Important notes

Section 2.3.1 (Water)

In the book "The pyramids and temples of Gizeh", which is the notes of Sir William Matthew Flinders Petrie, he wrote following observations during his excavations and examinations of the pyramid and the surroundings:

"The levels as well as the temperatures of water in the wells of the plain close to the pyramid, and in the Nile in the distance, should also be measured through a full twelvemonth interval."  (S5-[S5]-P6-[C3]-L26)

My comments to this:
Remember the pyramid is placed on a plateau above the Nile. If Sir William Matthew Flinders Petrie could find wells with water in the surroundings, then I may assume the Pit in the Subterranean Chamber could have been filled water from time to time or even constantly 4500 years ago - maybe from the same source (groundwater).

Sir William Matthew Flinders Petrie continues when his crew had removed the sand and rubble from the entrance above the descending passageway:
"Soon after passing this granite, we got into the lower part of the entrance passage, which was clear nearly to the bottom. Here a quantity of mud had been washed in by the rains, from the decayed limestone of the outside of the Pyramid, thus filling the last 30 feet of the slope."  (S5-[S13]-P16-[C3]-L31)

Mr. Edgar had this comment in the book Great Pyramid Passages Vol I :
”Because of a statement made by Pliny (79 A.D.) to the effect that: “ Within the largest Pyramid there is a Well; the river is supposed to have been let in by it,” the Well-shaft was believed by some to lead to a reservoir of water. Doubtless it is from Pliny´s account that this most peculiar shaft became known as” The Well.“ (S1-[S547]-P279-L19)

My comments:
It seems the Subterranean Chamber and especially the pit had been wet from time to time. If you look at the dark areas in my drawing underneath, you van see the Pit at the left and a peculiar construction at the right side. 

Set ovenfra Subchamber Closest look med mlJPG

   Mr. Edgar has taken a photo of the the grey area to the right and its surroundings:

Source: The Great Pyramid Passages And Chambers" Volume 1 by John and Morton Edgar, 1910

The boy is leaning over the ridge looking into a recess and with his right arm into the grey area. It looks like an unfinished chamber, but I am not so sure. Under his right hand the corner is well done.  The grey area looks like a cavity made by purpose. I have the same argument concerning the recess. The corners and the lines are too nice made.

Underneath there is another photo taken from the same angle but from the south - east corner of the chamber. Behind the right mound is the grey marked cavity. Between the two mounds you can see a trench, please observe the trench has a slope down from the western wall towards the Pit in the bottom of the picture:

Source: The Great Pyramid Passages And Chambers" Volume 1 by John and Morton Edgar, 1910

Please see the picture underneath I have taken during my travel through Egypt in January 1988.  The mounds (above) looks like the banks of the Nile. Maybe the reason the gravel has not been removed from the two mounds is to make the complete scenario more realistic .
Nilen Jan 1988JPG

Section 2.3.2 (Light)

In the notes of work in the book  "The Pyramids And Temples of Gizeh" of Sir William Matthew Flinders Petrie in 1880 concerning the Descending passage:
... "In the passage we soon came down on the big granite stone which stopped Prof. Smyth when he was trying to clear the passage, and also sundry blocks of limestone appeared. The limestone was easily smashed then and there, and carried out piecemeal; and as it had no worked surfaces it was of no consequence. But the granite was not only tough, but interesting, and I would not let the skillful hammer-man cleave it up slice by slice as he longed to do; it was therefore blocked up in its place, with a stout board across the passage, to prevent it being started into a downward rush. It was a slab 20,6 thick worked on both faces, and one end, but rough broken around the other three sides; and as it lay flat on the floor, it left us 27 inches of height to pass down the passage over it."   (S5-[S13]-P16-[C2]-L30)

..."This slab is 32 inches wide to the broken end, and 20,6 thick; and, strangely, on one side edge is part of a drill hole, which ran through the 20,6 thickness."                                      (S5-[S13]-P16-[C3]-L5)

..."what could be the use of a 4-inch hole right through the slab?"  (S5-[S13]-P16-[C3]-L27)

My comments: 
20,6 inches = 1 cubits RM (Thickness)
32 inches = 1½ cubits RM (Width)
4 inches = 1/5 cubits RM (Hole size)
The height of the Descending passage is 2 1/4 cubits RM.  

In the book "Great Pyramid Passages" volume I by Edgar in 1910 :

"It is wonderful how much light enters the passage right to the lower end. Notwithstanding the fact that quite two-thirds of its height is cut off by the granite block on which the iron grill-door is fixed, one evening at twenty minutes to six, when we were sitting at the junction of the Descending Passage, we found it possible to read the time. As Petrie´s granite block intercept the rays of light long the floor and axis of the passage, we found it necessary to hold the watch close to the roof, against the flat square end of the passage.When we did so, we discerned the time without much difficulty. If the granite block were removed, it is probable that the light, which is very strong in Egypt, would penetrate to the lower end of the passage sufficiently to enable one to read a clearly-printed newspaper."  (S1-[S318]-P164-L3)

Section 2.3.3 (Mould - and soil?)

In the book "Great Pyramid Passages" volume I by Edgar in 1910 :
"The little horizontal passage, which leads southward from the Subterranean Chamber...  The floor of this little passage is covered throughout with a dark earthy material like mould, two to three inches deep."  (S1-[S305]-P157-L14)

My comments:
It is not likely the mould can grow on a floor of rock in a layer of 3 inches = 7,6 cm. There might have been a layer of organic material like wood or soil on which the mould could grow.

Section 2.3.4
The wooden bridge theory is supported by the source S24:
"Ship Timber and the Reuse of Wood in Ancient Egypt*" by Pearce Paul Creasman, University of Arizona in Journal of Egyptian History 6 (2013) 152–176