Bisya Area Site - Al Hajar Project: Conservation Report
Richard and Helena Jaeschke Nov - Dec 2011
The conservation work began on 28th Nov 2011 and finished on 17th Dec 2011.
Following from the report submitted to
the Ministry for Heritage and Culture in
2010: Conservation Proposals for the
Site of the Al-Hajar Project, two trial
areas were chosen for stabilisation
treatment, both in Building 4/7 on the
Sallut plain.
Building 4/7 on the Sallut plain
There had been some damage to the site from the recent flood waters, which had
entered the excavated areas on both the eastern and western sides. An area of the
southern baulk, approximately 2.5 metres long by 40 cm wide had collapsed, along
crack lines which had been evident during the investigation visit in March 2010.
Southern edge - March 2010
Southern Edge – Nov 2011
Dried sediments on the base of the
excavated areas and on the upper
surfaces of most of the stone blocks
showed that a considerable depth of
flood water had entered the site,
depositing silts. A safety tape was laid
out along the fragile baulks to warn
people not to stand too close to the edge.
Laying out the warning tape
The two walls chosen for treatment were the wall of mixed large and small stones
which runs west-east just south of the circular wall of the platform monument, and
the most southerly exposed wall, made of very large blocks. Each wall was treated
up to the edge of the exposed areas, where adjacent tumbles of stones and
sediment have not yet been excavated.
Wall 1 of mixed large and small stones Nov 2011
Southern wall (Wall 2) Nov 2011
Wall 1 after initial clearance
Wall 2 after plant clearance
In addition, four small test walls were
constructed outside the fenced area of
the monument, using stones which were
lying piled nearby to the east of building
4/7. These were erected using the silt
from the surface of the plain, mixed with
water, as a rough mortar, to mimic the
earth packing found in the walls. It is not
yet clear whether mud mortar was used
during any phases of construction in the
monument itself; many of the stones
appear to have cracked when further blocks were placed on top of them, which
implies that they were lying unsupported on the uneven surface of the lower courses.
Building the first test wall
Construction is much easier if a mud mortar is used, but requires a considerable
amount of water.
Plants, which had grown extensively in
the excavated areas of the site, were
cleared by a team of 6 workmen from
Bisya, provided by the Ministry for
Heritage and Culture. They also
constructed three of the test walls,
copying the first wall which was built by
the conservators.
Materials for the conservation work were
gathered and taken to the site – washed
Workmen construction 3 more test walls
sand and sarooj (6 bags of each, kindly provided by Ahmed Al-Tamimi and Ali
Adawy from the stores at Bahla fort) and hydrated lime (2 bags purchased from
Construction Materials & Industries at Al Amerat, near Muscat) as well as a water
tank, filled by the local water station just outside Bisya.
An area of the wall approximately 1 metre wide by 1 metre high was treated by each
conservator at a time. The loose, cracked silts on the upper edges of the blocks were
cleared away using pointing tools and brushes and any remaining earth packing
behind removed to a depth of about 7 – 10 centimetres, depending on the size of the
blocks and the gap between them.
Tools for clearing the silt from between the stones
Clearing the silt and some of the earth packing
The area was then sprayed lightly with water and filled with a mortar made from
sand, sarooj and hydrated lime. The proportions for the mortar were those
recommended by the Dept of Civil Engineering at Sultan Qaboos University after
testing1: 3 parts sand, 3 parts sarooj, 1 part hydrated lime and approximately 2 parts
water, measured by volume. The sarooj was sifted to remove large pebbles and
unbroken lumps and mixed with the dry sand and lime before the water was added.
Enough water was added to provide a mixture which was soft enough to bind
together when compressed, but not too wet as this would allow the material to slump
too much and cause the finer particles to migrate to the surface.
Seiving the sarooj to remove lumps
Mixng the sieved sarooj with washed sand
Mixing in the hydrated lime
Adding the water
The correct consistency of mortar
Tools for applying the mortar
The mortar was firmly packed into the joints of the stonework, using spatulas and
small trowels. It was built up to within about 1 centimetre of the surface in small
joints and areas around small stones, and about 2-3 centimetres of the surface in
wider joints and areas between very large blocks. A considerable amount of
judgement needs to be exercised when determining the best level to fill the mortar,
according to the shape and arrangement of the stones in each part of the wall. Care
was taken to make sure that as little mortar as possible was applied to the front
surface of the stones. Any small spatters which fell on the surface of the stones were
immediately removed and sponges dipped in water were used to wipe away any wet
smears.
Packing the mortar into the cleaned joints
Spraying the walls with water
Tarpaulin to keep the area cool
As the mortar was drying it was sprayed lightly with water at intervals. A tarpaulin
was arranged over the work area to keep it shaded and cooler, allowing the mortar to
set more slowly. This allows the mortar to achieve greater strength. The tarpaulin
was held in place with ropes tied to the fence posts and weighted down with loose
stones brought in from outside the
fenced area. Each loose stone was
marked “O” for “outside” with a
waterproof, fade proof black marker
pen.
The next day the mortar was stiff, but
still soft enough for the surface to be cut
back a few millimetres to a level
surface, using flat-ended spatulas and a
Tools for cutting back the mortar surface
fine steel wire brush. This neatens the surface of the mortar and exposes the larger
sand grains, providing a more natural appearance.
Finishing the mortar surface
The finished mortar surface
The work progressed from the bottom to the top of the wall, in approximately 1 metre
squares. Adjacent areas were left untreated until the mortar was strong enough to be
load-bearing and support the stones, then the remaining areas were treated. Each
conservator could clear and fill approximately 1 square metre per day and the 15.28
square metres of the first wall were completed in less than 8 days. The mortar will
continue to harden for several months as it absorbs carbon dioxide from the
atmosphere.
The second wall, built from much larger blocks, with larger gaps between the stones,
required a slight modification of the technique. Some of the joints were so wide and
tall that the mortar would slump and sag if attempts were made to fill the entire area
with one application. Instead, the joint was filled in stages, applying a layer of mortar
approximately 8 – 10 centimetres deep and allowing it to harden for an hour before
applying the next layer. This wall, approximately
13 square meters, was completed in less than 5
days.
Wall 2- large blocks with wide joints
Building up the mortar in steps
Parts of broken blocks were correctly
positioned using small wooden wedges to
hold them in place until the mortar was
applied and had hardened. Once the
mortar was firm, the wedges were
removed and the small holes filled with
mortar.
Displaced broken fragment of a block
Fragment held in place with wedges
Fragment correctly positioned with mortar
A small piece of partially carbonised wood was found between
two blocks at the eastern end of Wall 2 as the loose silts were
being cleaned away. This was kept as a possible sample for
Carbon 14 dating.
Materials used
The conservation of 28 square metres of wall used 5 bags of
sand, 4.5 bags of sarooj and 1.66 bags of hydrated lime, mixed
into approximately 42 batches of mortar. Approximately 189 litres
of water were used in the mortar and 192 litres were used for
spraying the wall to keep the mortar cool and moist while it was
hardening.
Wall 1 Before treatment
Wall 1 After treatment
Wall 2 Before treatment
Wall 2 After treatment
Wall 2
The completed walls after treatment
Wall 1
The test walls were treated with different mixes of mortar: one was left with just the
mud mortar fill as a reference. The mortars used in the other walls were:
Wall Sarooj Sand Lime Water
2
3
3
1
2.25
3
2
3
1
2
4
4
3
1
2.5
2 3 4
No mortar
The mortars will be examined next year to
see how they have aged during the
intervening months. If necessary, the
mortar mix can then be adjusted for use in
the treatment of further areas of the
monuments.
A safety enclosure was designed by the conservators and
made from steel by Kausar Ahmad at Healty Tools and
Metals in Bahla. This will be used when taller or less stable
sections of wall are being treated. The enclosure allows the
conservators to work at the base of the wall and will protect
them from possible falling fragments of masonry, allowing
them time to scramble clear if any large blocks detach from
the top of the walls.
Flood protection
Flood water entry on the western side
Flood water entry around the southeast corner
The conservators had hoped to create two berms or low walls of earth and stone
using material from the old spoil heaps near the site, to divert floodwaters away from
the excavated area. The workmen created a short berm on the northwest side of the
site, but the JCB did not arrive to construct the longer berm on the east side of the
site. It is hoped that the Ministry of Heritage and Culture may undertake this work
before the next rainy season.
The authors would like to thank the
Directors of the Hajar Project, Jeffery
and Jocelyn Orchard for enabling them
to undertake this work, Dr Biubwa Ali
Al-Sabry for granting permission for the
work and Ahmed Al-Tamimi and Ali
Adawy for their support with materials.
Hago, A. W., Al-Rawas, A. and Al-Harthi, A.S. “Determination of Optimum Volume
Proportions for Sarooj (Pozzolan) Lime Mixes” in Environmental and Engineering
Geoscience Vol VI, No 2, May 2000 pp 171-176
1