattern s cons stent, an oun ar es

w ere one pattern c anges to anot er. Boun ar es etween ra n- age patterns often signify geological boundaries.

n Note part cu ar y areas w ere ra nage s a sent n cat ng g ypermeable soils), or consists of parallel streams or straight streamcourses (rock is controlling the stream pattern and is therefore nearthe surface).

iii) Vegetation and land use.Although it is not possible at normal air photo scales to distinguish indi-vidual plant species such as gravel indicator plants, plant communities

o a type t at may conta n n cator p ants are o ten ent a e.iv) Human features

Farm oun ar es, m n ng compan es ences an u ngs g ve n ca-t ons o t e extent o propert es an w ere t e owners may e contacte .

Based on all of the information gathered above, as well as that gatheredfrom maps, identify possible prospecting sites.

Dur ng t e es stu y:i) Identify a spot on the aerial photograph that appears to be most typical

o t e w o e s te, or t at seems to o er t e est prospect o n ngmaterial.

ii) Identify routes of access to the prospecting sites if these are not clearfrom the topographic map.

iii) Mark all interpreted features on the base map or directly onto thep otos us ng a t n wax penc

Construction Material Survey

Documents

Transcript of Construction Material Survey